WO1995034181A1

WO1995034181A1 - Communications system

Info

Publication number: WO1995034181A1
Application number: PCT/US1995/007362
Authority: WO
Inventors: Albert J. Mallinckrodt
Original assignee: Celsat America, Inc.
Priority date: 1994-06-07
Filing date: 1995-06-07
Publication date: 1995-12-14
Also published as: WO1995034177A1; WO1995034138A1

Abstract

Seamless hand-over of a user between nodes (516) of a communications system is effected by periodically recomputing an algorithm which determines a preferred node for communication with the user. When the algorithm indicates that another node is preferred, a call initiation handshake is established between the user and the preferred node, while still maintaining communication with the original node. Communication between the user and the first node is not interrupted until the user establishes a communication 'lock' with the second node.

Description

COMMUNICATIONS SYSTEM

BACKGROUND

This invention relates to improvements in mobile wireless communication systems.

More particularly, the invention relates to communications systems such as a cellular mobile communications system having integrated satellite and ground nodes.

In yet another respect the invention pertains to a multi-node wireless communications systems provided with methods and protocols for seamless hand-over of a user from one node to another .

The cellular communications industry has grown at a fast pace in the United States and even faster in some other countries. It has become an important service of substantial utility and because of the growth rate, saturation of the existing service is of concern. High density regions having high use rates, such as Los Angles, New York and Chicago are of most immediate concern.. Contributing to this concern is the congestion of the electromagnetic frequency spectrum which is becoming increasingly severe as the communication needs of society expand. This congestion is caused not only by cellular communications systems but also by other communications systems. In the cellular communications industry alone, it is estimated that the number of mobile subscribers will increase on a world-wide level by an order of magnitude within the next ten years. The radio frequency spectrum is limited and in view of this increasing demand for its use, means to more efficiently use it are continually being explored.

Mobile communications system such as Specialized Mobile Radio (SMR) , the planned Personal Communications Service (PCS) and existing cellular radio are primarily aimed at providing mobile telephone service to automotive users in developed metropolitan areas. For remote area users, airborne users, and marine users, AIRFONE and INMARSAT services exist but coverage is incomplete and/or service is relatively expensive. Mobile radio satellite systems in an advanced planning stage will probably provide improved direct-broadcast voice channels to mobile subscribers in remote areas but still at significantly higher cost in comparison to existing ground cellular service. The ground cellular and planned satellite technologies complement one another in geographical coverage in that the ground cellular communications service provides voice and data telephone service in relatively developed urban and suburban areas but not in sparsely populated areas, while the planned earth orbiting satellites will serve the sparsely populated areas. Although the two technologies use the same general area of the RF spectrum, they are basically separate and incompatible by design as they presently exist. At present, if a user needs both forms of mobile communications coverage, he must invest in two relatively expensive subscriber units, one for each system. To meet the world's demand for additional mobile communications capabilities, more communications systems will need to be employed. An additional technology that is anticipated to find widespread application in mobile wireless communications systems is the spread spectrum communications technique. The spread spectrum communications technique is a technology that has found widespread use in military applications which must meet requirements for security, minimized likelihood of signal detection, and minimum susceptibility to external interference or jamming. Due to its inherent advantages, it is anticipated that the spread spectrum technique will be used for commercial applications in the coming decade. In a spread spectrum system, the data modulated carrier signal is further modulated by a relatively wide-band, pseudo-random "spreading" signal so that the transmitted bandwidth is much greater than the bandwidth or rate of the information to be transmitted. Commonly the "spreading" signal is generated by a pseudo-random deterministic digital logic algorithm which is duplicated at the receiver. By further modulating the received signal by the same spreading waveform, the received signal is remapped into the original information bandwidth to reproduce the desired signal. Because a receiver is responsive only to a signal that was spread using the same unique spreading code, a uniquely addressable channel is possible. Also, the power spectral density is low and without the unique spreading code, the signal is very difficult to detect, much less decode, so privacy is enhanced and interference with the signals of other services is reduced. The spread spectrum signal has strong immunity to multipath fading, interference from other users of the same system, and interference from other systems.

The demand for mobile telephone service is steadily expanding and with the expansion of the service, the problem of serving an increased number of subscribers who are travelling from one region to another has become of primary importance. Cellular communications systems divide the service areas into geographical cells, each served by a base station or node typically located at its center. The central node transmits sufficient power to cover its cell area with adequate field strength. If a mobile user moves to a new cell, the radio link is switched to the new node provided there is an available channel. However, if the mobile user travels into a region where all channels are busy, or that is not served by cellular service, or, in some cases, into an area served by a different licensee/provider, then his call may be abruptly terminated.

Handoff between cellsites is a fairly simple process in principle, but becomes very complex as all the interactions between the network elements and the mobile units are considered. The following describes the basic handoff process and does not enter into the more complex handoff processes which are generally network equipment manufacturer dependent. Further, the processes for inter-cell and inter-sector handoff are substantially similar. Accordingly, though the description below makes reference only to inter-cellular handoff, the described protocol also describes the typical inter-sectoral handoff.

In the handoff between cellsites of a typical analog cellular system, such as the present day AMPS system, the cellsite voice channels continuously track their own respective mobile's received signal strength level (RSSL) . When a target mobile's received signal strength level drops below a pre-defined threshold (known as HOTL, the Hand-off Threshold Level) for that voice channel, then a message is sent back to the cellsite's respective cellsite controller (CSC) , indicating that the target mobile has a low received signal strength and may need to be handed off to an adjacent cellsite. The cellsite controller then polls the locate receivers (LCR's) on all the cellsites adjacent to the target cellsites (known as "Candidate" cellsites) , to obtain the received signal strength level and Signalling Audio Tone (SAT) for that voice channel frequency. (The signal audio tone is one of three sub-modulated "digital color codes" or DCC's, mainly used to help the locate receivers discriminate between co-channel mobiles, which would be confused in dense urban environments) . The cellsite controller also polls the target cellsite's locate receiver . The cellsite controller then compares the candidate cellsites' received signal strength level to the target cellsite's received signal strength level. Based on the received signal strength level from the target and candidate cellsites, the cellsite controller then decides whether to handoff the mobile and which adjacent cellsite and voice channel should handle the call, depending on voice channel availability at that cellsite. The cellsite which has the best received signal strength level and meets all the handoff criteria is then defined to be the "assigned" cellsite by the cellsite controller.

In order to continue the call in the new cellsite, the cellsite controller sends a control message to the assigned cellsite to set the newly assigned voice channel to "in-service" mode. The cell then sends a forward voice channel (FVC) in-band signaling hand-off request (HREQ) message to the target mobile telling it to change its transmit and receive parameters to those of the new voice channel, including mobile frequency, transmitter power level and digital color code.

