Transmit diversity schemes have gained recent attention due to the promise of increased capacity and improved performance. We shall review  transmit diversity schemes and metric-based code searches with a focus on space-time trellis coded modulation. Recent designs for space-time block codes (STBCs) have exploited the structure of the codes (unitary, orthogonal) to enable systematic construction. Code sets found by exhaustive search to optimize cost functions can often achieve performance gains of several dB; however, such searches are not feasible for large block or constellation sizes. We introduce non-linear hierarchical codes (NHCs), where coding gain is optimized at each layer of the hierarchy and codes of arbitrary block-sizes/constellations can be designed. A fundamental difference between the structured codes (unitary, orthogonal) and NHCs is that in NHCs, the relationship between codewords is manipulated rather than properties of the individual codeword. In many cases, STBCs alone are insufficient to provide the needed protection from the fading wireless channel. We design a multiple trellis coded modulation scheme which directly matches the distance spectrum of the constituent codes (in this case, the previously noted HNCs) to the trellis structure and thus are able to directly optimize the coding gain. The proposed coding system is highly systematic and can gracefully trade off between rates, block sizes and complexity. With limited growth in complexity, the proposed designs can achieve more than 3.5dB of gain over current MTCM designs.

Urbashi Mitra received the B.S. and the M.S. degrees from the University of California at Berkeley in 1987  and 1989 respectively, both in Electrical Engineering and Computer Science. From 1989 until 1990 she worked as a Member of Technical Staff at Bellcore in Red Bank, NJ.  In 1994, she received her Ph.D. from Princeton University in Electrical Engineering.   From 1994 to 2000, Dr. Mitra was an Assistant Professor in the Department of Electrical Engineering at The Ohio State University, Columbus, Ohio.   She became an Associate Professor in 2000 and currently holds that position in the Department of Electrical Engineering at the University of Southern California, Los Angeles.   She was an Associate Editor for the IEEE Transactions on Communications from 1996-2001.  Dr. Mitra is currently a member of the IEEE Information Theory Board of Governors.  She is the recipient of:  Texas Instruments Visiting  Professor (Fall 2002, Rice University), 2001 Okawa Foundation Award, 2000 Lumley Award for Research (OSU College of Engineering), 1997 MacQuigg Award for Teaching (OSU College of Engineering), 1996 National Science Foundation (NSF) CAREER Award, 1994 NSF International Post-doctoral Fellowship, 1998 Lockheed Leadership Fellowship, 1987 California Microelectronics Fellowship. Along with Steve McLaughlin of Georgia Tech, she co-chaired the IEEE Communication Theory Symposium at ICC 2003 in Anchorage, AK.