HK1074922A - Communications in an asynchronous wireless network - Google Patents

Description

Communication within asynchronous wireless networks

Cross Reference to Related Applications

This application has priority under 35 U.S.C. p1 § 119(e) for provisional application serial No. 60/337472, filed 2001, 11/9, the contents of which are incorporated herein by reference.

Background

FIELD

The present invention relates to communication systems, and more particularly, to systems and techniques for synchronizing communication devices to asynchronous network access points.

Background

Modern communication systems are designed to allow multiple users to share a common communication medium. One such communication system is a Code Division Multiple Access (CDMA) system. CDMA communication systems are modulation and multiple access schemes based on spread spectrum communications. In a CDMA communication system, multiple signals share the same frequency spectrum and thereby provide increased user capacity. This is achieved by transmitting each signal with a different code that modulates a carrier, thus spreading the spectrum of the signal waveform. The transmitted signals are separated in the receiver by a correlator which despreads the signal spectrum using a corresponding code. Unwanted signals whose codes do not match are not spread across the bandwidth and are treated as noise only.

In a CDMA communication system, a user may access a network or communicate with other users through a network access point. A network access point typically includes a radio network controller supporting multiple nodes. As disclosed herein, the term "node" is used to refer to a node B, a base station, or any other similar communication station. Each node is assigned to serve all users in an area commonly referred to as a cell or sector. Within any given area, a user may communicate with any number of neighboring nodes as well as the nodes of the service area.

In some CDMA communication systems, the nodes are synchronized with each other. For example, the Navstar global positioning satellite navigation system is often used to synchronize nodes to the same time reference. As a result, once a user acquires and synchronizes to a node, it can synchronously communicate with other nodes as it goes from one area to another. This is in contrast to asynchronous CDMA communication systems, which require users to re-synchronize to different nodes as they traverse different coverage areas. The resynchronization process should be fast to implement to minimize potential interruptions in communications that may be perceived by the user. In addition, it is desirable to minimize the time for the user to implement the resynchronization procedure, as this reduces the risk of losing radio communication links outside the reference nodes and makes the position estimation based on propagation delay measurements more accurate.

SUMMARY

In one aspect of the invention, a method of communication includes establishing a reference corresponding to the timing of a received signal from a first source, determining the timing for each received signal from a plurality of second sources, adjusting the reference to the timing of the received signal from one of the second sources, the timing of the received signal being used to adjust the reference that is closest in time to the unadjusted reference, and synchronizing the signal to the reference for transmission.

In another aspect of the invention, an apparatus includes a searcher to establish a reference corresponding to a timing of a received signal from a first source, determine a timing for each received signal from a plurality of second sources, adjust the reference for the timing of the received signal from one of the second sources, the timing of the received signal to adjust the reference that is closest in time to the unadjusted reference, and synchronize the signal to the reference for transmission.

In another aspect of the invention, a computer readable medium embodying a program of computer executable instructions implements a method of communication, the method comprising establishing a reference corresponding to the timing of a received signal from a first source, determining the timing for each received signal from a plurality of second sources, adjusting the reference to the timing of the received signal from one of the second sources, the timing of the received signal being used to adjust the reference that is closest in time to the unadjusted reference, and synchronizing the signal to the reference for transmission.

In another aspect of the invention, an apparatus comprises reference means for establishing a reference corresponding to the timing of a received signal from a first source, means for determining the timing for each received signal from a plurality of second sources, adjustment means for adjusting the reference for the timing of the received signal from one of the second sources, the timing of the received signal being used to adjust the reference that is closest in time to the unadjusted reference, and means for synchronizing the signal to the reference for transmission.

It is understood that other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described only exemplary embodiments of the invention for purposes of illustration. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modifications in various other respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive.

