Multimedia Support for Wireless W-CDMA with Dynamic Spreading

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Multimedia Support for Wireless W-CDMA with
Dynamic Spreading

• By
• Ju Wang
• Mehmet Ali Elicin
• And
• Jonathan C.L. Liu

Lecture by Seth Caplan
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Key Words

• CDMA (Code Division Multiple Access)
• BER (Bit Error Rate)
• QoS (Quality of Service)
• SF (Spreading Factor)

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Introduction

• Allow cell phone to do voice, audio and
  conventional data (ex. email, ftp,) transfers.
  
• Support multiple users with a guaranteed
  quality.
• Do all this without connection re- establishment
  . (ex. Sending an email while talking on the
  phone)

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Traffic Types Supported

• The table above shows the acceptable BER that is
  required to be able to process that traffic
  type.
• Minimum BER must be satisfied.

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Spreading Factor

• Increasing the spreading factor can decrease the
  BER for a given number of users.
• Spreading factor is the key variable in
  determining user data rate and BER.
• Increasing spreading factor can increase the
  desired signal strength linearly.

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Changes to CDMA

• System will dynamically change the spreading
  factor to allow more users and for certain
  traffic types.
• Changes to spreading factor will occur
  according to BER.
• Once a call is accepted the performance will be
  guaranteed for its lifetime.
• Mobile and Base stations need to change
  parameters to allow the above to work
  correctly.

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Mobile Stations
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Mobile Stations

• Has three types of request
• OPEN - a new connection
• ALTER - change the traffic type
• CLOSE - ends the connection
• Protocol defines 4 traffic types. (Voice, Audio,
  Video and Data)
• The base station determines weather the
  connection is allowed, if the connection is
  denied it will retry after waiting a random
  time period. Connection is determined by two
  items
• The BER requirement and min data rate.
• Interference by other users.

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Base Station
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Base Station

• For all connections the station calculates the
  average BER corresponding to the number of
  users.
• If the BER is high, the system will increase the
  spreading factor and sees if the data rate can
  now be met by the system. This is done for all
  connections.
• There are two possibilities after the above step
  is taken.
• If spreading factor is not increased then the
  mobile station with OPEN is immediately ACK.
• If the spreading factor is increased then the
  base station must broadcast an UPDATE to all
  its connections to tell them about this
  increase. When all the existing connections
  have ACK to the UPDATE, then the station with
  the OPEN is then ACK and may start to transmit.

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Fixed SF vs. Dynamic SF

• Dynamic spreading factor maintained the BER,
  and good overall performance.
• Where the fixed spreading factor could not
  maintain good BER past five users.

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Total Time to Establish a Connection

• Tendtoend Tm Tp Td Tupdate Ta Ts
• Tm - time to send request from mobile station to
  the base station.
• Tp - base station processing time such as finding
  the spreading factor and checking all existing
  connections.
• Td - time to notify and receive ACK from
  destinations.
• Tupdate - time needed to broadcast UPDATE
  messages and receive ACKs. Not done all the
  time.
• Ta - is the time used to send an ACK to the
  mobile station.
• Ts - the time when mobile updates channel
  parameters and gets ready for transmitting.

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How do we Improve Tendtoend ?
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Improving Tp
Tp was improved by calculating one spreading
factor for each type of traffic before doing the
review. This was done instead of doing the
calculation for each of the connections. As a
result, 25 reduction was achieved on Tp.
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Problems Associated with Tupdate

• Must wait until a ACK is received from all mobile
  stations that are connected.
• Only one access channel is used and there will be
  extra delays due to possible collisions.
• Will grow exponentially and then settle back to
  small delay.
• Only used when UPDATE is called.

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How to Improve Tupdate

• Can be improved if a collision prevention/resoluti
  on algorithm is used.
• Increasing the number of access channels (2
  access channels gave 49 reduction, 4 access
  channels gave 58 reduction)
• As access channels increase so does the
  interference between users which will also cause
  BER to increase, so 2 access channels is
  recommended.
• Higher spreading factor (128 or 160) are more
  tolerable to having more access channels.

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Dynamic Scheduling for Multimedia IntegrationCan
This Improve Ts?

• How do we continue to support voice users with
  guaranteed quality and integrate multimedia
  traffic?
• We implement a dynamic spreading algorithm.
• What's the difference between fixed scheduling
  and dynamic scheduling algorithms?

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The Fixed Spreading Factor Scheduling Algorithm

• Algorithm (FSF_Scheduling) Algorithm delays the
  transmission until the BER is acceptable.
  Algorithm will reduce the frequency of
  retransmission.
• Contact the base station for the current traffic
  load.
• Select an unfinished Email request r(i), check to
  see if the addition of this request will satisfy
  the BER requirement.
• If the predicted BER exceeds any of the existing
  connection, or the BER of r(i), the r(i) is not
  accepted time frame.
• Go back to 1. and check for other traffics.
  Otherwise, r(i) is scheduled at the next time
  frame.

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Dynamic Spreading Factor Scheduling Algorithm

• This algorithm improves on the fixed scheduling
  algorithm by assigning traffic types dynamically.
• Input parameters Nf, Ne, Na, Ni represent the
  number of pending FTP, Email, Audio and Image
  data request respectively.
• FTP1,,Nf array of pending FTP requests, the
  value of entry FTPk represents the remaining
  data amount yet to be transmitted.
• EMAIL1,Ne array of pending Email requests.
• AUDIO1,Na array of pending Audio requests.
• IMAGE1,Ni array of pending Image requests.
• RBER4 the BER requirement of the four
  traffics.
• BER5150 the predicated BER given the
  number of active users and the number of active
  users and the spreading factor. The first,
  second, up to fifth row correspond to BERs for
  SF 32, 64, 96, 128 and 160.

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Dynamic Spreading Factor Scheduling Algorithm
Continued

• This algorithm shows what would occur for audio
  traffic (The other algorithm traffic is similar)
  
• Find the minimum spreading factor SFv and Sfa
  such that
• BERSFvkNa lt RBER1 and
• BERSFakNa lt RBER4
• If both SFv and SFa are found
• All audio requests are assigned with spreading
  factor of Sfa.
• Go to step (8) to update audio traffics.
• If SFv cannot be found
• Use SFv 160 as the voice spreading factor.
• Locate the maximum audio traffic number Na such
  that
• BERSFvk Na lt RBER1
• Find the minimum Sfa that satisfies
• BERSFak Na lt RBER4
• Decide which subset of audio traffics will be
  chosen if Na lt Na. This should be based on fair
  strategy so that there is equal chance for all
  traffics.
• For each of the selected audio traffics i
• Calculate Tf as the length of time frame.
• Reduce their remaining data amount AUDIOi-
  Tf 4.096/Sfa.
• UPDATE array AUDIO and Na by deleting
  finished requests.
• 9. Continue with other traffic types.

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Benefits of Dynamic Spreading Factor Algorithm

• No starvation of data traffic and reasonable
  response time.
• Voice communication is guaranteed to be
  uninterrupted.
• Improved performance on Ts.

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Conclusion

• W-CDMA can support multiple traffic types.
• Only new calls admitted are calls that can be
  guaranteed.
• Dynamically spreading factor provides much needed
  improvement over fixed spreading factor.
• By using W-CDMA we are able to maintain an
  acceptable BER throughout the entire system.

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Questions?