MIMO-OFDM Systems

Background

Multiple-input multiple-output (MIMO) systems are systems with multiple transmit antennas and multiple receive antennas. Compared with single-input single-output (SISO) systems, the technique of using multiple transmit and receive antennas can improve the performance and increase the capacity of mobile communication systems. The improvement depends on the employed number of transmit and receive antennas as well as on the propagation environment. The complexity of MIMO systems increases proportional to the number of transmit and receive antennas.

Orthogonal frequency division multiplexing (OFDM) is a special form of multi-carrier modulation with densely spaced sub-carriers and overlapping spectra. OFDM is the most appropriate modulation technique for wireless data transmission to overcome the influence of multi-path propagation. Moreover, the OFDM modulator and demodulator can efficiently be implemented by using the inverse discrete Fourier transform in the transmitter and the discrete Fourier transform in the receiver. The computational complexity is significantly reduced by the application of fast Fourier transform (FFT) techniques. Nowadays, OFDM has been chosen as the transmission method in digital audio broadcasting (DAB), terrestrial digital video broadcasting (DVB-T), high performance radio local area network type 2 (HiperLAN/2), digital radio mondiale (DRM), and asymmetric digital subscriber line (ADSL) systems, only to name a few.

To take profit from the advantages of the two techniques above, MIMO-OFDM systems are proposed. With these systems, the problem of intersymbol interference caused by multi-path propagation can be combated and the capacity can be increased. MIMO-OFDM systems are promising candidates for the transmission schemes of future wireless local area networks (WLAN) and other future high data rate mobile communication systems.

Planned Research – A MIMO-OFDM system with MT transmit and MR receive antennas using space-time coding is shown in Figs. 1a and 1b. At the transmit side, an input binary data sequence is encoded to MT different signals. Each of these signals forms an OFDM signal, which is then transmitted by the corresponding antenna over a MIMO channel. At the receive side, MR different signals are received. After the OFDM demodulation, MR demodulated signals are decoded to an output binary data sequence. In this area, our planned research topics include:

• System analysis and parameter optimization
• Channel estimation and synchronization
• Interference cancellation
• Space-time coding
• Implementation of MIMO-OFDM systems in digital signal processors (DSP).

Fig. 1a: MIMO-OFDM system consisting of a transmitter with MT antennas.

Fig. 1b: MIMO-OFDM system consisting of a receiver with MR antennas.

Collaboration

Within the frame of this project, we are co-operating with:

Shandong University, Jinan, China
International University, Bremen, Germany

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