Brief Description of Drawings

Aspects of the present invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which:

FIG. 1 is a conceptual overview of an exemplary asynchronous CDMA communication system;

fig. 2 illustrates an exemplary illustration of a linear link frame structure for a node transmit channel in an asynchronous CDMA communication system, wherein the channel has: a Synchronization Channel (SCH), a primary common control physical channel (P-CCPCH), and a common pilot channel (CPICH);

FIG. 3 is a functional block diagram illustrating the generation of channels by an exemplary node in an asynchronous CDMA communication system, such channels having a Synchronization Channel (SCH), a primary common control physical channel (P-CCPCH), and a common pilot channel (CPICH);

fig. 4 is a conceptual overview of an example user equipment operating within asynchronous CDMA communications;

fig. 5 is a functional block diagram of an example user equipment operating within asynchronous CDMA communications;

FIG. 6 is a functional block diagram of an example searcher that may be used with the user device of FIG. 5.

Detailed Description

The following detailed description, taken in conjunction with the accompanying drawings, serve as a description of exemplary embodiments in which the present invention may be practiced. The word "exemplary" is used throughout this description to mean "serving as an example, instance, or illustration," and should not be construed as optimal or advantageous over other embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form in order to avoid obscuring the concepts of the present invention.

Although the various methods of the present invention may be described in the context of a CDMA communication system, those skilled in the art will appreciate that these aspects are also applicable in the context of a variety of other communication environments. Accordingly, any reference to a CDMA communications system is intended only to illustrate the inventive aspects of the present invention, with the understanding that such inventive aspects have a wide range of applications.

Fig. 1 is a functional block diagram of an asynchronous CDMA communication system. The radio network controller 102 may be used to provide an interface between the network 104 and all nodes 106a-d dispersed throughout a geographic region. The geographic region is generally divided into regions known as cells or sectors. A node is typically assigned to serve all users within an area. The user equipment 108 may access the network 104 or communicate with other user equipment through one or more nodes under the control of a radio network controller. In the case where the user equipment 108 communicates with more than one node, the user equipment 108 selects a reference node as the synchronization source that it sends to all nodes. Since the user equipment 108 is moved away from the reference node, the user equipment may eventually need to select a new node as the synchronization source. The user equipment may select the node that requires the least amount of slewing (sliding) to resynchronize its transmissions. Fast direction refers to the process of adjusting the timing of the transmitted signal within the user equipment to synchronize it with the new synchronization source. To minimize the slewing, the user equipment may select a node whose signal reception time is closest to the signal previously received from the reference node.

An exemplary asynchronous CDMA communication system may be designed to support FDD mode of standard operation as proposed by an association known as the third generation partnership project (3GPP) and incorporated by reference in a set of documents including document nos. 3GPP TS21.101, 3GPP TS 25.211, 3GPP TS 25.212, 3GPP TS 25.213 and 3GPP TS 25.214, referred to herein as the WC-DMA standards. The W-CDMA standard is expressly incorporated herein by reference. The W-CDMA specification promulgated by 3GPP is a public report and is well known in the art. W-CDMA (also known as UTRA-FDD) is adopted and released as a regional standard by a number of standards organizations (e.g., the European Telecommunications Standards Institute (ETSI)). The 3GPP specification describes the use of a combination of physical channels SCH and CPICH transmitted by each node in a W-CDMA communication system. The SCH and CPICH may be used by the user equipment for synchronization to different nodes as the user equipment moves within the coverage area.

Fig. 2 is a diagram illustrating a downlink frame structure of physical channels SCH, CPICH, and P-CCPCH transmitted by a node in a W-CDMA communication system. Frame 202 may be of any duration depending on the particular application and overall design constraints. In the described example W-CDMA communication system, the frame duration is ten milliseconds and includes 38400 chips. A frame may be divided into 15 slots 204 of 2560 chips each. Each time slot 204 may be further divided into ten portions 206 of 256 chips each.

