CN101569140A - Enhanced coexistence beacon protocol (ECBP) for precise inter-cell synchronization of overlapping wireless base stations - Google Patents

Abstract

A method for synchronizing overlapping cells covered by different base stations, comprising: obtaining a propagation delay between the first base station and the second base station; scheduling, by the first base station, a selected one of first users located in a first cell covered by the first base station to transmit a first data packet as part of a transmission data frame; transmitting, at the selected one of the first users, a transmission offset indicating a difference in a time domain between a start of the transmission data frame and the first data packet; recording, in response to receipt of the first data packet, a receive offset at a selected one of the second users located in a second cell covered by the second base station, the receive offset indicating a difference in a time domain between a start of a received data frame and the first data packet; transmitting the reception offset to the second base station at the selected one of the second users; calculating a frame slip at the second base station by adding the transmit offset and a propagation delay and subtracting the receive offset; and synchronizing the first cell and the second cell based on the calculated frame slips.

Description

Enhanced Coexistence Beacon Protocol (ECBP) for precise inter-cell synchronization of overlapping radio base stations

technical field

The present invention relates to a method for synchronizing overlapping cells over the air, and more particularly, to a method for precisely synchronizing overlapping cells by taking into account the propagation delay between a transmitter and a receiver.

Background technique

Time-division duplexing (TDD) uses time-division multiplexing to separate outgoing and return signals. Similarly, frequency division duplexing (FDD) uses frequency division multiplexing to separate outgoing and return signals. For both FDD and TDD systems, it is recommended that all base stations (BS) be time synchronized with the common network timing signal. In the event of loss of the network timing signal, the BS should continue to operate and should automatically resynchronize with the network timing signal when the network timing signal is restored.

Self-coexistence occurs when multiple wireless systems of the same type exist simultaneously (ie, coexist). In the case of IEEE 802.22, self-coexistence means the coexistence of multiple overlapping IEEE802.22 cells.

Accurate network time synchronization is important in general TDD systems, especially for self-coexistence situations. For example, when two units of Customer Premises Equipment (Custom Premises Equipment), CPE_a and CPE_b, are located within the overlapping cell range between two BSs BS_a and BS_b, if the two BSs are not synchronized, then between CPE_a and BS_a The uplink transmission between CPE_b and BS_b will destroy the downlink reception between CPE_b and BS_b. In addition, precise network time synchronization is required to support soft handover, mobility management, inter-cell communication and positioning. Moreover, in the IEEE 802.22 system, for reliable spectrum detection, it is expected that adjacent cells are simultaneously silent in order to establish a system-wide silent period. The more accurate the neighbor cell synchronization is, the less guard time is required to align the system wide silent periods or coexistence slots.

In IEEE 802.22, a method for synchronizing over-the-air overlapping base stations is disclosed. However, in this method it is assumed that the propagation delay between two units of user equipment is negligible. However, this assumption is not always reasonable. In fact, the 10Km distance between two units of user equipment may cause a propagation delay of about 33 microseconds, which obviously cannot be ignored. Therefore, it is necessary to take propagation delay into account when performing time synchronization.

Contents of the invention

technical problem

It is therefore an object of the present invention to provide an improved method and arrangement for data transmission.

Another object is to provide improved methods and arrangements for data transmission to achieve precise time synchronization especially in intra-BS and inter-BS communications.

Another object is to provide a method and apparatus capable of enhancing self-coexistence and synchronization between overlapping base stations.

According to an aspect of the present invention, there is provided a method for synchronizing overlapping cells, wherein, at a first node, a first data packet is sent and a sending offset is indicated at the start of sending a data frame and at the first A difference in the time domain between data packets, the first node being one of the first base station and the first user. The first user is located in a first cell covered by a first base station. In response to reception of the first data packet, a reception offset is recorded at a second user located in a second cell covered by the second base station, the reception offset indicating the start of the frame at the second base station and the first The difference in the time domain between data packets. Then, the reception offset is sent at the second user to the second base station. The propagation delay between the first node and the second user is then obtained. The frame slide is calculated at the second base station by adding the transmit offset and the propagation delay and subtracting the receive offset. Finally, the first cell and the second cell are synchronized based on the calculated frame slip.

The step of obtaining the propagation delay between the first node and the second user may comprise: sending at the first node a data packet carrying information about the geographic location of the first node; calculating a propagation delay of the data packet at the second user based on the location of a node; and sending the propagation delay at the second user to a second base station.

When one of the first node and the second user does not have knowledge of its own geographic location, round-trip communication can be used to calculate the propagation delay between the first node and the second user without knowledge of the geographic location of the first node Information.

The step of obtaining the propagation delay between the first node and the second user may comprise: generating and broadcasting a location database containing geographic location information of a plurality of users associated with respective identifiers located within the first cell; The user sends a data packet carrying the identifier of the first user; in response to the receipt of the data packet, the geographic location information of the first user is retrieved from the location database according to the identifier at the second user; calculating a propagation delay from the information of the location of the first user; and transmitting the propagation delay at the second user to the second base station.

The step of obtaining the propagation delay between the first node and the second user may comprise selecting, by the first base station, a subset of the first users for data transmission, the first users being located in a first cell covered by the first base station In the first time interval, a subset of first users competes for sending the first reference data packet; the first reference data packet is sent by a specific first user who wins the competition; selected by the second base station for a subset of second users for which data is received, the second users being located in a second cell covered by a second base station; in response to the receipt of the first reference data packet, selected second users during a second time interval A subset of is competing for sending a second reference data packet; the second reference data packet is sent by a specific second user who wins the competition; at the specific one first user, the number of times between the specific one first user and a propagation delay between the specific one of the second users; and sending the propagation delay at the specific one of the first users to the second base station. The specific one first user may send a third reference data packet. A first user selected when establishing cell synchronization can compensate for the processing delay of the first data packet.

The step of obtaining the propagation delay between the first node and the second user may include: scheduling the first user by the first base station to transmit the first reference data packet; scheduling the second user by the second base station to receive the first reference data packet ; the second user is scheduled by the second base station to send a second reference data packet; and in response to receiving the second reference data packet, calculating the propagation delay of the reference data packet at the first user; The user sends the propagation delay to the second base station.

The first cell and the second cell may be overlapping.

