US20090028324A1 - Method and system for providing scrambled coded multiple access (scma) - Google Patents
Method and system for providing scrambled coded multiple access (scma) Download PDFInfo
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- US20090028324A1 US20090028324A1 US12/056,144 US5614408A US2009028324A1 US 20090028324 A1 US20090028324 A1 US 20090028324A1 US 5614408 A US5614408 A US 5614408A US 2009028324 A1 US2009028324 A1 US 2009028324A1
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M13/00—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes
- H03M13/29—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes combining two or more codes or code structures, e.g. product codes, generalised product codes, concatenated codes, inner and outer codes
- H03M13/2957—Turbo codes and decoding
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M13/00—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes
- H03M13/29—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes combining two or more codes or code structures, e.g. product codes, generalised product codes, concatenated codes, inner and outer codes
- H03M13/2903—Methods and arrangements specifically for encoding, e.g. parallel encoding of a plurality of constituent codes
Definitions
- the present invention relates to communication systems, and more particularly to coded systems.
- FDMA Frequency Division Multiple Access
- TDMA Time Division Multiple Access
- CDMA Code Division Multiple Access
- FDMA Frequency Division Multiple Access
- TDMA Time Division Multiple Access
- CDMA Code Division Multiple Access
- System performance is also aided by error control codes.
- Nearly all communication system relies on some form of error control for managing errors that may occur due to noise and other factors during transmission of information through a communication channel.
- These communications systems can include satellite systems, fiber-optic systems, cellular systems, and radio and television broadcasting systems.
- Efficient error control schemes implemented at the transmitting end of these communications systems have the capacity to enable the transmission of data including audio, video, text, etc. with very low error rates within a given signal-to-noise ratio (SNR) environment.
- SNR signal-to-noise ratio
- Powerful error control schemes also enable a communication system to achieve target error performance rates in environments with very low SNR, such as in satellite and other wireless systems where noise is prevalent and high levels of transmission power are costly, if
- CDMA Code Division Multiple Access
- turbo code provides coding gain needed to close the link.
- CDMA also allows multiple users sharing the bandwidth at the same time.
- CDMA systems typically need a large bandwidth expansion factor to function properly.
- CDMA systems require all signals accessing the same spectrum at the same time to be of equal power; provision for power control makes CDMA system more complicated to implement. The inherent long propagation delay of a satellite link makes it even more difficult.
- SDMA scrambled division multiple access
- a method comprises scrambling a first bit stream from a first terminal according to a first scrambling signature.
- the method also comprises scrambling a second bit stream from a second terminal according to a second scrambling signature, wherein the first bit stream and the second bit stream are encoded using a low rate code.
- the first scrambling signature and the second scrambling signature are assigned, respectively, to the first terminal and the second terminal to provide a multiple access scheme.
- an apparatus comprises a plurality of encoders, each encoder being configured to encode a first data stream from a first terminal and a second data stream from a second terminal using a low code rate.
- the apparatus also comprises a first scrambler configured to scramble the first bit stream from a first terminal according to a first scrambling signature.
- the apparatus further comprises a second scrambler configured to scramble the second bit stream from a second terminal according to a second scrambling signature.
- the first scrambling signature and the second scrambling signature are assigned, respectively, to the first terminal and the second terminal to provide a multiple access scheme.
- a method comprises applying joint detection and interference cancellation on a received composite signal, wherein the composite signal includes one or more encoded bit streams having a low code rate.
- the method also comprises estimating the encoded bit streams, descrambling the estimated bit streams, and decoding the descrambled bit streams.
- the method further comprises modifying the composite signal based on the decoded bit stream, and iteratively decoding bit streams of the modified composite signal.
- a system comprises a joint detector and interference canceller configured to detect and cancel interference from a received composite signal, wherein the composite signal includes one or more encoded bit streams having a low code rate.
- the system also comprises a demodulator configured to estimate the encoded bit streams, a plurality of descramblers configured to descramble the estimated bit streams, and a plurality of decoders configured to decode the bit streams.
- the system further comprises a plurality of scramblers configured to re-scramble the decoded bit streams, wherein the joint detector and interference canceller is further configured to modify the composite signal for subsequent iterative decoding by the decoders.
- FIGS. 1A and 1B are communications systems capable of providing a scrambled division multiple access (SDMA) scheme, according to various exemplary embodiments;
- SDMA scrambled division multiple access
- FIG. 2 is a diagram of a transmitter configured to operate in the systems of FIGS. 1A and 1B ;
- FIG. 3 is a diagram of a receiver configured to operate in the systems of FIGS. 1A and 1B ;
- FIGS. 4A and 4B are, respectively, a diagram of a system capable of supporting multiple transmitters using a SDMA scheme employing low rate turbo codes, and a flowchart of an associated scrambling process, according to an exemplary embodiment
- FIG. 5 is a flowchart of a process for joint detection/interference cancellation in the system of FIG. 4A , according to an exemplary embodiment
- FIG. 6 is a diagram of a scrambler, in accordance with various exemplary embodiments.
- FIG. 7 is a diagram of a turbo code encoder configured to use constituent encoders, in accordance with various exemplary embodiments
- FIG. 8 is a flowchart of a process for turbo code encoding, according to an exemplary embodiment
- FIGS. 9 and 10 are diagrams of constituent encoders configured to provide low rate turbo codes, in accordance with various exemplary embodiments.
- FIG. 11 is a graph illustrating the performance of the SDMA system of FIG. 4A .
- SCMA scrambled coded multiple access
- low-rate turbo codes Although certain embodiments of the present invention are described with respect to low-rate turbo codes, it is contemplated that these embodiments have applicability to low-rate codes in general (e.g., low density parity check (LDPC)).
- LDPC low density parity check
- FIGS. 1A and 1B are communications systems capable of providing a scrambled division multiple access (SDMA) scheme, according to various exemplary embodiments.
- a digital communications system 100 includes one or more transmitters 101 that generate signal waveforms across a communication channel 103 to one or more receivers 105 (of which one is shown).
- the transmitter 101 has a message source that produces a discrete set of possible messages; each of the possible messages has a corresponding signal waveform.
- These signal waveforms are attenuated, or otherwise altered, by communications channel 103 .
- coding is utilized. For example, forward error correction (FEC) codes can be employed.
- FEC forward error correction
- FEC Forward error correction
- RF radio frequency
- exemplary design objectives include selecting modulation formats, error control schemes, demodulation and decoding techniques and hardware components that together provide an efficient balance between system performance and implementation complexity. Differences in propagation channel characteristics, such as between terrestrial and satellite communication channels, naturally result in significantly different system designs. Likewise, existing communication systems continue to evolve in order to satisfy increased system requirements for new higher rate or higher fidelity communication services.
- Code rate is an important factor that has a significant effect on the error performance of the code.
- the choice of which code rate to operate depends on the SNR of the environment in which the codes will be deployed.
- SNR the SNR of the environment in which the codes will be deployed.
- Low SNR environments require the use of low code rates (i.e., more redundancy), whereas high SNR environments can enable the utilization of higher code rates.
- turbo codes When considering turbo codes and LDPC codes, irregular LDPC codes have been demonstrated to achieve superior performance over turbo codes for high code rates, whereas turbo codes have been demonstrated to be superior for lower code rates in low SNR environments.
- turbo codes For very low codes such as 1/6 or lower, the coding industry has focused on classical turbo code design, which can, in essence, be improved.
- turbo codes have traditionally been designed to maximize the minimum Hamming weight of systematic codewords (where the information part of the codeword has a Hamming weight of two), it is recognized that further coding improvements can be made. At relatively high SNR, this approach yields good codes since two codewords are most easily confused when their information part is differed by two bits, owing to the recursive nature of their constituent codes.
