WO2007107895A1 - A method and apparatus of channel accessing for use in a wireless communication system - Google Patents

A method and apparatus of channel accessing for use in a wireless communication system Download PDF

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Publication number
WO2007107895A1
WO2007107895A1 PCT/IB2007/050700 IB2007050700W WO2007107895A1 WO 2007107895 A1 WO2007107895 A1 WO 2007107895A1 IB 2007050700 W IB2007050700 W IB 2007050700W WO 2007107895 A1 WO2007107895 A1 WO 2007107895A1
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WIPO (PCT)
Prior art keywords
timeslot
message
mobile terminal
access
testing
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Application number
PCT/IB2007/050700
Other languages
French (fr)
Inventor
Yan Zhang
Dan Shang
Lin Du
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Koninklijke Philips Electronics N.V.
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Application filed by Koninklijke Philips Electronics N.V. filed Critical Koninklijke Philips Electronics N.V.
Publication of WO2007107895A1 publication Critical patent/WO2007107895A1/en

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W74/00—Wireless channel access
    • H04W74/08—Non-scheduled access, e.g. ALOHA
    • H04W74/0808—Non-scheduled access, e.g. ALOHA using carrier sensing, e.g. carrier sense multiple access [CSMA]
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L43/00—Arrangements for monitoring or testing data switching networks
    • H04L43/50—Testing arrangements
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W52/00—Power management, e.g. TPC [Transmission Power Control], power saving or power classes
    • H04W52/04—TPC
    • H04W52/38—TPC being performed in particular situations
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W74/00—Wireless channel access
    • H04W74/08—Non-scheduled access, e.g. ALOHA
    • H04W74/0866—Non-scheduled access, e.g. ALOHA using a dedicated channel for access

Definitions

  • the present invention relates generally to wireless communication systems, and more particularly, to a method and apparatus of channel accessing for use in a wireless communication system having P2P communication function.
  • any communication between two mobile terminals must be routed via a base station's (BS's) relay whether they are far away from or close to each other, i.e., an uplink-BS-downlink mode is employed to carry out communication.
  • BS base station's
  • P2P Pear-to-Pear
  • a method for establishing a P2P communication link in a cellular communication system was disclosed in the patent application document with PCT publication No. WO2004/077920, entitled "Method and system for radio link establishment and maintenance with P2P communication in wireless communication".
  • two mobile terminals intended for P2P communication may use an allocated wireless channel to probe each other and determine whether a P2P communication link can be established. More specifically, a first mobile terminal transmits a probing message to a second mobile terminal with a predetermined initial transmission power on the allocated wireless channel, receives a feedback message from the second mobile terminal, and adjusts the transmission power.
  • the two mobile terminals During the probing procedure, if the transmission powers of the both mobile terminals do not exceed a predetermined upper limit, the two mobile terminals enters into P2P mode and starts P2P communication once the probing procedure ends, otherwise, the conventional uplink-BS-downlink mode is employed to carry out communication. Furthermore, the mobile terminal in P2P mode may keep on monitoring its communication channel, so as to switch to the conventional mode to continue communication when the communication quality deteriorates.
  • the present invention provides a different method of channel accessing for P2P communication in a wireless communication system.
  • the present invention provides a method of channel accessing for use in a wireless communication system and executed in a mobile terminal.
  • the method according to the present invention comprises steps of: overhearing a plurality of timeslots to select a timeslot and a channel in the timeslot to be used for P2P communication with a target mobile terminal; transmitting a probing message in an access timeslot to notify the target mobile terminal and mobile terminals communicating in the selected timeslot to carry out a channel access testing in the access timeslot; carrying out an access testing, in the access timeslot, for the selected channel and communicating channels in the selected timeslot to obtain a testing result indicating the interference among the channels; and overhearing a feedback message, which is based on the testing result, from any of the mobile terminals communicating in the selected timeslot, so as to determine whether to employ the selected channel for P2P communication with the target mobile terminal.
  • the present invention further provides a method of channel accessing for use in a wireless communication system and executed in a mobile terminal, the method comprising steps of: overhearing a probing message in an access timeslot, and judging whether a selected timeslot for P2P communication indicated by the probing message is identical to the timeslot in which it is communicating; when the selected timeslot is identical to the timeslot in which it is communicating, transmitting a testing message to a receiver with which it is communicating and receiving a feedback message from the receiver in the access timeslot, and adjusting the transmission power based on the received feedback message while calculating the Signal-to-Interference Ratio of the feedback signal; and when the Signal-to-Interference Ratio is lower than a predetermined threshold, transmitting an access deny message to the mobile terminal which transmits the probing message, to indicate the mobile terminal to give up accessing the selected timeslot; otherwise, adjusting the transmission power at which it is communicating in the selected timeslot to the power adjusted at the end of the testing .
  • Another object of the present invention is to provide an apparatus convenient for a mobile terminal, which intends to carry out P2P communication with another mobile terminal, to access a wireless communication system.
  • the present invention provides an apparatus of channel accessing for use in a wireless communication system and executed in a mobile terminal, the apparauts comprising: an selecting unit for overhearing a plurality of timeslots to select a timeslot and a channel in the timeslot to be used for P2P communication with a target mobile terminal; a probing unit for transmitting a probing message in an access timeslot to notify the target mobile terminal and terminals communicating in the selected timeslot to carry out a channel access testing in the access timeslot; a testing unit for carrying out, in the access timeslot, an access testing to the selected channel and communicating channels in the selected timeslot to obtain a testing result indicating the interference among the channels; and a deciding unit for overhearing a feedback message, which is based on the testing result, from any of the mobile terminals communicating in the selected timeslot, so as to decide whether to employ the selected channel for P2P communication with the target mobile terminal.
  • the present invention further provides an apparatus of channel accessing for use in a wireless communication system and executed in a mobile terminal, the apparatus comprising: an overhearing unit for overhearing a probing message in an access timeslot, and judging whether a selected timeslot for P2P communication indicated by the probing message is identical to the timeslot in which it is communicating; a testing unit for, when the selected timeslot is identical to the timeslot in which it is communicating, transmitting a testing message to a receiver with which it is communicating and receiving a feedback message from the receiver in the access timeslot, and adjusting the transmission power based on the received feedback message while calculating the Signal-to-Interference Ratio of the feedback signal; and a feedback unit for, when the Signal-to-Interference Ratio is lower than a predetermined threshold, transmitting an access deny message to the mobile terminal which transmits the probing message to indicate the mobile terminal to give up accessing the selected timeslot; otherwise, adjusting the transmission power at which it is communicating in the selected timeslot to the power adjusted at the end
  • the method provided in the present invention uses an access timeslot to simulate a timeslot to be used for P2P communication and meanwhile tests all the communicating channels and the selected channel. It can reduce risk of imposing negative impact on the timeslot to the accessed by performance predicting based on the result of simulative testing and thus improve system performance as a whole.