To confirm that the target mobile has received the handoff request message, the mobile sends back a 50 msec burst of signaling tone (T) to the cellsite voice channel, which tells the target cellsite's voice channel that the "handoff request is acknowledged" (HACK) . The mobile then changes its frequency and other relevant parameters as ordered by the cellsite controller and effects the handoff.

The assigned voice channel on the new cellsite then detects that the mobile has arrived on its frequency by detecting that the correct signal audio tone (Digital Color Code) is present. Once signal audio tone is detected, the assigned voice channel sends a handoff complete (HOCOMP) message to the cellsite controller to tell it that the target mobile has arrived.

After receiving the handoff complete HOCOMP message from the assigned cellsite, the cellsite controller then switches over the voice path from the old voice channel on the original cellsite to the assigned voice channel on the new cellsite. The entire handoff process takes less than 6 seconds. However, the subscriber just notices a slight blanking (250msec) in the audio path as the voice path is switched from the old cellsite to the new. The basic hand-off flowchart for a typical AMPS cellular system is shown in FIG. 5.

An additional cellular communication system that is finding acceptance is time division multiple access (TDMA) . The basic process for handoff of TDMA mobiles is similar to AMPS mobiles with the following exceptions. A dual-mode TDMA/AMPS mobiles transmits in bursts and therefore a TDMA digital locate receiver (DLR) must be used by candidate cellsites to make received signal strength level measurements and detect the digital color code (DVCC) on a TDMA mobile. Further, the TDMA mobile is able, upon command from the cellsite, to take measurements of the candidate cellsite's control channel power level and associated DVCC and send this data back to the cellsite controller. This comparison of received signal strength level data allows more accurate handoff's to be achieved and is called "mobile assisted handoff" (MAHO) .

The process for AMPS to TDMA and TDMA to AMPS handoffs is similar to the above, except that the candidate cellsite must have locate receivers that are compatible with the target mobile's technology. Otherwise, the candidate cellsite cannot measure the mobile's received signal strength level and digital color .en¬

code/signal audio tone. Accordingly, where the mobile unit is incompatible, the cellsite controller is unable to handoff the target mobile from the old cellsite to the candidate. Thus a digital locate receiver is employed to scan a particular timeslot and frequency in order to provide a measurement to the cellsite controller. When handing off from an AMPS to a TDMA traffic channel, the handoff request message to the AMPS mode mobile indicates (via the Timeslot indicator field) that the mobile has to switch to TDMA mode and synchronize to a particular timeslot. The mobile usually synchronizes within 1 superframe (~40us) , so there is no noticeable delay in cutting over the voice path from one technology to another .

When handing off from a digital traffic channel to an analog channel, the candidate cellsites must be installed with digital locate receivers in order to measure the target mobile. The process is generally the same, except that the target cellsite may request that the target mobile measures the candidate cellsite received signal strength and digital color codes. The cellsite controller uses this mobile assisted handoff information as part of its selection of the best server in the handoff process and sends an instruction to the target mobile as to where to handoff. Handoff between adjacent operator's networks is achieved with the aid of a signaling system (SS) which links adjacent cellular operator's mobile switching center (MSC) . When a target mobile is leaving an operator's cellular network, the target cellsite controller in the home system sends a locate receiver received signal strength level request to the adjacent cellsites in the system as well as to the adjacent cellular operator's MSC (via the SS links).

The adjacent operator's MSC then sends a message from its cellsite controller and the cellsite controller then requests a received signal strength level and digital color code measurement from the adjacent system candidate cells (which are normally border cells) . These measurements then come back through the adjacent operator's controller and MSC to the home system MSC and cellsite controller over the signaling system links and the home system cellsite controller decides which cellsites is the best server to handoff to and then assigns the voice channel on the home system (or the visitor system through the signaling system links) and issues the hand-off command to the target mobile.

The inter-system handoff process is limited because the typical standard protocol restricts the handoff algorithm to certain standard procedures, whereas the single system handoff algorithms can be tailored by the vendor to the operator's exact handoff requirements. Also the standard inter-system handoff requires more (MSC-MSC, MSC-cellsite controller and cellsite controller to cellsite) message signaling and data transfer than the single switch handoff process and this slows the overall handoff process, so that the entire handoff process (from the target mobile dropping below the handoff threshold on the serving cellsite to the time when the assigned voice channel on the new cellsite sends a "Handoff Complete" message to the cellsite controller, may take as much as 10 seconds to complete. This standard handoff procedure does however allow different vendor's equipment to handoff mobile users with the same or differing technology between them.

Unfortunately, each of the above referenced handover systems include inherent problems. For example, typically systems offer a "break before mate" protocol. In a "break before mate" handover, the communication is broken with the first cellsite prior to reestablishing communication with the new cellsite. Generally, this results in breaks of communication of 250 milliseconds or longer. Not only is this breaking annoying to those using the system for voice communication, but the breaks can result in the loss of information where the cellular communication system is being used for data transfer such as facsimile or computer modem hookup. An additional disadvantage of the typical protocol for cellular handover is that the determination of handover is based primarily upon measurements of received signal strength either at the cellsite or the mobile unit. Unfortunately, signal strength is effected strongly by the local topography creating received signal strength variations, such as those caused by building, foliage and other obstructions. For example, signal strength path loss suddenly increases x dB due for example to the mobile user driving behind a building or other obstruction in the immediate vicinity of the mobile unit. This causes the signal strength as sensed by the users unit to decrease x dB. These signal fluctuations often result in the unnecessary handover over of a mobile user's signal to an adjacent cellsite. In many instances, communication could be more readily maintained by having the mobile unit maintain communication with the local cellsite and by controlling the local cellsite's and mobile unit's transmit power.

Further, variations in measured signal strength often result in the handover to an adjacent cellsite only to have the cellsite controller immediately handover the communications signal back the first cellsite. Variations in signal strength due to topography reflections, blockage and such often result in the relatively rapid and unnecessary handover of a mobile user's signal to an adjacent cellsite. This results in the continuous periods of blanking experienced by user's of the system.

Thus it would be desirable to provide a cellular communications system which integrates satellite nodes with surface nodes to provide coverage of greater surface areas without requiring the use of two different systems with attendant expense and hardware requirements. Additionally, it would be desirable to provide a cellular communications system using a spread spectrum technique which can make more efficient use of existing frequency spectrum resources and result in increased privacy in communications.

Additionally, it would be desirable to permit the use of a relatively low power, compact and mobile user handset having a small, non-directional antenna, one which can communicate with both the land-based stations and the satellite-based stations.

Further, it would be desirable to develop a protocol for seamless handover of a user from one system node to another. Preferably, the system would use a "mate before break" protocol to reduce the likelihood that a communication link is accidentally ceased and to eliminate blanking on the system. Finally, it would be desirable that the protocol take into account factors other than received signal strength level in the determination of handover in an effort to reduce unnecessary signal handover.