- FIG. 1B is a diagram of an exemplary meshed network capable of supporting communication among terminals with varied capabilities, according to an embodiment of the present invention.
- Satellite communications system 120 includes a satellite 121 that supports communication among multiple satellite terminals (STs) 123 , 125 and a hub 127 .
- the hub 127 may assume the role of a Network Operations Control Center (NOCC), which controls the access of the STs 123 , 125 to the network 120 and also provides element management functions and control of the address resolution and resource management functionality.
- NOCC Network Operations Control Center
- the satellite 121 in an exemplary embodiment, operates as a packet switch (e.g., at a data link layer) that provides direct unicast and multicast communication among the STs 123 , 125 .
- the STs 123 , 125 provide connectivity to one or more hosts 129 , 131 , respectively.
- the system 120 has a fully meshed architecture, whereby the STs 123 , 125 may directly communicate.
- the common air interface needs to support a discovery of the terminial's capabilities profile (or context information).
- These capabilities can include encryption scheme, compression scheme, segmentation and reassembly (SAR) scheme, automatic repeat request (ARQ) scheme, Quality-of-Service (QoS) parameters, power levels, modulation and coding schemes, power control algorithms, and link adaptation capabilities.
- terminal profile can be readily exchanged over a network with a star topology where no peer-to-peer communication exists.
- capabilities profiles include a packet data protocol (PDP) context and a mobility management context.
- PDP packet data protocol
- mobility management context the concepts of PDP context and mobility management context are combined and the term packet data protocol (PDP) context is used in general to refer to terminal capabilities. It is recognized that these terminals can be mobile as well as non-mobile.
- this PDP context for example, which can provide information about the encryption algorithm, compression algorithm, modes of data link layer communication, and physical layer transfer capabilities is combined by the transmit ST with the Quality of Service (QoS) of a pending data flow to determine a packet transfer context to use in transmission of the flow. If a PDP context has been previously established, then the sending ST can autonomously create the packet transfer context, which both satisfies the QoS of the data flow and is compatible with the receive ST capabilities.
- QoS Quality of Service
- the exchange of terminal profile can be executed over a meshed network, in a peer-to-peer manner.
- the STs 123 , 125 support the use of a negotiation procedure to determine the optimal configuration for transmission and reception of data. If a protocol implements control procedures or options in newer versions (i.e., flow-control/rate-control), older protocol versions are able to detect the initiation as a new unsupported procedure and report the same to the peer with minimal disruption in the flow of traffic.
- the ST-ST protocol advantageously takes into account that even for peers of the same version, some capabilities may not necessarily be always supported due to local temporal processing/memory/congestion-related constraints. Additionally, the ST-ST protocol design provides for rapid developments in data communication technology.
- Incompatibility between two STs is detected by the terminal that originates the traffic.
- potential misconfigurations or software incompatibilities can at least be identified, without requiring communication at the service level of the more capable ST.
- one of the STs 123 , 125 may need to be reconfigured in order to communicate with compression disabled in order to allow communication with an ST that does not support compression.
- the capability is not necessarily a function of solely configuration or software compatibility, but may also be a function of current traffic load.
- each ST 123 , 125 there exist some configuration information, including network configuration, network service provider (NSP) configuration, software configuration, and user configuration, as indicated by the NOCC 127 .
- NSP network service provider
- These configurations relate to the features that the ST 123 , 125 supports and offers to the user, and have a direct bearing on the transmission and reception capabilities.
- a packet transfer context is employed. Such a common feature set depends on the PDP contexts of the two STs 123 , 125 ; further, this common feature set may also depend on the QoS of the flow, as well as the loading and status of the two STs at that point of time.
- the packet transfer context is unidirectional and valid only for the transmit ST to send packets to the specified receive ST; thus, the packet transfer context may be unique to a given pair of STs.
- FIG. 2 is a diagram of a transmitter configured to operate in the systems of FIGS. 1A and 1B .
- a transmitter 200 is equipped with a channel encoder (e.g., turbo encoder) 201 that accepts input from an information source and outputs coded stream of higher redundancy suitable for error correction processing at the receiver (as shown in FIG. 3 ).
- the information source generates k signals from a discrete alphabet, X.
- the channel encoder 201 may utilize a combination of a constituent encoder that uses one or more constituent codes and an interleaver 203 to implement the channel coding procedure.
- Turbo codes are produced by parallel concatenation of two codes (e.g., convolutional codes) with an interleaver in between the encoders (as seen in FIG. 4 ).
- the encoder 201 generates signals from alphabet Y to a channel scrambler 203 , which scrambles the alphabet. That is, the channel scrambler 203 pseudo-randomizes the code symbols.
- the scrambled signals are fed to a modulator 205 , which maps the encoded messages from encoder 201 to signal waveforms that are transmitted to a transmit antenna 207 .
- the antenna 207 emits these waveforms over the communication channel 103 . Accordingly, the encoded messages are modulated and distributed to a transmit antenna 207 .
- the transmissions from the transmit antenna 207 propagate to a receiver, as discussed below.
- FIG. 3 is a diagram of a receiver configured to operate in the systems of FIGS. 1A and 1B .
- a receiver 300 includes an antenna 301 that receives the waveforms emitted over the channel 103 .
- the receiver 300 provides a demodulator 303 that performs demodulation of the received signals. After demodulation, the received signals are forwarded to a channel descrambler 305 to unscramble the symbols.
- a decoder 307 then attempts to reconstruct the original source messages.
- the above transmitter 200 and receiver 300 can be deployed in within a single wireless terminal, in which case a common antenna system can be shared.
- the wireless terminal can for example be configured to operate within a satellite communication, a cellular system, wireless local area network (WLAN), etc.;
- FIGS. 4A and 4B are, respectively, a diagram of a system capable of supporting multiple transmitters using a SDMA scheme employing low rate turbo codes, and a flowchart of an associated scrambling process, according to an exemplary embodiment.
- a communication system 400 supports multiple terminals (i.e., users) configured with respective encoders 401 a - 401 n and scramblers 403 a - 403 n .
- these terminals can be the transmitter 200 and the receiver 300 of FIGS. 2 and 3 , respectively, operating in the satellite system 120 of FIG. 1B .
- the system 400 provides a multiple access scheme, such as SCMA, which achieves good performance with relatively lower receiver complexity compared to CDMA (as the number of users that share the same channel increases).
- SCMA multiple access scheme
- each user sharing the transmission channel is separated by user specific and scramblers 403 a - 403 n .
- a fraction of a satellite transponder is needed under the SCMA scheme, thereby lowering the operating cost.
- SCMA Interleave-Division Multiple Access
- IDMA Interleave-Division Multiple Access
- the low-rate turbo decoders in SCMA are much more straightforward to implement since turbo-Hadamard codes require the decoding of Hadamard codes in addition to the decoding of turbo-like codes.
- all the users can utilize the same scrambler hardware with different initial vector (also known as “seed”), instead of different interleaver design. Further, using scrambling sequences as signatures is simpler than random interleaver-based signatures.
- each of turbo encoders 401 a - 401 n utilizes the same turbo codes.
- the turbo encoded sequences are then fed to the respective user-specific scramblers 403 a - 403 n .
- the scrambled sequences are then transmitted over channel 405 to a receiver 300 , which includes a joint detector/interference canceller unit 407 that interacts with the turbo decoders 413 a - 413 n to iteratively produce an estimate of the received codewords.
- the turbo decoder 413 a - 413 n produces a better estimate to the joint detector/interference canceller 407 for achieving better cancellation.