  • Fig.l is a schematic diagram showing the structure of a sub-frame in a Time Division
  • TDD CDMA Duplex Code Division Multiple Access
  • Fig.2 is a schematic diagram showing the structure of a sub-frame according to an embodiment of the present invention.
  • Fig.3 is a schematic flowchart showing the method of channel accessing according to an embodiment of the present invention.
  • Fig.4 is a block diagram showing a first apparatus of channel accessing according to an embodiment of the present invention.
  • Fig.5 is a block diagram showing a second apparatus of channel accessing according to an embodiment of the present invention.
  • the basic idea of the method of channel accessing provided in the present invention is using an access timeslot to simulate a timeslot to be used for P2P communication and meanwhile tests all the communicating channels and the selected channel. It can reduce risk of imposing negative impact on the timeslot to the accessed by performance predicting based on the result of simulative testing and thus improve system performance as a whole.
  • Fig.l shows the structure of a sub-frame in a TDD CDMA system.
  • the sub-frame includes seven traffic timeslots TS0-TS6, each of the traffic timeslots has a duration time of 864 chips.
  • TSO is a downlink timeslot, which is used by a mobile terminal to receive broadcast messages from BSs. There exists a switch point for uplink and downlink timeslots among TSl to TS6.
  • the sub-frame also includes, between timeslots TSO and TSl, a Downlink Pilot TimeSlot (DwPTS), an Uplink Pilot TimeSlot (UpPTS), and a Guard Period (GP).
  • DwPTS Downlink Pilot TimeSlot
  • UpPTS Uplink Pilot TimeSlot
  • GP Guard Period
  • the duration of DwPTS, UpPTS and GP are 96 chips, 160 chips and 96 chips respectively.
  • a secondary carrier may be used specifically for P2P communication. In this case, it is unnecessary for P2P mobile terminal to keep uplink synchronization with the BS. Therefore, the unused UpPTS timeslot in the secondary carrier can act as an access timeslot for channel accessing of P2P mobile terminal.
  • the GP between DwPTS and UpPTS is designed mainly to avoid the overlapping of the uplink and downlink signals due to the signal propagation delay, and its duration is set mainly in consideration of the maximum support radius for a cell of a cellular system.
  • the maximum radius for a cell corresponding to a 96-chip GP is 11 kilometers.
  • the P2P mobile terminal operating on the secondary carrier has a reliable communication distance of several hundred meters, which is much shorter than the above-predetermined maximum radius for a cell.
  • the duration of the GP between DwPTS and UpPTS on the secondary carrier can be decreased.
  • a GP with 16 chips is large enough to support a P2P communication distance of 3.75km, i.e., to prevent the signals in DwPTS and UpPTS from overlapping due to propagation delay.
  • the remaining 80 chips may form the access timeslot together with UpPTS, so as to make full use of the existing resource.
  • Fig.2 is schematic diagram showing the structure of a sub-frame according to an embodiment of the present invention.
  • the duration time of the GP between DwPTS and UpPTS is decreased from 96 chips to 16 chips, the remaining 80 chips and the UpPTS are merged to form the access timeslot (Access TS), and the other traffic timeslots TS0-TS7 and DwPTS have the same position and duration on the sub-frame as the corresponding parts of fig.1.
  • Fig.3 is a schematic flowchart showing the method of channel accessing according to an embodiment of the present invention. As shown in fig.3, the method comprises establishing a call link (step SIlO), selecting a channel (step S120), probing (step S130), simulative testing (step S 140), and determining whether to access (step S 150), etc. A detailed description is given blow in conjunction with Fig.3.
  • a source mobile terminal (UEl) and a target mobile terminal (UE2) which are to establish a P2P communication, establish a call link using a primary carrier with the aid of the BS.
  • UEl transmits a call request message to the BS through an uplink access channel of the primary carrier, indicating its intention for P2P communication with UE2; the BS transmits a paging message to UE2 after receiving the call request; UE2 returns a paging reply to the BS after receiving the paging message; after receiving the paging reply, the BS transmits a acknowledge massage to UEl and UE2 respectively, and designates a secondary carrier for
  • UEl overhears all traffic timeslots on the secondary carrier, and selects a channel to be used for P2P communication according to the detected interference level in the timeslots. This channel is determined by the secondary carrier frequency, the timeslot, and spread code.
  • UEl transmits one or more probing messages with a predetermined power and a known channelization code in the access timeslot of the secondary carrier.
  • the probing message includes identifiers of UEl and UE2, the timeslot and spread code corresponding to the selected channel to be used for P2P communication, and a period for the simulative testing.
  • UE2 calculates a Signal-to-Interference Ratio based on the received signal power to estimate the condition of the link quality. If the link quality is satisfied, UE2 transmits a probing acknowledge message to UEl, otherwise, it keeps silent. If receiving the acknowledge message within a predetermined period, UEl makes a preparation for the simulative testing in the next time period, otherwise it gives up the attempt for P2P communication.
  • the messages transmitted between UEl and UE2 actually can be regarded as broadcast messages. In this case, all the other mobile terminals overhearing the access timeslot may receive the broadcast messages.
  • the mobile terminals such as UE3 and UE4, communicating in the timeslot selected by the source mobile terminal UEl and to be used for P2P communication will respectively prepare to participate the following simulative access testing.
  • the access timeslot is used to carry out a simulative access testing for the timeslot selected by UEl and to be used for P2P communication.
  • the mobile terminals such as UE3 and UE4 communicating in the selected timeslot participate the simulative testing together with UEl and UE2.
  • the simulative testing communication performs close-loop power control for UE1/UE2 and UE3/UE4 for several times, respectively, so as to make the transmission power converge step by step.
  • UEl transmits a testing message to UE2 one by one, the testing message including a training sequence used for synchronizing UEl with UE2; UE2 evaluates the link based on the received signal's quality after receiving the testing message, and transmits a testing feedback message to UEl; after receiving the feedback message from UE2, UEl adjusts, based on the feedback message, its transmission power to meet the communication quality requirement.
