FPGA Implementation of MIMO-OFDM STBC Systems

FPGA Implementation of MIMO-OFDM STBC Systems

Multiple-input multiple-output (MIMO) combined with Orthogonal Frequency Division Multiplexing (OFDM) techniques has been received a great attention in recent years. It is also well-known that Space-Time Block Coding (STBC) is the typical approach to significantly increase diversity gain. In this project, we present design of MIMO-OFDM STBC systems based on VLSI platform which consists of Verilog HDL and Matlab Program. The project is designed based on MIMO OFDM STBC system using Radix-4 256 Point FFT and 4QAM modulation with different numbers of receiver antennas in MIMO multipath fading environments. Radix-4 FFT is used to achieve low power design. The design is simulated using Matlab with Modelsim and performance is evaluated based of BER for various inputs such as image and text.

MIMO STBC

A block diagram of a MIMO STBC system with 2 transmit antennas and NR receive antennas is shown in below Fig.

 

Block diagram of a MIMO STBC system

At transmitter side, input data is encoded by Alamouti STBC as follows:

x₁(T)=s(T)

x₂(T)=s(T+1)                                                                                                  (3.1)

x₁(T+1) =-s*(T+1)

x₂(T+1) =s*(T)                                                                                               (3.2)

Where s(T) and s(T+1) denote input data symbols at symbol duration T and T+1, x(T) and x(T+1) denote encoded data symbols at the symbol duration T and T+1. Encoded data then are transmitted through two transmit antennas.

At the receive side, received signals at the symbol duration T and T+1 are given by:

r_i(T)=r_i1.x₁(T)+h_i2.x₂(T)+n_i(T)

       =h_i1s(T)+h_i2s(T+1) +n_i(T)                                                                                     (3.3)

r_i(T+1) =h_i1.x₁(T)+h_i2.x₂(T+1) +n_i(T+1)       

            =h_i1s*(T+1) +h_i2s*(T) +n_i(T+1)                                                                      (3.4)

where yi(T) and yi(T+1) denote received signals obtained at the ith receive antenna at the duration T and T+1, hij denotes the channel response from the jth transmit antenna to the ith receive antenna, ni(T) and ni(T+1) denote AWGN noise at the ith receive antenna at the duration T and T+1.

After simply exchanging in (3) and (4), we can get following equations:

h_i1*r_i(T). h_i2*r_i(T+1)=(| h_i1|²+| h_i2|²)s(T)+ h_i1*n_i(T)+ h_i2*n_i(T+1)                                   (3.5)

h_i2*r_i(T). h_i1*r_i(T+1)=(| h_i1|²+| h_i1|²)s(T+1)+ h_i2*n_i(T)+ h_i1*n_i(T+1)                               (3.6)

Here, we can see the advantage of the STBC technique is that we can get transmitted signals without any inter-symbol interference with a simple process at transmitter side and receive side. The signal to noise at the ith receive antenna is given by:

γ_i= (| h_i1|²+| h_i2|²)P_s/P_n                                                                                                                             (3.7)

The summation of signal to noise ratio of NR receive antennas in the system is thus given by:

                                                                                         (3.8)

 

MIMO-OFDM STBC block diagram

A block diagram of MIMO-OFDM STBC systems is shown in above figure. At the transmit side, a high-speed input data stream is divided into many sub-streams according to the number of subcarriers of the systems. On each subcarrier, input data will be encoded by STBC encoder, then data on each transmit antenna will be processed by IFFT before transmitting. At the receive side, received signals will be done by an FFT process and applied STBC decoder on each subcarrier. The decoded data will be multiplexed to obtain transmitted data. MIMO OFDM STBC is divided into two,

1. Transmitter Side
2. Receiver Side

 Block diagram of 2x2 MIMO-OFDM STBC hardware system

Simulation Video Demo