OFDM Transmitter And Reciver Using QPSK

Orthogonal Frequency Division Multiplexing

When a signal with high bandwidth traverses through a medium, it tends to disperse more compared to a signal with lower bandwidth.

A high-bandwidth signal comprises a wide range of frequency components. Each frequency component may interact differently with the transmission medium due to factors such as attenuation, dispersion, and distortion. OFDM combats the high-bandwidth frequency selective channel by dividing the original signal into multiple orthogonal multiplexed narrowband signals. In this way it, overcomes the inter-symbol interferences (ISI) issue.

OFDM converts frequency-selective channel to multiple (M) frequency-flat channels.

Block Diagram:

In OFDM, modulation and demodulation are performed using IFFT and FFT.

At transmitter side:

At receiver side:

In OFDM, Information is signalled in the frequency domain. It is based on 1-D transform in frequency domain (IFFT/FFT). Orthogonality among the subcarriers is the key.

Effect of high Doppler in OFDM

In presence of high Doppler, subcarriers lose orthogonality. This results in inter-carrier interference (ICI).

Causes severe degradation in bit error performance for high Doppler (error floors). Channel estimation and equalization in high Doppler channels is difficult.

To avoid ISI while transmitting many parallel low bandwidth signals, the individual subcarriers must be orthogonal to each other. Avoiding ISI by transmitting many orthogonal low bandwidth subcarriers motivates OFDM. An OFDM modulator converts a high-rate serial stream of symbols into many parallel low-rate streams. Each orthogonal low-rate stream encounters a relatively flat channel with minimal ISI, and can be easily equalized.

To demonstrate, consider a pulse of duration Tsym=0.25 sec, a symbol data rate Rsym=1 / Tsym=8 Hz, and additional pulses translated in frequency by Rsym, 2Rsym, and 3Rsym. The frequency-translated pulses are called subcarriers.

Rsym is the symbol rate of each of the low-rate QAM streams
Tsym=1 / Rsym (Tsym is the pulse duration of each

An OFDM modulator sums all these subcarriers together to form its output signal. Here, the subcarriers are baseband modulated using the QAM-method. Mathematically, the sampled modulator output signal s(k) is given by

Where, am,n is QAM symbol stream and it is a QAM-modulated symbol of the mth subcarrier in the nth OFDM time symbol

‘k’ indicates kth position in a input symbol
N is the number of subcarriers

OFDM Transmitter

1. Data Encoding:Convert the input data stream into symbols suitable for transmission (e.g., using QAM, PSK, or other modulation schemes).
2. Serial-to-Parallel Conversion: Group the symbols into blocks, and then convert these serially arranged symbols into parallel data streams, one for each subcarrier.
3. IFFT (Inverse Fast Fourier Transform): Apply the IFFT algorithm to convert the time-domain parallel data streams into frequency-domain OFDM symbols. The IFFT operation converts the data from the frequency domain to the time domain.
4. Cyclic Prefix Addition: Add a cyclic prefix to each OFDM symbol to mitigate the effects of multipath delay spread. The cyclic prefix is a copy of the end part of the OFDM symbol that is appended at its beginning.
5. Digital-to-Analog Conversion: Convert the time-domain OFDM symbols into analog signals.
6. Upconversion and Transmission: Upconvert the analog OFDM signal to the desired carrier frequency and transmit it over the wireless channel.

OFDM Receiver

1. Signal Reception:Receive the transmitted OFDM signal after it has traveled through the wireless channel.
2. Downconversion:Downconvert the received signal to baseband or an intermediate frequency.
3. Analog-to-Digital Conversion: : Convert the analog signal into a digital form suitable for processing.
4. Cyclic Prefix Removal: Remove the cyclic prefix from each OFDM symbol.
5. FFT (Fast Fourier Transform): Apply the FFT algorithm to convert the time-domain OFDM symbols back into the frequency domain. The FFT operation recovers the original frequency-domain OFDM symbols.
6. Parallel-to-Serial Conversion: Convert the frequency-domain symbols back into a serial data stream.
7. Data Demodulation: : Demodulate the received symbols to recover the original data stream.
8. Channel Equalization: Apply channel equalization techniques to mitigate the effects of channel impairments, such as frequency-selective fading.
9. Data Decoding: Decode the received symbols to obtain the original transmitted data.