According to another aspect of the present invention, a wireless communication system may be configured to have: a first base station defining a first cell; and a second base station defining a second cell. The second base station is synchronized with the first base station through the following steps: the first base station schedules a first user selected from a plurality of first users located in the first cell to send the first data as a part of the transmission data frame grouping; sending an indication at a selected first user place at the beginning of the transmission data frame and the transmission offset of the difference in the time domain between the first data packet; in response to the reception of the first data packet, A reception offset is recorded at a selected one of the plurality of second users located in the second cell, the reception offset indicating the distance between the start of the frame of the second cell and the first data packet difference in the time domain; sending the receive offset at said selected one second user to a second base station; obtaining said selected one first user and said selected one second user Propagation delay between; calculating frame slip at the second base station by adding the transmit offset to the propagation delay and subtracting the receive offset; and synchronizing the first cell and the second cell based on the calculated frame slip .

When said one second user has knowledge of its own geographical position, said one first user may send a second data packet carrying information about the distance between said one first user and said one second user .

According to yet another aspect of the present invention, there is provided a method for synchronizing overlapping cells, wherein a first data packet is sent by a first node, the first data packet has location information of the first node and indicates that in the first cell The transmit offset of the difference in the time domain between the start of the transmit data frame and the first data packet. When the second node receives the data packet, the second node records a reception offset indicating the difference in the time domain between the start of the frame and the first data packet in the second cell. Then, a propagation delay between the first node and the second node is obtained based on the location information of the first node. Finally, the frame slip is calculated by the second node by adding the transmit offset and the propagation delay and subtracting the receive offset.

According to still another aspect of the present invention, a wireless communication system may be configured to have: a first node constructing a first cell; and a second node constructing a second cell. The second node is synchronized with the first node by the following steps: a first data packet is sent by the first node, the first data packet has position information of the first node and indicates that in the first cell at the beginning of the transmitted data frame and The transmit offset of the difference in the time domain between the first data packets; the data packets are received by the second node and the receive offset is recorded, indicating the start of the frame in the second cell The difference in the time domain between the position and the first data packet; obtain the propagation delay between the first node and the second node based on the position information of the first node; and send the offset and the second node by the second node The frame slip is calculated by adding the propagation delay and subtracting the receive offset.

Description of drawings

A more complete understanding of the invention, and its numerous advantages which follow, will become apparent and better understood by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings, in which like reference characters indicate the same or like elements, in which:

Figure 1 illustrates an example of a typical cellular network (including IEEE 802.22) deployment configuration;

Figure 2 illustrates the structure of the CBP packet defined in IEEE 802.22;

Figure 3 illustrates a method for synchronizing overlapping base stations, which does not take into account the propagation delay between the CBP transmitter and the CBP receiver;

Figure 4 illustrates the establishment of synchronization between overlapping base stations;

Figure 5 illustrates communication between two synchronized overlapping cells;

Figure 6 illustrates the scenario of coexistence transmission;

Figure 7 illustrates a method for precise synchronization of overlapping cells;

Figure 8 illustrates another scenario for coexistence transmission;

Figure 9 illustrates a method for synchronizing overlapping cells according to an embodiment of the invention; and

Figure 10 illustrates a specific example for synchronizing overlapping cells.

Detailed ways

Aspects, features, and advantages of the invention will become apparent from the following detailed description, which illustrates a number of specific examples and implementations, including the best mode contemplated for carrying out the invention. The invention is also capable of other and different embodiments, and its several details can be modified in various obvious respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and descriptions are to be regarded as illustrative in nature and not restrictive. The present invention is illustrated by way of example and not limited by the accompanying drawings.

This application is incorporated by reference into IEEE 802.22: Part 22 of the Draft Standard for Wireless Area Networks.

It can be noticed that in §6.21.2 of the IEEE 802.22 standard, unlike other IEEE 802 standards, which only consider the self-coexistence (self-coexistence) problem after the specification is basically completed, IEEE 802.22 takes a proactive approach (as in its requirements document ) and requires that MAC should include self-coexistence protocols and algorithms as part of the initial standard concepts and definitions. As shown in Figure 1, multiple 802.22BSs and CPEs can operate in the same neighborhood, and self-interference can render an 802.22 system useless if proper measurements are taken at the air interface layer. Even with directional antennas at the CPE (although this may be implementation dependent), the self-coexistence problem is not fully overcome. This is exacerbated by the fact that 802.22 can have a coverage range of up to 100Km, so its interference range and impact on other 802.22 deployed cells is greater than any other unlicensed technology that exists.

Figure 1 illustrates the scenario of coexistence transmission. Customer Premises Equipment a (CPE_a) 105 and CPE_b 106 are located in an overlapping area between Cell_a 103 covered by Base Station a (BS_a) 101 and Cell_b 104 covered by BS_b 102. CPE_a 105 sends a coexistence beacon protocol (CBP) packet in response to the direction of BS_a 101. CPE_b 106 receives the CBP packet and relays the message to its BS_b. The propagation delay between BS_a 101 and CPE_a 105 is τ1; the propagation delay between BS_b 102 and CPE_b 106 is τ2; and the propagation delay between CPE_a 105 and CPE_b 106 is τ3.

There are two types of beacon frames that can be used for intra-BS and inter-BS communication, and are defined as superframe headers, sent by the BS at the beginning of each new superframe. BS beacons are sent only by BSs. For inter-BS (alternatively inter-WRAN) communication, CBP packets are utilized and can be sent by CPE and BS.

CBP packets are sent for the purpose of improving self-coexistence in neighboring IEEE 802.22 cells. These packets are sent under the control of the BS during the active self-coexistence window and share the same beacon MAC header. Since the purpose of CBP packets is to improve self-coexistence, these CBP packets are sometimes referred to as self-coexistence beacons.

In general, CBP packets provide information about the current cell and CPE traffic flow to the BS. In particular, it is the responsibility of the Beacon IE to convey information about the CPE's traffic flow to its BS, which is carried in the CBP packet payload and should be present in every CBP packet sent by the CPE.

A CBP packet can carry one or more information elements in the payload. Since the Beacon IE provides the basic information needed to enable self-coexistence, the CBP packet sent by the CPE should carry at least one Beacon IE in its payload. The CBP packet sent by the CPE may also carry a single DC/US boundary IE. The CBP packet sent by the BS should carry at least one DS/US boundary IE, and may also carry one or more beacon IEs.