SUMMARY OF THE INVENTION

The invention provides improvements in wireless communications systems. While various aspects of the invention will be explained by reference, for example, to a cellular communications system using spread spectrum waveforms, it will be apparent to those skilled in the art that these techniques are applicable to similar forms of wireless communications systems, such as, for example, Specialized Mobile Radio (SMR) , the planned Personal Communications Service (PCS) and existing cellular radio systems.

The invention provides improvements in such wireless communications systems, for example, a cellular communications system using spread spectrum wavefor s. The spread spectrum system makes possible the use of very low rate, highly redundant coding without loss of capacity to accommodate a large number of users within the allocated bandwidth.

Briefly, in one aspect, the invention is directed to a wireless communications system which includes node means and a plurality of user units, each said user unit including a means for establishing selective communication between the node and the user unit. According to another aspect of the invention, seamless handover of a mobile user from one system node to another is provided. Briefly, according to this aspect of the invention, in the operation of a wireless communications system, which system includes a plurality of nodes, a plurality of user units and means for establishing selective communication between said first one of said nodes to a second one of said nodes, the improvement comprising establishing an algorithm for determining a preferred node for said selective communication at any selected time, periodically recomputing said algorithm, establishing communication between said first node and said second node when said algorithm computation indicates that said second node is the preferred node, establishing call-initiation handshaking between said user and said second node, while maintaining said selective communication with said first node, establishing communications lock between said user and said second node, and interrupting communication between said user and said first node when said lock is established.

In a more detailed aspect of the invention, the cellular system includes a method and protocol for seamless handover of a mobile user from a first cellsite to second cellsite, a first sector to a second sector or a first operator to a second operator . To reduce signal blanking during handover, preferably, the system is a "mate before break" protocol in which the mobile user is connected to the second desired cellsite prior to breaking connection with the first cellsite.

Additionally, the handover protocol includes a uniform algorithm agreed upon by the operator of a cellular system or operators of adjoining cellular systems. The algorithm is used by the mobile user or cellular operator to determine which cellsite would provide the preferred service. The agreed upon algorithm determines service preference as a function of any or all of the following: mobile user location, present loading and spare capacity status of the cellular and satellite system, received signal strength level at either the mobile user or cellsite, and/or received signal quality at either the mobile user or cellsite.

Preferably, the system includes means for determining the position of a selected user unit by providing a timing signal to the selected user unit from the node, providing a timing response signal from the selected user unit from the node, providing a time response signal from the selected user unit in response to each timing signal, receiving the timing response signal by the node, measuring the response time of the user unit to the timing signal based on receipt of the timing response signal, and determining the position of the user unit based on the round trip time of transmission of the timing signal and receipt of the timing response signal.

In a more detailed aspect of the invention, the position means comprises means for measuring the response times of the user unit to respective timing signals transmitted by at least two nodes and for determining the position of the selected user unit based on the round trip times from each timing signal transmitting surface node.

In yet another aspect, the position means comprises means for determining the position of the selected user unit by measuring at a plurality of nodes the response time of the user unit to a timing signal transmitted by at least one of the nodes and determining the position of the selected user unit based on the times of receipt by the nodes of the timing response signal from the user unit.

In another aspect, the position means may store a priori information about the selected user unit and may determine the position of the selected user unit by providing a timing signal to the user unit from a node, measuring the response time of the user unit to the timing signal at the node, and determining the position of the user unit based on such measurement and on the a priori information. Additionally, the position means also determines in which cell a selected user unit is and indicates the location of the cell.

In an additional embodiment, the transfer of the user's receiver is synchronized to breaks in the forward direction utterances (transmissions from the cellsite) to minimize user perceived disruption. Similarly, after the subscriber unit achieves lock and sync on the new pilot, his reverse transmission transfer is delayed to the next break in his utterances (transmissions to the cellsite) . In this manner, it may be that the several tens of milliseconds required for transfer from one cellsite to a second cellsite can be made essentially transparent to both user and other party.

More particularly, the additional improvement for effecting seamless handover of a user from a first node to a second node comprises: establishing an algorithm for determining a preferred node, periodically computing the algorithm, establishing communication between the first node and the second node when the algorithm computation indicates that the second node is the preferred node, transmitting channel parameters of the second node such as frequency and coding, etc. from the first node to the user, initiating forward transmission between the second node and the user in parallel with the continued forward transmission from the first node and the user, synchronizing a transfer of the user's receiver from the first node to the second node with breaks in forward direction transmissions in order to minimize user perceived disruption, and synchronizing a transfer of the user's transmitter from the first node to the channel parameters of the second node with breaks in reverse direction transmissions from the user to minimize user perceived disruption.

Other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, illustrating by way of example the features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) -(c) are diagrams showing an overview of the principal elements of typical communications systems which embody the principles of the invention;

FIG. 2 is a diagram of the frequency sub-bands of the frequency band allocation for a mobile system, e.g., a cellular system;

FIG. 3 is an overview block diagram of a communications system in accordance with the principles of the invention without a network control center;

FIG. 4 is a diagram showing the interrelationship of the cellular hierarchial structure of the ground and satellite nodes in a typical section and presents a cluster comprising more than one satellite cell;

FIG. 5 is a block diagram of a typical prior art handoff;

FIG. 6 is a block diagram of one embodiment of a satellite signal processing system;

FIG. 7 is a functional block diagram of a user transceiver showing an adaptive power control system; FIG. 8 is a diagram showing the interrelationship of cellular nodes, cells, central controller and mobile switching center of a cellular communications system;

FIG. 9 is a diagram showing the interrelationship of cellular nodes, cells, central controller and mobile switching center of home and adjacent cellular communications system;

FIG. 10 depicts a method of control hierarchy for a hybrid satellite and ground based mobile communication system; and

FIG. 11 is a diagram of a protocol for handover of a mobile user from one node to another .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As is shown in the exemplary drawings, the invention is embodied in a mobile system, e.g., a cellular communications system utilizing integrated satellite and ground nodes both of which use the same modulation, coding, and spreading structure and both responding to an identical user unit.

With reference to FIGS. 5 and 8, the present invention is a seamless handover protocol that provides for the handover of a mobile user from a first cellsite to second cellsite, a first sector to a second sector, or a first operator to a second operator . In a preferred embodiment, the system uses a "mate before break" protocol that reduces blanking on the system and reduces the likelihood that a communication link is accidentally ceased. Further, in additional embodiments, the handover protocol takes into account factors other than received signal strength level, such as mobile user location and signal quality, in an effort to reduce unnecessary signal handover.

