Space sensing based random access in SIMO-OFDM systems

The problem of cross-layer medium access control (MAC) in a SIMO-OFDM system is addressed and a space sensing based random access (SSRA) scheme is proposed. In SSRA scheme, the transmit slot is divided into three parts: space sensing period, channel estimation period and packets transmission period. Without orthogonal training sequences, users can sense the possible spatial collisions in the following way. Each user sends a short random sequence to the access point (AP) and the AP forwards the receive signal to the users with an amplify-and-forward form. Each user detects the receive signal to determine whether a collision in packets transmission period will appear. After space sensing, the user sensing no collision sends a common training sequence to AP for channel estimation. Multiple nonoverlapping pilot subslots are set and the user randomly selects one for training sequence transmission. Failing channel estimation means failing packet reception. Thus, SSRA gives a more practical multipacket reception (MPR) model. The throughput performance of the proposed SSRA scheme is analyzed analytically and evaluated through simulations.