The SCH and P-CCPCH are time multiplexed. The SCH is transmitted only in the first portion of each slot 204 and the P-CCPCH is transmitted only in portions 2 through 10 of each slot. The CPICH is transmitted in parallel with the SCH and the P-CCPCH. The frame timing of SCH, P-CCPCH and CPICH are the same. The SCH is subdivided into a primary SCH, which carries a Primary Synchronization Code (PSC) sequence, and a second SCH, which carries a Second Synchronization Code (SSC) sequence. The PSC and SSC sequences are orthogonal to each other. They are generated using generic-level Golay sequences and hadamard sequences and are transmitted on top of each other (top). The PSC sequence is the same sequence for each slot and each node in the coverage area. The SSC sequence may be one of sixteen possible sequences in each slot. The P-CCPCH carries broadcast data such as the identity of the transmitting node and other information common to all user equipment communicating with the node. The CPICH is transmitted continuously in parallel with the SCH and the P-CCPCH. The CPICH carries a pilot signal that is known a priori. The pilot signal may be used by the user equipment to synchronize to the node and serve as a phase reference once the user equipment synchronizes to the node and successfully completes an access attempt to the system to coherently demodulate the data sent to the user equipment.

The pilot signal does not contain any data and is characterized as an unadjusted spread spectrum signal. The pilot signal from each node is typically spread with the same orthogonal code but scrambled with a different node-specific primary scrambling code. The primary scrambling code is truncated at the end of each CPICH frame and then repeated at the beginning of each frame. In the exemplary W-CDMA communication system, there are 512 possible primary scrambling codes for a given code. And the P-CCPCH is scrambled with the primary scrambling code. The primary scrambling code used by the node is not known a priori by the user equipment.

FIG. 3 is a functional block diagram illustrating the generation of SCH, P-CCPCH and CPICH by a node. The PSC generator 302 can be configured to generate a PSC sequence, comprising a predetermined 256 chip sequence, for use in slot timing acquisition for a node in a user equipment. The SSC generator 304 can be used to generate SSC sequences. The SSC sequence has two functions. First, the SSC sequence is used in the user equipment to represent the frame timing of the node. Second, the SSC sequence also provides a code group identifier that identifies the eight possible primary scrambling codes. The W-CDMA communication system uses 512 possible primary scrambling codes, with sixty-four code group identifiers. The SSC generator 304 first maps the group identifier to one of sixty-four possible fifteen-element codewords and then maps each codeword element, which may have sixteen different possible values, to one of sixteen possible 256-chip sequences. Each of the sixteen possible 256-chip sequences and the PSC sequence are orthogonal to each other. A summer 306 can be used to combine each of the fifteen 256-chip SSC sequences with the PSC sequence. A puncturing (puncturing) element 308 can puncture the PSC and SSC sequences from the adder 306 by a user into a first portion of each slot. A primary scrambling code generator 310 may be used to generate a primary scrambling code for the code. The broadcast data may then be scrambled with the primary scrambling code using a multiplier 312 and punctured into 2-10 portions of each slot using a puncturing element 308. An orthogonal code generator 314 may be used to generate the CPICH. The CPICH is then scrambled with the primary scrambling code using a multiplier 316. The scrambled CPICH may be combined with the SCH and the scrambled P-CCPCH from the puncture processing element 308, as well as other downlink channels using a summer 318.

Fig. 4 is a functional block diagram of an example user device operating within a W-CDMA communications environment. An RF Analog Front End (AFE)402 is coupled to one or more antennas 404, which may be used to support a radio communication link node. In receive mode, signals transmitted from the node are coupled from the antenna 404 to the AFE 402. The AFE filters and amplifies the signal, downconverts the signal to baseband, and digitizes the baseband signal.

The digital baseband signal may be provided to a searcher 406 for acquisition and synchronization. Acquisition of node slot timing involves searching the digital baseband signal for the PSC sequence embedded in the SCH. This may be achieved by correlating the digital baseband signal with locally generated OSC sequences. In a manner to be described in detail below, the frame timing can be extracted from the SSC sequence and used to determine which of the sixty-four possible code groups is subject to the primary scrambling code belonging to the node. Knowing the code group of the primary scrambling code, the searcher 406 can determine which primary scrambling code is actually used by the node. This can be obtained by correlating the digital baseband signal with eight a priori known versions of the pilot signal, which are generated by scrambling the eight possible primary scrambling codes of the code group and selecting the one with the highest energy at the correlation output. With information about slot timing, frame timing and primary scrambling code, the user equipment can use the CPICH for channel estimation and coherent demodulation of data sent to the user equipment.

