Analysis of the LTE Access Reservation Protocol for Real-Time Traffic

LTE is increasingly seen as a system for serving real-time Machine-to-Machine (M2M) communication needs. The asynchronous M2M user access in LTE is obtained through a two-phase access reservation protocol (contention and data phase). Existing analysis related to these protocols is based on the following assumptions: (1) there are sufficient resources in the data phase for all detected contention tokens, and (2) the base station is able to detect collisions, i.e., tokens activated by multiple users. These assumptions are not always applicable to LTE - specifically, (1) due to the variable amount of available data resources caused by variable load, and (2) detection of collisions in contention phase may not be possible. All of this affects transmission of real-time M2M traffic, where data packets have to be sent within a deadline and may have only one contention opportunity. We analyze the features of the two-phase LTE reservation protocol and asses its performance, when assumptions (1) and (2) do not hold.

I. Introduction

An access reservation protocol is instrumental in any multi-user communication system in order to enable users to connect asynchronously or transmit intermittently [1]. The Long Term Evolution (LTE) system [2] uses an access protocol consisting of two phases: a contention phase, where each user contends by activating a particular reservation token chosen from the set of available tokens; and a data phase, where the reservation tokens (i.e., token holders) detected by the base station (BS) get assigned resources for the data transfer. The asynchronous access in LTE gains importance as the needs to support traffic related to Machine-to-Machine (M2M) communications gets increasingly important. In many cases, M2M traffic is a real-time traffic, where data packets become obsolete after a deadline and thus may undergo only a single contention and data phases, i.e., unsuccessful transmissions cannot be postponed for later contention or scheduled to a later data phase.