The Beacon IE provides necessary and sufficient information about the traffic of the CPE reserved for the BS (the CPE does not have to send CBP packets without traffic reserved for the BS). At least one Beacon IE shall be included in every CBP packet sent by the CPE. Stations (CPE or BS) that belong to other IEEE 802.22 BSs and receive CBP packets can then improve coexistence in BSs through various mechanisms such as interference-free scheduling.

In order to deal with the severe self-interference problem that may occur in actual deployment scenarios, the CBP protocol should be utilized. CBP is the best-of-breed protocol based on the transmission of coexistence beacons. Assuming this mechanism is used for synchronization of overlapping BSs and assuming the fact that the CPE does not continuously maintain a lock on the BS, the successful delivery of the coexistence beacon transmission has a high probability.

Various mechanisms can be implemented on top of CBP, such as lessee/provider algorithms and rules for channel allocation.

Figure 2 shows the structure and transmission of a CBP packet. As shown in FIG. 2, a CBP packet 100 includes a preamble 121, an SCH 122, and a CBP MAC protocol data unit (PDU) 123. The CBP packet 100 starts with a preamble 121 which should be common throughout the IEEE802.22 network and is different from the preamble of a superframe. After the preamble 121, a SCH 122 transmission follows. By sending both the SCH 121 (which contains information about IEEE 802.22 cells) and the CBP MAC PDU 123 (which contains information about the CPE's reservations to its BS), the sent CPE conveys all the necessary information to allow better self-coexistence .

In CBP, 802.22 entities (ie, CPE and BS) can send beacons that provide enough information to receivers to achieve satisfactory and good coexistence in overlapping 802.22 cells. These beacons are intended for inter-cell communication and carry specific information about the CPE's attached cell and downstream/upstream bandwidth allocation to the BS.

In CMAC, coexistence beacons are scheduled by using coexistence interval usage codes (IUCs) (both passive and active) that can be specified in upstream (US)-MAP and downstream (DS)-MAP messages. When scheduling coexistence beacons, the Connection ID contained in the MAP IE indicates which CPE should send the beacon within the specified scheduling time. The connection ID can be unicast (eg, the primary connection ID of the CPE), multicast (ie, the multicast management connection ID), or even a broadcast ID. In the case of multicast, the BS can implement a cluster algorithm that improves spectrum utilization and maximizes the efficiency of coexistence beacons, since multiple CPEs will send coexistence beacons during the same scheduled time. Regardless of the type of connection ID, the CPE should always verify whether the connection ID specified by the BS includes itself. This will determine the behavior of the CPE during the scheduled coexistence IUC.

Coexistence IUC defines the time period in which channel access is contention based. In other words, during this time, the CPE should use the contention access mechanism (see 0) to gain access to the medium and send coexistence beacons. The contention-based access mechanism is preferred for sending coexistence beacons because it maximizes spectrum utilization. In most cases, it is expected that the BS will not schedule only a single CPE to send a beacon, but also will not attempt to send a beacon by scheduling multiple CPEs in the same time span (a multicast management connection can be used for this purpose) to improve coexistence. Furthermore, when combined with the clustering algorithm, the contention-based efficiency is maximized, since for the same coexisting IUC, the BS will only schedule CPEs that do not belong to the same physical cluster.

In order to maximize the probability of receiving co-existing beacons from other configured 802.22 cells, the CPE does not maintain a lock on the BS all the time during a frame. The CPE will lock to the BS only when it is scheduled to receive/transmit data from/to the BS, as indicated by US-MAP and DS-MAP messages. At other times during this frame, the CPE will listen to the medium and search for coexistence beacons. Thus, the probability of success and the efficiency of CBP is greatly increased. In case the CPE loses synchronization to the BS while listening/receiving the coexistence beacon, it should re-acquire synchronization at the beginning of the following frame, which causes fewer, if any, side effects.

Another mechanism that can be used by the BS to find the CBP beacon is to schedule the coexistence UIC in passive mode. Essentially, passive mode defines a time where the CPE does not perform any transmissions but just listens to the medium while looking for CBP packets and BS SCH beacons (if possible).

It is important to note that in order to increase the efficiency of CBP, the downstream/upstream bandwidth allocation made by the BS to the CPE in certain frames should not change for many consecutive frames. This ensures that the information carried in the coexistence beacon is valid for at least a minimum duration, allowing the recipient of the coexistence beacon sufficient time to implement the self-interference mitigation mechanisms described below. Additionally, any time the BS has to allocate bandwidth to a CPE, it should always seek to allocate bandwidth to the CPE based on previous allocations (if any). That is, the BS should always allocate bandwidth to CPEs using approximately the same combination of time slots and logical channels. This will reduce the number of coexistence beacons that need to be sent by the CPE, since its neighbors will already have information about the assignment since it has not been changed by the BS. Other optimizations may also improve the efficiency of CBP and make the transmission of coexistence beacons less frequent.

Once a CPE receives coexistence beacons from other configured CPEs belonging to different cells, it can use this information in a number of ways in order to improve coexistence. The first thing the CPE wishes to do is communicate the received information to the BS. In turn, the BS will implement a so-called "no-interference" scheduling algorithm that schedules various upstream/downstream traffic to and from a CPE in such a way that these allocations do not intersect allocations of that CPE interfering with other CPEs. Another use of this information is for bandwidth request purposes. In this case, the CPE may include constraints when requesting upstream bandwidth allocation from the BS, providing the information that the BS will need to avoid allocating time to this CPE that interferes with other configured CPEs.

Yet another option is that the CPE does not send anything to the BS. Here, the BS will have to send a Traffic Restriction Request (TRC-REQ) message exclusively to the CPE requesting any restrictions (that it may have on this allocation). Other uses than these are also possible.

As discussed later, CBP packets are used in CMAC for multiple purposes, such as establishing and maintaining synchronization, and for self-coexistence. Therefore, the process at the BS to decide when and in which mode (ie, passive or active) to schedule self-coexisting IUC is mostly implementation dependent and is recommended to be done every frame during normal operation.

However, CMAC can provide necessary information to assist the BS in the decision process. In this case, it is recommended to use the CPE statistics report as the basis for triggering the enforcement of CBP. For example, the decision criteria may be defined such that if the Packet Error Rate (PER) experienced by one or more CPEs (clustering may be used here) exceeds a predetermined threshold per CBP, the BS is triggered to assume at least the The corresponding CPE schedules the coexistence IUC.

