Wireless traffic theory and handoff
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The document discusses various topics related to mobile and wireless communication including: 1) Cell capacity and reuse, defining how capacity increases as the cluster size decreases but interference also increases. 2) Traffic theory, defining concepts like arrival rate, holding time, load, and blocked calls. Formulas for blocking probability and Erlang B & C are provided. 3) Channel assignment strategies including fixed and dynamic assignment and how their impact on performance.
Wireless traffic theory and handoff
- 1. ADVANCE MOBILE AND WIRELESS COMMUNICATION Dr. Munaf Rashid (cell capacity and reuse, Traffic Theory, Channel assignment strategies, Hand off strategies)
- 2. CELL CAPACITY AND REUSE • Consider a cellular system with s duplex channel • Suppose each cell is allocated with K channels .let these S channels can be divided among N cells (cluster). S=KN • If a cluster of N cell is replicated M times in the system, the total number of duplex channel, C, can be used as a measure of the system capacity. C=MKN=MS
- 3. CELL CAPACITY AND REUSE • If a cluster size N is reduced keeping the cell size fixed, more clusters are required to cover the entire area of interest, i.e., M C • Smaller N (higher capacity) implies larger co-channel interference which may results in low Quality of service (QOS).
- 4. DEFINITION OF SOME TERMS • Setup Time: the time required to allocate the radio channel to a requesting user. • Blocked call: A call that cannot be completed at the time of request due to congestion (lost call). • Holding Time: the average duration of typical call • Request Rate: Average number of calls per unit time (ʎ) • Traffic Intensity: Measure of channel time utilization (Erlang) • Load: Traffic intensity across the entire radio spectrum • A channel kept busy for one hour is defined as having a load of one Erlang.
- 5. DEFINITION OF SOME TERMS • Grade Of Service (GoS) : A measure of congestion which is specified as a probability. • The Probability of call being Blocked (Erlang B). • The probability of call being delayed beyond a certain amount of time (Erlang C).
- 6. TRAFFIC THEORY • Average no of MSs requesting service (request/time) Average arrival rate= ʎ • Average time for which MS requires service Average Hold time= T • Offered load a= ʎ T (Erlangs) • E.g, In a cell with 100 MSs on an average 30 requests are generated during an hour (3600 sec) with average hold time T=360 seconds (6 minutes) • Then arrival rate ʎ= 30/3600 request/sec. • A channel kept busy for one hour is defined as one Erlang • Offered load
- 7. TRAFFIC THEORY • Average arrival rate during a short interval t is given by ʎt . • Assuming Poison Distribution of service request the probability P(n,t) for n calls to arrive in an interval of length t is given by • Assuming µ to be the service rate, probability of each call to terminate during interval t is given by µ t. • Thus probability of given call requires service of each time t or less is given by
- 8. TRAFFIC THEORY • Probability of arriving call being blocked is Where S is the number of channels in a group • Probability of an arrival call being delayed is Where c(s,a) is the probability of an arriving call being delayed with ‘a’ load and ‘s’ channels. Erlang B formulae Erlang C formulae
- 10. TRAFFIC THEORY (Example 1) • Consider a cell with – S=2 channels – 100 mobile stations – Generating on an average 30 requests/hour – Average holding time T=360 seconds (6 minutes) Load a= (30 × 6)/60 = 3 Erlangs Blocking Probability B(S , a)=0.53 Total number of rerouted calls =30 × 0.53=16 Efficiency =3(1-0.53)/2=0.7
- 12. ERLANG B SYSTEM
- 13. Traffic Theory (Example 2) • Consider a system with – 100 cells – Each cell has S=20 channels – The users average ʎ=2 calls/hour – The average duration of each call (T) is 3 min – How many number of users can be supported if the allowed probability of blocking is 2%? • From Erlang B chart total carried traffic=13 Erlangs • Traffic Intensity per user= ʎT=0.1 Erlangs • Total number of isers that can be supported per cell = 13/0.1=130 users/cell. • Total number of users that can be supported =13,000
- 14. Traffic Theory (Example 3) • Consider another system with – 100 cells – Each cell has S=20 channels – The users average ʎ=2 calls/hour – The average duration of each call (T) is 3 min – How many number of users can be supported if the allowed probability of blocking is 0.2%? • From Erlang B chart total carried traffic=10 Erlangs • Traffic Intensity per user= ʎT=0.1 Erlangs • Total number of users that can be supported per cell = 10/0.1=100 users/cell. • Total number of users that can be supported =10,000
