Compressive Channel Division Multiple Access for MTC under Frequency-Selective Fading

Abstract

In this paper, we study random access for machine type communications (MTC) based on a multiple access scheme that exploits different channel state information (CSI) from active devices under frequency-selective fading, which is called channel division multiple access (ChDMA). In ChDMA, in order to use a low-complexity compressive sensing (CS) algorithm for multiuser detection (MUD) with a high transmission rate, we propose to use index modulation. Since the CSI is used as a signature for MUD,the CSI estimation becomes crucial. For the CSI estimation, pilot transmission can be considered where each active device randomly chooses a pilot from a set of pre-determined pilots.Since the CSI estimation suffers from pilot collision in this case, we propose multiple pilot transmissions and show that a low probability of collision can be achieved. A CS-based approach is derived not only to estimate the CSI, but also to detect randomly selected multiple pilot signals by exploiting the sparsity of active devices over frequency-selective fading channels.

Existing System

Sparse Representation, Convex Optimization.

Proposed System

In this paper, we consider a compressive random access scheme over frequency-selective fading channels that can mitigate some problems in conventional compressive random access schemes. However, if the number of devices is large, it may not be possible to assign a unique code for each device due to various reasons (e.g., if orthogonal codes are used, the number of devices is limited by the length of codes). In the proposed compressive random access scheme, the CSI of active device is used as a signature for MUD. As a result, the proposed compressive random access scheme can support any number of devices as long as there are few active devices and any active device’s CSI is different from each other. For a higher transmission rate, we employ index modulation for data transmission. For the CSI estimation, we consider a conventional approach as the RACH procedure with a pilot pool of finite size. In this case, unfortunately, the channel estimation suffers from pilot collision. To mitigate this, we consider multiple transmissions of pilots in the proposed compressive random access scheme. As a result, the proposed scheme can support a large number of devices of sparse activity with a negligible probability of pilot collision. This paper is based on for the channel estimation, while the use of channels as signatures for multiple access and index modulation. Since the size of the pilot pool is not necessarily large, there are also the following salient features in the proposed compressive random access scheme: i) a good set of pilots or pilots (such as Zadoff-Chu (ZC) sequences [24]) can be used; ii) the complexity of CS algorithms for the CSI estimation can be low.

Architecture

Structure of a Packet