Referring now to FIG. 1(a), an overview of a communications system 10 is presented showing the functional inte -relationships of the major elements. The system network control center 12 directs the top level allocation of calls to satellite and ground regional resources throughout the system. It also is used to coordinate system-wide operations, to keep track of user locations, to perform optimum allocation of system resources to each call, dispatch facility command codes, and monitor and supervise overall system health. The regional node control centers 14, one of which is shown, are connected to the system network control center 12 and direct the allocation of calls to ground nodes within a major metropolitan region. The regional node control center 14 provides access to and from fixed land communication lines, such as commercial telephone systems known as the public switched telephone network (PSTN) . The ground nodes 16 under direction of the respective regional node control center 14 receive calls over the fixed land line network, encode them, spread them according to the unique spreading code assigned to each designated user, combine them into a composite signal, modulate that composite signal onto the transmission carrier, and broadcast them over the cellular region covered.

Satellite node control centers 18 are also connected to the system network control center 12 via status and control land lines and similarly handle calls designated for satellite links such as from PSTN, encode them, spread them according to the unique spreading codes assigned to the designated users, and multiplex them with other similarly directed calls into an uplink trunk, which is beamed up to the designated satellite 20. Satellite nodes 20 receive the uplink trunks, frequency demultiplex the calls intended for different satellite cells, frequency translate and direct each to its appropriate cell transmitter and cell beam, and broadcast the composite of all such similarly directed calls down to the intended satellite cellular area. As used herein, "backhaul" means the link between a satellite 20 and a satellite node control center 18. In one embodiment, it is a K-band frequency while the link between the satellite 20 and the user unit 22 uses an L-band or an S- band frequency.

As used herein, a "node" is a communication site or a communication relay site capable of direct one or two- way radio communication with users. Nodes may include moving or stationary surface sites or airborne or satellite sites.

User units 22 respond to signals of either satellite or ground node origin, receive the outbound composite signal, separate out the signal intended for that user by despreading using the user's assigned unique spreading code, de-modulate, and decode the information and deliver the call to the user. Such user units 22 may be mobile or may be fixed in position. Gateways 24 provide direct trunks that is, groups of channels, between satellite and the ground public switched telephone system or private trunk users. For example, a gateway may comprise a dedicated satellite terminal for use by a large company or other entity. In the embodiment of FIG. 1, the gateway 24 is also connected to that system network controller 12.

All of the above-discussed centers, nodes, units and gateways are full duplex transmit/receive performing the corresponding inbound (user to system) link functions as well in the inverse manner to the outbound (system to user) link functions just described.

FIGs. 1(b) and 1(c) represent systems with space only and ground only nodes. Certain aspects of this invention relate to these two systems as well as the "hybrid" system previously described.

Referring now to FIG. 2, the allocated frequency band 26 of a communications system is shown. The allocated frequency band 26 is divided into 2 main sub- bands, an outgoing sub-band 25 and an incoming sub-band 27. Additionally the main sub-bands are themselves divided into further sub-bands which are designated as follows:

OG Outbound Ground 28 (ground node to user) OS Outbound Satellite 30 (satellite node to user) OC Outbound Calling and Command 32 (node to user) IG Inbound Ground 34 (user to ground node) IS Inbound Satellite 36 (user to satellite node) IC Inbound Calling and Tracking 38 (user to node)

All users in all cells use the entire designated sub-band for the described function. Unlike existing ground or satellite mobile systems, there is no necessity for frequency division by cells; all cells may use these same basic six sub-bands. This arrangement results in a higher frequency reuse factor as is discussed in more detail below.

In a typical cellular system, a mobile user's unit 22 will send an occasional burst of an identification signal in the IC sub-band either in response to a poll or autonomously. This may occur when the unit 22 is in standby mode. This identification signal is tracked by the regional node control center 14 as long as the unit is within that respective region, otherwise the signal will be tracked by the satellite node or nodes. In another embodiment, this identification signal is tracked by all ground and satellite nodes capable of receiving it. This information is forwarded to the network control center 12 via status and command lines. By this means, the applicable regional node control center 14 and the system network control center 12 remain constantly aware of the cellular location and link options for each active user 22. An intra-regional call to or from a mobile user 22 will generally be handled solely by the respective regional node control center 14. Inter-regional calls are assigned to satellite or ground regional system resources by the system network control center 12 based on the location of the parties to the call, signal quality on the various link options, resource availability and best utilization of resources.

A user 22 in standby mode constantly monitors the common outbound calling frequency sub-band OC 32 for calling signals addressed to him by means of his unique spreading code. Such calls may be originated from either ground or satellite nodes. Recognition of his unique call code initiates the user unit 22 ring function. When the user goes "off-hook", e.g., by lifting the handset from its cradle, a return signal is broadcast from the user unit 22 to any receiving node in the user calling frequency sub-band IC 38. This initiates a handshaking sequence between the calling node and the user unit which instructs the user unit whether to transition to either satellite, or ground frequency sub-bands, OS 30 and IS 36 or OG 28 and IG 34.

A mobile user wishing to place a call simply takes his unit 22 off hook and dials the number of the desired party, confirms the number and "sends" the call. Thereby an incoming call sequence is initiated in the IC sub-band 38. This call is generally heard by several ground and satellite nodes which forward call and signal quality reports to the appropriate system network control center 12 which in turn designates the call handling to a particular satellite node 20 or regional node control center 14. The call handling element then initiates a handshaking function with the calling unit over the OC 32 and IC 38 sub-bands, leading finally to transition to the appropriate satellite or ground sub-bands for communication.

Referring now to FIG. 3, a block diagram of a communications system 40 which does not include a system network control center is presented. In this system, the satellite node control centers 42 are connected directly into the land line network as are also the regional node control centers 44. Gateway systems 46 are also available as in the system of FIGS, l(a-c), and connect the satellite communications to the appropriate land line or other communications systems. The user unit 22 designates satellite node 48 communication or ground node 50 communication by sending a predetermined code.

Alternatively, the user unit could first search for one type of link (either ground or satellite) and, if that link is present, use it. If that link is not present, use the alternate type of link.

Referring now to FIG. 4, a hierarchial cellular structure is shown. A pair of clusters 52 of ground cells 54 are shown. Additionally, a plurality of satellite cells 56 are shown. Although numerals 54 and 56 point only to two cells each, this has been done to retain clarity in the drawing. Numeral 54 is meant to indicate all ground cells in the figure and similarly numeral 56 is meant to indicate all satellite cells. The cells are shown as hexagonal in shape, however, this is exemplary only. The ground cells may be from 3 to 15 km across although other sizes are possible depending on user density in the cell. The satellite cells may be approximately 200-500 km across as an example depending on the number of beams used to cover a given area. As shown, some satellite cells may include no ground cells. Such cells may cover undeveloped areas for which ground nodes are not practical. Part of a satellite cluster 58 is also shown. The cell members of such a cluster share a common satellite node control center 60.

