Space-Frequency Block-Coded Communication Systems for Time-Varying Wireless Fading Channels

Abstract

Fading is one of the most challenging phenomena in wireless to establish a reliable communication path between transmitter and receiver. Fading is very severe when there is no line-of-sight component, which is the most occurring case in urban and suburban cities. Generally, the multipath fading amplitude distribution is modeled with Rayleigh PDF. But when the fading is very severe, the Rayleigh model fails to characterize the exact channel characteristics. So, a more accurate model, named Nakagami-m model may use to represent the channel.

When channel bandwidth is smaller than the signal bandwidth, the channel creates frequency selective fading. This frequency selective fading causes inter symbol interference (ISI). There are several ways to combat ISI. The equalization is one way that tries to invert the effects of the channel. In OFDM, each subcarrier undergoes flat fading since its bandwidth is less than the coherence bandwidth.

OFDM converts a frequency selective channel into a collection of flat fading channels by the use of cyclic prefix, but sub-band of OFDM are exposed to deep fading and may cause complete loss of sub-band information. To cope with this problem space time block coding is applied in OFDM, however it also introduces redundancy. To avoid the problem of fast channel variations in time, the symbols of an orthogonal design can be transmitted on neighboring subcarriers of the same OFDM symbol rather than on the same sub-carrier at subsequent time interval this is basic idea behind “Space Frequency Block Coded OFDM (SFBC-OFDM)”

Therefore, we have made an effort to study STBC-OFDM and evaluation of cyclic prefix (CP) single carrier transmission with SFBC-OFDM working under various fading wireless environment. By appending CP at beginning of OFDM symbol block, the linear convolution associated with the channel impulse becomes a circular convolution, therefore inter-block interference is prevented. The SFBC system codes across two antennas and over two adjacent subcarriers instead of two consecutive symbol intervals and therefore becomes robust against fast fading distortion in frequency nonselective fading environments.

Simulation results are presented to demonstrate that in mobile environments where fading is relative slow such as Rayleigh (Nakagami-m, m=1), SFBC-OFDM does not give any significant performance over STBC-OFDM; more over STBC-OFDM mitigate slow fading (m > 1) more efficiently than SFBC-OFDM. It is also shown that under fast fading (m < 1) conditions SFBC-OFDM outperform STBC-OFDM. For poor SNR (SNR < 11dB), fast fading (m=0.75) is mitigated more efficiently than slow fading (m=1.25) by SFBC-OFDM.