Another simple strategy for the BS is to implement a pseudo-random process, where the self-coexistence window is statistically scheduled with a certain frequency, but the mode (ie, whether it is passive or active) is pseudo-random. This process is called pseudo-random because it enables other statistics, such as traffic patterns, to be considered in the decision process.

A key aspect that can undermine all existing solutions for coexistence in reservation-based wireless systems is the lack of synchronization among co-channel overlapping BSs. Synchronization is a difficult problem to solve, but the benefits gained from having reliable synchronization are significant and worth pursuing.

Traditionally, synchronization among overlapping BSs has been resolved over the backhaul. This simplifies both the PHY and MAC design, but it has one major drawback, which is the fact that it relies on third parties. In the specific case of IEEE802.22, another key flaw includes the fact that the technology would utilize license-exempt operations, and therefore the existence of a common backbone among competing operators serving a given location is extremely unlikely and impossible is assumed. This situation is further exacerbated by the greater coverage expected from IEEE 802.22 cells.

Since coexistence is key in IEEE 802.22, synchronization becomes very important in order to allow IEEE 802.22 systems to operate at their peak performance. In the case of IEEE 802.22, synchronization benefits both cases of incumbent protection and self-coexistence. In the case of obligatory protection, synchronization is beneficial because it allows silent periods of overlapping BSs to be synchronized. This will further enhance the obligation detection probability, which would otherwise be compromised if the overlap occurred randomly. In the case of self-coexistence, synchronization will make the self-coexistence mechanism more efficient, thereby providing efficient sharing of radio resources of overlapping IEEE802.22 cells.

A mandatory scheme that allows overlapping BSs to synchronize by aligning their frames in time solves this problem using an over-the-air approach, in the sense that the scheme does not rely on any kind of fixed backhaul infrastructure. However, that's not to say that the scheme can't work on the backhaul.

For any synchronization scheme to be efficient, certain constraints should be imposed on the overall frame timing. In the specific case of CMAC, superframes should have the same and fixed length in terms of time, or at least should be integer multiples of each other. Frames within a superframe should also have the same and fixed size, which should at least be an integer multiple of each other. This will not only help to establish synchronization in overlapping cells, but more importantly, will maintain synchronization with lower overhead.

Assume no GPS device is available for IEEE 802.22BS. If such devices are available, synchronization can be accomplished by imposing the additional requirement that the BS should only initiate superframes at certain points in time.

An IEEE 802.22 cell shall actively seek other overlapping IEEE 802.22 cells in order to establish synchronization, and provide techniques by which other configured cells can find it. In addition to the ability of the CPE to scan for beacons whenever the CPE is not transmitting or receiving, two other mechanisms should also be used for this purpose, which will greatly increase the probability of successful synchronization: Self-coexistence quiet periods and self-coexistence windows.

In addition to the silent period for obligation detection, the BS should also schedule the silent period for the purpose of self-coexistence, and it is referred to here as the self-coexistence silent period. In general, however, these silent periods need not be as frequent as the obligatory detection silent periods, although the BS has complete freedom to choose their occurrence. In practice, self-coexisting silent periods should be scheduled during low peak times, such as nighttime, without causing any impact on system performance and responsiveness. During this period, both the CPE and the BS will search for CBP or SCH packets sent by overlapping IEEE 802.22 terminals belonging to other IEEE 802.22 cells. As long as the BS is powered on, it should never schedule a self-coexistence window (ie, CBP transmission) in aggressive mode before at least one self-coexistence quiet period. This is done to ensure that there is a high probability that a new IEEE 802.22 cell should first synchronize with any other configured IEEE 802.22 cell before announcing its existence via a CBP packet.

Self-coexistence quiet periods should always be scheduled within the boundaries of a superframe, and should be scheduled in a random manner to increase the probability of overlapping BSs successfully detecting each other. The duration of the silent period will typically be one frame. The BS shall randomly pick a frame number between [0, FS-1], and the superframe number shall be obtained from [0, NSTQP], where NSTQP is the number of superframes in the obligatory silence period. NSIQP can be easily derived from the TTQP field. By doing this, we should enforce self-coexistence quiet periods as often as obligatory quiet periods. Obviously, this can be dynamically changed by the BS if the increase or decrease in the number of overlapping BSs can be estimated (eg PER statistics reported via CPE or backhaul signaling).

A self-coexistence window should also be used for this purpose and should always be scheduled by the BS at the end of a US subframe. The first key difference between the self-coexistence quiet period and the self-coexistence window is the temporal granularity. The former will generally take up at least an entire frame, while the latter occurs within portions of a frame. The second and probably most notable difference is that the CPE and BS do not perform any type of transmission during the self-coexistence quiet period, but only sense the channel. However, during the self-coexistence window, the CPE can send CBP packets if scheduled by the BS (in case of self-coexistence in proactive mode). A decision procedure can be used in the BS to determine whether to schedule a passive or active self-coexistence period.

For each CBP or SCH packet received, the BS and CPE should record the frame offset as it was received. The accuracy of this record is critical to a successful synchronization. Figure 3 illustrates the relationship between the transmit offset and receive offset fields for a frame size FS (in symbols) where the propagation delay between BS1 and BS2 is sufficiently small to be ignored. In Fig. 3, the transmit offset is marked relative to the start of the (super)frame of BS1 and the receive offset is marked relative to the start of the (super)frame of BS2. These fields are important to establish synchronization between two overlapping cells.

In order for this fully distributed synchronization process to converge within an acceptable duration, the BS should apply convergence rules before any synchronization attempt. Correct application of this convergence rule should ensure network convergence in all scenarios. Mathematically, the BSi responsible for cell i should only try to synchronize with the BSj responsible for neighboring cell j if and only if Math. 1 is satisfied.

