Talk:Multi-carrier code-division multiple access

MC-CDMA
MC-CDMA was born in February 1993, Nathan Yee proposed the original MC-CDMA system in \cite{Yee/Linnartz/Fettweis:1993} where serial to parallel converted bits are directly fed into the input of IFFT processor after coding applied, such that the number of subcarriers in the OFDM system equals to the code length of applied code. It can be seen that the same data symbol is duplicated in multiple carriers of OFDM symbol with the application of assigned code. User $u_{0}$ take the code sequence $\textbf{C}_{0}$ where $\textbf{C}_{0} {}={} [c_{0,0},c_{0,1},\ldots,c_{0,M-1}]$ and the number of subcarriers of OFDM symbol $M_{d}$ in this case is equal to $M$. — Preceding unsigned comment added by Nuwankumara (talk • contribs) 08:51, 1 August 2011 (UTC)

CDMA/OFDM
In September 1993 Fazel proposed a new MC-CDMA scheme \cite{Fazel:1993} and it was called CDMA/OFDM. It was implemented with a high end OFDM multiplexer compared to the complexity of spreading code were used. It was given two examples, in the first one it was considered $B$ length of data block from each user with $M_{d}$ number of subcarriers of OFDM multiplexer with $M_{d} = B \times G_{MC}$ where $G_{MC}$ is the processing gain due to spreading which is equal to the length of the spreading code $K$ in our notations.

MC-DS-CDMA
In order to increase the data rate in DS-CDMA, it is necessary to reduce the chip duration, so that chip level synchronization becomes even more difficult specially for reverse link (uplink). Dasilva proposed a new scheme in \cite{Dasilva/Sousa:1993} to use DS-CDMA with multicarrier so called MC-DS-CDMA to increase the data rate without reducing the chip duration.

MT-CDMA
In MT-CDMA transmitter it only spread the signal after IFFT conversion, which was proposed by Vandendorpe \cite{Vandendorpe:1995}. — Preceding unsigned comment added by Nuwankumara (talk • contribs) 08:54, 1 August 2011 (UTC)

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.

Set of orthogonal codes are used to spread the signal in both frequency domain and time domain. For example if we consider a $N_{F}$ length of frequency domain spreading code for one particular input stream or user then  $N_{F}$ number of orthogonal codes in frequency domain can be maximum use for other data streams or users. Similary in time domain for $N_{T}$ length of time domain spreading code, maximum number of users can be  $N_{T}$ achieved. Figure 3.1 show code assignment for $N = N_{T} \times N_{F}$ number of users or data streams with  $N_{F} = 2$ and  $N_{T} = 4$. — Preceding unsigned comment added by Nuwankumara (talk • contribs) 08:57, 1 August 2011 (UTC)

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