Virtual MIMO Beamforming for Opportunistic Cooperative Time Division Multiple Access

The conventional studies on virtual multiple-input multiple-output based cooperative time division multiple access (TDMA) have been focused on the cooperative retransmission of failed data. In this paper, we propose an opportunistic cooperative TDMA (OC-TDMA) scheme where a source user transmits its own data and the cooperative users opportunistically transmit low-data-rate users' data. To do this, time slots are divided into non-cooperative and cooperative time slots. The non-cooperative time slots are located at earlier time slots, in which weak users, who cannot satisfy the required signal-to-interference-plus-noise ratio (SINR), are scheduled. The cooperative time slots are located at later time slots, in which strong users, who can satisfy the required SINR, are scheduled. In the cooperative time slots, the source user transmits its own data and the other users (i.e., cooperative users) assist to transmit the weak users' data which was transmitted at the non-cooperative time slots. The optimal and sub-optimal OC-TDMA algorithms are analyzed and evaluated in terms of the required excess capacity, which indicates the capacity additionally required to satisfy the SINR constraints of all the users after all the transmission is finished. Throughout the simulation and numerical results, it is demonstrated that OC-TDMA schemes outperform conventional TDMA schemes.

I. Introduction

In most wireless communication systems, medium access control (MAC) is an essential mechanism to share radio resources among multiple users, efficiently. Time division multiple access (TDMA) has been widely utilized as a reliable MAC mechanism [1]. In conventional TDMA, a dedicated time slot is allocated to each user. Each user then transmits its own data at the assigned time slot. When the user does this in a dedicated time slot, the neighboring users can monitor and overhear the data. In the cooperative TDMA studies, these neighboring users can decode the received data and retransmit the failed data for assisting the source user. If the antennas of the neighboring users constitute a virtual antenna array, the TDMA system can achieve spatial diversity, which results in the throughput improvement of the TDMA system [2]–[6].