  • UE3 and UE4 participate, in the same access timeslot, the simulative testing using their allocated spread-spectrum code in the selected timeslot. They perform close-loop power control similar to that of UE1/UE2 for several times, calculate corresponding Signal-to-Interference Ratio, and estimate the influence to their communication quality after UEl is accessed while adjust the power to meet the communication quality requirement.
  • step S 150 for determining whether to access UEl overhears the simulative testing feedback information on the access channel within a predetermined period, so as to determine whether to start P2P communication.
  • UEl overhears the simulative testing feedback information on the access channel within a predetermined period, so as to determine whether to start P2P communication.
  • other sub-steps corresponding to two cases as shown in fig.3 are also included in this step.
  • the mobile terminal such as UE3 participating the testing finds that the current Signal-to-Interference Ratio is higher than or equal to a predetermined threshold through calculation. This means that the access of UEl will not cause the communication quality of UE3 and UE4 to deteriorate to an unacceptable degree, and thus UE3 keeps silent to accept the timeslot selected for the access of UEl. In this case, UEl will not overhear any access deny message for the simulative testing within a predetermined period, and thus carry out P2P communication with UE2 on the selected channel taking the power adjusted during the testing procedure as the initial transmission power (step S 154).
  • the mobile terminal such as UE3 participating the testing finds that the current Signal-to-Interference Ratio is lower than a predetermined threshold through calculation. This means that the accessing of UEl will cause the communication quality of UE3 and UE4 to deteriorate to an unacceptable degree.
  • UE3 transmits an access deny message to UEl, to indicate UEl to give up accessing the selected timeslot (step S 156).
  • UEl when having overheard the access deny message for the simulative testing within a predetermined period, UEl gives up the selected timeslot to be used for P2P communication, and reselects a channel to be used for P2P communication, or returns to the primary carrier and notifies the BS to give up the current P2P communication request (step S158).
  • the period for the probing procedure, simulative testing procedure, and overhearing procedure of testing feedback may generally be predetermined based on the number N ue of mobile terminals which meets the P2P supporting distance in a cell, sub-frame length T f , and the Poisson arrival rate ⁇ for P2P call, so as to reduce the possible signal conflicts on the access channel. For example, if N ue is 500, ⁇ is 1/120 (I/second), then the number of the P2P mobile terminals arriving within 1 second is:
  • the probing procedure, the simulative testing procedure, and the overhearing of testing feedback procedure are set to have 4, 20, and 4 sub-frames, respectively.
  • the step SIlO for establishing a call link in the above-mentioned method of channel accessing may be an optional step.
  • the source mobile terminal which is to carry out P2P communication directly overhears each of the timeslots on the carrier for P2P traffic to select a channel to be used for P2P communication with UE2, and implements the call link establishment and the probing function simultaneously on the access timeslot in the probing step S 130.
  • the method of channel accessing provided in the present invention can also be used in an ad hoc network, in which no BS or central controller is provided and a CDMA technique is employed.
  • the method of channel accessing described above in conjunction with Fig.3, which uses the access timeslot to simulate the timeslot to be used for P2P communication, and carries out simulative testing for all the communicating channels and channels to be accessed in that timeslot, can be implemented by means of software, or hardware, or combination of both.
  • Fig.4 is a schematic block diagram showing an accessing apparatus 100 for a mobile terminal according to an embodiment of the present invention.
  • the accessing apparatus resides in the source mobile terminal UEl, which initiates a P2P call, comprising a calling unit 110, a selecting unit 120, a probing unit 130, a testing unit 140, and a deciding unit
  • the calling unit 110 is used for establishing a call link with a target mobile terminal
  • the calling unit 110 comprises a first transmitting unit for transmitting a P2P call request to the UE2 via the BS, and a first receiving unit for receiving a P2P paging reply from the UE2 via the BS and obtaining an information indicating a secondary carrier for P2P communication from the BS.
  • the selecting unit 120 is used for overhearing the interference levels in all the traffic timeslots on the secondary carrier, and selecting the channel to be used for P2P communication with the UE2 based on the interference levels in the timeslots, wherein this channel is determined by the secondary carrier frequency, the timeslot, and the spread- spectrum code.
  • the probing unit 130 is used for, during a first predetermined time period, broadcasting a probing message and receiving the corresponding probing acknowledge message in the access timeslot.
  • the probing message indicates the identifiers of UEl and
  • the timeslot and spread- spectrum code corresponding to the selected channel to be used for the P2P communication so as to notify UE2 and other mobile terminals, such as
  • the probing unit 130 comprises a second transmitting unit for transmitting a probing message to UE2 and a second receiving unit for receiving a probing acknowledge from UE2.
  • the testing unit 140 is used for, during a second predetermined time period, simulating the selected timeslot with the access timeslot and carrying out simulative testing for the channels to be accessed and the accessed channels in this timeslot.
  • the testing unit 140 comprises a third transmitting unit for transmitting a simulative testing message one by one in the access timeslot by using the spread- spectrum code for the selected channel; a third receiving unit for receiving the simulative testing feedback information from UE2; and a power control unit for adjusting, based on signal quality of the received feedback message, the transmission power for the next testing message to meet the communication quality requirement.
  • a third transmitting unit for transmitting a simulative testing message one by one in the access timeslot by using the spread- spectrum code for the selected channel
  • a third receiving unit for receiving the simulative testing feedback information from UE2
  • a power control unit for adjusting, based on signal quality of the received feedback message, the transmission power for the next testing message to meet the communication quality requirement.
  • communicating in the selected timeslot also transmit the testing message and receive the feedback message in the access timeslot by using their spread- spectrum code allocated in the selected timeslot, and adjusts the transmission power based on the communication quality of the received message. Therefore, the signal quality between UEl and UE2 and the signal quality between UE3 and UE4 may cause influence to each other. In order to guarantees the communication quality, each of the mobile terminals must adjust the transmission power to make it converge step by step according to the change of the quality of the received signal. Thus, during the testing procedure, the evaluation of the signal quality may be based on the calculation for the SINR of the received signal.
  • the deciding unit 150 is used for, during a third predetermined time period, overhearing in the access timeslot the feedback message from any of the mobile terminals communicating in the selected timeslot, to determine whether to employ the selected channel for P2P communication.
