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MIMO-OFDM modem for WLAN

The future 802.11n WLAN standard that will emerge from the TGn group work should be based on a MIMO-OFDM physical layer. This paper gives an introduction to this promising technology and to 11n standardization process. Then, we highlight the changes required to extend an OFDM WLAN modem to a MIMO-OFDM modem. Finally, we focus our study on the MIMO-OFDM decoding block, giving performance and complexity results.

Introduction

Multiple Input Multiple Output (MIMO) is an advanced physical layer technology that uses multiple antennas at both transmit and receive sides. It is well known that antenna diversity offers robustness and gain over single antenna schemes. MIMO also improves the transmission?s spectral efficiency. Indeed, different signals can be transmitted simultaneously (i.e. time- and frequency-wise) by the different transmit antennas, and be correctly decoded by the multi-antenna receiver. Even more, the capacity of MIMO systems can linearly increase with the number of transmit antennas under a rich multipath environment [1-2].

On the other hand, Orthogonal Frequency Division Multiplexing (OFDM) is a spectrally efficient modulation technique invented in the 1960?s and now widely used in such popular communication systems as WLAN, DVB, etc. In a nutshell, this technique allows squeezing a (usually) large number of complex sine waves (also called subcarriers) in a limited bandwidth. Since these subcarriers are mutually orthogonal, they don?t interferer with one another. On the transmitter side, OFDM signals are generated using the fast Fourier transform (FFT).

On the receiver side, the equalization of the received signals, which are generally distorted by a frequency-selective channel (due to multipath), is easily performed in the frequency domain: each subcarrier is indeed equalized by a single complex coefficient, which makes this technique very robust against multipath fading.

The worldwide success of the OFDM based 802.11a/g WLAN standards [3-4] along with the astonishingly high spectral efficiencies achieved by systems implementing the MIMO technology have naturally led engineers to envisioning high date rate, OFDM, MIMO-based, WLAN standards to supersede existing ones.

First, this paper gives an overview of this MIMO based WLAN standardization process. Then, in section 3, we present the general architecture of a WLAN MIMO modem. Main functionalities are described. In section 4, we focus our study on the specific MIMO detection block. We describe and compare existing decoding algorithms in terms of performance and complexity.

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