【Mathematical formula 1】

$\left|\begin{array}{c}<span\; class="notranslate">\; (</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; FS</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; frame</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Number</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; FDC</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; +</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; FD</span>}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; Reception</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Offset</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\\ <span\; class="notranslate">\; (</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; FS</span>}}_{\mathrm{<span\; class="notranslate">\; j</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; frame</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Number</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; j</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; FDC</span>}}_{\mathrm{<span\; class="notranslate">\; j</span>}}<span\; class="notranslate">\; +</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; FDC</span>}}_{\mathrm{<span\; class="notranslate">\; j</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; Transmission</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Offset</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; )</span>\end{array}\right|<span\; class="notranslate">\; \&le;</span>\frac{{\mathrm{<span\; class="notranslate">\; FS</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; FDC</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; GuardBand</span>}<span\; class="notranslate">\; \&times;</span>\mathrm{<span\; class="notranslate">\; SymbolSize</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}$

where Frame_Number _i is the frame number in which the CBP packet is received, Frame_Number _j is the frame number in which the CBP packet is sent and should be -1 in case of SCH, GuardBand is the guard band to accommodate e.g. propagation delay, FDC is the frame Duration code, and FS is the number of frames per superframe.

Given the following requirements: FSi = FSj = FS and FDCi = FDCj = FDC, we can further simplify this equation to Math 2:

【Mathematical formula 2】

$\left|\begin{array}{c}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; frame</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Number</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; j</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; frame</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Number</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&times;</span>\mathrm{<span\; class="notranslate">\; FDC</span>}<span\; class="notranslate">\; +</span>\\ \mathrm{<span\; class="notranslate">\; Transmission</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Offset</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; Reception</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; Offset</span>}\end{array}\right|<span\; class="notranslate">\; \&le;</span>\frac{\mathrm{<span\; class="notranslate">\; FS</span>}<span\; class="notranslate">\; \&times;</span>\mathrm{<span\; class="notranslate">\; FDC</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; GuardBand</span>}<span\; class="notranslate">\; \&times;</span>\mathrm{<span\; class="notranslate">\; SymbolSize</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}$

Therefore, the BSi should apply this convergence rule to each and every available synchronization replacement. Only those that satisfy this rule are able to proceed to the next stage.

Even after applying the convergence rule to all possible simultaneous alternatives, it is unlikely, though unlikely, that multiple choices will still satisfy the convergence rule. The BS should attempt to synchronize with each overlapping BS one at a time. Unless the BS recognizes that it is already synchronized with the overlay network corresponding to the selected packet (i.e., slippage equal to 0 or FS - as shown below - or billed), the BS should immediately construct and send an FSL-REQ message as a response broadcast by all CPEs in the cell, and no additional active mode self-coexistence intervals should be scheduled before the time of the scheduled frame slip.

During construction of the FSL-REQ message, the Slip Count and Slip Amount fields should be configured as shown in Figure 4. The slip count should be equal to the number of frames just before the superframe start of the overlapping BS, and the slip amount will be equal to the number of slots to the superframe start of the overlapping BS. More specifically, the slip field and the direction field in the FSL-REQ message should be set according to the following formula 3 rules:

【Mathematical formula 3】

Mathematical formula 4

【Mathematical formula 4】

On the CPE side, it should report back to the BS about any received CBP or SCH packets, unless it simultaneously receives a FSL-REQ message from the BS, in which case it should terminate the self-coexistence procedure and return to normal operation. Regardless of whether the BS itself detects the CBP or SCH packets, or if these are received as a result of a CPE report, the BS should proceed in the same manner as above during an attempted synchronization. The key difference at the BS is due to the fact that CPEs receiving the same SCH and CBP packets experience different propagation delays, so they will report different values for the reception offset. In this case, one group is selected depending on the direction of the BS, since these groups refer to the same network. In order to cope with different propagation delays, this essentially consists of guard bands allowing equations (1) and (2).

The BS should always behave the same whenever the frame is swiped left or right. That is, at the scheduled sliding time, the BS should initiate the transmission of a new superframe (shown in FIG. 4 ). Thus, frame slippage does not disrupt any data communication, and the BS scheduler should take this slippage into account when scheduling US and DS transmissions.

Frame sliding operations may result in incomplete frames as shown in Figure 4. It is up to the BS to use the incomplete frame where appropriate. For example, the BS can use this time as a silent period. Another possibility is that the scheduler at the BS has the ability to schedule data transmission during this time so that no broadcast time is wasted. Yet another option would be to reserve the incomplete frame as idle time, which is a simple strategy when the incomplete frame is only a few slots.

Once synchronization is complete, maintenance is a simpler process. Once the FSL-REQ message is valid and the frame is shifted, the BS should schedule the self-coexistence window at a certain period and always at the end of the US subframe.

Confirmation and maintenance of synchronization is performed by periodic CBP packet transmission and reception during the self-coexistence window. Synchronization is done and confirmed once the first CBP packet is successfully received from the overlapping cell. At this point, the BS should continue to schedule the self-coexistence window, but now the main purpose is to get better self-coexistence through the exchange of traffic restrictions. Of course, a positive side effect of sending and receiving CBP packets is that overlapping cells can more easily maintain synchronization. This will effectively provide the best performance gain for all synchronized IEEE 802.22 cells. Figure 5 shows an example of how two synchronized 802.22 cells communicate over the air through a self-coexistence window.

Given the large propagation delays in IEEE 802.22 networks, synchronization from the BS point of view does not imply exact synchronization to all CPEs. This is due to different propagation delays experienced by different CPEs. To account for this inconsistency in propagation delay, and to accommodate preamble transmission and line contention compensation intervals, the BS should schedule the self-coexistence window with an appropriate guard band, which is suggested to be at least 3 slots. In this way, the BS can provide a guard band and take into account the worst-case scenario of transmission/reception of at least one CBP packet.

If frame slipping within a cell is the result of reports from one or more CPEs, those CPEs should be responsible, together with the BS, for keeping track of synchronization. During the maintenance phase, the CPE should periodically send/receive CBP packets to/from synchronized cells in order to confirm continued synchronization. During this time, the CPE should not report each and every received CBP packet to the BS. Instead, it should limit its control information exchanges with the BS to only those necessary for better self-coexistence and to achieve "interference-free scheduling".

