User talk:Nuwankumara

OFCDMA
The broadband orthogonal frequency code division multiple access systems with two-dimensional spreading is the promising candidate for $4^{th}$ generation mobile communication networks. OFCDMA can achieve a peak data rate of 1Gbps in low mobility environment and 100Mbps in outdoor environment with full mobility. Also the bandwidth of the future 4G system should be much larger than 20 MHz to transmit a peak rate of 1 Gbps. Therefor new wireless access technique using 50$-$100 MHz bandwidth is needed to cater above requirement.\cite{Zhou/Tung/Wang/Higuchi/Sawahashi:2008} OFCDM provides not only all advantages of OFDM, but also additional benefits by means of 2D spreading. For example, frequency diversity gain can be achieved through frequency domain despreading thanks to the different fading experienced by subcarriers in a broadband channel and with the introduction of time domain spreading, the system can provide flexible transmission rates. OFCDM has been tested using OFCDM and a bandwidth of 100 MHz, a data transmission rate of 100 Mbps in the downlink has been achieved without the use of MIMO in outdoor environments at a moving speed of 20 km/h.

In order to achieve good frequency diversity gain, $N_{F}$ interleaved subcarriers are employed instead of $N_{F}$ consecutive subcarriers. The 2D spreading in OFCDM is different from the conventional spreading in CDMA, which expands the signal bandwidth. Instead, it is more like a coding scheme carrying the same data information in $N = N_{T} \times N_{F}$ frequency- time blocks.

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