Referring to FIG. 8, a terrestrial cellular communications system is shown. The cellular system includes a plurality of cellular nodes, 400, 402, 404,

406, 408, 410, and 412, forming cellular cells 414, 416, 418, 420, 422, 424, and 426, respectively. Controlling receipt and transmission of signals is the cellsite controller (CSC) 430 and the mobile switching center (MSC) 432. It is anticipated for the cellular system to include a plurality of user units. A single user unit 440 is shown for example only. FIG. 9 shows cellular communication system 500 of FIG. 8 adjoining a similar cellular communication system 502 forming system boundary 504. Handoff between adjacent operator's networks is achieved with the aid of data link 510 which links adjacent cellular operator's MSC 508. Typically, when a target mobile is leaving an operator's cellular network, the target cellsite controller 512 in the home system 500 sends a locate receiver received signal strength level request to the adjacent cellsites in the system as well as to the adjacent cellular operator's MSC 508 (via the data links 510) .

The adjacent operator's MSC 508 then sends a message from its cellsite controller 514 requesting received signal strength level measurements from the adjacent system candidate cells 516 (which are normally border cells) . These measurements then come back through the adjacent operator's controller 514 and MSC 508 to the home system MSC 506 and cellsite controller 512 over the data links and the home system 500 decides which cellsite is the best server to handoff. The home system then assigns the voice channel on the home system (or the visitor system through the signaling system links) and issues the handoff command to the target mobile. As will be described in greater detail, the present invention provides an improved method of seamless handover between cellsites, satellite to terrestrial systems, or landbased systems.

Ideally suited for systems using the protocol of the present invention is use of spread spectrum multiple access. By employing spread spectrum, adjacent cells are not required to use different frequency bands. All ground-user links utilize the same two frequency sub- bands (OG 28, IG 34) and all satellite-user links use the same two frequency sub-bands (OS 30, IS 36) . This obviates an otherwise complex and restrictive frequency coordination problem of ensuring that frequencies are not reused within cells closer than some minimum distance to one another (as in the FM approach) , and yet provides for a hierarchial set of cell sizes to accommodate areas of significantly different subscriber densities.

Referring again to FIG. 1 as well as to FIG. 4, the satellite nodes 20 make use of large, multiple-feed antennas 62 which in one embodiment provide separate, relatively narrow beamwidth beams and associated separate transmitters for each satellite cell 56. For example, the multiple feed antenna 62 may cover an area such as the United States with, typically, about 100 satellite beams/cells and in one embodiment, with about 200 beams/cells. As used herein, "relatively narrow beamwidth" refers to a beamwidth that results in a cell of 500 km or less across. The combined satellite/ground nodes system provides a hierarchical geographical cellular structure. Thus within a dense metropolitan area, each satellite cell 56 may further contain as many as 100 or more ground cells 54, which ground cells would normally carry the bulk of the traffic originated therein. The number of users of the ground nodes 16 is anticipated to exceed the number of users of the satellite nodes 20 where ground cells exist within satellite cells. Because all of these ground node users would otherwise interfere as background noise with the intended user-satellite links, in one embodiment the frequency band allocation may be separated into separate segments for the ground element and the space element as has been discussed in connection with FIG. 2. This combined, hybrid service can be provided in a manner that is smoothly transparent to the user . Calls will be allocated among all available ground and satellite resources in the most efficient manner by the system network control center 12.

Referring now to FIG. 7, a functional block diagram of a typical user unit 22 is shown. The user unit 22 comprises a small, light-weight, low-cost, mobile transceiver handset with a small, non-directional antenna 68. The single antenna 68 provides both transmit and receive functions by the use of a circulator/diplexer 104 or other means. It is fully portable and whether stationary or in motion, permits access to a wide range - 34 -

of communication services from one telephone with one call number. It is anticipated that user units will transmit and receive on frequencies in the 1-3 GHz band but can operate in other bands as well.

The user unit 22 shown in FIG. 7 comprises a transmitter section 106 and a receiver station 108. For the transmission of a voice communication, a microphone couples the voice signal to a voice encode 110 which performs analog to digital encoding using one of the various modern speech coding technologies well known to those skilled in the art. The digital voice signal is combined with local status data, and/or other data, facsimile, or video data forming a composite bit stream in digital multiplexer 112. The resulting digital bit stream proceeds sequentially through forward error encoder 114, symbol or bit interleaver 116, symbol or bit, phase, and/or amplitude modulator 118, narrow band IF amplifier 120, wideband multiplier or spreader 122, wide band IF amplifier 124, wide band mixer 126, and final power amplifier 128. Oscillators or equivalent synthesizers derive the bit or baud frequency 130, pseudo-random noise or "chip" frequency 132, and carrier frequency 134. The PRN generator 136 comprises deterministic logic generating a pseudo-random digital bit stream capable of being replicated at the remote receiver. The ring generator 138 on command generates a short pseudo-random sequence functionally equivalent to a "ring" .

The transceiver receive function 108 demodulation operations mirror the corresponding transmit modulation functions in the transmitter section 106. The signal is received by the non-directional antenna 68 and conducted to the circulator 104. An amplifier 142 amplifies the received signal for mixing to an IF at mixer 144. The IF signal is amplified 146 and multiplied or despread 148 and then IF amplified 150 again. The IF signal then is conducted to a bit or symbol detector 152 which decides the polarity or value of each channel bit or symbol, a bit or symbol de-interleaver 154 and then to a forward error decoder 156, the composite bit stream from the FEC decoder 156 is then split into its several voice, data, and command components in the de-multiplexer 158.

Finally a voice decoder 160 performs digital to analog converting and results in a voice signal for communication to the user by a speaker or other means. Local oscillator 162 provides the first mixer 144 LO and the bit or symbol detector 152 timing. A PRN oscillator 164 and PRN generator 166 provide the deterministic logic of the spread signal for despreading purposes. The baud or bit clock oscillator 168 drives the bit in the bit detector 152, forward error decoder 156 and the voice decoder 160. The bit or symbol interleaver 116 and de-interleaver 154 provide a type of coded time diversity reception which provides an effective power gain against multipath fading to be expected for mobile users. Its function is to spread or diffuse the effect of short burst of channel bit or symbol errors so that they can more readily be corrected by the error correction code.

As an alternative mode of operation, provision is made for direct data or facsimile or other digital data input 170 to the transmitter chain and output 172 form the receiver chain.

A command decoder 174 and command logic element 176 are coupled to the forward error decoder 156 for receiving commands or information. By means of special coding techniques known to those skilled in the art, the non-voice signal output at the forward error decoder 156 may be ignored by the voice decoder 160 but used by the command decoder 174. An example of the special coding techniques are illustrated in FIG. 7 by the MUX 112 and DEMUX 158.