In the synchronization method shown in Figure 6, CPE_a 105 transmits CBP contained within a superframe. In addition, CPE_a 105 marks the "transmit offset" (Tx offset) of the CBP packet at the beginning of the superframe relative to Cell_a 103. The information of sending offset can be delivered by CPE_a 105 (or BS_a 101). The transmission offset may include information in a MAC header, or an information element, or a protocol data unit (PDU) of a CBP packet, or a predetermined signal. The transmit offset can indicate an offset (in slots) relative to the start of the first slot in which the PHY PDU (including the preamble) of the current frame is transmitted. After detecting the CBP packet, CPE_b 106 marks a "Receive Offset" (Rx Offset) at the beginning of the superframe relative to Cell_b 104. CPE_b 106 reports this event to BS_b 103. Given the following two assumptions: (1) both transmit and receive offsets are used cell-wide; and (2) the propagation delay between CPE_a 105 and CPE_b 106 is negligible, then BS_b 102 knows the transmit offset The difference between the amount and the received offset, so that the superframe timing of Cell_a is used to calibrate the superframe timing of Cell_b. For the first assumption, although the CBP packet is sent by CPE_a 105, CPE_a 105 has ensured that the send offset is relative to BS_a 101. Therefore, BS_b 102 can always assume that the transmit offset is defined relative to BS_a 101. The same concept applies to receive offsets. That is, although the reception offset is measured by CPE_b 106 and reported to BS_b 102, BS_b 102 can always ensure that this reception offset is defined relative to itself, because BS_b 102 or CPE_b 106 can take into account the propagation delay between them so that the final reception offset Shifts are relative to BS_b102. This is typically achieved through a ranging process. In other words, regardless of how the CBP packet is transmitted and received, both the transmission offset and the reception offset are assumed to be relative to the corresponding base station.

FIG. 7 shows another example of a synchronization method of overlapping cells. As shown in Figure 7, this does not involve CPE. BS_a 101 sends CBP packet 100 directly to BS_b 102. A CBP packet 100 is sent by BS_a 101 with a send offset and received by BS_b 102 after a propagation delay of τ. BS_b 102 measures reception offset. The sum of transmit offset and propagation delay τ is equal to the sum of receive offset and frame slip Δ. Although the propagation delay (ie τ) between the CBP sender and CBP receiver is small in Figure 3, the propagation delay τ is sufficiently large and not negligible in this case. Lacking knowledge of the propagation delay τ between the CBP sender and the CBP receiver, BS_b 102 cannot calculate the frame slip Δ, so BS_b 102 cannot synchronize its transmission with BS_a. Thus, synchronization accuracy is directly limited by such propagation delays. In other words, the second assumption is not always justified. If the distance between two base stations is 30Km, the propagation delay may be around 100 μsec. In another case, BS_b can directly obtain CBP packets from CPE_a. In this case, the propagation delay can easily be tens of microseconds, eg, a 10Km cell radius can produce a synchronization error of 33μsec.

Another scenario of coexistence transmission is illustrated in FIG. 8, where CPE_a 105 is located in the overlapping area between the cell Cell_a 103 covered by BS_a 101 and the cell Cell_b 104 covered by BS_b 102. CPE_a 105 sends a CBP packet in response to the direction of BS_a 101. BS_b 102 receives the CBP packet. The propagation delay between BS_a 101 and CPE_a 105 is τ1; and the propagation delay between BS_b 102 and CPE_a 105 is τ2. Using existing methods, synchronization errors can easily be tens of microseconds. Therefore, BS_b 102 cannot synchronize its transmission with BS_a 101 without knowledge of the propagation delay τ2.

The present invention provides a method of precise network synchronization by taking into account propagation delay when sending CBP packets.

When two base stations (BSs) are synchronized with each other, the propagation delay between the two BSs must be calculated and removed. This can be done by specifying that for example BSa and BSb can get their positions from a central database. An alternative is to include the sender's location information in the CBP packet. The receiver should already know its own position.

When the sender of the CBP packet is a CPE, the receiver can be a CPE or a BS. Then the propagation delay can be solved by send-feedback handshake.

It should be mandatory for a BS to know the location of neighboring BSs in order to support over-the-air synchronization of cells. This information can be obtained by provision during cell installation. However, there is a problem if cells from different vendors are required to be synchronized when the vendors do not want to share location information.

Another solution includes the information of the location of the BS in each SCH sent by the BS, so that neighboring BSs can calculate and remove the propagation delay between the BS and the neighboring BS to achieve cell synchronization. The problem with this approach is that each SCH has to carry the overhead of location information.

In either case of the present invention, propagation delays are taken into account when synchronizing overlapping cells.

In an embodiment according to the invention, the transmitter sends a CBP packet carrying the location information of the transmitter, so that the receiver can calculate and remove the propagation delay to achieve cell synchronization. The location information can be delivered by SCH (Superframe Control Header) including Location Configuration Information Element (IE), or by CBP including Location IE. The receiver can then report the calculated propagation delay to the BS covering or serving the receiver. Alternatively, the receiver can report the transmitter's location information to the BS covering or serving the receiver, and the BS can calculate the propagation delay based on the location information. If the BS also needs to know the location information of the receiver, the receiver can transmit the location information of both the receiver and the transmitter. This approach requires the transmitter to have a location solution, eg using the Global Positioning System (GPS).

If a location database is available, the sender ID can be included in the CBP message instead of the exact location. The receiver can then use the ID to retrieve the sender's location from the location database.

Furthermore, the overhead of including location information in the CBP message can be clipped, assuming the receiver knows the exact location of the receiver, and the receiver knows that the sender is within a certain radius (e.g., the sender and receiver are within 50Km of each other ). In other words, the least significant bit is enough to deduce the exact location of the transmitter.

In another embodiment according to the invention, the sender measures the inter-cell delay using two-way communication and carries information about the propagation delay in CBP packets. The inter-cell delay may be τ3 for the scenario illustrated in FIG. 6 , or τ2 for the scenario illustrated in FIG. 8 . In this method, no matter whether the inter-cell delay occurs between two CPEs or between a BS and a CPE, a new transmission procedure for measuring the inter-cell delay is required. Basically, this measurement requires immediately following the transmission in order to measure the propagation delay.

