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Introduction to CDMA
       Presented by:
  Syed Talha Umair Bukhari
              &
         Yasir Munir
Wireless Local Loop (WLL)
Code Division Multiple Access

 Code Division
 Code Division Multiple Access subscribers share a common channel
 (frequency).CDMA is a "spread spectrum" technology, allowing many users to
 occupy the same time and frequency allocations in a given band/space.
 All users are on the same frequency at the same time, they are divided however by
 codes.
CDMA System Block Diagram
Steps in Generating a CDMA Signal
 Analog to Digital Conversion (Signal is digitized).

 Vocoding (Voice Compression).

 Encoding & Interleaving.

 Channelizing of Signal.

 Conversion from Digital Signal to Radio Frequency(RF)
 Signal.
Steps in Generating a CDMA Signal
 Analog to Digital Conversion:-

       Analog Signal (Voice signal) is converted into Digital Signal by using digital
       modulation technique i.e.; Pulse Code Modulation (PCM).


 Vocoding (Voice Compression):-

       The vocoder is used to compress the digital signal from the Codec
       (Code/Decode). The vocoder, used in a CDMA system, compresses the
       voice signal into various data rates. The data rate is dynamically determined
       by the users speech activity. When we talk, we pause between syllables and
       words. CDMA takes advantage of these pauses in speech activity.

       The voice is compressed in the vocoder into either one of four rates (Full,
       1/2, 1/4, or 1/8 rate). CDMA systems can use either an 8 kbps or 13 kbps
       vocoder.

                *The vocoders are located at the BSC and in the phone.
Steps in Generating a CDMA Signal
  Encoding & Interleaving:-
      Encoding:
                 The purpose of the encoder is to build redundancy into the signal.
                 Redundancy is the number of bits used to transmit a message
                 minus the number of bits of actual information in the message.
      Example:A simple coding scheme is shown in this example. The
      digital message consists of four bits (A,B,C,D) of vocoded data. Each
      of the bits is repeated (encoded) 3 times. These encoded bits are
      called symbols.




      Decoding at the receiver uses a majority logic rule (two out of three wins). If
      an error occurs the redundancy can help recover the lost information.
Steps in Generating a CDMA Signal
 Encoding & Interleaving:-
       Interleaving:
                 Interleaving is a simple, but powerful, method of reducing the
                 effects of burst errors and recovering bits when burst errors occur.
       Example: In this example the symbols from each group are interleaved in a
       pattern that the receiver knows.




       Deinterleaving at the receiver will spread any burst errors that occur during
       transmission and make it simpler to decode.

    * The Encoder & interleaver is located at the BTS and in the phone.
Steps in Generating a CDMA Signal
 Channelizing of Signal:-
      The encoded voice data is encoded using a code that will separate it from other
      encoded voice data. This process, known as channelization, will spread the
      encoded symbols over the entire bandwidth of the CDMA channel.




       Coding Types:
                * Walsh Codes:
                  => Orthogonal (Walsh) codes are used on the forward link to
                      channelize users.
                  => 64 Walsh codes are there.
                * Pseudorandom Noise (PN) Codes:
                  => Pseudorandom noise (PN) code is used on the reverse link
                      to channelize users.
                  => yields about 4.4 trillion codes.
Steps in Generating a CDMA Signal
   Conversion from Digital Signal to Radio Frequency(RF) Signal :-

         The channelized data from all calls will be combined into a one signal that is
         converted to an analog RF signal which is transmitted over the air.



*At Reception side Reverse Process is done
Forward Link Channels in CDMA
 On the forward link there are 4 channels used to transmit control and
 voice data to the mobile. These code channels are:

 Pilot Channel
 Synchronize Channel
 Paging Channel
 Traffic Channel
Forward Link Channels in CDMA
Code Channels
   A channel is a stream of data designated for a specific use or person and is
   separated by a code. A channel may be voice data or overhead control data.
Forward Link Code channels in CDMA are:

   Pilot Channel:-
           The pilot channel is constantly transmitted by the BTS. The mobile uses the
   pilot signal to acquire the system. After the mobile has acquired the system the pilot
   is used for signal strength measurement. The strength of the pilot is used to
   determine the power required for mobile transmit.
   The pilot contains no information but it is the strongest signal on the forward
   link, containing at least 20% of the total power on the forward link.