  • the mobile terminal such as UE3 participating the testing finds that the current SINR is lower than a predetermined threshold through calculation, i.e., it is predicted that the accessing of UEl will cause the communication quality for UE3 and UE4 to deteriorate to an unacceptable degree, then UE3 transmits an access deny message to UEl, in order to indicate UEl to give up accessing the selected timeslot; otherwise, UE3 keeps silent.
  • UEl When UEl has overheard the access deny message within a predetermined period, it gives up the selected channel to be used for P2P communication, and reselects a channel to be used for the P2P communication, or returns to the primary carrier and notifies the BS to give up the current P2P communication request. Otherwise, if UEl does not receive any access deny message within the predetermined period, UEl and UE2 start P2P communication in the selected timeslot.
  • the transmitting units and the receiving units mentioned above have similar functions, and may be implemented with a same structure. Moreover, since the transmitting units and the receiving units operate in different time periods, they may be implemented with a same transmitting unit and a same receiving unit respectively.
  • the apparatus provided in the present invention may be used for a mobile terminal that establishes a call link without a BS and a central controller.
  • the accessing apparatus comprises all the function units other than the calling unit 110 in the accessing apparatus 100.
  • the selecting unit 120 directly overhears timeslots on the carrier for P2P traffic to select a channel to be used for P2P communication with UE2, while the probing unit S 130 accomplishes the call link establishment and the probing functions simultaneously.
  • Fig.5 is a schematic block diagram showing a second accessing apparatus 200 for use in the mobile terminal UE3 to cooperate with UEl to carry out the access testing, according to another embodiment of the present invention.
  • the accessing apparatus 200 comprises an overhearing unit 160, a testing unit 170, and a feedback unit 180.
  • the overhearing unit 160 is used for, during a first predetermined time period, overhearing a probing/probing acknowledge message in the access timeslot.
  • the probing message is transmitted by the source mobile terminal UEl which requests access, and indicates the identifiers of the source mobile terminal UEl and the target mobile terminal UE2, and the timeslot and spread- spectrum code corresponding to the selected channel to be used for the P2P communication; and the probing feedback message is transmitted by UE2, acknowledging the probing message sent by the first mobile terminal.
  • the testing unit 170 is used for, during a second predetermined time period, cooperating with UEl and UE2 to carry out simulative channel access testing in the access timeslot when the timeslot to be used for P2P communication is identical to the timeslot in which it is communicating. Specifically, in the access timeslot, it transmits a testing message to its corresponding receiver UE4 by using its spread- spectrum code allocated in the timeslot to be used for the P2P communication, receives a feedback message from UE4, and adjusts the transmission power based on the received feedback message.
  • the feedback unit 180 is used for evaluating the channel quality during the simulative access testing procedure. When its SINR is lower than a predetermined threshold, it transmits an access deny message to UEl in a third predetermined time period, so as to indicate UEl to give up accessing the selected timeslot; otherwise, it will keep silent to accept the selected timeslot for the access of UEl.
  • the method and apparatus provided in the present invention may be used not only in a cellular communication system but also in a Wireless Local Area Network (WLAN) communication system and other wireless communication systems that have P2P function and employ CDMA technique.
  • WLAN Wireless Local Area Network

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Abstract

The present invention provides a method and apparatus of channel accessing for use in a wireless communication system and executed in a mobile terminal. The method according to the present invention comprises steps of: overhearing a plurality of timeslots to select a timeslot and a channel in the timeslot to be used for P2P communication with a target mobile terminal; transmitting a probing message in an access timeslot to notify the target mobile terminal and mobile terminals communicating in the selected timeslot to carry out a channel access testing in the access timeslot; carrying out an access testing, in the access timeslot, for the selected channel and communicating channels in the selected timeslot to obtain a testing result indicating the interference among the channels; and overhearing a feedback message, which is based on the testing result, from any of the mobile terminals communicating in the selected timeslot, so as to determine whether to employ the selected channel for P2P communication with the target mobile terminal.

Description

A METHOD AND APPARATUS OF CHANNEL ACCESSING FOR USE IN A WIRELESS COMMUNICATION SYSTEM
FIELD OF THE INVENTION The present invention relates generally to wireless communication systems, and more particularly, to a method and apparatus of channel accessing for use in a wireless communication system having P2P communication function.
BACKGROUND OF THE INVENTION In conventional cellular mobile communication systems, any communication between two mobile terminals must be routed via a base station's (BS's) relay whether they are far away from or close to each other, i.e., an uplink-BS-downlink mode is employed to carry out communication. But in some cases, when two users within a same cell are close to each other, it is more suitable to employ P2P (Pear-to-Pear) communication mode to carry out communication, which may also save wireless resources and improve communication quality.
A method for establishing a P2P communication link in a cellular communication system was disclosed in the patent application document with PCT publication No. WO2004/077920, entitled "Method and system for radio link establishment and maintenance with P2P communication in wireless communication". According to the method provided by the application, based on an established signaling link with help of base station, two mobile terminals intended for P2P communication may use an allocated wireless channel to probe each other and determine whether a P2P communication link can be established. More specifically, a first mobile terminal transmits a probing message to a second mobile terminal with a predetermined initial transmission power on the allocated wireless channel, receives a feedback message from the second mobile terminal, and adjusts the transmission power. During the probing procedure, if the transmission powers of the both mobile terminals do not exceed a predetermined upper limit, the two mobile terminals enters into P2P mode and starts P2P communication once the probing procedure ends, otherwise, the conventional uplink-BS-downlink mode is employed to carry out communication. Furthermore, the mobile terminal in P2P mode may keep on monitoring its communication channel, so as to switch to the conventional mode to continue communication when the communication quality deteriorates.
The present invention provides a different method of channel accessing for P2P communication in a wireless communication system.
OBJECT AND SUMMARY OF THE INVENTION
Among others, it is an object of the present invention to provide a method convenient for a mobile terminal, which intends to carry out P2P communication with another mobile terminal, to access a wireless communication system.