As shown in Figure 9, CBP packets are scheduled by the BS through a self-coexistence window. The self-coexistence window defines the time period in which channel access is contention-based in order to maximize spectrum utilization. In other words, during this time, the CPE should use the contention for access mechanism to gain access to the medium and send CBP packets. First, BS_a selects some CPEs (eg, CPE_a1, CPE_a2, and CPE_a3) for sending CBP packets in a contention manner (eg, using a contention window of [0~7]). That is, CPE_a1, CPE_a2, and CPE_a3 compete for transmission of CBP packets. Alternatively, BS_a may allocate only one CPE (such as CPE_al) to send CBP packets. BS_b selects some CPEs (eg, CPE_b1, CPE_b2 and CPE_b3) for receiving CBP packets. Whoever receives the CBP packet should resend the follow message (ie another CBP packet) in a similar competitive manner. In other words, CPE_bl, CPE_b2 and CPE_b3 compete for the transmission of the following message. The original CBP sender receives this feedback (ie, follows the message) and calculates the propagation delay between sender and receiver. As a result, precise cell synchronization is established. Optionally, the CPE_ai that won the competition to send the first CBP packet may send another follow message to CPE_bi.

Another example of this procedure is illustrated in FIG. 10 , where BS_a selects CPE_a1 , CPE_a2 and CPE_a3 located in cell Cell_a to compete for transmission of the first CBP packet in contention window 201 . As a result of the contention, CPE_al wins the contention and sends the first CBP packet at time _t0 . The packets arrive at CPE_b (eg, CPE_b1, CPE_b2, and CPE_b3) of cell Cell_b at slightly different times after a certain propagation delay, shown as _τ3 in the figure. Assume that the packet is received by CPE_b2 at time t ₁ . After receiving the first CBP packet, CPE_b1 , CPE_b2 and CPE_b3 compete in the contention window 202 for the transmission of the second CBP packet (ie, following message). As a result of the contention, CPE_b2 wins the contention and sends the second CBP packet at time _t2 . The packet arrives at CPE_a1 after a certain propagation delay, again _τ3 shown in the figure. That is, the second CBP packet is received at time t ₄ CPE_al. It is worth noting that other CPE_a in the cell Cell_a will also receive the second CBP packet, but they cannot use the second CBP packet to calculate the propagation delay because they have not sent the first CBP packet. CPE_a1 calculates the propagation delay between CPE_a1 and CPE_b2; the calculated propagation delay is (t ₄ -t ₀ )/2. As a result, precise cell synchronization is accomplished. As an alternative embodiment, the first CBP sender, ie CPE_al, may send another follow-up message, ie a third CBP packet, at time _t5 . Assume that CPE_b2 receives this third CBP packet at _t6 . CPE_b2 is then able to calculate the propagation delay between CPE_b2 and CPE_a1; the resulting propagation delay is ( _t6 - _t2 )/2.

Additionally, the BS and CPE may have some processing delay during sending and receiving CBP and SCH packets. Such delays should be compensated for if they are significant. Generally, they do not occur in practice. Specifically, round-trip communication is used to calculate the propagation delay between CPE_a and CPE_b. At t ₀ CPE_a sends a packet, at t ₁ CPE_b receives the packet, at t ₂ CPE_b sends another packet, and at t ₃ CPE_a receives the packet. After processing, CPE_a measures t ₃ -t ₀ . The value of t ₂ -t ₁ is the processing delay at CPE_b. If the processing delay is negligible, the value t ₃ -t ₀ is the round-trip propagation delay between CPE_a and CPE_b. The one-way propagation delay between CPE_a and CPE_b is equal to half of the round-trip propagation delay. Therefore, t ₂ -t ₁ affects the accuracy of the propagation delay and thus the accuracy of the inter-BS synchronization. In general, this processing delay is negligible if we are talking about obtaining 25 μs accuracy for inter-BS synchronization.

As mentioned above, propagation delay needs to be measured and reported in CBP messages to synchronize overlapping cells. The intra-cell delay can be measured through a ranging procedure. That is, all CPEs in the cell calibrate their transmissions via the ranging process with the coordinates of the BS. In other words, the BS measures the propagation delay of each CPE relative to the BS. Thus, in the scenario shown in FIG. 8 , τ ₁ is known through the normal "ranging" procedure between BS_a 101 and CPE_a 105 , but τ ₂ is not, since CPE_a 105 is not associated with BS_b 102 . Similarly, in the scenario shown in Figure 6, τ ₁ and τ ₂ are known, but τ ₃ is not.

As shown in FIG. 7, even though the sender and receiver of the CBP packet are CPEs, the sending offset is relative to BS_a 101, and the receiving offset is relative to BS_b 102. The reason is that the propagation delays between BS_a and CPE_a, BS_b and CPE_b are known and removed from the equation.

Claims (25)