As shown, acquisition, control and tracking circuitry 178 are provided in the receiver section 108 for the three receive side functional oscillators 162, 164, 168 to acquire and track the phase of their counterpart oscillators in the received signal. Means for so doing are well known to those skilled in the art.

The economic feasibility of a mobile telephone system is related to the number of users that can be supported. Two significant limits on the number of users supported are bandwidth utilization efficiency and power efficiency. In regard to bandwidth utilization efficiency, in either the ground based cellular or mobile satellite elements, radio frequency spectrum allocation is a severely limited commodity. Accordingly, it is anticipated that the invention incorporate measures to maximize bandwidth utilization efficiency such as the use of code division multiple access (CDMA) technology which provides an important spectral utilization efficiency gain and higher spatial frequency reuse factor made possible by the user of smaller satellite antenna beams. In regard to power efficiency, which is a major factor for the satellite-mobile links, the satellite transmitter source power per user is minimized by the use of forward- error-correcting coding, which in turn is enabled by the above use of spread spectrum code division multiple access (SS/CDMA) technology and by the use of relatively high antenna gain on the satellite. CDMA and forward- error-correction coding are known to those skilled in the art and no further details are given here. In addition to the above listed advantages, the Code Division Multiplex system has the following important advantages in the present system. Blank time when some of the channels are not in use reduces the average interference background. In other words, the system overloads and underloads gracefully. The system inherently provides flexibility of base band rates; as opposed to FDM system, signals having different baseband rates can be multiplexed together on an ad-hoc basis without complex preplanned and restrictive sub-band allocation plans. Not all users need the same baseband rate. Satellite antenna sidelobe control problems are significantly reduced. The above mentioned numerical studies of out-of-cell interference factors show that secondary lobe responses may effectively be ignored. Co- code reassignment (that is reuse of the same spreading code) is feasible with just one beam separation. However, because there are effectively (i.e., including phasing as a means of providing independent codes) an unlimited number of channel codes, the requirements on space division are eased; there is no need to reuse the same channel access i.e., spreading code.

Fig. 10 depicts a hierarchial control division along geographical and ground vs. satellite elements of a mobile system. In this diagram solid lines denote traffic flow, dotted lines command, status and control flow. The total number of national, much less worldwide circuits is so vast that maximum decentralization of control is desired and accomplished by this control hierarchy invention. Every command and allocation decision should be made at the lowest level at which all the necessary information to make the decision is available. Thus it is anticipated that the bulk of the service requests and handovers coming upward into a particular level will be resolved at that level, with only those involving higher or lateral coordination being passed on up the line. A numerical example of a system using this control hierarchy follows.

A typical ground cell 310 is assumed to comprise two 1.25 MHz subbands, each serving up to 54 circuits for a total of 108. These are the common ground cells.

Geographically they may be thought of as from 3 to 12 miles in diameter. About 6 of the 108 circuits are reserved for calling channels. Power control functions with ground cellular users are the only functions handled here. No other switching or control functions are performed here, but all traffic lines are trunked on up the line along with station status.

Depending on local demographics, up to about 50 (more typically 20) such ground cell trunks comprise a GND METRO control 311, or Mobile Telephone Switching Office (MTSO) . This would correspond roughly with a metropolitan area such as greater Los Angeles. Local calls and handovers within the ground metro area would be resolved and controlled at this level. As a mobile user travels towards the edge of the metro area this will be recognized by the fact that he is being served by one of the outer rank of "edge" cells. For any user in these cells any signal drop requiring handover will be coordinated at regional level with the appropriate adjacent metro or satellite cell. Generally the GND METRO regions will be made coterminous with the satellite cells provided there are any ground cells within the satellite cell.

The SATELLITE CELLS corresponding to satellite beams, might be about 200 mi (normal to beam) in diameter, and provide about 741 circuits of which some 200 are calling channels. Control functions correspond to those of the Ground cell.

The REGIONAL control 312 areas in one embodiment are coterminous with the satellite "Clusters", typically about 10 satellite cells or about 600 miles diameter. Each may handle 1 to 100 (typically 15) METRO regions.

The SATELLITE CLUSTER CONTROL as part of the regional control handles about 10 satellite cells. The facility is collocated with the Ground Regional Control facility. The NATIONAL NET CONTROL 314 handles about 15 Regional centers for the case of a United States National system. This comprises all of the facilities envisioned in the present application.

Control functional allocation among these various control levels can be as follows:

Ground Cell 310 and Satellite Cell 315: Power control, Time-of-Arrival measurement and reporting as assigned (basis for position determination) , detect, monitor and report up all current standbys and call requests, and call terminations in coverage area, handle traffic as assigned including handshaking and call establishment, and disconnect. Each cell has a level at which saturation occurs, i.e., a limit on how many bits of information can be communicated through that node.

The instantaneous information being transmitted through any node can be measured by the instantaneous output power level at each of the transmitters associated with each node, and/or the instantaneous received power level at each of the receivers associated with each node.

Alternatively, the number of calls being instantaneously handled is known at the control centers. A measure of this information can be sent to any or all users such that they could delay transmission until a time when the use is low and hence receive more favorable rates. The information can be displayed by lamps or LCD or other means to permit manual decision making, e.g., whether or not to place a call. In an alternative embodiment, the information could be automatically used to enable transmission, e.g., for data or fax transmission.

Ground Metro Control 311: Coordinates soft handovers between ground cells, TELCO interface for ground links.

Regional Control 312: Provides TELCO interface to satellite links, tracks position of all active or standby units in region; assigns traffic handling facility and subband (coordinating exclusion areas) , and forwards up requests for handovers out of the region and requests for additional resources for position fixing.

National Control 314: in one embodiment provides; Satellite status monitoring; orbit maintenance, power; management, spares control; satellite housekeeping; coordinates position fixing resources as requested; and coordinates interregional handovers.

One embodiment of a method and protocol for the handover of a mobile user from one operator to another is described below (with reference to FIG. 11) :

1. There is established an agreed upon uniform algorithm whereby either the user or the operator could determine which would be the preferred service for any user at any time as a function of any or all of the following:

a) User's present location;

b) Present loading and spare capacity status of PCS and satellite systems (signalled between systems) ;

c) User preferences, as established by his service option choices, which would be a set of properties, (a service "mask") in his subscriber database;

d) Present mobile user signal quality; and or;

e) Present cellular node signal quality.

2. This algorithm is periodically (e.g., every few seconds) tracked and recomputed by the current, or "old" service provider .