A digital baseband signal may also be provided to the receiver 407. The receiver includes a demodulator 408 and a decoder 410. The demodulator 408 may be implemented in a variety of ways. By way of example, a rake receiver may be used in a W-CDMA communication system, or any other type of communication system that uses diversity techniques to combat fading. Rake receivers typically use independent fading of the resolvable multipaths to obtain diversity gain. This can be achieved by a combined effort of the searcher 406 and the rake receiver. In particular, the searcher 406 may be used to identify strong multipath arrivals of the pilot signal. Fingers may then be assigned by searcher 406 to identify timing offsets for the multipaths. Fingers may be used by the rake receiver as a timing reference for correlating the traffic for each expected multipath reflection. The separate correlations may then be coherently combined and provided to a decoder 410 for de-interleaving, decoding, and frame check functions.

The user equipment may also include a transmitter 411 to support the transmit mode. The transmitter 411 includes a data queue 412, an encoder 414, and a modulator 416. The data queue 412 may be used to buffer data for the user equipment to send to the node. The searcher-derived frame timing information may be used to release traffic from the data queue 412 with a time offset from the corresponding downlink frame. In a W-CDMA communication system, data is released from the data queue 412 by a first detectable multipath of the downlink from the reference node relative to transmission of a corresponding frame of a corresponding uplink in a manner that establishes a 1024 chip offset for frame reception. However, any offset may be used, depending on the particular application and the overall design parameters.

Data from the data queue 412 may be provided to an encoder 414 for encoding, interleaving, and frame check functions. The encoded data from encoder 414 may then be provided to a modulator 416, which spreads the data with an orthogonal code. The modulated data is then provided to AFFE 402, where it is filtered, upconverted, amplified, and coupled to an antenna 404.

Fig. 5 is a diagram of user equipment operating in a multi-node W-CDMA communication environment. In a W-CDMA communication environment, four nodes 502a-d are shown. Each node 502a-d transmits a pilot signal through its respective coverage area 504 a-d. The pilot signals transmitted by each node 502a-d may be spread with the same orthogonal code but scrambled with a different primary scrambling code. The primary scrambling codes allow the pilot signals to be different from each other so that the originating nodes 502a-d can be distinguished. The user equipment 506 is shown moving within different coverage areas with a set of broken lines. Assume that the user equipment 506 attempts to establish communication with the network. From the first coverage area 504a, the user equipment 506 searches the SCH and CPICH for acquisition and synchronization, as described above. After this point is completed, the user equipment 506 determines the wireless communication link quality to the node by measuring the strength of the pilot signal from the node. When the strength of the pilot signal exceeds the threshold, the user equipment 506 attempts to access the node 502a in the case where the pilot signal is from the first node 502 a. Depending on the available resources, the node 502a may establish a radio communication link to the user equipment 506 for downlink traffic transmission. Upon successful completion of the access attempt, the user equipment 506 joins the node 502a to its active set and establishes a wireless communication link to send traffic to the node. At this point, only node 502a is a member of the active set of user device 506. The user equipment 506 also uses this node 502a as a reference for synchronizing its uplink frames.

As the user device 506 moves into an area where the first and second coverage areas 504a-b overlap, the strength of the pilot signal from the second node 502b increases until it exceeds a threshold. As a result, the second node 502b may be added to the active set of the user equipment 506 and establish another radio communication link. In this case, the user equipment 506 communicates with the first and second nodes 502a-b, but the transmission of uplink frames by the user equipment 506 remains synchronized with the reference node 502 a.

