Hybrid OFDM-OFCDM Modulation Explained

There are three primary multiple access techniques: FDMA, TDMA, and CDMA. All of these techniques share common resources, such as frequency, time slots, and PN codes, respectively, among multiple users.

Various hybrid combinations of these access techniques exist, including OFDM with CDMA, hybrid TDMA-FDMA, and more. This article will focus on a hybrid OFDM-based technique in combination with CDMA, referred to as the OFCDM modulation technique.

Why Hybrid OFDM?

OFDM and CDMA each possess distinct advantages and disadvantages. Hybrid OFDM techniques leverage the benefits of both technologies.

For example, in OFDM with CDMA, the information to be transmitted is spread across several OFDM subcarriers. Orthogonal codes can be used for this spreading. This effectively avoids narrowband fading. This frequency diversity effect, combined with the high data rate of OFDM technology, makes this hybrid OFDM approach superior to pure OFDM-based systems.

There are three main versions of hybrid OFDM combined with CDMA:

• MC/DS-CDMA
• MC-CDMA
• OFCDM

These techniques are detailed below.

MC/DS-CDMA

In the MC/DS-CDMA scheme, data, after serial-to-parallel conversion, is modulated using DSSS (Direct-Sequence Spread Spectrum). This is accomplished through a spreading code. This time-domain spreading process results in narrowband sub-channels in the frequency domain. These sub-channels are then transmitted in parallel over multiple, distinct subcarriers. Ideally, under slow, selective frequency fading conditions in a synchronized downlink scenario, MAI (Multiple Access Interference) will be absent.

MC-CDMA

In the MC-CDMA scheme, multiple copies of a single data symbol are multiplied by a chip specific to the user. These modified data symbols are then transmitted simultaneously using different subcarriers. These carriers are orthogonal to each other.

Frequency diversity is achieved by transmitting duplicate copies of the same symbol across different sub-carriers, effectively mitigating the impact of frequency-selective fading. However, MAI may be present in the downlink direction in this MC-CDMA scheme.

OFCDM

The OFCDM scheme is derived from MC-CDMA. It capitalizes on both the transmission of low-rate subcarriers over a broadband channel (inherent to OFDM) and the spreading of data using coded signals across parallel, multiple subcarriers (characteristic of CDMA).

This allows OFCDM to exploit both multipath fading and frequency diversity. OFCDM achieves better throughput performance in broadband channel environments.

The problem of inter-code interference can be avoided by maintaining orthogonality through despreading.