HF Messaging with OFDM_b – Telecommunications, Navigation & Electronics

Introduction

In previous posts I examined the possible use of OFDM for HF messaging (Ref.1) and for voice (Ref.2). I introduced several channel models using Scicos. In this post I will expand my Scicos 8 channel model to a 64 channel model generated by IDFT.

OFDM IFFT/FFT 64Ch Model

In order to obtain the benefits of OFDM to combat multipath and selective fading over long HF ionospheric paths, the baud rate has to be somewhere in the range of 18.75(53.3msec) to 150(6.7msec) (Ref.2). This requires the use of many more channels to allow for a reasonable channel modulation format. Consider a 64 channel OFDM system with baud rate of 37.5Bd that sends 9600bps:

BW = 2400Hz (300,2700)Hz
Composite Bit Rate = 9600bps
Channels = 64
Channel Data Rate = 150bps
Channel Modulation = 16(2^4)QAM
Channel Baud Rate = 37.5Bd (26.7msec)
This would be very difficult to build in a discrete way, but can be successfully implemented using the IFFT/FFT DSP algorithms.

Scicos OFDM IFFT/FFT 64Ch Simulation

Fig.3 ScicosLab Random I,Q (64×40) Array & IFFT Output

Fig.7 Transmit Real IFFT & Imag IFFT = Receive Real IFFT & Imag IFFT

Figure 1 shows the model used similar to the previous post. Data enters the modem at 9600bps and is converted into 64 parallel streams at 150bps. For 16QAM modulation, this is equivalent to I & Q streams at 37.5bd. Figure 2 shows the ScicosLab code used to generate the random data. The “rand” function is used to generate a random vector (64,4) consisting of 40 columns of 64channels. The interval is divided into 4 regions to give a random matrix with possible values of (-3,-1,1,3) as before for 16QAM. One column of 64 values for I & Q is fed into the IFFT function which generates 64 samples as shown in Figures 3 & 4. These are shifted out at 37.5*64sps. Each IFFT value is over sampled 10x to effectively create a DAC. The samples create structures A1(real_xn) and A2(imag_xn) which feed the I & Q transmitter shown in Figure 5. The receiver reinserts the carrier and integrates over the sample time then holds that value as shown in Figure 6. Figure 7 shows the first transmit I & Q xn samples and the same received samples.