- turbo decoders 413 a - 413 n The information exchanged between turbo decoders 413 a - 413 n and the joint detector/interference canceller 407 is scrambled or descrambled via scramblers 409 a - 409 n or de-scramblers 411 a - 411 n , respectively. Once “good” estimates of the decoded sequences are produced, they are output from the turbo decoders 413 a - 413 n.
- the joint-detection/interference canceller 407 does not require all the signals accessing the same spectrum at the same time to be of equal power. In fact, the performance is better when the signals are of different power level. Thus, no tight power controls are needed. Also due to joint-detection/interference cancellation, the system 400 provides a scheme that is much more robust against Rician fading, which makes it particularly more attractive for small mobile terminals experiencing Rician multipath fading.
- the system 400 requires less power to transmit data at the same speed vis-à-vis a CDMA system.
- the system 400 can be operated in a random access manner and does not require reservation of time slots, which minimize the delay to one satellite round trip. Additionally, the system 400 , as mentioned, does not require tight power control, minimizing the coordination needed between transmitter 200 and receiver 300 .
- potential applications will be for mobile or aeronautical terminals. It may also have applications to enable direct broadcast satellite (DBS) operators to provide return link over satellite via a commercial satellite using existing antenna systems.
- DBS direct broadcast satellite
- Each user encodes its data with, for example, a rate 1/n FEC, where n is an integer larger than 3.
- the coded bits are then scrambled with a unique scrambling sequence and transmitted.
- the number of unique sequences are virtually unlimited with common sequence generators, such as the Gold sequences.
- the same generator can generate all the sequences, which are differentiated by the initial vector. It is noted that other low rates can be utilized, m/n (e.g., less than 1/3).
- the scrambling sequence can be generated by selecting a pseudorandom number sequence (e.g., Gold sequence) whose period is greater than the code block.
- a pseudorandom number sequence e.g., Gold sequence
- the respective user uses the corresponding de-scrambler and a rate 1/n decoder to retrieve its data.
- the signals are modulated by the same type of modulation, e.g., QPSK, of the same bandwidth, centered at the same frequency and transmitted at the same time (e.g., similar to CDMA).
- the antennas can be shared.
- each terminal encodes data using the corresponding turbo encoder (e.g., 401 a - 401 n ).
- the encoded data is then scrambled by the respective scramblers 1 . . . U (e.g., 403 a - 403 n ) and transmitted to the receiver 300 , per steps 423 and 425 .
- the received signal is processed by the joint detector/interference canceller 407 and undergoes descrambling and re-scrambling, as in step 427 .
- the descrambling and re-scrambling is performed in conjunction with the decoding process, which outputs decoded data (step 429 ).
- FIG. 5 is a flowchart of a process for joint detection/interference cancellation in the system of FIG. 4A , according to an exemplary embodiment.
- a key enabler for this communication system 400 is the joint-detection/interference cancellation receiver.
- This receiver 300 includes the descramblers 409 a - 409 n and the decoders 413 a - 413 n as well as all the signal estimators and interference reducers for each of the individual signal paths.
- the receiver 300 includes a buffer (not shown) to store a complete block of the composite signal.
- the receiver 300 employs joint detection/estimation; it is contemplated that any joint-detection/estimation technique may be used.
- the receiver 300 operates iteratively to output the bit streams represented by the composite signal.
- the receiver 300 first uses the first descrambler (e.g., descrambler 409 a ) and a turbo decoder (e.g., decoder 413 a ) to estimate the first bit-stream.
- the composite signal is modified accordingly.
- only one pass of the turbo decoding is performed.
- the interference reducer then operates on the stored waveform given the result of the first pass decoding of the first signal.
- the receiver 300 uses the second descrambler (e.g., descrambler 409 b ) and turbo decoder (e.g., decoder 413 b ) to estimate the second bit-stream, and so on.
- the receiver 300 uses the second descrambler (e.g., descrambler 409 b ) and turbo decoder (e.g., decoder 413 b ) to estimate the second bit-stream, and so on.
- the receiver 300 uses the second descrambler (e.g., descrambler 409 b ) and turbo decoder (e.g., decoder 413 b ) to estimate the second bit-stream, and so on.
- all the paths can be processed in-parallel for each pass; this approach may entail more passes than the above process.
- Signal estimation via a demodulator (not shown), plays an important role. In most applications of interest, this demodulator must operate at very low signal-to-noise plus interference ratio. In one embodiment, the demodulator is aided by two features: synchronization, and joint detection.
- the initial synchronization involves use of a known pilot, which can be introduced using anyone of the techniques known in the art. For example, known pilot symbols can be introduced by multiplexing them into the data stream, or pilot symbols may be introduced by puncturing the encoder output. Just as each encoder 401 a - 401 n employs a different scrambling signature, each may employ a different pilot symbol pattern, thereby minimizing interference effects.
- the signals are transmitted in a burst mode.
- the demodulator is configured to detect the burst arrival times by using, for example, a “Unique Word” pattern. It is recognized that any other well-known techniques may be used for this purpose.
- the Unique Word patterns of the various encoders may or may not be distinct.
- this process involves iterative refinement of the demodulation. As the iteration progresses, the demodulation is improved through two techniques. First, as interference is removed, the estimation of signal parameters (e.g., frequency, symbol timing, carrier phase) is improved. Secondly, as more reliable estimates of the data symbols become available from the turbo decoders 413 a - 413 n , these are used to improve the demodulator performance.
- signal parameters e.g., frequency, symbol timing, carrier phase
- FIG. 6 is a diagram of a scrambler, in accordance with various exemplary embodiments.
- a scrambler 601 receives a codeword (e.g., “0 0 1 0 1 0”) and a scrambling sequence (e.g., “1 0 1 0 1 1”).
- the scrambling sequence can be a Gold sequence or any pseudorandom number sequence.
- Gold codes exhibit a number of characteristics. In addition to being simple to generate, these sequences contain roughly an equal number of zeros and ones, and are approximately orthogonal when delayed or shifted. Also, they are orthogonal to other codes.
- Gold sequences can be generated using feedback shift registers, whose outputs are added to produce the Gold codes.
- the codeword and scrambling sequence are combined by adder 603 to output a transmitted sequence (e.g., “1 0 0 0 0 1”).
- scramblers as opposed to interleavers
- use of scramblers reduces complexity.
- seed initial vector
- FIG. 7 is a diagram of a turbo code encoder configured to use constituent encoders, in accordance with various exemplary embodiments.
- turbo encoder 201 employs two constituent encoders 701 , 703 and an interleaver 705 . Although two encoders 701 , 703 are described in this scenario, the encoder 201 can provide more than two encoders to achieve various code rates. As seen in FIG. 7 , the turbo encoder 201 can optionally output the information (i.e., systematic) bit as part of the output code, depending on the design of the constituent encoders 701 , 703 and the code rates. The operation of the turbo code encoder 201 is explained with respect to FIG. 8 , as follows.
- FIG. 8 is a flowchart of a process for turbo code encoding, according to an exemplary embodiment.
- Information bits that are to be turbo code encoded are fed to both constituent encoders 701 and 703 (step 801 ).
- the bits that are fed to constituent encoder 703 are, however, interleaved by interleaver 705 prior to being input to constituent encoder 703 , as in steps 803 - 807 .
- steps 809 and 811 once both bit streams are encoded, the output of constituent encoders 701 , 703 is punctured to achieve the desired code rate.
- the output of turbo encoder 201 can contain the unpunctured bits at the output of constituent encoders 701 , 703 and, alternatively, the information bits that have not undergone any processing, as illustrated in FIG. 7 .