As the user equipment 506 moves out of the first coverage area 504a, the pilot signal strength from the first node 502a decreases until it falls below a threshold, causing the reference node 502a to be removed from the active set of the user equipment 506. In at least one embodiment, the reference node is not removed from the active set immediately after the pilot signal strength is below the threshold. Rather, the pilot signal strength should remain below the threshold for a predetermined time and then the reference node is removed from the active set. This approach reduces the likelihood of the reference node being removed from the active set of the user equipment due to spurious signal level fluctuations. Once the reference node 502a is removed from the active set of the user equipment 506, the radio communication link between the two is torn down and the user equipment 506 wants to resynchronize the timing of its uplink frame to the downlink frame from the second node 502b using the frame timing information extracted from the PSC and SSC sequences embedded in the SCH channel and the multipath timing estimated from the CPICH of the second node 502 b. The second node 502b now becomes the reference node.

The user equipment 506 moves further towards its final destination and it enters an area where the second, third and fourth coverage areas 504b-d overlap. Within this region, the strength of the pilot signals from the third and fourth nodes 502c-d increases until they each exceed the threshold. As a result, the third and fourth nodes 502c-d may be added to the active set of the user equipment 506 and a radio communication link is established between the user equipment 506 and each of the third and fourth nodes 502 c-d. In this case, the user equipment 506 communicates with the second, third and fourth nodes 502b-d, but the transmission of uplink frames remains synchronized with the second node 502 b.

In the case where the user equipment 506 is synchronized to the second node 502b, ambiguity may arise in which of the two nodes 502c-d the user equipment 506 selects as the reference node when moving out of the second coverage area 504 b. This ambiguity can be resolved in a number of ways. For example, the user equipment 506 may resynchronize to the node that requires the least amount of slewing. In particular, the user equipment 506 may resynchronize to a node whose first multipath arrival at the beginning of the downlink frame is closest in time to the first multipath arrival beginning of the same downlink frame previously received from the second node 502 b.

FIG. 6 is a block diagram of an example searcher that may be used within the user device of FIG. 4. The searcher includes a PSC detector 602. Since the PSC sequence is the same for each slot, the PSC detector 602 may estimate slot timing by correlating the received digital baseband signal with a locally generated replica of the PSC sequence by methods known in the art.

The SSC detector 604 can be used to decode the SSC sequence by methods known in the art. In particular, the SSC detector 604 correlates the SSC in each slot (which may be one of sixteen possible sequences) over one or more frames to determine sixteen codeword elements. Based on the generated codeword, the SSC detector 604 can determine the first slot within the frame and, with slot timing information from the PSC detector 602, can determine the frame timing. The SSC detector 604 can also remap the code words to the code group identifier of the node primary scrambling code.

Pilot detector 606 may be used to correlate the digital baseband signal with a locally generated scrambled orthogonal code. Orthogonal code generator 608 may be used to generate eight possible scrambled orthogonal codes for the code group to which the node is assigned based on the code group identifier from SSC detector 604. One or more time slots of the received digital baseband signal may be correlated with each of the eight possible orthogonal codes until a pilot signal is detected, by means known in the art.

As a result of this process, multiple copies of the pilot signal from a node may be detected at different times due to multipath reflections. The timing generator 610 may be used to detect multipath of the pilot signal and assign the fingers to corresponding rake receivers (not shown). In communications involving multiple nodes, the frame timing for each node in the active set may be provided to the selector 612. The timing generator 610 may be used to select a frame timing for the first multipath arrival from the reference node. The selected frame timing is provided to an offset generator 614 to delay the transmission of the uplink from the reception of the corresponding downlink frame. In the example embodiment described, the delay is 1024 chips, although any delay may be used based on the particular application and overall design constraints. Offset generator 614 may be used to delay the release of data from data queue 412 (see fig. 4) accordingly. The offset generator 614 should be set to be slightly less than the desired delay to account for the processing delays of the searcher 406, encoder 414, modulator 416, and AFE 412 (see fig. 4).

