JP2008541601A - Adaptive MAC protocol based on multi-user detection - Google Patents

Abstract

  In MC-CDMA based WLAN, CDMA based, or other multi-channel wireless systems, dynamically determine whether to transmit in parallel with the current transmission or select other frequency channels in the communication system A system or method for providing a station, a mobile station, or otherwise. The example device is well suited to a CDMA based wireless system using MUD. The systems and methods provide a means for estimating signal interference on the same frequency channel determined by the MUD. Individual stations in the system may decide in a distributed manner whether to change their communication link to another frequency channel or to continue on the same frequency channel in parallel with the current communication channel. it can.

Description

  Multi-carrier code division multiple access (MC-CDMA) has attracted considerable attention in recent years and has become a promising candidate for future wireless high-capacity communication networks. Typically, multicarrier technology is robust against multipath fading and provides high spectral efficiency and interference cancellation capability. MC-CDMA has several other advantages such as spectral diversity and resistance to frequency selective fading and impulse noise.

  Referring to FIG. 1, each symbol 1, 2, 3, 4 of one user's MC-CDMA data stream is multiplied by each coefficient of the same spreading code, and then several narrowband subcarriers 6a, 6b, 6c, 6d. The chips 8 are not sequential but instead are transmitted in parallel on different subcarriers (eg 6a). In MC-CDMA, a single data symbol is frequency spread. Such a system of spreading factor (SF) 4 is shown in FIG. FIG. 1 is an example of MC-CDMA with SF = 4.

  Then, referring to FIG. 2, the station 20 to be transmitted must select a code channel (CCh1, CCh2, CCh3, CCh4). Two methods are possible for this code channel selection. One is to select a code channel before every packet transmission. Initially, this selection is made randomly. For later transmissions, a station will use a code channel that has already been reserved by another station (according to the standard setting the network assignment vector (NAV) for the channel occupied by the station considered). Do not select. A second method of code channel selection includes selecting the code channel with the least traffic and maintaining this code channel for the entire duration of the communication.

  FIG. 2 is an example of an RTS / CTS access mechanism. Prior to accessing the medium, the station should detect whether the medium is idle during a period referred to as the distributed interframe space (DIFS). Once the station detects that the medium is idle, the station notifies the intended data transfer by sending an RTS packet 22 as shown.

  All stations that are not intended receivers (eg, STA3 and STA4) that receive the RTS control packet set their NAV timer 24, suspend their backoff down counts, and The medium is postponed so as not to interfere. If the receiving side of RTS 22 (eg, STA2) is idle, i.e., can receive data, it responds to CTS packet 26 after a period called short interframe space (SIFS) 28. If the RTS receiver is busy, the RTS transmission is repeated after a new backoff. The mobile station that receives the CTS also sets its NAV timer. The transmitting side 20 can transmit the data packet 32 after the SIFS 30. The reception side (for example, STA2) confirms the successful reception by the confirmation (ACK) 34 after the period of the SIFS 36 after the data frame 32 ends. The standard distributed control function (DCF) procedure 38 is performed on every code channel for each data transmission.

  Referring to FIG. 3, an example MUD receiver is shown. Multi-carrier CDMA systems, like most asynchronous CDMA systems, require a so-called multi-user detector MUD 40. The reason is that in an asynchronous multi-access CDMA system, the received signal consists of all active user data. Timing mismatch destroys the orthogonality of spreading codes of different users, resulting in multiple access interference (MAI). For this reason, the MUD 40 must be applied to the receiving side (for example, STA2). One example of such a MUD is a linear detector based on the minimum mean square error (MMSE) criterion. The MMSE receiver combines both high performance and simple implementation.

ここでｂ_ｋは、ｋ番目のユーザのシンボルである。 As can be seen from the MUD diagram in FIG. 3, in the linear multi-user detector, the demodulator output ym is multiplied by a decision variable wm, which optimizes the detector decision based on the transmitted symbols. Used to reduce channel effects. For MMSE MUD, the optimal weight matrix for a given set of delay τ _K and fading parameter β _km is selected to minimize the mean square error of the detector,

Where b _k is the symbol of the k th user.

