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Spatial Reuse Through Adaptive Interference Cancellation in Multi-Antenna Wireless Networks

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Spatial Reuse Through Adaptive Interference Cancellation in Multi-Antenna Wireless Networks Spatial Reuse through Adaptive Interference Cancellation in Multi-Antenna Wireless Networks A. Singh, P. Ramanathan and B. Van Veen Department of Electrical and Computer Engineering University of Wisconsin-Madison [email protected], [email protected], [email protected] Abstract— Efficient medium access control in wireless net- streams between a transmit-receive pair (spatial multiplex- works has been a challenging task. While the IEEE 802.11 ing). Reference [7] proposes a MAC protocol that utilizes standard coordinates contention effectively, it severely limits the controlled spatial multiplexing to ensure network fairness. number of concurrent communications. This results in reduced throughput and efficiency. Recent research has focused on em- Another MAC protocol that works for multipath but leverages ploying multiple antennas to increase throughput in a multipath on the interference cancellation capability of multi-antenna environment by enabling multiple streams between a transmit- systems to enhance spatial reuse is presented in [8]. It uses receive pair. In this paper we show that exploiting multiuser deterministic nulling and thus requires knowledge of channels diversity to enable concurrent communications has certain ad- to the interferers and weights used by them. This necessiates vantages over multiple streaming. We propose a medium access control (MAC) protocol that uses adaptive interference cancella- extra control overhead and a separate control channel, and is tion with multiple antennas to increase network throughput and able to suppress interference only from packets that can be to provide better fairness, while requiring minimal change to the decoded. widely-deployed 802.11 MAC structure. In this paper, we focus on developing a MAC protocol that employs adaptive interference cancellation to increase network I. INTRODUCTION throughput and fairness, while making minimal changes to Wireless networks are becoming an integral part of the the widely deployed 802.11 MAC structure. We model the information infrastructure. Applications of wireless LANs physical and MAC layers in detail and compare our results to (Local Area Networks) formed between laptops, PDAs etc. 802.11a and a spatial multiplexing based MAC with respect include conferencing, disaster relief and military operations. to the tradeoff of energy vs. throughput, and fairness. However, throughput in these networks is limited by the need Cross-layer design of MAC and physical layers is necessary to coordinate multiuser access to the shared wireless medium. to leverage the full benefits of multi-antenna systems for Current medium access control (MAC) protocols like IEEE setting up concurrent transmissions in a wireless network. 802.11 [1] that allow a single communication in a contention Resolution of multiuser contention for the medium and desti- region (region around a transmit-receive pair that overlaps with nation necessitates the need for an omni-directional control the transmission or sensing ranges of other nodes) are likely exchange before the beamformed data packet transmission. to be insufficient to meet the throughput demands of emerging Thus we retain the 802.11 control structure of RTS (Request applications and systems. to Send) and CTS (Clear to Send) which additionally carry Devices equipped with multiple antennas have been envi- pilot symbols for estimating the channel state information sioned as a means to increase throughput in wireless networks. (CSI) between the transmitter and receiver. After the control Antenna arrays can have either a pre-determined beampat- exchange, beamformed DATA and ACK transmission can take tern such as in switched or steered directional arrays, or place. The transmitter uses the CSI for coupling into the best adapt their beampattern in a smart fashion by changing the spatial channel between the source-destination pair, while the weighted combination of signals from the multiple antennas. receiver minimizes asynchronous interference from other users Refs. [2]–[6] present MAC protocols that utilize fixed-beam by adapting its receive weights to achieve maximum SINR directional antennas to concentrate the energy in a particular (Signal to Interference plus Noise Ratio). Another node can direction, thus enabling simultaneous non-interfering directed start its control exchange soon after the first node completes communications. While these protocols work for a line of sight its RTS-CTS exchange, thus establishing concurrent DATA environment, practical scenarios are often characterized by rich transmissions. This requires modifying IEEE 802.11 physical scattering in which signals propagate along multiple paths be- and virtual carrier sensing to sensing for only omni-directional tween a transmit-receive pair. In multipath environments smart packets and a modified network allocation vector (NAV), antenna techniques can allow multiple users, each coupling respectively. We