Multimedia Support for Wireless W-CDMA with Dynamic Spreading By Ju Wang Mehmet Ali Elicin And Jonathan C.L. Liu Lecture by Seth Caplan

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

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

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.

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.

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.

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.

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. 1.If spreading factor is not increased then the mobile station with OPEN is immediately ACK. 2.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.

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.

Total Time to Establish a Connection T endtoend = T m + T p + T d + T update + T a + T s T m - time to send request from mobile station to the base station. T p - base station processing time such as finding the spreading factor and checking all existing connections. T d - time to notify and receive ACK from destinations. T update - time needed to broadcast UPDATE messages and receive ACKs. Not done all the time. T a - is the time used to send an ACK to the mobile station. T s - the time when mobile updates channel parameters and gets ready for transmitting.

How do we Improve T endtoend ?

Improving T p T p 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 T p.

Problems Associated with T update 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.

How to Improve T update Can be improved if a collision prevention/resolution 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.

Dynamic Scheduling for Multimedia Integration Can This Improve T s ? 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?

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. 1.Contact the base station for the current traffic load. 2.Select an unfinished request r(i), check to see if the addition of this request will satisfy the BER requirement. 3.If the predicted BER exceeds any of the existing connection, or the BER of r(i), the r(i) is not accepted time frame. 4.Go back to 1. and check for other traffics. Otherwise, r(i) is scheduled at the next time frame.

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, , Audio and Image data request respectively. FTP[1,…,Nf]: array of pending FTP requests, the value of entry FTP[k] represents the remaining data amount yet to be transmitted. [1,…Ne]: array of pending requests. AUDIO[1,…Na]: array of pending Audio requests. IMAGE[1,…Ni]: array of pending Image requests. RBER[4]: the BER requirement of the four traffics. BER[5][1:50]: 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.

Dynamic Spreading Factor Scheduling Algorithm Continued This algorithm shows what would occur for audio traffic (The other algorithm traffic is similar): 1.Find the minimum spreading factor SF v and Sf a such that: BER[SFv][k+N a ] < RBER[1] and BER[SFa][k+N a ] < RBER[4] 2.If both SF v and SF a are found: All audio requests are assigned with spreading factor of Sf a. Go to step (8) to update audio traffics. 3.If SF v cannot be found: 4.Use SF v = 160 as the voice spreading factor. 5.Locate the maximum audio traffic number Na such that: BER[SF v ][k + N a ] < RBER[1] 6.Find the minimum Sfa that satisfies: BER[SF a ][k + N a ] < RBER[4] 7.Decide which subset of audio traffics will be chosen if N a < N a. This should be based on fair strategy so that there is equal chance for all traffics. 8.For each of the selected audio traffics i: Calculate T f as the length of time frame. Reduce their remaining data amount AUDIO[i]- = T f * 4.096/Sf a. UPDATE array AUDIO[] and N a by deleting finished requests. 9.Continue with other traffic types.

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.

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.

Questions?