MUD is not only applied to many CDMA systems, but may also be applied in other systems such as MIMO systems. Further information regarding the background of the present invention can be found in the following references included here by reference as appropriate.
G. Orfanos, J. Habetha, L Liu, "MC-CDMA based IEEE 802.11 wireless LAN," Proc. IEEE MASCOTS 2004, Oct. 2004; J. Proakis, Digital Communications. McGraw-Hill, Singapore, 4th Ed., 2001 ; S. Hara, R. Prasad, "Overview of multicarrier CDMA," IEEE Comm. Magazine, vol. 35, issue 4, pp. 104-108, April 1997; B. Walke, Mobile Radio Networks. Willey & Sons Ltd, Chichester, Sussex, UK, 2nd Ed., 2001; J. Linnartz, "Performance analysis of synchronous MC-CDMA in mobile rayleigh channel with both delay and doppler spreads," IEEE Trans. On Vehicular Technology, vol. 50, issue 6, Nov. 2001; IEEE 802.11 WG, Draft supplement to standard for information technology telecommunications and information exchange between systems-LAN / MAN specific requirements-Part 11: Wireless medium access control (MAC) and physical layer (PHY) specifications: High-speed physical layer in the 5GHz band, IEEE Std. 802.11a, 1999; F. Schreiber, "Effective Control of Simulation Runs by a New Evaluation Algorithm for Correlated Random Sequences, "in AEU International Journal of Electronics and Communications, vol. 42, no. 6, pp. 347-354, 1988; B. Walke, Mobifunknetze und ihre Protokolle, Stuttgart, Germany: BGTeubner, Band2, 2. Auflage, 2000 ; D.Qiao, S. Choi, A. Jain, K. Shin, "Adaptive Transmit Power Control in IEEE 802.11a Wireless LANs," in Proc. IEEE VTC 2003-Spring, April 2003; M. Lott, Entwurf eines drahtlosen multihop Ad-hoc-Funknetzes mit Dienstguteunterstutzung, PhD Thesis, Mainz, Germany: Aachener Beitrage zur Mobil- und Telekommunikation, 2001; K.Wang, P.Zong, Y.Bar-Ness, "A reduced complexity partial sampling MMSE receiver for asynchronous MC -CDMA systems, "IEEE Proc.GLOBECOM'01, 2001; and S.Yi, C.Tsimenidis, O.Hinton, B.Sharif," Adaptive minimum bit error rate multi-user detection for asynchronous MC-CDMA systems in frequency selective rayleigh fading channels, "IEEE Proc. PIMRC 2003, Sept. 7-10, 2003.

  Embodiments of the present invention may transmit to a stream receiver in parallel with a current transmission in a communication network including a plurality of devices including at least a data stream transmitter indicative of the current transmission power, or Including a method of dynamically determining whether to select another frequency channel, wherein a receiver of a stream of data packets calculates an average interference level of at least one previous data packet by a multi-user detector and The side calculates the maximum achievable SINR according to its interference conditions, maximum allowable transmission power, and path loss from the transmitter to the receiver, and the receiver calculates the maximum achievable SINR calculated above for the transmission Whether it is smaller than intended or not is determined with respect to the frequency change, and the receiver side avoids the interference of stations changing channels in parallel. Therefore, perform a change to another frequency channel with probability P, or transmit in parallel with the current transmission with probability 1-P, and the receiver will set a small value in the duration field of the frame. By using it, the transmitting side is notified of the frequency change, and the receiving side encodes the frequency channel id in the period field of the frame.

  In addition, an embodiment of the present invention provides a transmitter and receiver that uses packets prior to a data frame and / or in the data frame itself to exchange information about transmit power and interference conditions at the current station. The packet to be transmitted in the signal including the side is equivalent to a request for transmission (RTS) and transmission permission (CTS) packet according to the standard IEEE 802.11 or a newer revision of the standard (such as 802.11n). The probability P of performing a frequency change for a packet, or any management frame, increases with the amount of packets that trigger a frequency change procedure on the receiving or transmitting side, and the communication network is capable of multi-carrier code division multiple access ( MC-CDMA), multiple input multiple output (MIMO), orthogonal frequency division multiple access (O DMA) techniques, time division multiple access (TDMA), frequency division multiple access (FDMA), or using a combination thereof.

  The above summary of the present invention is not intended to represent each embodiment or every aspect of the present invention.

  A more complete understanding of the method and apparatus of the present invention can be obtained by reference to the following detailed description, taken in conjunction with the accompanying drawings, in which:

  Code division multiple access (CDMA) networks suffer from perspective problems. The perspective problem occurs, for example, when receiver A is blocked by a nearby transmitter B when operating with different spreading codes in the same frequency channel at the same time. Embodiments of the present invention provide a mechanism or means for controlling the amount of parallel transmission in MC-CDMA WLAN to avoid perspective problems. By providing a mechanism or means for controlling the parallel transmission of different stations in MC-CDMA WLAN, the performance of the communication network is improved, thus providing more efficient use of the transmission channel, higher signal throughput and The result is a shorter delay.

