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Hybrid OFDM-OFCDM modulation scheme

There are three types of multiple access techniques viz. FDMA, TDMA, CDMA. All these techniques share some common resource such as frequency, time and PN codes respectively among multiple users or subscribers. Various hybrid combinations of these access techniques exist. It incudes OFDM with CDMA, hybrid TDMA-FDMA and more. We will focus on hybrid OFDM based technique in combination with CDMA. It is referred as OFCDM modulation technique.

OFDM and CDMA have got their advantages as well as disadvantages. Taking into consideration benefits of both these technologies Hybrid OFDM techniques have been devised.

Hybrid OFDM

In these techniques for example, OFDM with CDMA information to be transmitted is spread across several OFDM subcarriers. Orthogonal codes can be used for spreading. Using this narroe band fading can easily be avoided. This frequency diversity effect in addition to high data rate of OFDM technology make this form of hybrid OFDM better compare to the pure OFDM based systems.

There are three versions of hybrid OFDM along with CDMA. These are MC/DS-CDMA, MC-CDMA and OFCDM. The hybrid techniques are explained below.


In this MC/DS-CDMA scheme, data after conversion from serial to parallel form are modulated using DSSS technique. This is done using spreading code. This time domain spreading process leads to narrow band sub channels in the frequency domain. These are transmitted in parallel over multiple different subcarriers. Here there will not be MAI (Multiple Access Intereference) in synchronized downlink under slow selective frequency fading environment.


In MC-CDMA scheme, multiple copies of the one data symbol is multipled each with one chip specific to the user. These are transmitted using different subcarriers simultaneously. These carriers are orthogonal to one another. Here frequency diversity effect can be achieved due to transmission of duplicate copies of the same symbol on different sub-carriers. Hence frequency selective fading can be eliminated. MAI may exist in the downlink direction in this MC-CDMA scheme.


This OFCDM scheme is derived initially from MC-CDMA. It takes advantage of both transmitting low rate subcarriers over broadband channel due to OFDM and spreading the data using coded signals over parallel multiple subcarriers because of CDMA. Hence it is able to make use of both multipath fading as well as frequency diversity effect.

OFCDM achieves better throughput performance in broadband channel environment. Problem of inter code interference can be avoided by maintaining orthogonality using despereading.



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RF and Wireless Terminologies

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