Multicarrier CDMA: A Comprehensive Introduction

This page provides a tutorial on Multicarrier CDMA, encompassing CDMA and OFDM modulation/multiple access techniques.

Description

Future mobile communication systems demand high data rates and superior Quality of Service (QoS). The desired high data rate can be realized through multi-carrier systems like OFDM. Multicarrier CDMA strategically combines the principles of DS-CDMA with OFDM. This synergy allows Multicarrier CDMA to achieve enhanced data rates while maintaining optimum spectrum efficiency. Furthermore, Multicarrier CDMA showcases robustness in multipath fading environments, making it a prominent candidate for future 4G technologies.

CDMA

CDMA relies on the spread spectrum concept and functions as both a modulation and multiple access technique.

In CDMA, the message signal is multiplied by a pseudo-noise (PN) sequence, which spreads the time-domain message over a wider bandwidth. This process reduces the signal power to below the noise floor, making demodulation/decoding difficult without the correct PN sequence. At the receiver, only the appropriate PN sequence can decode the message signal. Each subscriber is assigned a unique PN sequence for identification.

Multi-Carrier Modulation

OFDM, a multi-carrier modulation scheme, divides the incoming data stream into N parallel channels, each with a smaller data rate and narrower bandwidth. This division is accomplished using a Serial-to-Parallel converter. The parallel streams are then multiplied by orthogonal carriers and summed to create a spectrum that is bandwidth-efficient. To maintain orthogonality between carriers and mitigate Inter-Symbol Interference (ISI), a guard period is introduced. Typically, the guard period is set to exceed the delay spread of the channel.

The process of multi-carrier modulation can be easily implemented using Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform (FFT). IFFT is employed at the transmitter, and FFT is used at the receiver.

Let’s explore how multiple access is achieved using OFDM. The total number of subcarriers are distributed among different subscribers. Each set of subcarriers allocated to a subscriber forms a sub-channel. Thus, each subscriber possesses a distinct sub-channel for transmitting data/voice.

Figure mentions base station model of MC CDMA

As illustrated in the figure, the data ($d_m$) is modulated using a chosen modulation scheme, such as BPSK, QPSK, or 16QAM. The modulated set of complex data is then passed to a Serial-to-Parallel converter. The resulting parallel data is fed into various PN codes ($C_0$ to $C_n$), and this multiplied data is multiplied with an orthogonal set of carriers ($g_0$ to $g_n$) to generate the OFDM spectrum that constitutes a single symbol. The same process is repeated for all subscriber stations at the base station to form the Multicarrier CDMA signal spectrum.

For the Subscriber transmitter model, zeros are inserted in the positions where data is not available at the input of the IFFT block.