ALOHA With Collision Resolution (ALOHA-CR): Theory and Software Defined Radio Implementation

Abstract

A cross-layer scheme, namely ALOHA With Collision Resolution (ALOHA-CR), is proposed for high throughput wireless communications in a cellular scenario. Transmissions occur in a time-slotted ALOHA-type fashion but with an important difference: simultaneous transmissions of two users can be successful. If more than two users transmit in the same slot the collision cannot be resolved and retransmission is required. If only one user transmits, the transmitted packet is recovered with some probability, depending on the state of the channel. If two users transmit the collision is resolved and the packets are recovered by first over-sampling the collision signal and then exploiting independent information about the two users that is contained in the signal polyphase components. The ALOHA-CR throughput is derived under the infinite backlog assumption and also under the assumption of finite backlog. The contention probability is determined under these two assumptions in order to maximize the network throughput and maintain stability. Queuing delay analysis for network users is also conducted.The performance of ALOHA-CR is demonstrated on the Wireless Open Access Research Platform (WARP) test-bed containing five software defined radio nodes. Analysis and test-bed results indicate that ALOHA-CR leads to significant increase in throughput and reduction of service delays.keywords-multi-user system, blind source separation, MIMO systems, collision resolution, software defined radio scheme, i.e., Request To Send (RTS) followed by Clear To Send (CTS). However, in most protocols, collisions occur more frequently as the traffic load increases, in which case the RTS/CTS scheme becomes less effective due to collisions of the RTS reservation packets.Collision resolution can be viewed as multiuser separation. However, well known approaches that allow for multiuser separation, such as TDMA, FDMA, OFDMA, CDMA or use of multiple antennas, might not be a good fit for wireless networks. Wireless network traffic can be bursty, users operate on limited battery power, and in certain cases, wireless receivers have physical size limitations. TDMA, FDMA and OFDMA approaches are fixed resource allocation schemes and thus are not efficient for bursty traffic.The CDMA approach requires bandwidth expansion, which results in increased power consumption for each wireless network user. The use of multiple antennas, might not be feasible for compact wireless receivers. Wireless network-friendly approaches to achieve diversity include the NDMA protocol [6,19], ALLIANCES [10, 17] and ZigZag decoding [7]. In these protocols, collisions are resolved by combining collided packets and several retransmissions. In these schemes it is assumed that nodes transmit with the same power, and that there is no significant power decrease due to propagation in small-scale networks.For cases in which users transmit using different power levels, user separation could be achieved via successive interference cancellation (SIC) [14]. However, it might not be a good approach ...

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