A MIMO System with Backward Compatibility for OFDM-Based WLANs
- PDF / 710,214 Bytes
- 11 Pages / 600 x 792 pts Page_size
- 87 Downloads / 198 Views
A MIMO System with Backward Compatibility for OFDM-Based WLANs Jianhua Liu Department of Electrical and Computer Engineering, University of Florida, P.O. Box 116130, Gainesville, FL 32611-6130, USA Email: [email protected]
Jian Li Department of Electrical and Computer Engineering, University of Florida, P.O. Box 116130, Gainesville, FL 32611-6130, USA Email: [email protected] Received 16 December 2002; Revised 28 June 2003 Orthogonal frequency division multiplexing (OFDM) has been selected as the basis for the new IEEE 802.11a standard for highspeed wireless local area networks (WLANs). We consider doubling the transmission data rate of the IEEE 802.11a system by using two transmit and two receive antennas. We propose a preamble design for this multi-input multi-output (MIMO) system that is backward compatible with its single-input single-output (SISO) counterpart as specified by the IEEE 802.11a standard. Based on this preamble design, we devise a sequential method for the estimation of the carrier frequency offset (CFO), symbol timing, and MIMO channel response. We also provide a simple soft detector based on the unstructured least square approach to obtain the soft information for the Viterbi decoder. This soft detector is very simple since it decouples the multidimensional QAM symbol detection into multiple one-dimensional QAM symbol—and further PAM symbol—detections. Both the sequential parameter estimation method and the soft detector can provide excellent overall system performance and are ideally suited for real-time implementations. The effectiveness of our methods is demonstrated via numerical examples. Keywords and phrases: MIMO system, OFDM, WLAN, symbol timing, carrier synchronization, channel estimation.
1.
INTRODUCTION
Orthogonal frequency division multiplexing (OFDM) has been selected as the basis for several new high-speed wireless local area network (WLAN) standards [1], including IEEE 802.11a [2], IEEE 802.11g, and HIPERLAN/2. IEEE 802.11g and HIPERLAN/2 are very similar to IEEE 802.11a in terms of signal generation and detection/decoding. We use IEEE 802.11a to exemplify our presentation in this paper. The OFDM-based WLAN system, as specified by the IEEE 802.11a standard, uses packet-based transmission. Each packet, as shown in Figure 1, consists of an OFDM packet preamble, a signal field, and an OFDM data field. The preamble can be used to estimate the channel parameters such as the carrier frequency offset (CFO), symbol timing, as well as channel response. These parameters are needed for the data symbol detection in the OFDM data field. The preamble design adopted by the standard is specifically tailored to the single-input single-output (SISO) system case where both the transmitter and receiver deal with a single signal. This standard supports a data rate up to 54 Mbps.
Transmission data rates higher than 54 Mbps are of particular importance for future WLANs. Deploying multiple antennas at both the transmitter and receiver is a promising way to achieve a high transmission data rate for multipath
Data Loading...
