Bandwidth Efficient OFDM Transmitter Diversity Techniques

EURASIP Journal on Applied Signal Processing 2004:10, 1508–1519 c 2004 Hindawi Publishing Corporation
Bandwidth Efficient OFDM Transmitter Diversity Techniques King F. Lee Multimedia Architecture Lab, Motorola Labs, Schaumburg, IL 60196, USA Email: Douglas B. Williams School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0250, USA Email: Received 17 December 2002; Revised 2 September 2003 Space-time block-coded orthogonal frequency division multiplexing (OFDM) transmitter diversity techniques have been shown to be efficient means of achieving near-optimal diversity gain in frequency-selective fading channels. However, these known techniques all require a cyclic prefix to be added to the transmitted symbols, resulting in bandwidth expansion. In this paper, iterative space-time and space-frequency block-coded OFDM transmitter diversity techniques are proposed that exploit spatial diversity to improve spectral efficiency by eliminating the need for a cyclic prefix. Keywords and phrases: space-time coding, space-frequency coding, transmitter diversity, OFDM, channel estimation, pilot symbols. 1. INTRODUCTION The last decade has witnessed an explosive growth of wireless communications, especially in mobile communications and personal communications services (PCS). With the continuing expansion in both existing and new markets and the introduction of exciting new services such as wireless internet access and multimedia applications, the wireless communications market is expected to continue to grow at a rapid pace. Furthermore, the ever-increasing demand for faster and more reliable services to support new applications has created strong interests in developing high data rate wireless communications systems. With existing and emerging wireless applications, all competing for a limited radio spectrum, the development of high data rate wireless communications systems that are spectrally efficient is especially important. The main challenge in developing reliable high data rate mobile communications systems is to overcome the detrimental effects of frequency-selective fading in mobile communications channels. A number of space-time coded orthogonal frequency division multiplexing (OFDM) transmitter diversity techniques have recently been proposed for high data rate wireless communications [1, 2, 3, 4]. It has been shown in [3, 4] that space-time and space-frequency block-coded OFDM (STBC-OFDM and SFBC-OFDM) sys- tems are efficient means of achieving near optimum diversity gain in frequency-selective fading channels. These previously proposed OFDM transmitter diversity systems all require a cyclic prefix to be added to the transmitted symbols to avoid intersymbol interference (ISI) and interchannel interference (ICI) in the OFDM symbols, and the number of cyclic prefix symbols has to be equal to or greater than the order of the wireless channels [5]. The addition of the cyclic prefix causes bandwidth expansion if a desired data rate is to be maintained or a reduction in data rate if the transmission bandwidth is fixed. For many high data rate systems, the addition of a cyclic prefix can cause more than a 15% bandwidth expansion, which is a very significant loss of a valuable resource [6]. In this paper, we propose iterative space-time and spacefrequency block-coded OFDM (ISTBC-OFDM and ISFBCOFDM) transmitter diversity techniques that do not require a cyclic prefix and, therefore, are more bandwidth efficient than previously proposed systems. Computer simulations are used extensively to evaluate the performances of the various systems considered in this paper. The COST207 six-ray typical urban (TU) channel power delay profile [7] is used to model the frequencyselective fading channels in all the simulations. Furthermore, for the simulations in Sections 2 and 3, perfect estimates of the channel impulse responses (CIRs) are assumed to be available at the receiver. Bandwidth Efficient OFDM Transmitter Diversity Techniques 1509 100 Tx1 X(u) Serial to X(n) Transmitter diversity parallel encoder Parallel to serial
X(n) Diversity decoder Y(n)
1 (n) Λ
2 (n) Λ h1 (n) Rx Tx2 h2 (n) X2 (n)
X(u) IDFT & cyclic prefix IDFT & cyclic prefix Prefix removal & DFT 10−2 10−4 r(n) Channel estimator Figure 1: Block diagram of a two-branch OFDM transmitter diversity system utilizing a cyclic prefix. The remainder of the paper is organized as follows. In Section 2, a brief overview of OFDM transmitter diversity systems utilizing a cyclic prefix is provided. Section 3 gives a detailed description of the proposed bandwidth efficient ISTBC-OFDM and ISFBC-OFDM transmitter diversity systems. Section 4 considers channel estimation techniques for OFDM transmitter diversity systems without a cyclic prefix. Finally, Section 5 summarizes the results and outlines possible future research in this area. 2. Average BER X1 (n) OFDM TRANSMITTER DIVERSITY SYSTEMS UTILIZING A CYCLIC PREFIX A block diagram of a general two-branch OFDM transmitter diversity system with a cyclic prefix is shown in Figure 1. Let X(u) denote the input serial data symbols with symbol duration TS . The serial to parallel converter collects K serial data symbols into a data vector X(n) = [X(n, 0) X(n, 1) · · · X(n, K − 1)]T , which has a block duration of KTS .1 The transmitter diversity encoder codes X(n) into two vectors X1 (n) and X2 (n) according to an appropriate coding scheme as in [1, 2, 3, 4]. The coded vector X1 (n) is modulated by an inverse discrete Fourier transform (IDFT) into an OFDM symbol sequence. A length G cyclic extension is added to the OFDM symbol sequence and the resulting signal is transmitted from the first transmit antenna. Similarly, vector X2 (n) is modulated by an IDFT, cyclically extended, and transmitted from the second transmit antenna. Let h1 (n) denote the CIR between the first transmit antenna and the receiver and let h2 (n) denote the CIR between the second transmit antenna and the receiver. To avoid ISI and ICI, the length of the cyclic extension G is chosen to be greater than or equal to L, the maximum order of the CIRs, that is, G ≥ L [5]. At the receiver, the received signal vector first has the 1 Throughout the paper, we will use the notation that A(n, k) denotes the kth element of the vector A(n). 10−6 0 5 10 15 20 25 30 35 40 Average received SNR (dB) 4-QAM in flat Rayleigh fading channel (theoretical) 2-branch STBC-OFDM without a cyclic prefix (simulated) 2-branch STBC-OFDM with a cyclic prefix (simulated) Figure 2: Performance of STBC-OFDM without a cyclic prefix in a TU channel with TS = 2−20 second, K = 32, L = 5, and fD = 10 Hz. cyclic prefix removed and is then demodulated by a discrete Fourier transform (DFT) to yield the demodulated signal vector Y(n). Assuming the CIRs remain constant during the entire block interval, the demodulated signal is given by [3, 4] Y(n) = Λ1 (n)X1 (n) + Λ2 (n)X2 (n) + Z(n), (1) (...truncated)