In traffic communications between the node and the user equipment, the pilot detector 606 continues to search for new pilot signals. Once a new pilot signal with sufficient strength is detected, the originating node may be added to the active set. At the same time, pilot detector 606 continues to monitor the pilot signals from the active nodes using the particular primary scrambling code established for each pilot signal at the time of acquisition. If the pilot signal from any node falls below a predetermined threshold for an extended period of time, that node should be removed from the active set. In the event that the pilot signal from the reference node falls below a predetermined threshold, the pilot detector 606 may cause the timing generator 610 to select a new node from the active set as the timing reference. Assuming that two or more nodes remain active, the timing generator 610 should select the node that requires the least amount of slewing. This means that the timing generator 610 should select a node in which the first multipath arrival of the downlink frame is close in time to the first multipath arrival of the downlink frame previously sent from the previous reference node. This selection criterion should be used even if there is no more frame timing information from the SCH from the previous reference node. This can be done in a number of ways. In a user equipment with a rake receiver, finger assignments for a previous reference node may be used to select an appropriate node for resynchronization. Alternatively, the frame timing of the uplink transmission may be used to select the appropriate node for resynchronization. In particular, with respect to the latter, timing generator 610 may be used to establish a reference for frame timing for uplink transmissions. The reference may be advanced in time by 1024 chips to derive the first multipath arrival of the downlink frame from the previous reference node. This approach is an attractive solution because the frame timing of the uplink can be determined even when there is no longer a finger assignment for the previous reference node.

Regardless of the method used, timing generator 610 selects the new node as a reference to resynchronize its transmissions. A selector 612 may be used to select a frame timing for the first multipath arrival from the reference node. The selected frame timing may be provided to an offset generator 614 to delay the uplink transmission of the reception of the corresponding downlink frame from the new reference node. A trigger from the offset generator 614 may be used to release the traffic of the data queue 412 in the transmitter 411 (see fig. 4). The adjustment of the uplink transmission timing during resynchronization may be adjusted smoothly rather than instantaneously. Otherwise the node receiver cannot track the change in the transmission timing.

The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with: a general purpose processor, a Digital Signal Processor (DSP) or other processor, an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof, to implement the functions described herein. A general purpose processor is preferably a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary processor is preferably coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an application specific integrated circuit, ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal.

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (48)

1. A method of communication, comprising:

establishing a reference corresponding to the timing of the received signal from the first source;

determining a timing for each received signal from a plurality of second sources;

adjusting the timing reference of the received signal from a second source, the timing of the received signal being used to adjust the reference that is closest in time to the unadjusted reference; and

the signal is synchronized to the reference for transmission.

2. The method of claim 1, wherein the reference is adjusted after the signal from the first source is no longer received.

3. The method of claim 2, wherein the adjusting of the reference comprises: after no more received signals from the first source are received, the synchronized signal timing is used to determine the reference.

4. The method of claim 1, wherein the received signal from the first source comprises a frame and the unadjusted reference corresponds to a beginning of the frame.

5. The method of claim 4, wherein the received signals from the second sources each comprise a frame, and the adjusted reference corresponds to a start of frame of one of the second sources that is closest in time to the unadjusted reference.

6. The method of claim 5, wherein the synchronized signal is transmitted after the reference.

7. The method of claim 1, wherein the received signal from the first source comprises a plurality of multipath arrivals, and wherein the unadjusted reference corresponds to the first multipath arrival in time.

8. The method of claim 7, wherein the received signals from the second sources each comprise a plurality of multipath arrivals, and wherein the adjusted reference corresponds to the first multipath arrival of said one of the second sources that is closest in time to the unadjusted reference.

9. The method of claim 8, wherein the received signals from the first and second sources each comprise a frame, and wherein the unadjusted reference corresponds to a frame start of a first multipath arrival of the received signal from the first source, and the adjusted reference corresponds to a frame start of the first multipath arrival of the one of the second sources that is closest in time to the unadjusted reference.

10. The method of claim 9, wherein the synchronized signal is transmitted after the reference.

11. The method of claim 9 wherein the received signals from the first and second sources each include a pilot signal, the method further comprising determining the timing of multipath arrivals from the first and second sources using their respective pilot signals, and demodulating the received signals from the first and second sources using the determined timing, wherein adjusting the reference comprises using the timing of multipath arrivals to identify the first multipath arrival of said one of the second sources that is closest in time to the non-reference.

