Introduction Basic multiple access techniques Evolution of MC-CDMA Technologies in MC-CDMA Applications of MC-CDMA Advantages and problems of MC-CDMA Key challenges and issues of MC-CDMA

INTRODUCTION

• Code-division multiple access (CDMA) schemes have been considered as attractive multiple access schemes in both second-generation (2G) and third-generation (3G) wireless systems

•The evolution from 2G to 3G corresponds to adapting a new air interface i.e. change of focus from voice to multimedia.

•MC-CDMA scheme has become a promising access technique for 4G air interface.

Basic multiple access techniques

•FDMA•TDMA•CDMA•OFDM

FDMA

•In frequency division multiple access, the available bandwidth is subdivided into a number of narrower band channels. •FDMA is used as the primary breakup of large allocated frequency bands and is used as part of most multichannel systems.

TDMA

• TDMA divides the available spectrum into multiple time slots, by giving each user a time slot in which they can transmit or receive.

•TDMA is generally used in conjunction with FDMA to subdivide the total available bandwidth into several channels

CDMA

•CDMA is a spread spectrum technique that uses neither frequency channels not time slots.

•CDMA is a multiplexing technique where a number of users simultaneously and asynchronously access a channel by modulating and spread their information-bearing signals with pre-assigned signature sequences called pseudo random noise codes (PN codes).

•The above slide shows a simplified transmitter structure of DS-CDMA for one user support. DS-CDMA uses orthogonal code for spreading and channelization. •Pseudo random noise codes (PN codes) are used to detect the each multi path signal and to pick up the signal from the certain base station

OFDM• OFDM is a multicarrier transmission technique, which divides the available spectrum into many carriers, each one being modulated be a low rate data stream. •It has gained popularity because of its capability to transmit high data rate.

•OFDM is similar to FDMA in that the multiple user access is achieved by subdividing the available bandwidth into multiple channels that are then allocated to users. 

•In OFDM, a block of data is converted into a parallel form and mapped into each subcarrier. Thus, they become the frequency domain symbols.•To get the time domain signal again, inverse discrete Fourier transform or its fast version, IFFT, is applied. •Each subcarrier is orthogonal to each other while the frequency

EVOLUTION

1xRTT•First version of CDMA2000 provides data rates of 307 Kbps (downlink) and 153 Kbps (uplink) 1xEV (1xEV-DO and 1xEV-DV)•This provides higher speeds implemented in two phases.•Phase1 increases the downlink peak data rate to 2.4 Mbps.•Revision A supports IP packets, increases the downlink to 3.1 Mbps and boosts uplink dramatically to 1.2 Mbps.

• Phase 2 is Evolution-Data Voice (1xEV-DV), which integrates voice and data on the same carrier with rates up to 4.8 Mbps.

3x•CDMA2000 3x uses three 1.25 MHz CDMA channels. It is part of the CDMA2000 specification for countries that require 5 MHz of spectrum for 3G use. CDMA2000 3X is also known as "3XRTT," "MC-3X," and "IMT-CDMA Multicarrier 3X."

•MC-CDMA is a form of direct sequence CDMA, but after spreading a Fourier Transform (FFT) is performed.•MC-CDMA is a form of orthogonal frequency division multiplexing(OFDM) but we first apply an orthogonal matrix operation to the user bits, therefore •MC-CDMA is sometimes also called “CDMA-OFDM”.•MC-CDMA is a form of frequency diversity. Each bit is transmitted simultaneously on many different subcarriers.

•MC-CDMA applies spreading sequences in frequency domain. The original information becomes spreaded in frequency domain directly.

•After this spreading, the highly successful OFDM transmitter structure is borrowed.

•DS-CDMA is a method to share spectrum among multiple simultaneous users. DS-CDMA with a spread factor N can accommodate N simultaneous users only if highly complex interference techniques are used.•MC-CDMA can handle N simultaneous users with good BER using standard receiver techniques.

•To avoid excessive bit errors on subcarriers that are in a deep fade, OFDM typically applies coding.

•MC-CDMA replaces this encoder by an NxN matrix operation and results reveal an improved BER.

Technologies in MC-CDMA and Applications

• MC-CDMA is mainly used in multimedia services in 3G/4G networks(BCMCS).

•BCMCS provide point-to-multipoint transmission of multimedia data.

Architecture design for BCMCS

 

Content ProviderContent ServerPacket data serving nodeBCMCS Controller

Broadcast Packet Data Air Interface

The air interface of the high rate broadcast packet data system has a group of protocols called as the broadcast protocol suite. •Broadcast Framing protocol•Broadcast security protocol•Broadcast MAC protocol•Broadcast physical layer protocol•Broadcast control protocol          

•MC-CDMA is a suitable modulation technique in the indoor environment.

•Multiple access is achieved with different users transmitting at same set of subcarriers but with spreading codes that are orthogonal to the codes of others.  

•MC-CDMA transmission with a low complexity iterative receiver is proposed for the PLC (power line communication) channel.•A significant performance gain is observed for a realistic powerline channel and high power impulsive noise.

ADVANTAGES OF MC-CDMA

•Easier implementation for high data rate services than DS-CDMA by the increased signaling interval

•Suitable for indoor wireless environment: small delay spread and small Doppler spread

•Fading resistance using frequency diversity

•Possibility of quasi-synchronous operation in reverse link

•DS-CDMA has energy loss due to the limited number of rake receivers while MC-CDMA gathers nearly all energy scattered in subcarriers

PROBLEMS IN MC-CDMA

•High Peak-to-Mean Envelope Power Ratio(PMEPR) •Nonlinear amplification - spurious power •Power inefficient

•Sensitive to carrier frequency offset : Difficult to deploy for high speed vehicles

•Sensitive to phase noise

•Low frequency reuse factor than DS-CDMA