Intra-Frame Transmission Adaptation for Fast Fading MIMO-OFDMA Systems

Abstract

Abstract-We propose multiple antenna transmission methods for fast fading channel conditions. Proposed methods allocate alternative multiple antenna transmission modes depending on the SNR, modulation order and Doppler frequency to increase reliability, i.e., decrease bit-error-rate (BER). A major difference of our approach from previous works is to allow different transmission modes during a single frame by considering the channel variations within the frame for high mobility scenarios. First method effectively makes use of available channel knowledge by allocating beamforming (BF) as long as channel knowledge is not outdated and switches to space frequency block coding (SFBC) afterwards. Approximate BER expressions for BF and SFBC that are functions of SNR, modulation order, Doppler frequency and initial channel knowledge are derived to be used as a decision metric for mode allocation throughout the frame. Second method adapts rate and transmission mode across symbols in a frame by considering that average channel power is monotonically decreasing as a function of time within the frame when multiuser diversity is exploited. First method performs as good as the better of BF and SFBC over all SNR values. Second method provides additional performance gain over the first method due to more efficient use of better channel conditions. Index Terms-MIMO, OFDMA, time varying channel, nonuniform channel knowledge, statistical bit loading, transmission mode allocation.

An Adaptive MIMO-OFDMA Relay System

“…Rayleigh fading channel is assumed with L = 4 sample-spaced uncorrelated taps and a normalized 3-dB per tap decaying power delay profile. The log-mean and variance are μ dB = 0 and σ (8) and (10) reflects the simulation results better than those in (12) and (13). However, both yield essentially the same performance for the MIMO mode adaptation.…”

“…(The subscript of m is omitted for clarity when it appears as superscript but it can be deduced from the subscript of h.) The MGF of γ m ,n S i R k can be derived as [13] …”

“…In practical time-varying channels, the CSI acquisition overhead grows with mobile speed; the CSI delay and gradual degradation of CSI within a transmission frame occur which will inevitably affect the existing relaying strategies which assume quasi-static channels during the transmission frame. For non-relay systems, an adaptive transmission using beamforming and space-frequency block coding (SFBC) has been recently proposed in [13]. But for relay systems, a new relay transmission scheme to combat the CSI degradation issue in time-varying channels is needed.…”

Channel Knowledge Acquisition in Relay and Multipoint-to-Multipoint Transmission Systems

“…We assume that during the CSI acquisition and data transmission interval, the channels remain static but it can be relaxed for the data transmission interval as some transmission schemes [32] accommodate channel variations within a frame. The channel between I i,ν and J m,n is represented by a time domain vector h Ii,ν -Jm,n of size L Ii,ν -Jm,n × 1, and its corresponding frequency-domain channel gain vector is denoted by H Ii,ν -Jm,n whose kth element H Ii,ν -Jm,n [k] represents the channel gain on subcarrier k.…”

New Fractional Frequency Reuse Patterns for Multi-Cell Systems in Time-Varying Channels

“…However, when channels vary within a physical transmission frame, the CSIT exploitation gain is substantially suppressed [5], [6]. This issue is addressed for a single-cell system in [7], [8] and for multiple frequency division duplexing (FDD) based systems in [9]. However, such issue has not been studied in the context of FFR, and this problem is important since current and future wireless systems aim to support high mobility with high spectrum efficiency [10] which is a major challenge for outer coverage zones.…”