To this end the present invention provides a method of channel accessing for use in a wireless communication system and executed in a mobile terminal. The method according to the present invention comprises steps of: overhearing a plurality of timeslots to select a timeslot and a channel in the timeslot to be used for P2P communication with a target mobile terminal; transmitting a probing message in an access timeslot to notify the target mobile terminal and mobile terminals communicating in the selected timeslot to carry out a channel access testing in the access timeslot; carrying out an access testing, in the access timeslot, for the selected channel and communicating channels in the selected timeslot to obtain a testing result indicating the interference among the channels; and overhearing a feedback message, which is based on the testing result, from any of the mobile terminals communicating in the selected timeslot, so as to determine whether to employ the selected channel for P2P communication with the target mobile terminal.
The present invention further provides a method of channel accessing for use in a wireless communication system and executed in a mobile terminal, the method comprising steps of: overhearing a probing message in an access timeslot, and judging whether a selected timeslot for P2P communication indicated by the probing message is identical to the timeslot in which it is communicating; when the selected timeslot is identical to the timeslot in which it is communicating, transmitting a testing message to a receiver with which it is communicating and receiving a feedback message from the receiver in the access timeslot, and adjusting the transmission power based on the received feedback message while calculating the Signal-to-Interference Ratio of the feedback signal; and when the Signal-to-Interference Ratio is lower than a predetermined threshold, transmitting an access deny message to the mobile terminal which transmits the probing message, to indicate the mobile terminal to give up accessing the selected timeslot; otherwise, adjusting the transmission power at which it is communicating in the selected timeslot to the power adjusted at the end of the testing .
Another object of the present invention is to provide an apparatus convenient for a mobile terminal, which intends to carry out P2P communication with another mobile terminal, to access a wireless communication system.
To this end the present invention provides an apparatus of channel accessing for use in a wireless communication system and executed in a mobile terminal, the apparauts comprising: an selecting unit for overhearing a plurality of timeslots to select a timeslot and a channel in the timeslot to be used for P2P communication with a target mobile terminal; a probing unit for transmitting a probing message in an access timeslot to notify the target mobile terminal and terminals communicating in the selected timeslot to carry out a channel access testing in the access timeslot; a testing unit for carrying out, in the access timeslot, an access testing to the selected channel and communicating channels in the selected timeslot to obtain a testing result indicating the interference among the channels; and a deciding unit for overhearing a feedback message, which is based on the testing result, from any of the mobile terminals communicating in the selected timeslot, so as to decide whether to employ the selected channel for P2P communication with the target mobile terminal.
The present invention further provides an apparatus of channel accessing for use in a wireless communication system and executed in a mobile terminal, the apparatus comprising: an overhearing unit for overhearing a probing message in an access timeslot, and judging whether a selected timeslot for P2P communication indicated by the probing message is identical to the timeslot in which it is communicating; a testing unit for, when the selected timeslot is identical to the timeslot in which it is communicating, transmitting a testing message to a receiver with which it is communicating and receiving a feedback message from the receiver in the access timeslot, and adjusting the transmission power based on the received feedback message while calculating the Signal-to-Interference Ratio of the feedback signal; and a feedback unit for, when the Signal-to-Interference Ratio is lower than a predetermined threshold, transmitting an access deny message to the mobile terminal which transmits the probing message to indicate the mobile terminal to give up accessing the selected timeslot; otherwise, adjusting the transmission power at which it is communicating in the selected timeslot to the power adjusted at the end of the testing.
The method provided in the present invention uses an access timeslot to simulate a timeslot to be used for P2P communication and meanwhile tests all the communicating channels and the selected channel. It can reduce risk of imposing negative impact on the timeslot to the accessed by performance predicting based on the result of simulative testing and thus improve system performance as a whole. The above and other objects and features of the present invention will become more apparent and appreciated by referring to the following descriptions and claims taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS Fig.l is a schematic diagram showing the structure of a sub-frame in a Time Division
Duplex Code Division Multiple Access (TDD CDMA) system;
Fig.2 is a schematic diagram showing the structure of a sub-frame according to an embodiment of the present invention;
Fig.3 is a schematic flowchart showing the method of channel accessing according to an embodiment of the present invention;
Fig.4 is a block diagram showing a first apparatus of channel accessing according to an embodiment of the present invention; and
Fig.5 is a block diagram showing a second apparatus of channel accessing according to an embodiment of the present invention.
In the figures, the same reference number represents the same, similar or corresponding feature or function.
DETAILED DESCRIPTION OF THE INVENTION
The basic idea of the method of channel accessing provided in the present invention is using an access timeslot to simulate a timeslot to be used for P2P communication and meanwhile tests all the communicating channels and the selected channel. It can reduce risk of imposing negative impact on the timeslot to the accessed by performance predicting based on the result of simulative testing and thus improve system performance as a whole.
The sub-frame structure provided in the present invention and the method and apparatus for accessing system by using an access timeslot in the sub-frame will be described in detail below in conjunction with the drawings.
Fig.l shows the structure of a sub-frame in a TDD CDMA system. As shown in Fig.l, the sub-frame includes seven traffic timeslots TS0-TS6, each of the traffic timeslots has a duration time of 864 chips. Where, TSO is a downlink timeslot, which is used by a mobile terminal to receive broadcast messages from BSs. There exists a switch point for uplink and downlink timeslots among TSl to TS6. The sub-frame also includes, between timeslots TSO and TSl, a Downlink Pilot TimeSlot (DwPTS), an Uplink Pilot TimeSlot (UpPTS), and a Guard Period (GP). The duration of DwPTS, UpPTS and GP are 96 chips, 160 chips and 96 chips respectively. When enabling P2P communication in a TDD CDMA system, particularly in a multi-carrier TDD CDMA system, a secondary carrier may be used specifically for P2P communication. In this case, it is unnecessary for P2P mobile terminal to keep uplink synchronization with the BS. Therefore, the unused UpPTS timeslot in the secondary carrier can act as an access timeslot for channel accessing of P2P mobile terminal. On the other hand, in the sub-frame structure of Fig.l, the GP between DwPTS and UpPTS is designed mainly to avoid the overlapping of the uplink and downlink signals due to the signal propagation delay, and its duration is set mainly in consideration of the maximum support radius for a cell of a cellular system. For a TDD CDMA system with a low rate of 1.28 Mchip/s, the maximum radius for a cell corresponding to a 96-chip GP is 11 kilometers. Typically, due to the limitation of the P2P transmission model and interference, the P2P mobile terminal operating on the secondary carrier has a reliable communication distance of several hundred meters, which is much shorter than the above-predetermined maximum radius for a cell. Thus, the duration of the GP between DwPTS and UpPTS on the secondary carrier can be decreased. For example, a GP with 16 chips is large enough to support a P2P communication distance of 3.75km, i.e., to prevent the signals in DwPTS and UpPTS from overlapping due to propagation delay. In this case, the remaining 80 chips may form the access timeslot together with UpPTS, so as to make full use of the existing resource.