- FIGS. 9 and 10 are diagrams of constituent encoders configured to provide low rate turbo codes, in accordance with various exemplary embodiments.
- a constituent encoder 900 provides code rates of 1/14, 1/12, 1/10, 1/8, 1/6, and 1/4. When implemented in the turbo encoder 201 of FIG. 7 , the encoder 201 does not output the information bits given these code rates. For rate 1/14 turbo code, all seven parity bits of the constituent code are transmitted.
- Y 6 is punctured from both constituent codes, for rate 1/10 turbo code Y 5 and Y 6 are punctured, for rate 1/8 turbo code, Y 4 , Y 5 and Y 6 are punctured, for rate 1/6 turbo code Y 3 , Y 4 , Y 5 and Y 6 are punctured and for rate 1/4 turbo code, Y 2 , Y 3 and Y 4 , Y 5 and Y 6 are punctured from both constituent codes.
- the transfer function of the constituent encoder 900 is given as:
- G ⁇ ( D ) [ n 0 ⁇ ( D ) d ⁇ ( D ) , n 1 ⁇ ( D ) d ⁇ ( D ) , ... ⁇ ⁇ n 6 ⁇ ( D ) d ⁇ ( D ) ]
- the logic or circuitry for the encoder 900 encompasses adders 901 , 903 , 905 , 907 , 909 , 911 , 913 , and 915 and shift registers 917 , 919 and 921 .
- Modular adder 901 receives the data input, adding it together with the output of registers 917 and 921 .
- the output of adder 901 produces the parity bit Y 6 and is also fed into adders 903 , 911 , 909 and 905 .
- Modular adder 903 sums the signals from adder 901 and register 917 , resulting in the parity bit Y 4 ; the summed value is additionally provided to adder 907 .
- Adder 905 receives inputs from register 919 and adder 901 , and produces the parity bit Y 5 at its output.
- adder 907 sums the values from adder 903 and register 919 to generate parity bit Y 3 .
- Adder 909 receives inputs from register 919 and adder 901 to produce a value, which is then fed into adder 913 .
- Adder 911 adds the value from register 921 as well as the value from adder 901 ; the resultant value is the parity bit Y 2 .
- this adder 913 produces parity bit Y 0 from the summation of a value from adder 909 and register 921 .
- Adder 915 receives inputs from adder 913 and register 921 to generate parity bit Y 1 .
- FIG. 10 shows a block diagram of a constituent encoder 1000 for achieving varying code rates, such as 1/15.
- the transfer function of the constituent encoder 1000 is as follows:
- G ⁇ ( D ) [ n 0 ⁇ ( D ) d ⁇ ( D ) , n 1 ⁇ ( D ) d ⁇ ( D ) , ... ⁇ ⁇ n 6 ⁇ ( D ) d ⁇ ( D ) ]
- the constituent encoder 1000 has circuitry that includes modular adders 1001 , 1003 , 1005 , 1007 , 1009 , 1011 , 1013 , 1015 , and 1017 and shift registers 1019 , 1021 , and 1023 .
- modular adder 1001 adds the output of register 1023 with the data input and generates parity bit Y 0 , which is also supplied into adders 1005 , 1003 , 1011 , and 1007 .
- Adder 1003 receives input from adder 1001 and register 1019 , summing these inputs for a resultant value that is fed into adder 1013 .
- signals from adder 1001 and register 1019 are summed to yield parity bit Y 6 —which is also fed into adder 1009 .
- Adder 1007 adds the input from adder 1001 and the input from register 1021 , to provide parity bit Y 1 .
- Parity bit Y 5 can be generated through the summation, at adder 1009 , of the values from adder 1005 and register 1021 .
- Adder 1011 inputs from adder 1001 and register 1021 are added by adder 1011 and provided to adder 1015 .
- Adder 1013 receives input from adder 1003 and register 1023 to generate parity bit Y 4 .
- Parity bit Y 2 is formed by the addition of the result from adder 1011 and content of register 1023 .
- Adder 1017 receives inputs from adder 1015 and register 1019 and produces parity bit Y 3 at its output.
- the turbo encoder 201 of FIG. 7 outputs the systematic bit, by contrast to the constituent encoders 900 , 1000 of FIGS. 9 and 10 , respectively, in which no information bit is output by the turbo encoder 201 .
- the low rate turbo codes described herein have been developed by targeting minimum Hamming weight corresponding to information sequences with Hamming weight more than two. Using this approach, a search for new turbo codes was conducted using a uniform interleaving assumption and constraining the Hamming weight of input sequences to a maximum of six, for instance.
- FIG. 11 is a graph illustrating the performance of the SDMA system of FIG. 4 .
- the performance of system 400 is compared against Shannon's constrained capacity for QPSK modulation as a function of the aggregated signal power per symbol time over one-sided noise spectral density in an Additive White Gaussian Noise (AWGN) channel (e.g., using a rate 1/10 code).
- AWGN Additive White Gaussian Noise
- a relatively short 1000 information bit frame length is used. As seen, up to about five simultaneous overlapping transmission that are close to a theoretically best with unlimited processing.
- the corresponding energy per symbol over one-sided noise spectral density ratio (Es/No) for each individual user terminal and the energy per information bit to one-sided noise spectral density ratio (Ebi/No) to achieve 10 ⁇ 5 frame error rate is shown in Table 1, as the aggregated power are evenly divided to each user terminal.
- the approach of system 400 can be applied for both the forward link and the return link.
- the advantage is mainly the lower Ebi/No that is required by the lower rate code is lower than required by the rate 1/3 or rate 1/4 code previously used.
- the lower operating threshold allows the terminal to accept more adjacent satellite interference, which is often the dominating performance limitation for small terminals.
- the advantage of the low-rate code can be applied to reduced receive antenna size, or higher bit rate.
- the combination of low-rate code and SCMA allows the terminal to operate autonomously with minimum coordination with the hub. Unlike conventional CDMA, interference cancellation operates better when the terminals are not operating at exactly the same power, tight power control is in fact not desirable.
- the bandwidth expansion from the low-rate coding serves two purposes reduction of emission spectral density from regulator standpoint; and additional coding gain.
- the processes described herein for generating low rate codes and scrambling may be implemented via software, hardware (e.g., general processor, Digital Signal Processing (DSP) chip, an Application Specific Integrated Circuit (ASIC), Field Programmable Gate Arrays (FPGAs), etc.), firmware or a combination thereof.
- DSP Digital Signal Processing
- ASIC Application Specific Integrated Circuit
- FPGA Field Programmable Gate Arrays
Abstract
Description
- This application is related to, and claims the benefit of the earlier filing date under 35 U.S.C. § 119(e) of, U.S. Provisional Patent Application (Ser. No. 60/908,340) filed Mar. 27, 2007 (Attorney Docket: 115426-1147), entitled “Efficient USAT Transmission Using Low-Rate Turbo Codes and Scrambled Coded Multiple Access (SCMA) Techniques”; the entirety of which is incorporated herein by reference.
- The present invention relates to communication systems, and more particularly to coded systems.
- Multiple access schemes are employed by modern radio systems to allow multiple users to share a limited amount of bandwidth, while maintaining acceptable system performance. Common multiple access schemes include Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). System performance is also aided by error control codes. Nearly all communication system relies on some form of error control for managing errors that may occur due to noise and other factors during transmission of information through a communication channel. These communications systems can include satellite systems, fiber-optic systems, cellular systems, and radio and television broadcasting systems. Efficient error control schemes implemented at the transmitting end of these communications systems have the capacity to enable the transmission of data including audio, video, text, etc. with very low error rates within a given signal-to-noise ratio (SNR) environment. Powerful error control schemes also enable a communication system to achieve target error performance rates in environments with very low SNR, such as in satellite and other wireless systems where noise is prevalent and high levels of transmission power are costly, if even feasible.