   Synchronize Channel:-
         The sync channel is constantly transmitted by BTS providing critical timing
   information to the mobile. The mobile will decode the sync channel message during
   the power up sequence. Once the mobile is synchronized with the base station the
   synchronize channel is ignored.
Forward Link Channels in CDMA
   Paging Channel:-
          The paging channel is used to transmit overhead information (i.e. commands
   and pages) to the mobile. When a call is being set up the commands and traffic
   channel assignment are sent on the paging channel. Once a traffic channel is
   established the paging channel is ignored by the mobile.
   CDMA uses up to 7 Paging channels.

   Forward Traffic Channel:-
         The forward traffic channel is used when there is a call. Voice data and control
   overhead information, normally sent on the paging channel, are transmitted to the
   mobile on the traffic channel.
   CDMA uses 55 and 61 forward Traffic channels.

*Once the call is completed, mobile tunes back to Paging channel for commands
 and pages.
Reverse Link Channels in CDMA
On the reverse link there are 2 types of channels used to transmit control and
voice data to the mobile. These channels are:



   Access Channel
   Traffic Channel
Reverse Link Channels in CDMA
 Access Channel:-
       The access channel is used by the mobile when not assigned to a traffic
 channel. The access channel is used by the mobile to:
       * Register with the network,
       * Originate calls,
       * Respond to pages and commands from the base station,
       * Transmit overhead messages to the base station.

 Reverse Traffic Channel:-
       The reverse traffic channel is used when there is a call. Voice data and control
 overhead information, normally sent on the access channel, are transmitted to the
 base station (BTS) on this channel.
Call Processing in CDMA
There are 4 Call Processing modes in CDMA:


   Initializing Mode
   Idle Mode
   Access Mode
   Traffic Mode
Call Processing in CDMA
Initialization Mode:-
        ł Mobile acquires system via Pilot code channel.
        ł Mobile synchronizes with system via Sync code channel.

Idle Mode:-
      ł Mobile and base station communicate over Access and Paging code Channels
        when NOT involved in a call.
      ł Mobile obtains overhead information via the Paging code channel.

Access Mode:-
      ł Mobile accesses the network via the Access code channel

Traffic Mode:-
Land to mobile call
       ‫ ܦ‬When a call for a mobile is placed the mobile will receive a page on the
         Paging channel.
       ‫ ܦ‬The mobile responds on the Access channel and Traffic channels are
       established.
Mobile to land call
       ‫ ܦ‬When a call is placed from a mobile the call is placed using the Access channel.
       ‫ ܦ‬The base station responds on the Paging channel and Traffic channels are
         established.