Fig.2 is schematic diagram showing the structure of a sub-frame according to an embodiment of the present invention. Compared with the sub-frame structure of fig.l, the duration time of the GP between DwPTS and UpPTS is decreased from 96 chips to 16 chips, the remaining 80 chips and the UpPTS are merged to form the access timeslot (Access TS), and the other traffic timeslots TS0-TS7 and DwPTS have the same position and duration on the sub-frame as the corresponding parts of fig.1.
Fig.3 is a schematic flowchart showing the method of channel accessing according to an embodiment of the present invention. As shown in fig.3, the method comprises establishing a call link (step SIlO), selecting a channel (step S120), probing (step S130), simulative testing (step S 140), and determining whether to access (step S 150), etc. A detailed description is given blow in conjunction with Fig.3.
In step SIlO for establishing a call link, a source mobile terminal (UEl) and a target mobile terminal (UE2), which are to establish a P2P communication, establish a call link using a primary carrier with the aid of the BS. In a preferred embodiment, UEl transmits a call request message to the BS through an uplink access channel of the primary carrier, indicating its intention for P2P communication with UE2; the BS transmits a paging message to UE2 after receiving the call request; UE2 returns a paging reply to the BS after receiving the paging message; after receiving the paging reply, the BS transmits a acknowledge massage to UEl and UE2 respectively, and designates a secondary carrier for
P2P communication.
In the overhearing step S 120, UEl overhears all traffic timeslots on the secondary carrier, and selects a channel to be used for P2P communication according to the detected interference level in the timeslots. This channel is determined by the secondary carrier frequency, the timeslot, and spread code.
In the probing step S 130, UEl transmits one or more probing messages with a predetermined power and a known channelization code in the access timeslot of the secondary carrier. The probing message includes identifiers of UEl and UE2, the timeslot and spread code corresponding to the selected channel to be used for P2P communication, and a period for the simulative testing. After receiving the probing message, UE2 calculates a Signal-to-Interference Ratio based on the received signal power to estimate the condition of the link quality. If the link quality is satisfied, UE2 transmits a probing acknowledge message to UEl, otherwise, it keeps silent. If receiving the acknowledge message within a predetermined period, UEl makes a preparation for the simulative testing in the next time period, otherwise it gives up the attempt for P2P communication.
During the probing procedure described above, since the access timeslot is a common slot and a known channelization code is employed, the messages transmitted between UEl and UE2 actually can be regarded as broadcast messages. In this case, all the other mobile terminals overhearing the access timeslot may receive the broadcast messages. After having overheard the probing message and the probing acknowledge message, the mobile terminals, such as UE3 and UE4, communicating in the timeslot selected by the source mobile terminal UEl and to be used for P2P communication will respectively prepare to participate the following simulative access testing.
In the simulative testing step S 140, within a predetermined period, the access timeslot is used to carry out a simulative access testing for the timeslot selected by UEl and to be used for P2P communication. In this case, the mobile terminals such as UE3 and UE4 communicating in the selected timeslot participate the simulative testing together with UEl and UE2. During the simulative testing procedure, the simulative testing communication performs close-loop power control for UE1/UE2 and UE3/UE4 for several times, respectively, so as to make the transmission power converge step by step. In each process of the close-loop power controls, UEl transmits a testing message to UE2 one by one, the testing message including a training sequence used for synchronizing UEl with UE2; UE2 evaluates the link based on the received signal's quality after receiving the testing message, and transmits a testing feedback message to UEl; after receiving the feedback message from UE2, UEl adjusts, based on the feedback message, its transmission power to meet the communication quality requirement. Meanwhile, UE3 and UE4 participate, in the same access timeslot, the simulative testing using their allocated spread-spectrum code in the selected timeslot. They perform close-loop power control similar to that of UE1/UE2 for several times, calculate corresponding Signal-to-Interference Ratio, and estimate the influence to their communication quality after UEl is accessed while adjust the power to meet the communication quality requirement.
In step S 150 for determining whether to access, UEl overhears the simulative testing feedback information on the access channel within a predetermined period, so as to determine whether to start P2P communication. In addition to the overhearing sub-step, other sub-steps corresponding to two cases as shown in fig.3 are also included in this step.
In the first case (A): at the end of the testing procedure, the mobile terminal such as UE3 participating the testing finds that the current Signal-to-Interference Ratio is higher than or equal to a predetermined threshold through calculation. This means that the access of UEl will not cause the communication quality of UE3 and UE4 to deteriorate to an unacceptable degree, and thus UE3 keeps silent to accept the timeslot selected for the access of UEl. In this case, UEl will not overhear any access deny message for the simulative testing within a predetermined period, and thus carry out P2P communication with UE2 on the selected channel taking the power adjusted during the testing procedure as the initial transmission power (step S 154).
In the second case (B): at the end of the testing procedure, the mobile terminal such as UE3 participating the testing finds that the current Signal-to-Interference Ratio is lower than a predetermined threshold through calculation. This means that the accessing of UEl will cause the communication quality of UE3 and UE4 to deteriorate to an unacceptable degree. In this case, UE3 transmits an access deny message to UEl, to indicate UEl to give up accessing the selected timeslot (step S 156). Accordingly, when having overheard the access deny message for the simulative testing within a predetermined period, UEl gives up the selected timeslot to be used for P2P communication, and reselects a channel to be used for P2P communication, or returns to the primary carrier and notifies the BS to give up the current P2P communication request (step S158).
In the above mentioned accessing method, the period for the probing procedure, simulative testing procedure, and overhearing procedure of testing feedback may generally be predetermined based on the number Nue of mobile terminals which meets the P2P supporting distance in a cell, sub-frame length Tf , and the Poisson arrival rate λ for P2P call, so as to reduce the possible signal conflicts on the access channel. For example, if Nue is 500, λ is 1/120 (I/second), then the number of the P2P mobile terminals arriving within 1 second is:
N can = Nue - λ - At = 500 • 1/120 • 1 = 4.167
In this case, the period for the overall accessing procedure should be controlled within
240 milliseconds, i.e., within 48 sub-frames when the duration time of a sub-frame is 5 milliseconds. For example, the probing procedure, the simulative testing procedure, and the overhearing of testing feedback procedure are set to have 4, 20, and 4 sub-frames, respectively.
