As being introduced in the first chapter, LTE systems use OFDM as their main modulation method. In order to combat the multipath effects in wireless communication, many method have been proposed. One of the solutions to combat Inter Symbol Interference (ISI) is multicarrier modulation for data transmission, that is Orthogonal Frequency Division Multiplexing (OFDM). The analysis of Bit Error Rate (BER) performance suggests, OFDM is better than Code Division Multiple Access (CDMA) which is mostly incorporated in existing 3G systems.
The aim of OFDM is to divide the wide frequency selectivity of fading channels into multiple flat fading channels. The idea of using a Discrete Fourier Transform (DFT) for the generation and reception of OFDM signals eliminates the requirement of banks of analog sub carrier oscillators. Orthogonality property allows multiple information signals to be transmitted in parallel over a common channel and detected, without interference. In OFDM spectrum each subchannel has a peak at the subcarrier frequency and nulls evenly spaced with a frequency gap equal to the carrier spacing Δf = 1/Ts, where Ts is OFDM symbol duration. Another characteristic of orthogonality is that each carrier has an integer number of sine wave cycles in one bit period.
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Although OFDM enables simple equalization, it is sensitive to carrier frequency offset. The peak to average ratio (PAR) of the transmitted signal power is large. OFDM system performance can be improved by channel coding.
The OFDM system is modeled using MATLAB/SIMULUNK to allow various parameters of the system to be varied and tested. The following OFDM system parameters are considered for the simulation.
- Bit rate R = 1/T : 1 Mbps
- Data mapping : M-PSK and M-QAM
- IFFT, FFT size : 64-point
- Channel used : AWGN
- Guard Interval size : IFFT size/4 = 16 samples
- OFDM transmitted frame size: 64+16 = 80
The OFDM system is modeled using MATLAB/SIMULUNK. The system model for OFDM with M-PSK mapping is shown in Figure x, representing the following blocks. M-PSK block can be replaced by M-QAM block for 64-QAM comparison.
Binary source: Random data generator is used to generate a serial random binary data. This binary data stream models the raw information that going to be transmitted. The serial binary data is then fed into OFDM transmitter.
Data mapping: The input data stream is available serially, converted into parallel stream according to digital modulation scheme. The data is transmitted in parallel by assigning each data word to one carrier in the transmission. Once each subcarrier has been allocated symbols, they are phase mapped according to modulation scheme, which is then represented by a complex In-phase and Quadrature-phase (I-Q) vector.
Consider QPSK mapping in M-PSK block of proposed model, which maps 2 bits per symbol into phase. Each combination of 2 bits of data corresponds to a unique I-Q vector. By moving to higher order constellation, it is possible to transmit more bits per symbol in parallel resulting in high speed communication. The use of phase shift keying produces constant amplitude signal and reduce problems with amplitude fluctuation due to fading. M-QAM modulation can be considered as combination of ASK (Amplitude Shift Keying) and M-PSK. Digital M-PSK is a special case of M-QAM, where the amplitude of the modulated signal is constant. In M-QAM, constellation points are usually arranged in a square grid with equal horizontal and vertical spacing. If data rates beyond those offered by 8-PSK are required, it is more usual to move to M-QAM since it achieves a greater distance between adjacent points in the I-Q plane by distributing the points more evenly. In M-QAM the location of constellation points no longer indicate the same amplitude and so the demodulator must now correctly detect phase and amplitude, rather than just phase.
IFFT-Frequency domain to time domain conversion: The IFFT converts frequency domain data into time domain signal and at the same time maintains the orthogonality of subcarriers. The real signal output can be generated by arranging conjugate subcarriers.
In this stage, IFFT mapping, zero pad, and selector blocks are included. Zero pad block adds zeros to adjust the IFFT bin size of length L, as the number of subcarriers may be less than bin size. Selector block reorders the subcarriers. The IFFT block computes the Inverse Fast Fourier Transform (IFFT) of length L points, where L must be a power of 2.
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Guard period: The effect of ISI on an OFDM signal can be eliminated by the addition of a guard period at the start of each symbol. This guard period is a cyclic copy that extends the length of the symbol waveform. The guard period adds time overhead, decreasing the overall spectral efficiency of the system. Guard duration should be longer than channel delay spread. After the guard band has been added, the symbols are converted into serial form. One frame length duration T = Ts + Tg , where Ts = NT, N = number of carriers. This is the OFDM base band signal, which can be up converted to required transmission frequency.
An AWGN channel model is then applied to transmitted signal. The model allows for the Signal to Noise Ratio (SNR) variation. The receiver performs the reverse operation of the transmitter. The receiver consists of removal of guard band, FFT, removal of zero padding and demapping of data.