12. An apparatus, comprising:

a searcher for establishing a reference corresponding to the timing of the received signal from the first source, determining the timing for each received signal from the plurality of second sources, adjusting the reference to the timing of the received signal from one of the second sources, the timing of the received signal being used to adjust the reference that is closest in time to the unadjusted reference and to synchronize the signal to the reference for transmission.

13. The apparatus of claim 12, further comprising a receiver for receiving signals from the first and second sources.

14. The apparatus of claim 12, further comprising a transmitter for transmitting the synchronized signal.

15. The apparatus of claim 14, wherein the synchronized signal is transmitted after the reference.

16. The apparatus of claim 15, wherein the searcher further comprises an offset generator for generating a trigger that is delayed in time from the reference, the trigger for transmitting the synchronized signal.

17. The apparatus of claim 16, wherein the transmitter further comprises a data buffer for storing the synchronized signal, the data buffer for releasing the synchronized signal for transmission based on a trigger.

18. The apparatus of claim 12, wherein the searcher is further configured to adjust the reference after the signal from the first source is no longer received.

19. The apparatus of claim 18, wherein the searcher determines the adjustment of the reference from the timing of the synchronized signal after the received signal from the first source is no longer received.

20. The apparatus of claim 12, wherein the received signal from the first source comprises a frame, and wherein the searcher is configured to establish a reference corresponding to a start of the frame.

21. The apparatus of claim 20, wherein the received signals from the second sources each comprise a frame, and wherein the searcher is further configured to adjust the reference to start with the frame corresponding to said one of the second sources that is closest in time to the unadjusted reference.

22. The apparatus of claim 12, wherein the received signal from the first source comprises a plurality of multipath arrivals, and wherein the searcher is further configured to establish the reference to correspond to the first multipath arrival in time.

23. The apparatus of claim 22 wherein the received signals from the second sources each comprise a plurality of multipath arrivals, and wherein the searcher is further configured to adjust the reference to correspond to the first multipath arrival of said one of the second sources that is closest in time to the unadjusted reference.

24. The apparatus of claim 23 wherein the signals received from the first and second sources each comprise a frame, and wherein the searcher is further configured to establish the reference to correspond to the start of the frame of the first multipath arrival of the signal received from the first source and to adjust the reference to correspond to the start of the frame of the first multipath arrival of said one of the second sources that is closest in time to the unadjusted reference.

25. The apparatus of claim 23, wherein the received signals from the first and second sources each comprise a pilot signal, and wherein the searcher is further configured to determine the timing of multipath arrivals from the first and second sources using their respective pilot signals, the apparatus further comprising a demodulator configured to demodulate the received signals from the first and second sources using the timing of multipath arrivals determined by the searcher, and wherein the searcher is further configured to determine the adjustment of the reference signal using the timing of multipath arrival determination for said one of the second sources that is close in time to the unadjusted reference.

26. A computer readable medium embodying a program of instructions executable by a computer program for implementing a method of communication, the method comprising:

establishing a reference corresponding to the timing of the received signal from the first source;

determining a timing for each received signal from a plurality of second sources;

adjusting the timing reference of the received signal from a second source, the timing of the received signal being used to adjust the reference that is closest in time to the unadjusted reference; and

the signal is synchronized to the reference for transmission.

27. The computer-readable media of claim 26 wherein the reference is adjusted after the signal from the first source is no longer received.

28. The computer-readable medium of claim 27, wherein the adjusting of the reference comprises: after no more received signals from the first source are received, the synchronized signal timing is used to determine the reference.

29. The computer-readable medium of claim 26 wherein the received signal from the first source comprises a frame and the unadjusted reference corresponds to a start of the frame.

30. The computer-readable media of claim 29 wherein the received signals from the second sources each comprise a frame and the adjusted reference corresponds to the beginning of the frame of one of the second sources that is closest in time to the unadjusted reference.