3. Whenever the recomputed algorithm should call for a transfer from "old" to any other, "new" service provider, the following events would occur: a) Old provider sends a formatted message 316 to new provider, meaning:

"Request transfer to your system of User N at location XYZ, now connected to user K, code assignments UVW, whose traffic is hereby being bridged to you via landline circuit PDQ of trunk ABC" ... and any other information that may be useful in call setup on new system.

b) New system sends message 317 to old meaning:

"call can be accepted, assign calling channel L, code XY, subband S..."

c) Old system sends a command 318 to subscriber set:

"Initiate transfer to new system on new system calling channel L, code XY, subband S..." and any other information which would expedite the transfer .

d) New system assigns a termination unit to acquire user N on his calling channel L, code

XY, subband S... and makes the landline connection to circuit PDQ of trunk ABC... 319. e) Subscriber and new system do as much as possible of call initiation handshaking i.e., "initial setup", while maintaining traffic via old system. In one embodiment the other (non- handling-over) party is connected and the forward direction signals are being transmitted from both old and new systems simultaneously in parallel; the necessary user command signals (like new frequency, new spreading code, ...) have been sent to him and are stored in registers ready to be but not yet executed; and the new reverse direction receiver terminal unit is assigned, standing by, searching for the anticipated signal code, and its output bridged into the reverse direction landline to the other party.

f) When initial setup is all ready and standing by, new system messages 320 old system:

"Ready to transfer . "

g) Old system signals 321 to subscriber unit

"Execute" .

h) Subscriber unit while continuing reverse direction transmission to old system, instantly drops receive tracking of old forward signal, and commences receive search to new pilot signal in assigned new frequency band and code 322. When lock is achieved, normally several 20 ms frames later, transfers reverse transmission to new band and codes. In one embodiment the transfer command to the user is synchronized to breaks in the forward direction utterances to minimize user perceived disruption, and similarly, after the subscriber unit achieves lock and sync on the new pilot, his transmission transfer is delayed to the next break in his utterances. In this manner it may be that the several tens of milliseconds required for resync can be made essentially transparent to both user and other party.

i) When new system achieves lock-on to user's reverse transmission 323, new system signals old system to drop the connection.

Of importance, the transfer of the user's receiver is synchronized to breaks in the forward direction utterances (transmissions from the cellsite) . This minimizes user perception of signal disruption because the second party has ceased speaking during the transfer from the first cellsite to the second cellsite.

Similarly, after the subscriber unit achieves lock and synchronization on the new pilot, his reverse transmission transfer is delayed to the next break in his utterances (transmissions to the cellsite) . There is no perception of an signal disruption because the transfer is synchronized to a period when the user is not speaking. In this manner, it may be that the several tens of milliseconds required for transfer from one cellsite to a second cellsite can be made essentially transparent to both user and other party.

More particularly, the handover from a first node to a second node establishing communication between the first node and the second node when the algorithm computation indicates that the second node is the preferred node. Channel parameters of the second node such as frequency and coding, etc. are then transmitted from the first node to the user. Once complete, the second node initiates forward transmission to the user in parallel with the continued forward transmission from the first node to the user. Thereafter, a transfer of the user's receiver from the first node to the second node is synchronized with breaks in forward direction transmissions in order to minimize user perceived disruption. Similarly, a transfer of the user's transmitter from the first node to the channel parameters of the second node are synchronized with breaks in reverse direction transmissions from the user to minimize user perceived disruption. In this manner, any breaks in signal transmission are effectively camouflaged by pauses in the users voice communication.

As described above, handover determination may be made upon a function of mobile user location. Referring again to FIG. 7, an arrangement is provided for generating call requests and detecting ring signals. The ring generator 138 generates a ring signal based on the user's code for calling out with the user unit 22. For receiving a call, the ring signal is detected in a fixed matched filter 198 matched to a short pulse sequence which carries the user's unique code. As an option, the ring detect and call request signals may be utilized in poll/response mode to provide tracking information on each active or standby mode user .

For the precision location option, the user response signal time is accurately locked to the time of receipt of the polling or timing signal, to a fraction of a PRN chip width. Measurement of the round trip poll/response time from two or more nodes or time differences of arrival at several nodes provides the basic measurement that enables solution and provision of precise user position. For example, with reference to FIG. 8, round trip poll/response times from nodes 400, 402, and 406 to user unit 440 provides the measurement of distances 450, 452, and 454. Through simple trigonometric analysis, the central cellsite controller can determine the location of the user unit.

Ground transmitters and receivers duplicate the functions summarized above for the user units. Given a priori information, for example as to the route plan of a vehicle, a single round trip poll/response time measurement from a single node can yield valuable user position information. The position means may store a priori information about the selected user unit and may determine the position of the selected user unit by providing a timing signal to the user unit from a node, measuring the response time of the user unit to the timing signal at the node, and determining the position of the user unit based on such measurement and on the a priori information. An example of a priori information includes the sought to be travelled route of a user . By knowing the route of a selected user and the distance from a node, determined by application of the present invention, the central controller can determine the position of a selected user .

In another embodiment of position determination including the use of a priori information, the position of the user unit can be determined by distance determination from only two nodes. For example, the distances 452 and 454 of user unit 440 from nodes 402 and 406 combined the a priori information that the user unit is located cell sector 414 provides the necessary information to accurately determine the location of user unit 440. This a priori information may be determined by knowledge of the user unit's last known location or by analysis of the signal quality of the user unit's transmissions by cell nodes 400, 402, 404 and 406 to determine the user unit's location to be in cell sector 414. In another embodiment, once the user unit's location and present cell cite has been determined utilizing the three node trigonometric analysis described above, the cell site controller may switch to two node position determination thereby reducing computer computations.

Ideally suited for position determination is the use of code division multiple access (CDMA) technology which provides an important spectral utilization efficiency gain and higher spatial frequency reuse factor made possible by the use of smaller satellite antenna beams. Accurate position determination can be obtained through two-dimensional multi-lateration. Each CDMA mobile user unit's transmitted spreading code is synchronized to the epoch of reception of the pilot signal from its current control site, whether ground or satellite node. The normal mode of operation will be two-dimensional, i.e., based upon two receptions, at ground or satellite nodes of the user response code. In conjunction with a priori information inherent in a topographic database, e.g., altitude of the surface of the earth, position accuracy to within a fraction of a kilometer can be provided.

Other means are available for position location, such as GPS.

Means for determining the position of a mobile user relative to a multiplicity of known system nodes, either fixed on the ground or at known positions in space, are known to those skilled in the art. In a CDMA of system, any of these means are largely incidental to the function of transmitting and/or receiving the CDMA signal at multiple sites. The receiving function requires synchronization of the epoch of a local spread code generator to that of the received spread code, so that having achieved code synchronization, one inherently has a measure of the delay time and hence the range of the signal. Various references describe how this information can be used in several different geometrical configurations to provide the delay measurements necessary to provide hyperbolic, elliptical, spherical or hybrid multi-lateration position determination. By any of these means the mobile position can either be determined by the network controller or by the mobile user and relayed to the network controller . In another embodiment, the uniform agreed upon algorithm includes a parameter based upon received signal quality. Signal quality may be measured by either the mobile unit or at the cellsite. Where signal quality is measured by the mobile user, each mobile user receiver determines the quality of the received signal and provides a local quality signal to its associated transmitter in the respective transceiver indicative of that received signal quality. Each mobile user transmitter then transmits the local quality signal provided to it from its associated receiver to its presently linked cellsite. The cellsite, upon receiving the local quality signal, processes the local quality signal in the uniform algorithm to provide a determination of system handover .