The step SIlO for establishing a call link in the above-mentioned method of channel accessing may be an optional step. In this case, the source mobile terminal which is to carry out P2P communication directly overhears each of the timeslots on the carrier for P2P traffic to select a channel to be used for P2P communication with UE2, and implements the call link establishment and the probing function simultaneously on the access timeslot in the probing step S 130. Similarly, the method of channel accessing provided in the present invention can also be used in an ad hoc network, in which no BS or central controller is provided and a CDMA technique is employed.
The method of channel accessing described above in conjunction with Fig.3, which uses the access timeslot to simulate the timeslot to be used for P2P communication, and carries out simulative testing for all the communicating channels and channels to be accessed in that timeslot, can be implemented by means of software, or hardware, or combination of both.
Fig.4 is a schematic block diagram showing an accessing apparatus 100 for a mobile terminal according to an embodiment of the present invention. The accessing apparatus resides in the source mobile terminal UEl, which initiates a P2P call, comprising a calling unit 110, a selecting unit 120, a probing unit 130, a testing unit 140, and a deciding unit
150.
The calling unit 110 is used for establishing a call link with a target mobile terminal
UE2 with the aid of a BS. The calling unit 110 comprises a first transmitting unit for transmitting a P2P call request to the UE2 via the BS, and a first receiving unit for receiving a P2P paging reply from the UE2 via the BS and obtaining an information indicating a secondary carrier for P2P communication from the BS.
The selecting unit 120 is used for overhearing the interference levels in all the traffic timeslots on the secondary carrier, and selecting the channel to be used for P2P communication with the UE2 based on the interference levels in the timeslots, wherein this channel is determined by the secondary carrier frequency, the timeslot, and the spread- spectrum code.
The probing unit 130 is used for, during a first predetermined time period, broadcasting a probing message and receiving the corresponding probing acknowledge message in the access timeslot. The probing message indicates the identifiers of UEl and
UE2, the timeslot and spread- spectrum code corresponding to the selected channel to be used for the P2P communication, so as to notify UE2 and other mobile terminals, such as
UE3 and UE4 etc., communicating in the selected timeslot to participate the simulative access testing. The probing unit 130 comprises a second transmitting unit for transmitting a probing message to UE2 and a second receiving unit for receiving a probing acknowledge from UE2.
The testing unit 140 is used for, during a second predetermined time period, simulating the selected timeslot with the access timeslot and carrying out simulative testing for the channels to be accessed and the accessed channels in this timeslot. The testing unit 140 comprises a third transmitting unit for transmitting a simulative testing message one by one in the access timeslot by using the spread- spectrum code for the selected channel; a third receiving unit for receiving the simulative testing feedback information from UE2; and a power control unit for adjusting, based on signal quality of the received feedback message, the transmission power for the next testing message to meet the communication quality requirement. Meanwhile, all the mobile terminals such as UE3 and UE4 etc. communicating in the selected timeslot also transmit the testing message and receive the feedback message in the access timeslot by using their spread- spectrum code allocated in the selected timeslot, and adjusts the transmission power based on the communication quality of the received message. Therefore, the signal quality between UEl and UE2 and the signal quality between UE3 and UE4 may cause influence to each other. In order to guarantees the communication quality, each of the mobile terminals must adjust the transmission power to make it converge step by step according to the change of the quality of the received signal. Thus, during the testing procedure, the evaluation of the signal quality may be based on the calculation for the SINR of the received signal.
The deciding unit 150 is used for, during a third predetermined time period, overhearing in the access timeslot the feedback message from any of the mobile terminals communicating in the selected timeslot, to determine whether to employ the selected channel for P2P communication. At the end of the testing procedure, if the mobile terminal such as UE3 participating the testing finds that the current SINR is lower than a predetermined threshold through calculation, i.e., it is predicted that the accessing of UEl will cause the communication quality for UE3 and UE4 to deteriorate to an unacceptable degree, then UE3 transmits an access deny message to UEl, in order to indicate UEl to give up accessing the selected timeslot; otherwise, UE3 keeps silent. When UEl has overheard the access deny message within a predetermined period, it gives up the selected channel to be used for P2P communication, and reselects a channel to be used for the P2P communication, or returns to the primary carrier and notifies the BS to give up the current P2P communication request. Otherwise, if UEl does not receive any access deny message within the predetermined period, UEl and UE2 start P2P communication in the selected timeslot.
The transmitting units and the receiving units mentioned above have similar functions, and may be implemented with a same structure. Moreover, since the transmitting units and the receiving units operate in different time periods, they may be implemented with a same transmitting unit and a same receiving unit respectively.
The apparatus provided in the present invention may be used for a mobile terminal that establishes a call link without a BS and a central controller. In this case, the accessing apparatus comprises all the function units other than the calling unit 110 in the accessing apparatus 100. Wherein, the selecting unit 120 directly overhears timeslots on the carrier for P2P traffic to select a channel to be used for P2P communication with UE2, while the probing unit S 130 accomplishes the call link establishment and the probing functions simultaneously. Fig.5 is a schematic block diagram showing a second accessing apparatus 200 for use in the mobile terminal UE3 to cooperate with UEl to carry out the access testing, according to another embodiment of the present invention. The accessing apparatus 200 comprises an overhearing unit 160, a testing unit 170, and a feedback unit 180.
The overhearing unit 160 is used for, during a first predetermined time period, overhearing a probing/probing acknowledge message in the access timeslot. Wherein, the probing message is transmitted by the source mobile terminal UEl which requests access, and indicates the identifiers of the source mobile terminal UEl and the target mobile terminal UE2, and the timeslot and spread- spectrum code corresponding to the selected channel to be used for the P2P communication; and the probing feedback message is transmitted by UE2, acknowledging the probing message sent by the first mobile terminal.
The testing unit 170 is used for, during a second predetermined time period, cooperating with UEl and UE2 to carry out simulative channel access testing in the access timeslot when the timeslot to be used for P2P communication is identical to the timeslot in which it is communicating. Specifically, in the access timeslot, it transmits a testing message to its corresponding receiver UE4 by using its spread- spectrum code allocated in the timeslot to be used for the P2P communication, receives a feedback message from UE4, and adjusts the transmission power based on the received feedback message.