- Thus, a broad class of powerful error control schemes that enable reliable transmission of information have emerged including convolutional codes, low density parity check (LDPC) codes, and turbo codes. Both LDPC codes as well as some classes of turbo codes have been successfully demonstrated to approach near the theoretical bound (i.e., Shannon limit). Although long constraint length convolutional codes can also approach the Shannon limit, decoder design complexity prevents practical, wide spread adoption. LDPC codes and turbo codes, on the other hand, can achieve low error rates with lower complexity decoders. Consequently, these codes have garnered significant attention.
- For example, conventional data transmission to and from an ultra small terminal via satellite is usually based on Code Division Multiple Access (CDMA)
technique using rate 1/2 or 1/3 turbo codes. CDMA spreads bandwidth to reduce the interference to adjacent satellites, whereas the turbo code provides coding gain needed to close the link. CDMA also allows multiple users sharing the bandwidth at the same time. However, CDMA systems typically need a large bandwidth expansion factor to function properly. Additionally, CDMA systems require all signals accessing the same spectrum at the same time to be of equal power; provision for power control makes CDMA system more complicated to implement. The inherent long propagation delay of a satellite link makes it even more difficult. - Based on the foregoing, there is a need for an access scheme that can effectively utilize low code rates, while minimizing complexity.
- These and other needs are addressed by the present invention, wherein a scrambled division multiple access (SDMA) scheme is provided.
- According to one aspect of an exemplary embodiment, a method comprises scrambling a first bit stream from a first terminal according to a first scrambling signature. The method also comprises scrambling a second bit stream from a second terminal according to a second scrambling signature, wherein the first bit stream and the second bit stream are encoded using a low rate code. The first scrambling signature and the second scrambling signature are assigned, respectively, to the first terminal and the second terminal to provide a multiple access scheme.
- According to another aspect of an exemplary embodiment, an apparatus comprises a plurality of encoders, each encoder being configured to encode a first data stream from a first terminal and a second data stream from a second terminal using a low code rate. The apparatus also comprises a first scrambler configured to scramble the first bit stream from a first terminal according to a first scrambling signature. The apparatus further comprises a second scrambler configured to scramble the second bit stream from a second terminal according to a second scrambling signature. The first scrambling signature and the second scrambling signature are assigned, respectively, to the first terminal and the second terminal to provide a multiple access scheme.
- According to another aspect of an exemplary embodiment, a method comprises applying joint detection and interference cancellation on a received composite signal, wherein the composite signal includes one or more encoded bit streams having a low code rate. The method also comprises estimating the encoded bit streams, descrambling the estimated bit streams, and decoding the descrambled bit streams. The method further comprises modifying the composite signal based on the decoded bit stream, and iteratively decoding bit streams of the modified composite signal.
- According to yet another aspect of an exemplary embodiment, a system comprises a joint detector and interference canceller configured to detect and cancel interference from a received composite signal, wherein the composite signal includes one or more encoded bit streams having a low code rate. The system also comprises a demodulator configured to estimate the encoded bit streams, a plurality of descramblers configured to descramble the estimated bit streams, and a plurality of decoders configured to decode the bit streams. The system further comprises a plurality of scramblers configured to re-scramble the decoded bit streams, wherein the joint detector and interference canceller is further configured to modify the composite signal for subsequent iterative decoding by the decoders.
- Still other aspects, features, and advantages of the present invention are readily apparent from the following detailed description, simply by illustrating a number of particular embodiments and implementations, including the best mode contemplated for carrying out the present invention. The present invention is also capable of other and different embodiments, and its several details can be modified in various obvious respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.
- The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:
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FIGS. 1A and 1B are communications systems capable of providing a scrambled division multiple access (SDMA) scheme, according to various exemplary embodiments; -
FIG. 2 is a diagram of a transmitter configured to operate in the systems ofFIGS. 1A and 1B ; -
FIG. 3 is a diagram of a receiver configured to operate in the systems ofFIGS. 1A and 1B ; -
FIGS. 4A and 4B , are, respectively, a diagram of a system capable of supporting multiple transmitters using a SDMA scheme employing low rate turbo codes, and a flowchart of an associated scrambling process, according to an exemplary embodiment; -
FIG. 5 is a flowchart of a process for joint detection/interference cancellation in the system ofFIG. 4A , according to an exemplary embodiment; -
FIG. 6 is a diagram of a scrambler, in accordance with various exemplary embodiments; -
FIG. 7 is a diagram of a turbo code encoder configured to use constituent encoders, in accordance with various exemplary embodiments; -
FIG. 8 is a flowchart of a process for turbo code encoding, according to an exemplary embodiment; -
FIGS. 9 and 10 are diagrams of constituent encoders configured to provide low rate turbo codes, in accordance with various exemplary embodiments; and -
FIG. 11 is a graph illustrating the performance of the SDMA system ofFIG. 4A . - A method, system, and software for providing a scrambled coded multiple access (SCMA) scheme is described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It is apparent, however, that the invention may be practiced without these specific details or with an equivalent arrangement. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the invention.
- Although certain embodiments of the present invention are described with respect to low-rate turbo codes, it is contemplated that these embodiments have applicability to low-rate codes in general (e.g., low density parity check (LDPC)).
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FIGS. 1A and 1B are communications systems capable of providing a scrambled division multiple access (SDMA) scheme, according to various exemplary embodiments. Adigital communications system 100 includes one ormore transmitters 101 that generate signal waveforms across acommunication channel 103 to one or more receivers 105 (of which one is shown). In thisdiscrete communications system 100, thetransmitter 101 has a message source that produces a discrete set of possible messages; each of the possible messages has a corresponding signal waveform. These signal waveforms are attenuated, or otherwise altered, bycommunications channel 103. To combat thenoise channel 103, coding is utilized. For example, forward error correction (FEC) codes can be employed. - Forward error correction (FEC) is required in terrestrial and satellite systems to provide high quality communication over a radio frequency (RF) propagation channel, which induces signal waveform and spectrum distortions, including signal attenuation (freespace propagation loss) and multi-path induced fading. These impairments drive the design of the radio transmission and receiver equipment; exemplary design objectives include selecting modulation formats, error control schemes, demodulation and decoding techniques and hardware components that together provide an efficient balance between system performance and implementation complexity. Differences in propagation channel characteristics, such as between terrestrial and satellite communication channels, naturally result in significantly different system designs. Likewise, existing communication systems continue to evolve in order to satisfy increased system requirements for new higher rate or higher fidelity communication services.
- Code rate is an important factor that has a significant effect on the error performance of the code. The choice of which code rate to operate, in turn, depends on the SNR of the environment in which the codes will be deployed. Traditionally, low SNR environments require the use of low code rates (i.e., more redundancy), whereas high SNR environments can enable the utilization of higher code rates. There is a continual challenge to devise codes that edge closer to the Shannon limit, while minimizing complexity.
- When considering turbo codes and LDPC codes, irregular LDPC codes have been demonstrated to achieve superior performance over turbo codes for high code rates, whereas turbo codes have been demonstrated to be superior for lower code rates in low SNR environments. For very low codes such as 1/6 or lower, the coding industry has focused on classical turbo code design, which can, in essence, be improved. Because turbo codes have traditionally been designed to maximize the minimum Hamming weight of systematic codewords (where the information part of the codeword has a Hamming weight of two), it is recognized that further coding improvements can be made. At relatively high SNR, this approach yields good codes since two codewords are most easily confused when their information part is differed by two bits, owing to the recursive nature of their constituent codes.