  Embodiments of the present invention determine the interference level, thereby determining the SINR by MUD. The interference level and / or SINR is used by the station to determine whether the communication link should be changed to another frequency channel or to continue in parallel with the current link in the same frequency channel. Is done. In an exemplary embodiment, the transmitter and receiver use frames such as RTS and CTS to explicitly exchange information about transmit power. In yet another embodiment, the frequency change probability P increases with the amount of packets that trigger the frequency change.

ただし、Ｋはアクティブユーザの最大数、ａ_ｋはｋ番目のユーザのシンボルｂ_ｋの送信電力、Ｍはサブキャリアの数、ｐ（ｔ）は［０,Ｔ］に渡る矩形波パルス、τ_ｋはｋ番目のユーザの遅延、η（ｔ）は加算性ホワイトガウスノイズを示す。ｍ番目のサブキャリアで、ｋ番目のユーザに対するレイリーフェージングプロセスは、

のように記載される。ただしβ_ｋｍがレイリー分散であり、φ_ｋｍが［０，２π）に渡って一様である分散変数である。 A signal received in an asynchronous MC-CDMA system may be described by the following equation:

Where K is the maximum number of active users, a _k is the transmission power of the symbol b _k of the _kth user, M is the number of subcarriers, p (t) is a square wave pulse over [0, T], τ _k Is the delay of the kth user, and η (t) is additive white Gaussian noise. On the mth subcarrier, the Rayleigh fading process for the kth user is

It is described as follows. However, β _km is Rayleigh dispersion, and φ _km is a dispersion variable that is uniform over [0, 2π).

ここで行列Ｐ及びｐは、式１から得られ、ｗはＭＵＤの重みベクトルである。 In this case, the SINR in the MUD detector can be given as follows:

Here, the matrices P and p are obtained from Equation 1, and w is a MUD weight vector.

  It is clear from Equation 4 that a station using four correlators can calculate an SINR estimate according to Equation 4 by estimating the power received by other spreading codes. It is clear to use the analysis starting at. After the SINR estimation, the MS can estimate the average interference during the packet reception period for the known received power.

For algorithm stability, the average interference condition can be calculated at each receiver as follows according to all previously received packets:

  The interference between the last received frames is weighted by 50% because the last received frame is closest to the actual value. Using this weight, channel changes due to a single packet with deep fading can be avoided. Furthermore, the algorithm is fast enough to overcome the perspective problem. The weighting factor 50% is an exemplary value and may be applied to different operating conditions of the WLAN.

表１：適合型アルゴリズムのパラメータ In order to allow the exchange of interference conditions between stations required for frequency adaptive algorithms, the RTS 50 and CTS 51 frames, as illustrated in FIGS. 4 and 5 (beyond the IEEE 802.11a standard). ) Expanded with two additional fields, TxPow52 and IfPow54. FIG. 4 is an extended RTS frame 50. FIG. 5 is an expanded CTS frame 51. In field TxPow 52, the transmit power of the current frame is encoded, and IfPow 54 carries information about the last estimate of average interference for this station in the channel on which the data transfer takes place. The length of each field consists of 1 byte.

Table 1: Adaptive algorithm parameters

  FIG. 6 is a frequency adaptive algorithm or flowchart 60 according to one embodiment of the present invention. FIG. 6 provides an overview of the frequency adaptive algorithm based on the RTS-CTS-Data-Ack transmission cycle. Station 1 (S1) 62 indicates a transmitting side, and station 2 (S2) 64 indicates a corresponding receiving side. Further, the variables required for the algorithm are defined in Table 1. All values are given in dB.

Station S1 transmits RTS frame 66 using the extended frame format of FIG. In the RTS frame, the current values of P ^S1 _Tx and P ^S1 _IF are set and transmitted. At 70, station S2 receives an RTS frame with power P ^RTS _RX and decodes the values of P ^S1 _TX and P ^S1 _IF . S2 can calculate the path loss L between S1 and S2.

Thereafter, S2 calculates the maximum possible SINR for packet reception considering the actual average interference estimate P ^S2 _IF , the path loss L and the maximum allowable transmission power.

Station S2 can determine whether a packet transmission can occur in S2's current interference situation (P ^S2 _IF ).

  Two settings are possible.

First setting: max SINR <min SINR
Here, the min SINR indicates a setting threshold required according to the PHY mode to be used and a QoS target of the link, for example, in the form of a maximum tolerated packet error rate (PER). In this case, S2 64 initializes the frequency change with a certain probability. The extended CTS frame 72 is sent to S1 62 using a robust BPSK1 / 2 PHY mode. Fields P ^S2 _TX and P ^S2 _IF are filled with the current value of S2.

