2334 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 57, NO. 4, JULY 2008 Multicode MIMO Systems With Quaternary LCZ and ZCZ Sequences Jae-Dong Yang, Xianglan Jin, Kyoung-Young Song, Jong-Seon No, Member, IEEE, and Dong-Joon Shin, Member, IEEE Abstract—In this paper, we propose multicode multiple-input– multiple-output (MIMO) systems with quaternary low-correlation zone (LCZ) and zero-correlation zone (ZCZ) sequences as spread- ing codes. Quaternary LCZ and ZCZ sequences have very low correlation values when the time shifts between these sequences are within the predetermined correlation zone, and thus, the multi- user or multipath interference can be substantially reduced when the delay is within a few chips. The bit error probability of the proposed systems is theoretically analyzed, which is numerically confirmed. It is also numerically shown that the performance of the multicode MIMO systems with quaternary LCZ and ZCZ sequences is better than that of the conventional multicode MIMO systems with quaternary spreading codes constructed from pairs of binary Hadamard codes. Index Terms—Hadamard codes, low-correlation zone (LCZ) sequences, multicode multiple-input multiple-output (MIMO), wireless local area network (WLAN), zero-correlation zone (ZCZ) sequences. I. I NTRODUCTION M ULTIPLE-INPUT–multiple-output (MIMO) techniques have widely been used to increase the capacity of wireless communication systems [1]. By utilizing high spa- tial dimension of multiple antennas, high spectral efficiency can be achieved in the wireless communication systems. In code-division multiple-access (CDMA) systems, higher data- rate communication can be achieved by using multicode chan- nelization [2]. To accommodate the demand for various high data-rate services, we can construct a system by combining these two techniques, i.e., MIMO and multicode techniques, which is called a multicode MIMO system. Multicode MIMO systems are considered to be standard techniques for high-speed downlink packet access systems [3]. In conventional multicode Manuscript received April 9, 2007; revised September 15, 2007 and October 23, 2007. This work was supported in part by the Ministry of Education and Human Resources Development (MOE), by the Ministry of Commerce, Industry, and Energy (MOCIE), and by the Ministry of Labor (MOLAB), Korea, through the fostering project of the Laboratory of Excellence. The review of this paper was coordinated by Prof. H. Nguyen. J.-D. Yang, X. Jin, K.-Y. Song, and J.-S. No are with the Institute of New Media and Communications, Department of Electrical Engineering and Computer Science, Seoul National University, Seoul 151-744, Korea (e-mail: yjdong@ccl.snu.ac.kr, xianglan.jin@ccl.snu.ac.kr, sky6174@ccl.snu. ac.kr, jsno@snu.ac.kr). D.-J. Shin is with the Department of Electronics and Computer Engineering, Hanyang University, Seoul 133-791, Korea (e-mail: djshin@hanyang.ac.kr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TVT.2007.914059 MIMO systems, each transmit antenna uses the same set of spreading codes, and usually, a pair of binary Hadamard codes has been used to make a quaternary spreading code. Low-correlation zone (LCZ) and zero-correlation zone (ZCZ) sequences [4], [5] have very low autocorrelation and cross-correlation values when the time shifts between the sequences are within the predetermined correlation zone. Therefore, they are suitable for the quasi-synchronous CDMA systems and the multipath resolution for CDMA systems [6], [7]. In this paper, we assume that the delays are much smaller than the data symbol duration (or the period of spreading code) and propose multicode MIMO systems that use quaternary LCZ and ZCZ sequences instead of binary Hadamard codes as spreading codes. Because of 2-D (spatial and code domains) interference, we need 2-D successive in- terference cancellation (SIC) detection. However, it is shown that for the proposed systems, 1-D (spatial domain) SIC detec- tion shows negligible performance degradation compared with 2-D SIC detection. Also, the bit error probability (BEP) of the proposed systems is theoretically analyzed, which is numer- ically confirmed, and it is shown that the proposed systems outperform the conventional multicode MIMO systems. Thus, the proposed multicode MIMO system can be used for the high-speed data transmission with multipath resolution in the isolated cell environments, such as hotspots of wireless local area network (WLAN) and indoor wireless communication systems, etc. This paper is organized as follows: In Section II, the system model of multicode MIMO systems is presented, together with the construction methods of LCZ and ZCZ sequences and their detection scheme. The performance analysis and the numerical results of multicode MIMO systems with LCZ and ZCZ se- quences as spreading codes are presented in Section III. Finally, the conclusion is given in Section IV. II. SYSTEM MODEL The multicode MIMO system is assumed to have N t transmit antennas and N r receive antennas, such that each transmit antenna uses the same set of spreading codes {c 1 (t),c 2 (t),...,c K (t)}, where K denotes the number of spreading codes. Fig. 1 shows the multicode MIMO system. We assume that quaternary LCZ and ZCZ sequences are used as spreading codes, that is, complex spreading is used [8]. Spreading codes have the period G, which corresponds to the processing gain. A data stream is demultiplexed into N t groups, and each group is partitioned into K streams of data symbols. 0018-9545/$25.00 © 2008 IEEE