1. A method for synchronizing overlapping cells, the method comprising the steps of: transmitting a first data packet and a transmission offset indicating a difference in the time domain between a start of a transmitted data frame and said first data packet at a first node, the first node is one of a first base station and a first user, and the first user is located in a first cell covered by the first base station; In response to receipt of said first data packet, recording at a second user located in a second cell covered by a second base station a reception offset indicating the start of a frame at said second base station The difference in time domain between at and said first data packet; sending the receive offset to the second base station at the second user; obtaining a propagation delay between said first node and said second user; calculating frame slip at the second base station by adding the transmit offset and propagation delay and subtracting the receive offset; and The first cell and the second cell are synchronized based on the calculated frame slip. 2. The method of claim 1, wherein obtaining the propagation delay between the first node and the second user comprises: sending at said first node a data packet carrying information about the geographic location of said first node; in response to receipt of the data packet, calculating a propagation delay of the data packet at the second user based on information about the location of the first node; and The propagation delay is sent at the second user to the second base station. 3. The method of claim 2, comprising using round-trip communication to calculate the distance between the first node and the second user when one of the first node and the second user does not have knowledge of their geographic location. The propagation delay between said second users without information about the geographic location of the first node. 4. The method of claim 1, wherein obtaining a propagation delay between the first node and the second user comprises: generating and broadcasting a location database containing geographic location information of a plurality of users associated with respective identifiers located within said first cell; sending a data packet carrying an identifier of the first user via the first user; in response to receiving the data packet, retrieving, at the second user, geographic location information for the first user from the location database based on the identifier; computing a propagation delay at the second user from information about the location of the first user; and The propagation delay is sent at the second user to the second base station. 5. The method of claim 1, wherein obtaining a propagation delay between the first node and the second user comprises: selecting, by the first base station, a subset of a plurality of first users for data transmission, the plurality of first users being located in a first cell covered by the first base station; contending for transmission of first reference data packets by a subset of the plurality of first users in a first time interval; sending the first reference data packet by a specific one of the first users winning the competition; selecting, by the second base station, a subset of a plurality of second users for data reception, the plurality of second users being located in a second cell covered by the second base station; in response to receipt of the first reference data packet, contending for transmission of a second reference data packet during a second time interval by the selected subset of the plurality of second users; sending the second reference data packet by the specific one of the second users who won the competition; calculating, at said specific one first user, a propagation delay between said specific one first user and said specific one second user; and sending the propagation delay at the specific one of the first users to the second base station. 6. The method of claim 5, further comprising sending a third reference data packet by said particular one of the first users. 7. The method of claim 5, further comprising said selected one first user compensating for processing delay of said first data packet when cell synchronization is established. 8. The method of claim 1, wherein obtaining a propagation delay between the first node and the second user comprises: scheduling the first user to send a first reference data packet by the first base station; scheduling the second user to receive the first reference data packet by the second base station; scheduling the second user to send a second reference data packet by the second base station; and calculating a propagation delay of the reference data packet at the first user in response to receipt of the second reference data packet; The propagation delay is sent at the first user to the second base station. 9. The method of claim 8, further comprising the first user sending a third reference data packet. 10. The method of claim 1, comprising the first cell and the second cell being overlapping. 11. A wireless communication system comprising defining a first base station of a first cell; defining a second base station of a second cell; The second base station is synchronized with the first base station through the following steps: Scheduling, by the first base station, a selected first user among the plurality of first users located in the first cell to transmit a first data packet as part of a transmission data frame; transmitting at said selected one first user a transmit offset indicating a difference in the time domain between a start of a transmit data frame and said first data packet; In response to receiving the first data packet, recording a reception offset at a selected one of the plurality of second users located in the second cell, the reception offset indicating the difference in the time domain between the start of the frame of the cell and said first data packet; sending the receive offset to the second base station at the selected one of the second users; obtaining a propagation delay between said selected one first user and said selected one second user; calculating frame slip at the second base station by adding the transmit offset and propagation delay and subtracting the receive offset; and The first cell and the second cell are synchronized based on the calculated frame slip. 12. The wireless communication system of claim 11 , wherein the step of obtaining a propagation delay between said selected one first user and said selected one second user comprises: sending at said one first user located in said first cell a second data packet carrying information about the geographic location of said one first user; In response to receipt of the second data packet, calculating a propagation delay of the data packet at the one second user located in the second cell based on information about the location of the transmitter; and The propagation delay is sent at the one second user to the second base station. 13. The wireless communication system as claimed in claim 12, comprising: when said one second user has the knowledge of own geographic location, said one first user sends information carrying information about the distance between said one first user and said one A second data packet of information on the distance between the second users. 14. The wireless communication system of claim 11 , wherein the step of obtaining a propagation delay between the selected one first user and the selected one second user comprises: generating and broadcasting a location database containing geographic location information for a plurality of first users associated with respective identifiers located within said first cell; sending via said one first user a second data packet carrying an identifier of said one first user; in response to receiving the second data packet, retrieving, at the second user, geographical location information of the first user from the location database based on the identifier; calculating propagation delay at said one second user from information about the location of said one first user; and The propagation delay is sent at the one second user to the second base station. 15. The wireless communication system of claim 11 , wherein the step of obtaining a propagation delay between the selected one first user and the selected one second user comprises: selecting, by the first base station, a subset of the plurality of first users for data transmission; contending for transmission of first reference data packets by a subset of the plurality of first users in a first time interval; sending the first reference data packet by a specific one of the first users winning the competition; selecting, by the second base station, a subset of the plurality of second users for data reception; in response to receipt of the first reference data packet, competing for transmission of a second reference data packet during a second time interval by a selected subset of the plurality of second users; sending the second reference data packet by the specific one of the second users who won the competition; calculating, at said specific one first user, a propagation delay between said specific one first user and said specific one second user; and sending the propagation delay at the specific one of the first users to the second base station. 16. The wireless communication system according to claim 15, further comprising: said specific one first user sending a third reference data packet. 17. The wireless communication system of claim 11, further comprising said selected one first user compensating for processing delay of said first data packet when cell synchronization is established. 18. The wireless communication system of claim 11 , wherein the step of obtaining a propagation delay between the selected one first user and the selected one second user comprises: scheduling the one first user to send a first reference data packet by the first base station; scheduling, by the second base station, the one second user to receive a first reference data packet; scheduling, by the second base station, the one second user to send a second reference data packet; and calculating, at said one first user, a propagation delay of the reference data packet in response to receipt of the second reference data packet; The propagation delay is sent at the one first user to the second base station. 19. The wireless communication system of claim 18, further comprising: said one first user sending a third reference data packet. 20. The wireless communication system of claim 11, comprising the first cell and the second cell being overlapping. 21. A method for synchronizing overlapping cells, the method comprising the steps of: sending a first data packet by a first node, the first data packet having location information of said first node and indicating the time in the first cell between the start of a transmitted data frame and said first data packet The difference send offset in the field; receiving the data packet by the second node and recording a reception offset indicating the difference in the time domain between the start of the frame and said first data packet in the second cell; obtaining a propagation delay between the first node and the second node based on the location information of the first node; and Frame slip is calculated by the second node by adding the transmit offset and propagation delay and subtracting the receive offset. 22. The method of claim 21, further comprising synchronizing the first cell and the second cell based on the calculated frame slip. 23. The method of claim 21, wherein the first node is one of a first base station and a first user, and the first user is located in a first cell covered by the first base station, the second The second node is one of a second base station and a second user, and the second user is located in a second cell covered by the second base station. 24. A wireless communication system comprising constructing the first node of the first cell; constructing a second node of the second cell; The second node is synchronized with the first node through the following steps: sending a first data packet by the first node, the first data packet having location information of the first node and indicating the distance between the start of the transmitted data frame and the first data packet in the first cell The transmit offset of the difference in the time domain; receiving the data packet by the second node and recording a reception offset indicating the time domain between the start of the frame and the first data packet in the second cell difference; obtaining a propagation delay between the first node and the second node based on the location information of the first node; and Frame slip is calculated by the second node by adding the transmit offset and propagation delay and subtracting the receive offset. 25. The wireless communication system according to claim 24, wherein the first node is one of a first base station and a first user, and the first user is located in a first cell covered by the first base station, The second node is one of a second base station and a second user, and the second user is located in a second cell covered by the second base station.

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