Multiple-Antenna Interference Cancellation for WLAN with MAC Interference Avoidance in Open Access Networks

Hindawi Publishing Corporation EURASIP Journal on Wireless Communications and Networking Volume 2007, Article ID 51358, 11 pages doi:10.1155/2007/51358 Research Article Multiple-Antenna Interference Cancellation for WLAN with MAC Interference Avoidance in Open Access Networks Alexandr M. Kuzminskiy1 and Hamid Reza Karimi1, 2 1 Alcatel-Lucent, Bell Laboratories, The Quadrant, Stonehill Green, Westlea, Swindon SN5 7DJ, UK 2 Ofcom, Riverside House, 2a Southwark Bridge Road, London SE1 9HA, UK Received 31 October 2006; Accepted 3 September 2007 Recommended by Monica Navarro The potential of multiantenna interference cancellation receiver algorithms for increasing the uplink throughput in WLAN systems such as 802.11 is investigated. The medium access control (MAC) in such systems is based on carrier sensing multiple-access with collision avoidance (CSMA/CA), which itself is a powerful tool for the mitigation of intrasystem interference. However, due to the spatial dependence of received signal strengths, it is possible for the collision avoidance mechanism to fail, resulting in packet collisions at the receiver and a reduction in system throughput. The CSMA/CA MAC protocol can be complemented in such scenarios by interference cancellation (IC) algorithms at the physical (PHY) layer. The corresponding gains in throughput are a result of the complex interplay between the PHY and MAC layers. It is shown that semiblind interference cancellation techniques are essential for mitigating the impact of interference bursts, in particular since these are typically asynchronous with respect to the desired signal burst. Semiblind IC algorithms based on second- and higher-order statistics are compared to the conventional no-IC and training-based IC techniques in an open access network (OAN) scenario involving home and visiting users. It is found that the semiblind IC algorithms significantly outperform the other techniques due to the bursty and asynchronous nature of the interference caused by the MAC interference avoidance scheme. Copyright © 2007 A. M. Kuzminskiy and H. R. Karimi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. INTRODUCTION Interference at the radio receiver is a key source of degradation in quality of service (QoS) as experienced in wireless communication systems. It is for this reason that a great proportion of mobile radio engineering is exclusively concerned with the development of transmitter and receiver technologies, at various levels of the protocol stack, for mitigation of interference. Multiple-antenna interference cancellation (IC) at the receiver has been the subject of a great deal of research in different application areas including wireless communications [1– 3] and others. Despite the considerable interest in this area, IC techniques are typically studied at the physical (PHY) layer and in isolation from the higher layers of the protocol stack, such as the medium access control (MAC). However, it is clear that any gains at the system level are highly dependent on the nature of cross-layer interactions, particularly if multiple layers are designed to contribute to the interference mitigation process. This is indeed the case for the IEEE 802.11 family of wireless local area network (WLAN) systems [4], where the carrier sensing multiple-access with collision avoidance (CSMA/CA) MAC protocol is itself designed to eliminate the possibility of interference at the receiver from other users of the same system. Although the MAC layer CSMA/CA protocol may be very effective for avoidance of intrasystem interference in typical conditions, certain applications which experience significant hidden terminal problems and/or interference from coexisting “impolite” systems may also benefit from PHY layer IC. PHY/MAC cross-layer design is clearly required in such situations. One important example of the above is an open access network (OAN) where visiting users (VUs) are allowed to share the radio resource with home users (HUs) [5]. In many scenarios, VUs typically experience greater distances from an access point (AP) compared to HUs. This means that VUs may interfere with each other with higher probability compared to HUs, leading to throughput reduction for VUs or gaps in coverage. A multiple-antenna AP with IC may be a solution to this problem. A cross-layer design in such a system is required because the CSMA/CA protocol leads to an asynchronous 2 EURASIP Journal on Wireless Communications and Networking “Listen” “Backoff ” SIFS t DIFS Slot time MPDU ACK Figure 1: Transmission of MPDU and ACK bursts. Section 4 by a description of the conventional and semiblind IC receiver algorithms, along with a demonstration of their performance at the PHY layer. Section 5 provides a description of the simulation framework and the cross-layer simulation results in typical OAN scenarios with intra- and intersystem interference. Conclusions are presented in Section 6. 2. interference structure, where interference bursts appear with random delays during the desired signal data burst. One way to account for higher-layer effects is to develop interference models that reflect key features of cross-layer interaction and design PHY-layer algorithms that address these. This is the methodology adopted in [6–11], where semiblind space-time/frequency adaptive second- and higherorder statistic IC algorithms have been developed in conjunction with an asynchronous (intermittent) interference model. The second-order algorithm is based on the conventional least-squares (LS) criterion formulated over the training interval, regularized by means of the covariance matrix estimated over the data interval. This simple analytical solution demonstrates performance that is close to the nonasymptotic maximum likelihood (ML) benchmark [6, 7]. Further analysis is given in [8], which introduces nonstationary interval-based processing and benchmark in the asynchronous interference scenario. The regularized semiblind algorithms can be applied independently or as an initialization for higher-order algorithms that exploit the finite alphabet (FA) or constant modulus (CM) properties of communication signals. The efficiency of these algorithms has been compared to the conventional LS solution [1] by means of PHY simulations. These involve evaluation of metrics such as mean square error (MSE), bit-error rate (BER), or packeterror rate (PER), as a function of signal-to-interference ratio (SIR) for given signal-to-noise ratio (SNR), and a number of independent asynchronous interferers. Our goal in this paper is to evaluate cross-layer interference avoidance/cancellation effects for different algorithms and estimate the overall system performance in terms of throughput and coverage. The combined performance of different IC algorithms at the PHY (...truncated)