- However, for very low SNR where low code rates are traditionally used, an investigation of the erroneous turbo code frames reveals that the number of errors in the information part, is in general, more than two. This observation suggests that by targeting minimum Hamming weight corresponding to information sequences with Hamming weight more than two, the performance of low code rates, in principle, can be enhanced. With improved design, low rate turbo codes can approach the Sham-on limit more closely, resulting in a variety of advantages for communication systems such as extended battery lifetime within cellular networks, lower transmit power within satellite communication and broadcasting systems, etc.
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FIG. 1B is a diagram of an exemplary meshed network capable of supporting communication among terminals with varied capabilities, according to an embodiment of the present invention. -
Satellite communications system 120 includes asatellite 121 that supports communication among multiple satellite terminals (STs) 123, 125 and ahub 127. Thehub 127 may assume the role of a Network Operations Control Center (NOCC), which controls the access of theSTs network 120 and also provides element management functions and control of the address resolution and resource management functionality. Thesatellite 121, in an exemplary embodiment, operates as a packet switch (e.g., at a data link layer) that provides direct unicast and multicast communication among theSTs STs more hosts system 120 has a fully meshed architecture, whereby theSTs - As previously discussed, a system in which terminals are deployed, particularly a satellite system, incompatibility problems may arise if different “generations” of terminals exist, in which one ST employs older hardware and/or software technologies than the other.
- For newer, highly capable terminals to communicate with older (typically) less capable terminals, an exchange of information regarding the capabilities among the communicating terminals is needed. Specifically, the common air interface needs to support a discovery of the terminial's capabilities profile (or context information). These capabilities can include encryption scheme, compression scheme, segmentation and reassembly (SAR) scheme, automatic repeat request (ARQ) scheme, Quality-of-Service (QoS) parameters, power levels, modulation and coding schemes, power control algorithms, and link adaptation capabilities.
- Under a conventional approach, terminal profile can be readily exchanged over a network with a star topology where no peer-to-peer communication exists. For example, in the General Packet Radio Service (GPRS)/Universal Mobile Telecommunications System (UMTS) family of protocols, such capabilities profiles include a packet data protocol (PDP) context and a mobility management context. In an embodiment of the present invention, the concepts of PDP context and mobility management context are combined and the term packet data protocol (PDP) context is used in general to refer to terminal capabilities. It is recognized that these terminals can be mobile as well as non-mobile. In an exemplary embodiment, this PDP context, for example, which can provide information about the encryption algorithm, compression algorithm, modes of data link layer communication, and physical layer transfer capabilities is combined by the transmit ST with the Quality of Service (QoS) of a pending data flow to determine a packet transfer context to use in transmission of the flow. If a PDP context has been previously established, then the sending ST can autonomously create the packet transfer context, which both satisfies the QoS of the data flow and is compatible with the receive ST capabilities.
- According to one embodiment, the exchange of terminal profile can be executed over a meshed network, in a peer-to-peer manner. The
STs - The ST-ST protocol advantageously takes into account that even for peers of the same version, some capabilities may not necessarily be always supported due to local temporal processing/memory/congestion-related constraints. Additionally, the ST-ST protocol design provides for rapid developments in data communication technology.
- Incompatibility between two STs is detected by the terminal that originates the traffic. Thus, potential misconfigurations or software incompatibilities can at least be identified, without requiring communication at the service level of the more capable ST. For example, one of the
STs - For each
ST NOCC 127. These configurations relate to the features that theST - To facilitate the flow of data from one
peer ST 123 to anotherST 125 of possibly different generations equipped with different capabilities, a packet transfer context is employed. Such a common feature set depends on the PDP contexts of the twoSTs -
FIG. 2 is a diagram of a transmitter configured to operate in the systems ofFIGS. 1A and 1B . As seen inFIG. 2 , atransmitter 200 is equipped with a channel encoder (e.g., turbo encoder) 201 that accepts input from an information source and outputs coded stream of higher redundancy suitable for error correction processing at the receiver (as shown inFIG. 3 ). The information source generates k signals from a discrete alphabet, X. Thechannel encoder 201 may utilize a combination of a constituent encoder that uses one or more constituent codes and aninterleaver 203 to implement the channel coding procedure. Turbo codes are produced by parallel concatenation of two codes (e.g., convolutional codes) with an interleaver in between the encoders (as seen inFIG. 4 ). - Essentially, the
encoder 201 generates signals from alphabet Y to achannel scrambler 203, which scrambles the alphabet. That is, thechannel scrambler 203 pseudo-randomizes the code symbols. The scrambled signals are fed to amodulator 205, which maps the encoded messages fromencoder 201 to signal waveforms that are transmitted to a transmitantenna 207. - The
antenna 207 emits these waveforms over thecommunication channel 103. Accordingly, the encoded messages are modulated and distributed to a transmitantenna 207. - The transmissions from the transmit
antenna 207 propagate to a receiver, as discussed below. -
FIG. 3 is a diagram of a receiver configured to operate in the systems ofFIGS. 1A and 1B . At the receiving side, areceiver 300 includes anantenna 301 that receives the waveforms emitted over thechannel 103. Thereceiver 300 provides a demodulator 303 that performs demodulation of the received signals. After demodulation, the received signals are forwarded to achannel descrambler 305 to unscramble the symbols. Adecoder 307 then attempts to reconstruct the original source messages. - It is contemplated that the
above transmitter 200 andreceiver 300 can be deployed in within a single wireless terminal, in which case a common antenna system can be shared. The wireless terminal can for example be configured to operate within a satellite communication, a cellular system, wireless local area network (WLAN), etc.; -
FIGS. 4A and 4B , are, respectively, a diagram of a system capable of supporting multiple transmitters using a SDMA scheme employing low rate turbo codes, and a flowchart of an associated scrambling process, according to an exemplary embodiment. For the purposes of illustration, acommunication system 400 supports multiple terminals (i.e., users) configured with respective encoders 401 a-401 n and scramblers 403 a-403 n. In an exemplary embodiment, these terminals can be thetransmitter 200 and thereceiver 300 ofFIGS. 2 and 3 , respectively, operating in thesatellite system 120 ofFIG. 1B . - By way of example, the
system 400 provides a multiple access scheme, such as SCMA, which achieves good performance with relatively lower receiver complexity compared to CDMA (as the number of users that share the same channel increases). With SCMA, each user sharing the transmission channel is separated by user specific and scramblers 403 a-403 n. Also, due to lack of spreading factor and more efficient FEC coding, a fraction of a satellite transponder is needed under the SCMA scheme, thereby lowering the operating cost. - By using low rate codes, the
system 400 can achieve greater power efficiency while spreading the spectrum, whereas conventional CDMA does not. Additionally, SCMA is different from another multiple access technique called Interleave-Division Multiple Access (IDMA) which also spreads with low-rate turbo-Hadamard codes but uses random interleavers as user signature. The low-rate turbo decoders in SCMA are much more straightforward to implement since turbo-Hadamard codes require the decoding of Hadamard codes in addition to the decoding of turbo-like codes. Also, all the users can utilize the same scrambler hardware with different initial vector (also known as “seed”), instead of different interleaver design. Further, using scrambling sequences as signatures is simpler than random interleaver-based signatures. - In one embodiment, each of turbo encoders 401 a-401 n utilizes the same turbo codes. The turbo encoded sequences are then fed to the respective user-specific scramblers 403 a-403 n. The scrambled sequences are then transmitted over