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Cd

  • 1. Introduction to CDMA Presented by: Syed Talha Umair Bukhari & Yasir Munir
  • 3. Code Division Multiple Access Code Division Code Division Multiple Access subscribers share a common channel (frequency).CDMA is a "spread spectrum" technology, allowing many users to occupy the same time and frequency allocations in a given band/space. All users are on the same frequency at the same time, they are divided however by codes.
  • 5. Steps in Generating a CDMA Signal Analog to Digital Conversion (Signal is digitized). Vocoding (Voice Compression). Encoding & Interleaving. Channelizing of Signal. Conversion from Digital Signal to Radio Frequency(RF) Signal.
  • 6. Steps in Generating a CDMA Signal Analog to Digital Conversion:- Analog Signal (Voice signal) is converted into Digital Signal by using digital modulation technique i.e.; Pulse Code Modulation (PCM). Vocoding (Voice Compression):- The vocoder is used to compress the digital signal from the Codec (Code/Decode). The vocoder, used in a CDMA system, compresses the voice signal into various data rates. The data rate is dynamically determined by the users speech activity. When we talk, we pause between syllables and words. CDMA takes advantage of these pauses in speech activity. The voice is compressed in the vocoder into either one of four rates (Full, 1/2, 1/4, or 1/8 rate). CDMA systems can use either an 8 kbps or 13 kbps vocoder. *The vocoders are located at the BSC and in the phone.
  • 7. Steps in Generating a CDMA Signal Encoding & Interleaving:- Encoding: The purpose of the encoder is to build redundancy into the signal. Redundancy is the number of bits used to transmit a message minus the number of bits of actual information in the message. Example:A simple coding scheme is shown in this example. The digital message consists of four bits (A,B,C,D) of vocoded data. Each of the bits is repeated (encoded) 3 times. These encoded bits are called symbols. Decoding at the receiver uses a majority logic rule (two out of three wins). If an error occurs the redundancy can help recover the lost information.
  • 8. Steps in Generating a CDMA Signal Encoding & Interleaving:- Interleaving: Interleaving is a simple, but powerful, method of reducing the effects of burst errors and recovering bits when burst errors occur. Example: In this example the symbols from each group are interleaved in a pattern that the receiver knows. Deinterleaving at the receiver will spread any burst errors that occur during transmission and make it simpler to decode. * The Encoder & interleaver is located at the BTS and in the phone.
  • 9. Steps in Generating a CDMA Signal Channelizing of Signal:- The encoded voice data is encoded using a code that will separate it from other encoded voice data. This process, known as channelization, will spread the encoded symbols over the entire bandwidth of the CDMA channel. Coding Types: * Walsh Codes: => Orthogonal (Walsh) codes are used on the forward link to channelize users. => 64 Walsh codes are there. * Pseudorandom Noise (PN) Codes: => Pseudorandom noise (PN) code is used on the reverse link to channelize users. => yields about 4.4 trillion codes.
  • 10. Steps in Generating a CDMA Signal Conversion from Digital Signal to Radio Frequency(RF) Signal :- The channelized data from all calls will be combined into a one signal that is converted to an analog RF signal which is transmitted over the air. *At Reception side Reverse Process is done
  • 11. Forward Link Channels in CDMA On the forward link there are 4 channels used to transmit control and voice data to the mobile. These code channels are: Pilot Channel Synchronize Channel Paging Channel Traffic Channel
  • 12. Forward Link Channels in CDMA Code Channels A channel is a stream of data designated for a specific use or person and is separated by a code. A channel may be voice data or overhead control data. Forward Link Code channels in CDMA are: Pilot Channel:- The pilot channel is constantly transmitted by the BTS. The mobile uses the pilot signal to acquire the system. After the mobile has acquired the system the pilot is used for signal strength measurement. The strength of the pilot is used to determine the power required for mobile transmit. The pilot contains no information but it is the strongest signal on the forward link, containing at least 20% of the total power on the forward link. Synchronize Channel:- The sync channel is constantly transmitted by BTS providing critical timing information to the mobile. The mobile will decode the sync channel message during the power up sequence. Once the mobile is synchronized with the base station the synchronize channel is ignored.
  • 13. Forward Link Channels in CDMA Paging Channel:- The paging channel is used to transmit overhead information (i.e. commands and pages) to the mobile. When a call is being set up the commands and traffic channel assignment are sent on the paging channel. Once a traffic channel is established the paging channel is ignored by the mobile. CDMA uses up to 7 Paging channels. Forward Traffic Channel:- The forward traffic channel is used when there is a call. Voice data and control overhead information, normally sent on the paging channel, are transmitted to the mobile on the traffic channel. CDMA uses 55 and 61 forward Traffic channels. *Once the call is completed, mobile tunes back to Paging channel for commands and pages.
  • 14. Reverse Link Channels in CDMA On the reverse link there are 2 types of channels used to transmit control and voice data to the mobile. These channels are: Access Channel Traffic Channel
  • 15. Reverse Link Channels in CDMA Access Channel:- The access channel is used by the mobile when not assigned to a traffic channel. The access channel is used by the mobile to: * Register with the network, * Originate calls, * Respond to pages and commands from the base station, * Transmit overhead messages to the base station. Reverse Traffic Channel:- The reverse traffic channel is used when there is a call. Voice data and control overhead information, normally sent on the access channel, are transmitted to the base station (BTS) on this channel.
  • 16. Call Processing in CDMA There are 4 Call Processing modes in CDMA: Initializing Mode Idle Mode Access Mode Traffic Mode
  • 17. Call Processing in CDMA Initialization Mode:- ł Mobile acquires system via Pilot code channel. ł Mobile synchronizes with system via Sync code channel. Idle Mode:- ł Mobile and base station communicate over Access and Paging code Channels when NOT involved in a call. ł Mobile obtains overhead information via the Paging code channel. Access Mode:- ł Mobile accesses the network via the Access code channel Traffic Mode:- Land to mobile call ‫ ܦ‬When a call for a mobile is placed the mobile will receive a page on the Paging channel. ‫ ܦ‬The mobile responds on the Access channel and Traffic channels are established. Mobile to land call ‫ ܦ‬When a call is placed from a mobile the call is placed using the Access channel. ‫ ܦ‬The base station responds on the Paging channel and Traffic channels are established.