Space-Time Spreading Codes for a Multiuser MIMO System* Roya Doostnejad, 'Teng Joon Lim, Elvino S. Sousa Department of ECE: University of Toronto 10 King's college Road, Toronto, ON, M5S 3G4, Canada Email:{roya, limtj, sousa)@comm.toronto.edu Abstract We study the use of the spatial dimension in enabling multiple access for the down-link of a cellular communication system and propose a t.wo dimensional spreading code. The band- width efficiency is increased by minimum num- ber of transmit, and receive antennas, over a sin- gle antenna system, while diversity is provided at t.he receiver. A key feature of the proposed technique is that it simplifies the implementa- tion of joint space-time decoding and multiuser detection in each user station. The channel is supposed to be known only at the receiver. 1 Introduction A large amount of research has been performed to achieve very high data rate networks to sup port reliable transmission of video, data, and speech at high rates to many users. One way to increase the data rate in a wireless system is to use multiple transmit and/or receive an- tennas (AIIAIO structure). Indeed, it has been shown that the Shannon capacity of hlIRIO cliaimels (if the channel is known to the re- ceiver) grows linearly with min(NT. NR), where NT is the number of transmit antennas and NR is the nuniber of receive antennas [8]. It pro- vides niin(Arr, NR) spatial channels which one can transmit independent iiiformation symbols through. On the other hand in a system with NT transmit, and NR receive antennas, a di- versity order of ".NR can be provided for the system. In single user systems, there are several space time coding (STC) techniques to achieve 'This work is supported in part by Bell Canada, and Ontario Graduate Scholamhips in Science and Technol- ogy (OGSST). 0-7803-7576-9/02%17.00 0 2002 IEEE 1374 high data rates by taking advantage of multi- plexing gain of I\,IIhlO systems [5, 121, or to achieve diversity gain (11, 21, or both [l]. There are a few papers in space-time cod- ing for DS-CDMA systems [7, 9, 61. In all of these studies, spreading (de-spreading), space- time coding (decoding) and channel coding (de- coding) are done in separate blocks. Besides, there is no use of the additional space resource to design spreading codes for accommodating more users than single antenna systems. In [9]> space-time diagonal sequences are proposed for spreading the data over space and time. How- ever, the number of transmit and receive anten- nas is required to be the same as the processing gain. Also since joint MUD such as interference cancellation is applied at the receiver, each user needs to detect the symbols for all other users in the system as well as its own, which may not be feasible in the down-link. In a multiple-antenna system, the system de- sign can benefit from treating t.he extra space dimension as an analog of the time and fre- quency dimensions. We propose a space-time spreading matrix for each user, rather than a temporal spreading code vector. The band- width expansion of a system with a given num- ber of users may then be reduced by a factor of min(NT, NE). The remainder of this paper is organized as follow. The spreading matrix design is given in the next section. The detection algorithm at the receiver is discussed in section 3. Perfor- mance analysis is done in section 4 and simu- lation results are shown in section 5. Finally, sect.ion 6 concludes t.he paper.