channel 405 to areceiver 300, which includes a joint detector/interference canceller unit 407 that interacts with the turbo decoders 413 a-413 n to iteratively produce an estimate of the received codewords. With each iteration, the turbo decoder 413 a-413 n produces a better estimate to the joint detector/interference canceller 407 for achieving better cancellation. The information exchanged between turbo decoders 413 a-413 n and the joint detector/interference canceller 407 is scrambled or descrambled via scramblers 409 a-409 n or de-scramblers 411 a-411 n, respectively. Once “good” estimates of the decoded sequences are produced, they are output from the turbo decoders 413 a-413 n. - Unlike conventional CDMA systems, the joint-detection/
interference canceller 407 does not require all the signals accessing the same spectrum at the same time to be of equal power. In fact, the performance is better when the signals are of different power level. Thus, no tight power controls are needed. Also due to joint-detection/interference cancellation, thesystem 400 provides a scheme that is much more robust against Rician fading, which makes it particularly more attractive for small mobile terminals experiencing Rician multipath fading. - Therefore, the
system 400, as a SCMA system using low-rate FEC coding, requires less power to transmit data at the same speed vis-à-vis a CDMA system. In one embodiment, thesystem 400 can be operated in a random access manner and does not require reservation of time slots, which minimize the delay to one satellite round trip. Additionally, thesystem 400, as mentioned, does not require tight power control, minimizing the coordination needed betweentransmitter 200 andreceiver 300. By way of example, potential applications will be for mobile or aeronautical terminals. It may also have applications to enable direct broadcast satellite (DBS) operators to provide return link over satellite via a commercial satellite using existing antenna systems. - Each user encodes its data with, for example, a
rate 1/n FEC, where n is an integer larger than 3. The coded bits are then scrambled with a unique scrambling sequence and transmitted. The number of unique sequences are virtually unlimited with common sequence generators, such as the Gold sequences. The same generator can generate all the sequences, which are differentiated by the initial vector. It is noted that other low rates can be utilized, m/n (e.g., less than 1/3). - In an exemplary embodiment, the scrambling sequence can be generated by selecting a pseudorandom number sequence (e.g., Gold sequence) whose period is greater than the code block. On the receiver side, the respective user uses the corresponding de-scrambler and a
rate 1/n decoder to retrieve its data. The signals are modulated by the same type of modulation, e.g., QPSK, of the same bandwidth, centered at the same frequency and transmitted at the same time (e.g., similar to CDMA). Typically, for receivers located in a hub of a star-shaped network, the antennas can be shared. - The
system 400 operates as follows. Instep 421, each terminal encodes data using the corresponding turbo encoder (e.g., 401 a-401 n). The encoded data is then scrambled by therespective scramblers 1 . . . U (e.g., 403 a-403 n) and transmitted to thereceiver 300, persteps interference canceller 407 and undergoes descrambling and re-scrambling, as instep 427. The descrambling and re-scrambling is performed in conjunction with the decoding process, which outputs decoded data (step 429). -
FIG. 5 is a flowchart of a process for joint detection/interference cancellation in the system ofFIG. 4A , according to an exemplary embodiment. A key enabler for thiscommunication system 400 is the joint-detection/interference cancellation receiver. Thisreceiver 300 includes the descramblers 409 a-409 n and the decoders 413 a-413 n as well as all the signal estimators and interference reducers for each of the individual signal paths. In addition, thereceiver 300 includes a buffer (not shown) to store a complete block of the composite signal. Thereceiver 300 employs joint detection/estimation; it is contemplated that any joint-detection/estimation technique may be used. In an exemplary embodiment, thereceiver 300 operates iteratively to output the bit streams represented by the composite signal. - As seen in
FIG. 5 , in steps 501-507, once an entire block of composite waveform is sampled and stored in the buffer, thereceiver 300 first uses the first descrambler (e.g.,descrambler 409 a) and a turbo decoder (e.g.,decoder 413 a) to estimate the first bit-stream. Instep 509, the composite signal is modified accordingly. In this example, only one pass of the turbo decoding is performed. The interference reducer then operates on the stored waveform given the result of the first pass decoding of the first signal. Thereceiver 300 then uses the second descrambler (e.g.,descrambler 409 b) and turbo decoder (e.g.,decoder 413 b) to estimate the second bit-stream, and so on. When all the bit-streams have been estimated once (as determined in step 511), thereceiver 300 than returns to process the first bit-stream in a second pass. - When all the bit-streams have been processed for the required number passes (steps 513-517), all the bit-streams are completely estimated and decoded.
- Alternatively, in another embodiment, all the paths can be processed in-parallel for each pass; this approach may entail more passes than the above process.
- Signal estimation, via a demodulator (not shown), plays an important role. In most applications of interest, this demodulator must operate at very low signal-to-noise plus interference ratio. In one embodiment, the demodulator is aided by two features: synchronization, and joint detection. The initial synchronization involves use of a known pilot, which can be introduced using anyone of the techniques known in the art. For example, known pilot symbols can be introduced by multiplexing them into the data stream, or pilot symbols may be introduced by puncturing the encoder output. Just as each encoder 401 a-401 n employs a different scrambling signature, each may employ a different pilot symbol pattern, thereby minimizing interference effects.
- In one embodiment, the signals are transmitted in a burst mode. Accordingly, the demodulator is configured to detect the burst arrival times by using, for example, a “Unique Word” pattern. It is recognized that any other well-known techniques may be used for this purpose. The Unique Word patterns of the various encoders may or may not be distinct.
- With respect to joint detection, this process involves iterative refinement of the demodulation. As the iteration progresses, the demodulation is improved through two techniques. First, as interference is removed, the estimation of signal parameters (e.g., frequency, symbol timing, carrier phase) is improved. Secondly, as more reliable estimates of the data symbols become available from the turbo decoders 413 a-413 n, these are used to improve the demodulator performance.
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FIG. 6 is a diagram of a scrambler, in accordance with various exemplary embodiments. In this example, ascrambler 601 receives a codeword (e.g., “0 0 1 0 1 0”) and a scrambling sequence (e.g., “1 0 1 0 1 1”). The scrambling sequence (or signature) can be a Gold sequence or any pseudorandom number sequence. Gold codes exhibit a number of characteristics. In addition to being simple to generate, these sequences contain roughly an equal number of zeros and ones, and are approximately orthogonal when delayed or shifted. Also, they are orthogonal to other codes. Gold sequences can be generated using feedback shift registers, whose outputs are added to produce the Gold codes. The codeword and scrambling sequence are combined byadder 603 to output a transmitted sequence (e.g., “1 0 0 0 0 1”). - As mentioned, use of scramblers (as opposed to interleavers) reduces complexity. In a large system with numerous users, it is difficult to deploy a large number of interleavers that are prearranged between each pair of sender and receiver, whereas a common scrambler with different initial vector (also known as “seed”) can be used for each pair of sender and receiver. Such arrangement is substantially easier to implement.