- 15. Traffic Theory (Example 4) • Consider a systems with: – Total number of channels= 20 – Probability of blocking constraint=1% Approach 1: Divide 20 channels in 4 Trunks of 5 channels Traffic capacity for one trunk (5 channels)=1.36 Erlangs Traffic capacity for four trunks (20 channels)=5.44 Erlangs Approach 2: Divide 20 channels in 2 trunk of 10 channels Traffic capacity for one trunk (10 channels)=4.46 Erlangs Traffic capacity for 2 trunk (20 channels)=8.92 Erlangs Approach 3: Use 20 channels as such Traffic capacity for one trunk (20 channels)=12.00 Erlangs Allocation of channels has a major impact! Conclusion: It is better to have larger poll or trunk
- 16. CHANNEL ASSIGNMENT STARTEGIES • A scheme for increasing capacity and minimizing interference is required • Channel assignment strategies can be classified as either fixed or dynamic. • The choice of channel assignment strategy impacts the performance of the system how a call is managed when a mobile user is hand off from one cell to another cell
- 17. FIXED CHANNEL ASSIGNMENT • Each cell is assigned a predetermined set of voice channels. • Any call attempt within the cell can only be served by the unused channels in that particular cell • If all the channels in the cell are occupied the call is blocked the user does not get service • In variation of the fixed channel assignment a cell can borrow channels from it’s neighboring cell if it’s own channels are full
- 18. DYNAMIC CHANNEL ASSIGNMENT • Voice channels are not allocated to different cells permanently. • Each time a call request is made , the Bs request a channel from the Msc. • Msc allocates the channel to the requested call using an algorithm that takes into account • The likelihood of future blocking • The frequency of use of the candidate channels • The reuse distance of the channel and • Other cost functions.
- 19. DYNAMIC CHANNEL ASSIGNEMENT • To ensure the minimum QoS the Msc only allocates a given frequency if that frequency is not currently in use in the cell, or any other cell which falls within the limiting reuse distance. • DCA reduces the likelihood of blocking thus increasing the capacity of the system • DCA strategies requires the Msc to collect the real time data on channel occupancy and traffic distribution on a continuous basis.
- 20. HANDOFF • When a mobile moves in different cells while the call is in progress the Msc must automatically transfer a call to a new channel belonging to the new BS. • The Hand off operation involves indentifying new base station and the allocation of the voice and control signals associated with the new base station. • Handoff must be per formed successfully, as infrequently as possible, and must be imperceptible to the user.
- 21. HANDOFF REGION Signal Strength due to BS i Signal Strength due to BS j Pi(x) Pj(x) By Looking at the variation of Signal Strength from either Base station it is possible to decide on the optimum area where handoff can take place Minimum receiver sensitivity
- 23. Hand Off Region • Hand off is made when the received signal at the BS falls below a pre-specified threshold. • In deciding when to hand off it is important to ensure that the drop In the signal level is not due to the momentary fading. • In order to ensure this the Bs monitors the signal for a certain period of time before initiating hand off • The length of the time needed to decide if handoff is necessary depends on the speed at which the mobile is moving.
- 24. HANDOFF STRATEGIES • In the first generation Analog cellular system the signal strength measurement are made by Bs and are supervised by the MSC. • In the second generation systems that use TDMA technology , Mobile Assisted HandOff MAHO are used. • In MAHO every MS measures the received power from the surrounding BS and continually report these values to the corresponding Bs. • Handoff is initialized if the signal strength of neighboring BS exceeds that of current BS
- 25. SOFT HANDOFF • CDMA spread spectrum cellular system provides a unique hand off capability. • Unlike channelized wireless systems that assign radio channels during a handoff (called hard handoff) the spread spectrum Ms share the same channel in every cell. • The term handoff here implies that a different Bs handles the radio communication task • The ability to select between the instantaneous received signal from different Bs is called soft Hand off.