MC-CDMA systems for wireless networks

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MC-CDMA systems for wireless networks
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OUTLINE

• 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

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• 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.

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• Basic multiple access techniques
• FDMA
• TDMA
• CDMA
• OFDM

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• 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.

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• 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

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• 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).

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DS-CDMA Transmitter Structure
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• 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

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• 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. 

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OFDM Transmitter Structure
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• 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

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EVOLUTION
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• 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.

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• 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."

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• 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.

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• 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.

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MC-CDMA Transmitter Structure
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Comparison of MC-CDMA with DS-CDMA and OFDM

• 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.

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• 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.

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• 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.

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Architecture design for BCMCS
 
Content Provider Content Server Packet data
serving node BCMCS Controller
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• 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.
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• Broadcast Framing protocol
• Broadcast security protocol
• Broadcast MAC protocol
• Broadcast physical layer protocol
• Broadcast control protocol 
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MC-CDMA in indoor wireless radio networks

• 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.  

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Iterative solution for broadband over power line
communications

• 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.

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• 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

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• 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