Collision Resolution by Exploiting Symbol Misalignment

Soung Chang Liew Dept. of Information Engineering The Chinese University of Hong Kong soung@ie.cuhk.edu.hk

Shengli Zhang Dept. of Information Engineering The Chinese University of Hong Kong slzhang5@ie.cuhk.edu.hk

Abstract—This paper presents CRESM, a novel collision resolution method for decoding collided packets in random-access wireless networks. In a collision, overlapping signals from several sources are received simultaneously at a receiver. CRESM exploits symbol misalignment among the overlapping signals to recover the individual packets. CRESM can be adopted in 802.11 networks without modification of the transmitter design; only a simple DSP technique is needed at the receiver to decode the overlapping signals. Our simulations indicate that CRESM has better BER performance than the simplistic Successive Interference Cancellation (SIC) technique that treats interference as noise, for almost all SNR regimes. The implication of CRESM for random-access networking is significant: in general, using CRESM to resolve collisions of up to n packets, network throughput can be boosted by more than n times if the transmitters are allowed to transmit more aggressively in the MAC protocol. Index Terms—multi-packet reception, collision resolution, interference cancellation, 802.11
I. INTRODUCTION In wireless random-access networks, packet collisions are common. For example, in the popular IEEE 802.11 MAC, collisions occur when two or more stations decide to transmit to the access point (AP) simultaneously. At a station, a random backoff countdown process is used to decide when the station can transmit its packet. The most common cause of collisions is when two or more stations simultaneously count down to zero and transmit together. This can happen even when the stations can carrier-sense each other. Collisions can also happen due to the hidden-node phenomenon [1], wherein two stations that cannot carrier-sense each other transmit to the AP simultaneously. This paper presents a novel method to recover collided packets. We call our method CRESM (collision resolution by exploiting symbol misalignment). CRESM does not require symbol-level synchronization among the stations. In fact, it thrives on symbol misalignment among the stations, which occurs naturally. A fundamental concept underlying CRESM is that collided signals with symbol misalignment can be treated as the output from a virtual convolutional encoder. To the best of our knowledge, this is the first paper to use this concept to extract collided packets by means of (1) over-sampling and (2) an optimal Viterbi-like decoding algorithm.

Related Work Ref. [2] proposes the disabling of the carrier sensing mechanism in a carrier-sense multiple access (CSMA) network to increase the likelihood of simultaneous transmissions (collisions). Collided signals are modeled using higher order constellation maps, and the joint decoding method requires symbol-level synchronization. Ref. [3] makes use of interference cancellation techniques to resolve the collisions. CRESM, on the other hand, does not assume symbol alignment. Also, CRESM does not require de-activating carrier sensing and can be deployed in a CSMA or a non-CSMA random access network.
In general, however, we do not advocate the disabling of carrier sensing when we can resolve collisions. Although we would want to encourage simultaneous transmissions, it is not clear that disabling carrier sensing altogether is the best way to do so. Instead, we would have better control over the system using a higher carrier sensing threshold or by increasing the transmission probabilities of the stations [4] in a way that is commensurate with the degree of collisions (number of packets in collisions) that can be dealt with. Ref. [1] focuses on resolving collisions due to the hidden-node phenomenon. In Zig-Zag decoding [1], for example, several consecutive hidden-node collisions of the same group of packets are used to resolve the collided symbols. In practice, and in particular with the use of RTS/CTS, hidden-node collisions are not as common as backoff collisions. Furthermore, resolving hidden-node collisions does not boost the overall system throughput so much as it solves the unfairness problem induced by hidden nodes. Resolving backoff collisions, on the other hand, can potentially lead to much higher system throughput by allowing the stations to attempt to transmit more aggressively. CRESM can be used to deal with both backoff collisions and hidden-node collisions.
Recently, there have been intense research activities on using physical-layer network coding (PNC) [5, 6] to boost wireless network performance. The application domain of PNC is in relay netw

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