Coded modulation (usually with interleaving) is used in fading channel cornmunications to achieve good error performance. The major benefit froin using coded modulation in fading channels is achieved if each code symbol of a codeword (or coded sequence) suffers statistically different fading (preferably independent fading). However, in many applications of mobile communications (e.g. in a metropolitan environment), a low vehicle speed (and hence a small Doppler spread) is very common. With a small Doppler spread, ideal or close-to-ideal interleaving is no longer feasible and all code symbols of a codeword would suffer highly correlated fading especially in stationary fading (Doppler spread==O). Coded modulations will thus suffer seriously degraded perfolrmance. The transmitter diversity using intentional frequency offset between antennas can generate the necessary time-varying fading and maintain the effectiveness of the colded signaling scheme. This work found that proper selections of the intentional frequency offset and interleaving depth can achieve less correlated fading or even independent fading (if enough antennas are used). Previous performance analysis based on ideal interleaving is not sufficient for slow fading cases and this work establishes the performance analysis of coded modulation in fading channels for a more general case which permits correlated fading and pilot symbol assisted modulation. Also the prevalent union bouind is found of large bounding errors in slow fading environments and an improved union bound is presented to assess the close- to-actual error performance.

Date of this Version

November 1994