Accordingly, the handover protocol in accordance with the invention considers not only received signal strength level but also a measure of data loss or "signal quality" reported to it from another unit with which it is in communication. As used herein, "signal quality" refers to the accuracy or fidelity of a received signal in representing the quantity or waveform it is supposed to represent. In a digital data system, this may be measured or expressed in terms of a bit error rate, or, if variable, the likelihood of exceeding a specified maximum bit error rate threshold. Signal quality involves more than just signal strength, depending also on noise and interference level, and on the variability of signal loss over time. Additionally, "grade of service" as used herein is a collective term including the concepts of fidelity, accuracy, fraction of time that communications are satisfactory, etc., any of which may be used to describe the quality objectives or specifications for a communication service. Examples of grade of service objectives would include:

bit error rate less than one in 10³;

- ninety percent or better score on the voice diagnostic rhyme test; and

less than one-half percent probability of fade below threshold, although the exact numbers may vary depending on the application.

By virtue of the above discussed design factors the system in accordance with the invention provides a flexible capability of providing the following additional special services: high quality, high rate voice and data service; facsimile (the standard group 3 as well as the high speed group 4); two way messaging, i.e., data interchange between mobile terminals at variable rates; automatic position determination and reporting to within several hundred feet; paging rural residential telephone; and private wireless exchange. It is anticipated that the satellite will utilize geostationary orbits but is not restricted to such. The invention permits operating in other orbits as well. The system network control center is designed to normally make the choice of which satellite or ground node a user will communicate with. In another embodiment, as an option, the user can request his choice between satellite link or direct ground based link depending on which provides clearer communications at the time or request his choice based on other communication requirements.

While a satellite node has been described above, it is not intended that this be the only means of providing above-ground service. In the case where a satellite has failed or is unable to provide the desired level of service for other reasons, for example, the satellite has been jammed by a hostile entity, an aircraft or other super-surface vehicle may be commissioned to provide the satellite functions described above. The "surface" nodes described above may be located on the ground or in water bodies on the surface of the earth. Additionally, while users have been shown and described as being located in automobiles, other users may exist. For example a satellite may be a user of the system for communicating signals, just as a ship at sea may or a user on foot.

While several particular forms of the invention have been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited, except by the appended claims.

Having described the invention in such terms as to enable those skilled in the art to make and use it, and having identified the presently preferred best modes thereof, we claim:

Claims

1. In the operation of a cellular communications system, which system includes a plurality of nodes, a plurality of user units and means for establishing selective communication between a first one of said nodes and one of said user units, the improvement for effecting seamless handover of a user from said first one of said nodes to a second one of said nodes, said improvement comprising:

a) establishing an algorithm for determining a preferred node for said selective communication at any selected time, said algorithm including computation of a user's location;

b) periodically computing said algorithm;

c) establishing communication between said first node and said second node when said algorithm computation indicates that said second node is the preferred node;

d) establishing call initiation handshaking between said user and said second node, while maintaining said selective communication with said first node;

e) establishing communications lock between said user and said second node; and f) interrupting communication between said user and said first node when said lock is established.

2. In the operation of a cellular communications system, which system includes a plurality of nodes, a plurality of user units and means for establishing selective communication between a first one of said nodes and one of said user units, the improvement for effecting seamless handover of a user from said first one of said nodes to a second one of said nodes, said improvement comprising:

a) establishing an algorithm for determining a preferred node for said selective communication at any selected time, said algorithm including computation of a user's received signal quality level;

b) periodically computing said algorithm;

d) establishing call initiation handshaking between said user and said second node, while maintaining said selective communication with said first node; e) establishing communications lock between said user and said second node; and

f) interrupting communication between said user and said first node when said lock is established.

3. In the operation of a cellular communications system, which system includes a plurality of nodes, a plurality of user units and means for establishing selective communication between a first one of said nodes and one of said user units, the improvement for effecting seamless handover of a user from said first one of said nodes to a second one of said nodes, said improvement comprising:

a) establishing an algorithm for determining a preferred node for said selective communication at any selected time, said algorithm including computation of a cellsite's received signal quality level;

b) periodically computing said algorithm;

d) establishing call initiation handshaking between said user and said second node, while maintaining said selective communication with said first node; - 6 1 -

e) establishing communications lock between said user and said second node; and

4. In the operation of a cellular communications system, which system includes a plurality of nodes, a plurality of user units and means for establishing selective communication between a first one of said nodes and one of said user units, the improvement for effecting seamless handover of a user from said first one of said nodes to a second one of said nodes, said improvement comprising:

a) establishing an algorithm for determining a preferred node for said selective communication at any selected time, said algorithm including computation of the present loading of the cellular system;

b) periodically computing said algorithm;

5. In the operation of a cellular communications system, which system includes a plurality of nodes, a plurality of user units and means for establishing selective communication between a first one of said nodes and one of said user units, the improvement for effecting seamless handover of a user from said first one of said nodes to a second one of said nodes, said improvement comprising:

a) establishing an algorithm for determining a preferred node for said selective communication at any selected time;

b) periodically computing said algorithm;

d) establishing call initiation handshaking between said user and said second node, while maintaining said selective communication with said first node; e) synchronizing a first transfer command with breaks in reverse direction transmissions of the user to minimize user perceived disruption;

f) transferring communication in the reverse direction when said first transfer command is synchronized;

g) synchronizing a second transfer command with breaks in forward direction transmissions to the user to minimize user perceived disruption; and

h) transferring communication in the forward direction when said second transfer command is synchronized.

6. In the operation of a cellular communications system, which system includes a plurality of nodes, a plurality of user units and means for establishing selective communication between a first one of said nodes and one of said user units, the improvement for effecting seamless handover of a user from said first one of said nodes to a second one of said nodes, said improvement comprising:

b) periodically computing said algorithm;

d) transmitting channel parameters of said second node such as frequency and coding, etc. from said first node to said user;

e) initiating forward transmission between said second node and said user in parallel with the continued forward transmission of said first node and said user; f) synchronizing a transfer of said user's receiver from said first node to said second node with breaks in forward direction transmissions in order to minimize user perceived disruption; and

g) synchronizing a transfer of said user's transmitter from said first node to said channel parameters of said second node with breaks in reverse direction transmissions from said user to minimize user perceived disruption.