<<<All Things Wireless>>>
Simulation using MATLAB, Octave and Python.
64-QAM is an important component of 4G/5G Air Interface that promises higher data rates and spectral efficiencies. Combined with OFDM and MIMO it successfully combats the detrimental effects of the wireless channels and provides data rates in excess of 100Mbps (peak data rate). Here, we discuss a simple example of 64-QAM modulation with OFDM in an AWGN channel. We assume a bandwidth of 1.25MHz which corresponds to an FFT size of 128.
Read more64-QAM is an important modulation scheme being used in WiMAX and LTE. It allows for transmission of 6 bits symbol which results in higher bit rate and spectral efficiency. The calculation of bit error rate of 64-QAM is a bit tricky as there are many different formulas available with varying degrees of accuracy. Here, we first calculate the bit error rate (BER) of 64-QAM using a simulation and then compare it to the theoretical curve for 64-QAM. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % FUNCTION TO CALCULATE 64-QAM BER USING SIMULATION % n_bits: Input, number of bits % EbNodB: Input, energy per bit to noise […]
Read moreWe have previously looked at a simple OFDM modulation and demodulation scheme. We saw that the BER performance of OFDM in AWGN was the same as the BER performance of the underlying modulation scheme (QPSK in this case). We will now continuously improve upon our basic simulation to get a more realistic picture. In this regard we introduce the cyclic prefix which is used in OFDM to overcome Intersymbol Interference. The duration of the cyclic prefix is 0.8usec (16 samples at 20MHz) resulting in a symbol duration of 4usec (IEEE 802.11a). Given below is the code for OFDM modulation and demodulation […]
Read moreOFDM modulation works on the principle of converting a serial symbol stream to a parallel symbol stream with each symbol from the parallel set modulating a seperate carrier. The spacing between the carriers is 1/T where T is the duration of the OFDM symbols (without cyclic prefix). This guarantees orthogonality of the carriers i.e. there is no interference between carriers. The addition of orthogonal carriers modulated by parallel symbol streams is equivalent to taking the IFFT of the parallel symbol set. At the receiver the inverse operation of FFT is performed and the parallel symbol streams are converted to serial symbol streams. The main advantage of this scheme is that one […]
Read moreAverage Cell Throughput requires the following simulation results • Average SINR distribution table (system level result), which provides the SINR probability • Average throughput or spectral efficiency versus average SINR table (link level result) For urban channel model and a fixed inter-site distance of 1732m,downlink throughput for LTE for different values of SINR is shown below. MCS vs SINR Average Cell Throughput=Σ(Pi*Ri) where Pi=Probability of occurrence of a specific SINR value at cell edge obtained using simulations Ri=Average throughput corresponding to SINR range Let us consider the following distribution for the SINR at the cell edge: P1=0.5 (SINR=1.50-3.50 dB) P2=0.25 […]
Read moreShannon Capacity of LTE in AWGN can be calculated by using the Shannon Capacity formula: C=B*log2(1+SNR) or C=B*log2(1+P/(B*No)) The signal power P is set at -90dBm, the Noise Power Spectral Density No is set at 4.04e-21 W/Hz (-174dBm/Hz) and the bandwidth is varied from 1.25MHz to 20MHz. It is seen that the capacity increases from about 10Mbps to above 70Mbps as the bandwidth is varied from 1.25MHz to 20MHz (keeping the signal power constant). It must be noted that this is the capacity with a single transmit and single receive antenna (MIMO capacity would obviously be higher).
Read moreSo we have seen that multiple transmit antennas provide the same gain as multiple receive antennas if the channel state information can be fed back to the transmitter. But what if the channel state information cannot be fed back to the transmitter (or it can be done but not quickly enough). The solution to this problem is the so called “Alamouti Scheme”. In this scheme two symbols are simultaneously transmitted from two transmit antennas and in the next time slot phase shifted versions of these two symbols are transmitted over the two transmit antennas. The channel is assumed to be […]
Read moreWe saw that equal gain combining and maximal ratio combining result in tremendous improvement in bit error rate performance in a Rayleigh fading channel. These are receive diversity schemes i.e. schemes that work with multiple receive antennas. Now let us turn our attention to schemes that work with multiple transmit antennas. We know that the main aim of a combining scheme is to coherently add the signals. If the same signal is transmitted from multiple transmit antennas the resulting signals would not add up coherently when they arrive at the receiver (remember that each path introduces a random phase shift). […]
Read moreWe just saw the advantage an equal gain combiner (a combining scheme that just adds the signals after co-phasing them) provides in a Rayleigh fading channel. Lets now look at a variant of this scheme called maximal ratio combining (MRC). In MRC the signals arriving at the receivers are weighted by the channel gains i.e. a stronger signal is weighted more than a weaker signal before combining. It must be noted that in an actual system the received signals are both scaled and phase shifted thus an MRC receiver multiplies the received signals by the complex conjugate of the channel […]
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