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

A new scheme, namely ALOHA with Collision Resolution (ALOHA-CR) is proposed, which is a cross-layer approach for high throughput wireless communications in a cellular uplink scenario. Transmissions occur in a time-slotted ALOHA-type fashion but with an important difference: simultaneous transmissions of two users can be successful. In the beginning of each slot, users with non-empty queues contend with some probability after experiencing an intentional random delay. If more than two users transmit in the same slot, no attempt is made by the base station (BS) to separate the collision; the packets are discarded and the users are asked to retransmit at a later time. If one or two users transmit, then, depending on the state of the channel, the base station can recover the transmitted packets. The latter is achieved by oversampling the collision signal and exploiting independent information about the users that is contained in the signal polyphase components. The properties of the user intentional random delays are determined to maximize the probability of a second order collision being separable. The throughput of ALOHA-CR under both infinite and finite backlog assumptions is derived. Under the former assumption, the optimal contention probability is calculated to maximize the system throughput, while under the latter assumption, the region of stabilizing contention probabilities is determined based on the packet arrival rate. Queuing delay analysis for network users is also conducted. The performance of ALOHA-CR is demonstrated on the Wireless Open Access Research Platform (WARP) testbed containing five software defined radio nodes. The testbed results indicate that ALOHA-CR leads to significant increase in throughput and reduction of service delays as compared to ALOHA.

I. Introduction

In a wireless uplink scenario, collisions occur when two or more users transmit at the same time over the same channel. Traditionally, once a collision occurs, retransmissions are requested. Retransmissions lower throughput and waste power and bandwidth. Well-studied schemes for avoiding collisions include Carrier Sensing Multiple Access with Collision Avoidance (CSMA/CA) (e.g., see IEEE 802.11 [1]). In order to overcome the hidden terminal problem, IEEE 802.11 incorporates a positive acknowledgment 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.