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FIG. 7 is a diagram of a turbo code encoder configured to use constituent encoders, in accordance with various exemplary embodiments. In this example,turbo encoder 201 employs twoconstituent encoders interleaver 705. Although twoencoders encoder 201 can provide more than two encoders to achieve various code rates. As seen inFIG. 7 , theturbo encoder 201 can optionally output the information (i.e., systematic) bit as part of the output code, depending on the design of theconstituent encoders turbo code encoder 201 is explained with respect toFIG. 8 , as follows. -
FIG. 8 is a flowchart of a process for turbo code encoding, according to an exemplary embodiment. Information bits that are to be turbo code encoded are fed to bothconstituent encoders 701 and 703 (step 801). The bits that are fed toconstituent encoder 703 are, however, interleaved byinterleaver 705 prior to being input toconstituent encoder 703, as in steps 803-807. Insteps constituent encoders turbo encoder 201 can contain the unpunctured bits at the output ofconstituent encoders FIG. 7 . -
FIGS. 9 and 10 are diagrams of constituent encoders configured to provide low rate turbo codes, in accordance with various exemplary embodiments. Aconstituent encoder 900 provides code rates of 1/14, 1/12, 1/10, 1/8, 1/6, and 1/4. When implemented in theturbo encoder 201 ofFIG. 7 , theencoder 201 does not output the information bits given these code rates. Forrate 1/14 turbo code, all seven parity bits of the constituent code are transmitted. Forrate 1/12 turbo code, Y6 is punctured from both constituent codes, forrate 1/10 turbo code Y5 and Y6 are punctured, forrate 1/8 turbo code, Y4, Y5 and Y6 are punctured, forrate 1/6 turbo code Y3, Y4, Y5 and Y6 are punctured and forrate 1/4 turbo code, Y2, Y3 and Y4, Y5 and Y6 are punctured from both constituent codes. The transfer function of theconstituent encoder 900 is given as: -
- As seen, the logic or circuitry for the
encoder 900 encompassesadders shift registers Modular adder 901 receives the data input, adding it together with the output ofregisters adder 901 produces the parity bit Y6 and is also fed intoadders Modular adder 903 sums the signals fromadder 901 and register 917, resulting in the parity bit Y4; the summed value is additionally provided to adder 907.Adder 905 receives inputs fromregister 919 andadder 901, and produces the parity bit Y5 at its output. - Further,
adder 907 sums the values fromadder 903 and register 919 to generate parity bit Y3.Adder 909 receives inputs fromregister 919 andadder 901 to produce a value, which is then fed into adder 913.Adder 911 adds the value fromregister 921 as well as the value fromadder 901; the resultant value is the parity bit Y2. As for adder 913, this adder 913 produces parity bit Y0 from the summation of a value fromadder 909 and register 921.Adder 915 receives inputs from adder 913 and register 921 to generate parity bit Y1. -
FIG. 10 shows a block diagram of aconstituent encoder 1000 for achieving varying code rates, such as 1/15. The transfer function of theconstituent encoder 1000 is as follows: -
- To generate the parity bits according to this transfer function, the
constituent encoder 1000 has circuitry that includesmodular adders shift registers - As seen,
modular adder 1001 adds the output ofregister 1023 with the data input and generates parity bit Y0, which is also supplied intoadders Adder 1003 receives input fromadder 1001 and register 1019, summing these inputs for a resultant value that is fed intoadder 1013. Atadder 1005, signals fromadder 1001 and register 1019 are summed to yield parity bit Y6—which is also fed intoadder 1009. -
Adder 1007 adds the input fromadder 1001 and the input fromregister 1021, to provide parity bit Y1. Parity bit Y5 can be generated through the summation, atadder 1009, of the values fromadder 1005 andregister 1021. With respect toadder 1011, inputs fromadder 1001 and register 1021 are added byadder 1011 and provided toadder 1015.Adder 1013 receives input fromadder 1003 and register 1023 to generate parity bit Y4. Parity bit Y2 is formed by the addition of the result fromadder 1011 and content ofregister 1023.Adder 1017 receives inputs fromadder 1015 and register 1019 and produces parity bit Y3 at its output. Under this arrangement (i.e., code rate of 1/15), theturbo encoder 201 ofFIG. 7 outputs the systematic bit, by contrast to theconstituent encoders FIGS. 9 and 10 , respectively, in which no information bit is output by theturbo encoder 201. - It should be noted that, according to certain embodiments, the low rate turbo codes described herein have been developed by targeting minimum Hamming weight corresponding to information sequences with Hamming weight more than two. Using this approach, a search for new turbo codes was conducted using a uniform interleaving assumption and constraining the Hamming weight of input sequences to a maximum of six, for instance.
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FIG. 11 is a graph illustrating the performance of the SDMA system ofFIG. 4 . Specifically, ingraph 1100, the performance ofsystem 400 is compared against Shannon's constrained capacity for QPSK modulation as a function of the aggregated signal power per symbol time over one-sided noise spectral density in an Additive White Gaussian Noise (AWGN) channel (e.g., using arate 1/10 code). A relatively short 1000 information bit frame length is used. As seen, up to about five simultaneous overlapping transmission that are close to a theoretically best with unlimited processing. The corresponding energy per symbol over one-sided noise spectral density ratio (Es/No) for each individual user terminal and the energy per information bit to one-sided noise spectral density ratio (Ebi/No) to achieve 10−5 frame error rate is shown in Table 1, as the aggregated power are evenly divided to each user terminal. -
TABLE 1 No. of Users 1 2 3 4 5 6 7 Es/No (dB) −6.7 −6.3 −6.1 −5.7 −5.0 −3.9 −2.3 per user Ebi/No (dB) 0.3 0.7 0.9 1.3 2.0 3.1 4.7 per user - Conventional spectral spreading achieves emission reduction, but does not offer additional coding gain. When applied to a star-shaped satellite network, the approach of
system 400 can be applied for both the forward link and the return link. In the forward link, the advantage is mainly the lower Ebi/No that is required by the lower rate code is lower than required by therate 1/3 orrate 1/4 code previously used. The lower operating threshold allows the terminal to accept more adjacent satellite interference, which is often the dominating performance limitation for small terminals. The advantage of the low-rate code can be applied to reduced receive antenna size, or higher bit rate. - In the return link, the combination of low-rate code and SCMA allows the terminal to operate autonomously with minimum coordination with the hub. Unlike conventional CDMA, interference cancellation operates better when the terminals are not operating at exactly the same power, tight power control is in fact not desirable. The bandwidth expansion from the low-rate coding serves two purposes reduction of emission spectral density from regulator standpoint; and additional coding gain.
- The processes described herein for generating low rate codes and scrambling may be implemented via software, hardware (e.g., general processor, Digital Signal Processing (DSP) chip, an Application Specific Integrated Circuit (ASIC), Field Programmable Gate Arrays (FPGAs), etc.), firmware or a combination thereof.
- In the preceding specification, various embodiments have been described with reference to the accompanying drawings. It will, however, be evident that various modifications and changes may be made thereto, and additional embodiments may be implemented, without departing from the broader scope of the invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative rather than restrictive sense.
Claims (23)
Priority Applications (6)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US12/056,144 US20090028324A1 (en) | 2007-03-27 | 2008-03-26 | Method and system for providing scrambled coded multiple access (scma) |
US12/647,096 US8619974B2 (en) | 2007-03-27 | 2009-12-24 | Method and system for providing spread scrambled coded multiple access (SSCMA) |
US12/686,171 US20100122143A1 (en) | 2007-03-27 | 2010-01-12 | Method and system for providing low density parity check (ldpc) coding for scrambled coded multiple access (scma) |
US13/742,635 US9461765B2 (en) | 2007-03-27 | 2013-01-16 | Method and system for providing scrambled coded multiple access (SCMA) |
US15/254,814 US10263814B2 (en) | 2007-03-27 | 2016-09-01 | Method and system for providing scrambled coded multiple access (SCMA) |
US16/298,922 US10630512B2 (en) | 2007-03-27 | 2019-03-11 | Optimal forward error correction for non-orthogonal multiple access (NOMA) communications systems |
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US13/742,635 Continuation US9461765B2 (en) | 2007-03-27 | 2013-01-16 | Method and system for providing scrambled coded multiple access (SCMA) |
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