  The period field in the CTS frame is used to indicate the data transfer period in microseconds after the CTS frame ends. For frequency changes, S2 64 can use the period field to indicate a frequency change and to encode a new frequency channel id, since small values in the period field are not used by the standard.

After transmission of the CTS frame 72, S2 64 changes to a new frequency channel 74 and resets all its transmission related parameters.

  The stochastic change of the frequency channel is used to prohibit unnecessary frequency changes and collisions due to perspective problems in the new frequency channel. In cases where there is a heavy communication load between stations, there may be situations where the receivers of more than one station are blocked by transmissions from other stations, or the stations block each other's transmissions. If the link between stations is changed to another frequency channel, the interference condition can be changed immediately, and then all or most receivers can be able to operate without perspective problems. If this is not the case, further frequency changes can be initiated.

  In the embodiment of the present invention, the above-described stochastic change can be made as follows. Each station that calculates maxSINR <minSINR changes its frequency channel with a certain probability, which is set to 25% in this exemplary embodiment. The selection of a new frequency channel is determined randomly from another set of frequency channels in the system.

と、現在の干渉状況での最小達成可能ＳＩＮＲ

とを計算する。 At 76, S1 62 receives a CTS frame with Rx power P ^CTS _RX and decodes the values of P ^S2 _TX and P ^S2 _IF from the frame. Therefore, S1 is the path loss between S1 and S2.

And the minimum achievable SINR in the current interference situation

And calculate.

S1 compares the above calculated value of maxSINR with the minSINR to determine for frequency change. This operation is not required but provides other control mechanisms for the algorithm when information about the frequency change is already entered in the duration field of the CTS frame extended by S2. S1 62 changes to a new frequency channel 78 and resets its P ^S1 _TX and P ^S1 _IF parameters according to equations 7 and 8. Thereafter, S1 62 starts a new transmission with an RTS packet on the new channel.

  Second setting: maxSINR ≧ minSINR

Data transfer 80 may occur on the current frequency channel. S2 64 transmits an extended CTS frame 72 with actual values P ^S2 _TX and P ^S2 _IF, as shown in FIG. The period field includes a data transmission period after the CTS frame ends, as described in the standard.

  Since the CTS frame should be received correctly, S1 62 will send the actual data packet 80 according to the protocol. In the case of an erroneous reception, the R1 retransmission timer of S1's MAC protocol will approach zero and S1 62 will start a retransmission attempt with a new RTS packet by doubling the CW in advance. .

のように表現され得る。 To further support the embodiments, methods, and applications of the present invention, the following mathematical support is provided. The signal received on the nth subcarrier is

Can be expressed as:

In a linear multiuser detector, the demodulator output yn is used to optimize the detector decision based on the transmitted symbols and is multiplied by a decision variable wn that mitigates channel effects.

ここでｂ_ｋは、ｋ番目のユーザのシンボルである。 From FIG. 3, in the case of MMSE MUD 40, the optimal weight matrix for a given set of delay τ _K and fading parameter β km is selected to minimize the mean square error of the detector and can be calculated from the following equation: .

Where b _k is the symbol of the k th user.

のようにも表現され得る。ここでは、

となっている。 Equation 12 is

It can also be expressed as here,

It has become.

これは、式１７にも適用され得る。式１５は、ここで、行列表示で表現され得る。

ただし、

であり、ここでζは、共分散行列Γ_ＭｘＭ＝σ_ｎ ^２Ｉを持つホワイトガウスノイズベクトルを示す。 By analyzing Equation 15, an analytical form can be derived.

This can also be applied to Equation 17. Equation 15 can now be expressed in a matrix representation.

However,

Where ζ represents a white Gaussian noise vector having a covariance matrix Γ _MxM = σ _n ² I.

  As such, embodiments of the present invention may transmit in parallel with current transmissions for stations in MC-CDMA based WLAN or other CDMA based wireless systems or other in communication systems. A method for dynamically determining whether to select a frequency channel is provided. Embodiments of the present invention are well suited for CDMA based wireless systems using MUD. In an embodiment of the present invention, an estimate of signal interference in the same frequency channel is determined using the MUD. The station can decide in a distributed manner to change its communication link to another frequency channel or continue in the same frequency channel in parallel with the current communication link in the same frequency channel.