defer the details of the scheme to a later into the best spatial channel (as opposed to direction), to set section. up concurrent transmissions by canceling interference from In this paper we also derive a bound on the maximum other users. Alternatively, the linearly independent channels number of concurrent multiuser transmissions that can be created by multipath can be used to send multiple data established (Section IV) as a function of data rates. If a sufficient number of antennas are available, each user can the antenna inputs xj (t) and obtain the reconstructed signal further be allowed to send more than one stream. This can as be done easily along the lines suggested in [7] or [9]. It ∗ H sˆ(t)= w xj (t)= w x (3) should be noted that this would increase throughput but at Rj R Xj the expense of additional energy. A combined interference H H cancellation-opportunistic streaming approach would degrade The gain to the signal s(t) is thus given as wR H wT . more gracefully than spatial multiplexing when the channel C. Omni-directional Control Exchange between a transmit-receive pair looses diversity. The paper is organized as follows. Section II presents In the absence of CSI at the beginning of a communication, the physical layer processing required at the transmit and the RTS and CTS are transmitted omni-directionally. The RTS receive antennas to adaptively suppress interference and max- and CTS headers carry an identifier bit which indicates that the imize SINR. Section III proposes a MAC layer that achieves packet is an omni-directonal control packet. When idle, nodes multiuser coordination to enable setting up of concurrent listen for this identifier bit by decoding any signal that appears communications. Section IV presents the simulation results above the noise level at its receive array. If the omni-identifier and we conclude in Section V. bit is not detected in the header, the signal is identified as interference and whitened. The whitening prevents interference II. PHYSICAL LAYER due to ongoing beamformed transmissions from disrupting A. Channel model the decoding of an omni-packet. The RTS/CTS headers also The wireless physical medium is modeled as a flat or carry a pilot sequence that is transmitted using all antennas to frequency-independent Rayleigh fading model. In this model, enable the estimation of channels between the transmit-receive assuming equal number of antennas N at each node, the antennas using standard channel estimation methods [11], [12]. channel between a pair of nodes is given by a N N matrix The receiver now uses the channel information for enhancing × th SINR by maximal ratio combining (MRC) [13] to receive the H where Hij represents the effective fading between the i transmit antenna and the jth receive antenna. This model rest of the packet. This enables successful reception of the is appropriate for narrowband communication. Extensions to omni-packet despite interference from transmissions that may wideband environments will require conventional physical start during the packet. layer enhancements such as in [10]. We assume that there D. Beamformed DATA and ACK Transmissions is rich scattering so that all channels are independent and H • Design of Transmit Weights: DATA/ACK is transmitted is full rank. We also assume that the channel is symmetric so by beamforming to couple the energy into the best spatial that the receiver knows the transmit weights that are designed channel between the transmitter and receiver array, char- based on the channel estimate at the transmitter. 1 Further, it acterized by the largest singular value of the estimated is assumed that the channel remains constant over a packet channel matrix. exchange (RTS, CTS, DATA, ACK). This ensures that the 1 channel estimates from the RTS/CTS packets are valid for wT = u1 (4) DATA/ACK transmission and reception. σ1 where σ1 and u1 denote the largest singular value and B. Antenna Array Processing associated left singular vector. The modulated DATA/ACK signal s(t) to be transmitted is • Design of Receive Weights: The DATA/ACK receive passed through a transmit antenna array or beamformer with weights are designed to maximize the received SINR: a N 1 weight vector w . Thus the received signal vector T wH R w at the× receiver antenna array in the absence of interference is R ss R SINR = H (5) given as wR (Rii + Rnn)wR where terms R , R and R represent the correlation x = HH w s(t)+ n (1) ss ii nn T due to the signal, interference and noise, respectively. where n denotes additive white Gaussian noise. In the presence This expression can be reduced to a Rayleigh quotient of interferers, the received vector is modified as form that is maximized by setting the receive weights to H H w q x = H wT s(t)+ Hi wisi(t)+ n (2) R = max (6) XiI where qmax is the eigenvector associated with the largest −1 where the summation is over the number of other transmitting eigenvalue λmax of the matrix (Rii + Rnn) Rss [14]. nodes or interferers. Here wi and Hi represent the interferer’s With this choice of receive weights, a maximum SINR transmit weight vector and its channel to the receiver, respec- of λmax is obtained.
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