31. The computer-readable media of claim 26 wherein the received signal from the first source comprises a plurality of multipath arrivals, and wherein the unadjusted reference corresponds to the first multipath arrival in time.

32. The computer-readable media of claim 31 wherein the received signals from the second sources each comprise a plurality of multipath arrivals, and wherein the adjusted reference corresponds to the first multipath arrival of said one of the second sources that is closest in time to the unadjusted reference.

33. The computer-readable media of claim 32 wherein the received signals from the first and second sources each comprise a frame, and wherein the unadjusted reference corresponds to a start of frame of a first multipath arrival of the received signal from the first source, and the adjusted reference corresponds to a start of frame of the first multipath arrival of said one of the second sources that is closest in time to the unadjusted reference.

34. The computer-readable media of claim 32 wherein the received signals from the first and second sources each include a pilot signal, the method further comprising determining the timing of multipath arrivals from the first and second sources using their respective pilot signals, and demodulating the received signals from the first and second sources using the determined timing, wherein adjusting the reference comprises using the timing of multipath arrivals to identify the first multipath arrival of said one of the second sources that is closest in time to the non-reference.

35. An apparatus, comprising:

reference means for establishing a reference corresponding to the timing of the received signal from the first source;

means for determining a timing for each received signal from a plurality of second sources;

adjusting means for adjusting the timing reference of the received signal from a second source, the timing of the received signal being used to adjust the reference which is closest in time to the unadjusted reference; and

means for synchronizing the signal to the reference for transmission.

36. The apparatus of claim 35, further comprising means for receiving signals from the first and second sources.

37. The apparatus of claim 35, further comprising transmitting means for transmitting the synchronized signal.

38. The apparatus of claim 35, further comprising transmission means for transmitting the synchronized signal after the reference.

39. The apparatus of claim 38, further comprising a trigger offset in time from the reference, the trigger being used by the transmitting means for transmitting the synchronized signal.

40. The apparatus of claim 39, wherein the means for transmitting further comprises means for storing the synchronized signal, the means for buffering being configured to release the synchronized signal for transmission based on a trigger.

41. The apparatus of claim 35 wherein the adjusting means is further for adjusting the reference after the signal from the first source is no longer received.

42. The apparatus of claim 41 wherein the means for adjusting further comprises means for determining the adjustment of the reference from the timing of the synchronized signal after the received signal from the first source is no longer received.

43. The apparatus of claim 35, wherein the received signal from the first source comprises a frame, and wherein the reference means is for establishing a reference corresponding to a start of the frame.

44. The apparatus of claim 43 wherein the received signals from the second sources each comprise a frame, and wherein the adjusting means is further for adjusting the reference to begin with the frame corresponding to said one of the second sources that is closest in time to the unadjusted reference.

45. The apparatus of claim 35 wherein the received signal from the first source comprises a plurality of multipath arrivals, and wherein the searcher is further configured to establish the reference to correspond to the first multipath arrival in time.

46. The apparatus of claim 45 wherein the received signals from the second sources each comprise a plurality of multipath arrivals, and wherein the adjusting means is further for adjusting the reference to correspond to the first multipath arrival of said one of the second sources that is closest in time to the unadjusted reference.

47. The apparatus of claim 46 wherein the signals received from the first and second sources each comprise a frame, and wherein the searcher is further configured to establish the reference to correspond to the start of the frame of the first multipath arrival of the signal received from the first source and to adjust the reference to correspond to the start of the frame of the first multipath arrival of said one of the second sources that is closest in time to the unadjusted reference.

48. The apparatus of claim 46 wherein the received signals from the first and second sources each comprise a pilot signal, and the means further comprises means for determining the timing of multipath arrivals from the first and second sources using their respective pilot signals, means for demodulating the received signals from the first and second sources using the timing of multipath arrivals determined by the searcher, and wherein the means for adjusting is further for determining the adjustment of the reference signal using the timing of multipath arrival determination for said one of the second sources that is temporally close to the unadjusted reference.