The feedback unit 180 is used for evaluating the channel quality during the simulative access testing procedure. When its SINR is lower than a predetermined threshold, it transmits an access deny message to UEl in a third predetermined time period, so as to indicate UEl to give up accessing the selected timeslot; otherwise, it will keep silent to accept the selected timeslot for the access of UEl.
It is to be understood by those skilled in the art that the method and apparatus provided in the present invention may be used not only in a cellular communication system but also in a Wireless Local Area Network (WLAN) communication system and other wireless communication systems that have P2P function and employ CDMA technique.
It is to be understood by those skilled in the art that the embodiments herein intend to illustrate but not to limit the present invention. Various improvements and modifications can be made to the accessing method and accessing apparatus for use in wireless communication systems without departing from the basis of the present invention. Therefore, the scope of the present invention is to be defined by the attached claims herein.

Claims

CLAIMS:
1. A method of channel accessing for use in a wireless communication system and executed in a mobile terminal, comprising steps of: (a) overhearing a plurality of timeslots to select a timeslot and a channel in the timeslot to be used for P2P communication with a target mobile terminal;
(b) transmitting a probing message in an access timeslot to notify the target mobile terminal and mobile terminals communicating in the selected timeslot to carry out a channel access testing in the access timeslot; (c) carrying out an access testing, in the access timeslot, for the selected channel and communicating channels in the selected timeslot to obtain a testing result indicating the interference among the channels; and
(d) overhearing a feedback message, which is based on the testing result, from any of the mobile terminals communicating in the selected timeslot, so as to determine whether to employ the selected channel for P2P communication with the target mobile terminal.
2. The method as claimed in claim 1, further comprising a step of receiving a probing acknowledge message from the target mobile terminal before the step (c).
3. The method as claimed in claim 1, wherein the step (c) comprising: performing a plurality of close-loop power control processes, wherein in each process of power control, transmitting a testing message to the target mobile terminal and receiving a feedback message from the target mobile terminal in the access timeslot, and adjusting the transmission power based on the received feedback message to meet the communication quality requirement.
4. The method as claimed in claim 3, wherein each of the mobile terminals communicating in the selected timeslot respectively transmits the testing message to its corresponding receiver and receives the feedback message from the receiver in the access timeslot, and adjusts its transmission power based on the received feedback message to meet the communication quality requirement.
5. The method as claimed in claim 1, wherein in the step (d), giving up accessing the selected channel when having overheard an access deny message from any of the mobile terminals communicating in the selected timeslot; otherwise, starting P2P communication with the target mobile terminal on the selected channel with the power adjusted at the end of the testing as the initial transmission power.
6. The method as claimed in claim 1, wherein, before the step (a), further comprising steps of: transmitting a paging message to the target mobile terminal via a central controller in the wireless communication system; and receiving a paging acknowledge message from the target mobile terminal via the central controller in the wireless communication system, the message indicating a frequency carrier available for P2P communication.
7. The method as claimed in claim 6, wherein the frequency carriers used for paging the target mobile terminal and carrying out the access testing are a primary carrier and a secondary carrier respectively.
8. The method as claimed in claim 1, wherein, the wireless communication system is a Time Division Duplex Code Division Multiple Access (TDD CDMA) communication system having P2P function, and the access timeslot comprises a conventional uplink pilot channel and a part of the guard period between a downlink pilot channel and the uplink pilot channel.
9. The method as claimed in claim 1, wherein the wireless communication system is an ad hoc network using a Code Division Multiple Access technique.
10. An apparatus of channel accessing for use in a wireless communication system and executed in a mobile terminal, comprising: an selecting unit for overhearing a plurality of timeslots to select a timeslot and a channel in the timeslot to be used for P2P communication with a target mobile terminal; a probing unit for transmitting a probing message in an access timeslot to notify the target mobile terminal and terminals communicating in the selected timeslot to carry out a channel access testing in the access timeslot; a testing unit for carrying out, in the access timeslot, an access testing to the selected channel and communicating channels in the selected timeslot to obtain a testing result indicating the interference among the channels; and a deciding unit for overhearing a feedback message, which is based on the testing result, from any of the mobile terminals communicating in the selected timeslot, so as to decide whether to employ the selected channel for P2P communication with the target mobile terminal.
11. The apparatus as claimed in claim 10, wherein the probing unit further receives a probing acknowledge message from the target mobile terminal.
12. The apparatus as claimed in claim 11, further comprising a paging unit for transmitting a paging message to a target mobile terminal via a central controller in the wireless communication system, and receiving a paging acknowledge message from the target mobile terminal via the central controller in the wireless communication system, the message indicating a carrier available for P2P communication.
13. A method of channel accessing for use in a wireless communication system and executed in a mobile terminal, comprising steps of: (a) overhearing a probing message in an access timeslot, and judging whether a selected timeslot for P2P communication indicated by the probing message is identical to the timeslot in which it is communicating;
(b) when the selected timeslot is identical to the timeslot in which it is communicating, transmitting a testing message to a receiver with which it is communicating and receiving a feedback message from the receiver in the access timeslot, and adjusting the transmission power based on the received feedback message while calculating the Signal-to-Interference Ratio of the feedback signal; and
(c) when the Signal-to-Interference Ratio is lower than a predetermined threshold, transmitting an access deny message to the mobile terminal which transmits the probing message, to indicate the mobile terminal to give up accessing the selected timeslot; otherwise, adjusting the transmission power at which it is communicating in the selected timeslot to the power adjusted at the end of the testing .
14. An apparatus of channel accessing for use in a wireless communication system and executed in a mobile terminal, comprising: an overhearing unit for overhearing a probing message in an access timeslot, and judging whether a selected timeslot for P2P communication indicated by the probing message is identical to the timeslot in which it is communicating; a testing unit for, when the selected timeslot is identical to the timeslot in which it is communicating, transmitting a testing message to a receiver with which it is communicating and receiving a feedback message from the receiver in the access timeslot, and adjusting the transmission power based on the received feedback message while calculating the Signal-to-Interference Ratio of the feedback signal; and a feedback unit for, when the Signal-to-Interference Ratio is lower than a predetermined threshold, transmitting an access deny message to the mobile terminal which transmits the probing message to indicate the mobile terminal to give up accessing the selected timeslot; otherwise, adjusting the transmission power at which it is communicating in the selected timeslot to the power adjusted at the end of the testing.
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