  Embodiments of the invention may be applied to other systems with parallel streams of communication, such as multiple-input multiple-output (MIMO) and orthogonal frequency division multiple access (OFDMA) systems. Interference estimates for embodiments of the present invention may not be provided by the MUD, but instead may be calculated by channel measurements while the station or mobile station is idle. An exemplary protocol for implementing embodiments of the present invention is the IEEE 802.11a WLAN standard Media Access Control (MAC), which requires some modifications to support the exemplary CDMA physical layer (PHY layer). It should be understood that it may be based on a protocol.

  Many variations and embodiments of the above invention and method are possible. While only certain embodiments of the present invention and method are depicted in the accompanying drawings and described in the detailed description, the present invention is not limited to the disclosed embodiments, and is described and defined in the claims. It will be understood that additional reconfigurations, modifications, and substitutions are possible without departing from the invention. Accordingly, it should be understood that the scope and general understanding of the invention encompasses all such configurations and is merely limited by the claims.

FIG. 1 is a spreading factor (SF) 4 MC-CDMA system that may include an embodiment of the present invention. FIG. 2 is an MC-CDMA based MAC protocol with an RTS / CTS access mechanism that may include an embodiment of the present invention. FIG. 3 is a block diagram of the MMSE receiver that may include an embodiment of the present invention. FIG. 4 is an exemplary MAC RTS MAC frame format according to one embodiment of the present invention. FIG. 5 is an exemplary MAC CTS MAC frame format according to one embodiment of the present invention. FIG. 6 is an exemplary frequency adaptation flowchart and exemplary algorithm according to one embodiment of the present invention.

Claims (15)

  In a wireless multi-channel communication network, the station decides whether to transmit the data in parallel with other current transmissions or to select another frequency channel in the multi-channel communication network to transmit data. A method for dynamically determining comprising: a transmitting station displaying a current transmission power level in a data packet to a receiving station; and an average interference of at least one previous data packet received from the transmitting station. A level is calculated by the receiving station; a maximum achievable SINR is calculated by the receiving station based on the interference level; and if the maximum achievable SINR is less than a minimum allowable SINR, the receiving step A step of changing to another frequency channel, and a step of notifying the transmitting station of the step of changing to the other frequency channel by the receiving station. A method performed in at least one field of a packet and a CTS packet.   The method of claim 1, wherein the changing further comprises changing to the other frequency channel with a certain probability to avoid collision with the other current transmissions in the other frequency channel.   The method according to claim 1, further comprising: encoding by the receiving station a frequency channel Id of the other frequency channel in a period field of at least one of the RTS frame and the CTS frame.   The method of claim 1, wherein the displaying is performed on a packet provided by the transmitting station before the transmitting station transmits a data frame to the receiving station.   The method of claim 1, wherein the displaying is performed in a data frame by the transmitting station.   The method of claim 1, wherein the RTS packet and the CTS packet substantially conform to at least one of the IEEE 802.11 standards.   The method of claim 1, wherein the multi-channel communication network is a network that uses at least one of MC-CDMA, MIMO, OFDMA, FDMA, SDMA, and TDMA technologies.   A multi-channel wireless network comprising a plurality of stations in the multi-channel wireless network such that at least one station is a transmitting station and at least one other is a receiving station, the receiving station having a MUD The receiving station determines an associated SINR and signal interference level based on a signal determination from the MUD, and whether the receiving station changes the link with the transmitting station to another channel. Multi-channel wireless network that further determines based on part.   The multi-channel wireless network of claim 8, wherein the receiving stations exchange information about transmit power using at least one of an RTS frame and a CTS frame.   The multi-channel wireless network according to claim 9, wherein each of the RTS frame and the CTS frame has at least one of a transmission power field and an interference power field.   The multi-channel wireless network of claim 8, wherein the network incorporates at least one of MC-CDMA, MIMO, OFDMA, SDMA, FDMA, and TDMA technologies.   A transceiver for a multi-channel wireless network, means for receiving wireless communication from a transmitting station, means for determining SINR of the wireless communication, having means of MUD, and said SINR is below a predetermined level In some cases, the transceiver has means for determining whether the transceiver should change the communication link to another frequency channel and means for communicating with the transmitting station to provide a channel Id for the other frequency channel.   The transceiver of claim 12, wherein the transceiver is for use in a multi-channel wireless network that incorporates at least one of MC-CDMA, MIMO, SDMA, FDMA, TDMA, and OFDMA technologies.   The transceiver of claim 12, wherein the transceiver is part of a mobile station.   The transceiver of claim 12, wherein the wireless network incorporates a protocol based on one MAC protocol of the IEEE 802.11 standard.

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