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In this Lecture

In this Lecture. More on handoff Co-channel interference Trunking Cell Splitting. More on Handoff. When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station. Handoff operation

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In this Lecture

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  1. In this Lecture • More on handoff • Co-channel interference • Trunking • Cell Splitting

  2. More on Handoff • When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station. • Handoff operation • identifying a new base station • re-allocating the voice and control channels with the new base station. • Handoff Threshold • Minimum usable signal for acceptable voice quality (-90dBm to -100dBm) • Handoff margin cannot be too large or too small. • If is too large, unnecessary handoffs burden the MSC • If is too small, there may be insufficient time to complete handoff before a call is lost.

  3. Handoff for first generation analog cellular systems • 10 secs handoff time • is in the order of 6 dB to 12 dB • Handoff for second generation cellular systems, e.g., GSM • 1 to 2 seconds handoff time • mobile assists handoff • is in the order of 0 dB to 6 dB • Handoff decisions based on signal strength, co-channel interference, and adjacent channel interference. • IS-95 CDMA spread spectrum cellular system • Mobiles share the channel in every cell. • No physical change of channel during handoff • MSC decides the base station with the best receiving signal as the service station

  4. 2.5 Interference and System Capacity • Sources of interference • another mobile in the same cell • a call in progress in the neighboring cell • other base stations operating in the same frequency band • any noncellular system which inadvertently leaks energy into the cellular frequency band • Effects of interference • Interference on voice channels causes cross talk, where the subscriber hears interference in the background due to undesired transmission • On control channels, interference leads to missed and blocked calls due to errors in the digital transmission

  5. Two major cellular interference • co-channel interference • adjacent channel interference

  6. 2.5.1 Co-channel Interference and System Capacity

  7. 2.5.1 Co-channel Interference and System Capacity • To reduce co-channel interference, co-channel cell must be separated by a minimum distance. • When the size of the cell is approximately the same • co-channel interference is independent of the transmitted power • co-channel interference is a function of • R: Radius of the cell • D: distance to the center of the nearest co-channel cell • Increasing the ratio Q=D/R, the interference is reduced. • Q is called the co-channel reuse ratio • By increasing Q, the spatial separation between co-channel cells to the coverage distance of a cell is increased. Therefore, a large value of Q improves the transmission quality, due to smaller level of co-channel interference

  8. For a hexagonal geometry • A small value of Q provides large capacity • A large value of Q improves the transmission quality - smaller level of co-channel interference • A tradeoff must be made between these two objectives

  9. A “tier" of cells is the collection of co-channel cells that are more-or-less the samedistance away from the mobile in the serving cell. • When a hexagonal shape is assumed, the number of co-channel cells in the kth tier ofco-channel cells is 6k, regardless of the cluster size.

  10. Six effective interfering cells of cell-1

  11. Let be the number of co-channel interfering cells. The signal-to-interference ratio (SIR) for a mobile receiver can be expressed as S: the desired signal power : interference power caused by the ith interfering co-channel cell base station

  12. The SIR should be greater than a specied threshold for proper signal operation. • In the first-generation AMPS system, designed for voice calls, the desired performance threshold is SIR equal to 18 dB. • For the second-generation digital AMPS system (D-AMPS or IS-54/136), athreshold of 14 dB is deemed suitable. • For the GSM system, a range of 7-12 dB, depending on the study done, issuggested as the appropriate threshold.

  13. Propagation measurements in a mobile radio channel show that the average received signal strength at any point decays as a power law of the distance of separation between a transmitter and receiver. • The average received power at a distance d from the transmitting antenna is approximated by or close-in reference point n is the path loss exponent which ranges between 2 and 4.

  14. When the transmission power of each base station is equal, SIR for a mobile can be approximated as

  15. Decibels (reminder) • The decibel is a dimensionless unit used to express a power ratio • where P0 is the reference power level and P is the considered power level • Decibel (dB) • express the magnitude of a physical quantity relative to a reference level. • represent very large range of ratios • are easy to manipulate (e.g., consecutive amplifiers) • A ratio • can be expressed in decibels relative to 1 Watt (dBW) • is more frequently expressed in decibels relative to 1mW (dBm) • Example: • If the transmission power P0 is 10W and the received power P is 0.1W, the loss is 16

  16. Example: If the transmission power P0 is 0.01W and then P0 in terms of dBm (mili decibel) is 0.01W=10mW 10log10(10)=10dBm

  17. Approximation: Assume • Base stations are located in the centers of each cell. (b) The transmit power of each base station is equal. (c) Each cell transmits an independent signal, such that interfering signal powers maybe added. (d) The path loss exponent is the same throughout the coverage area. (e) All i0 interfering (co-channel) base stations are equidistant from the desired basestation and this distance is equal to the co-channel distance D.

  18. Consider only the first layer of interfering cells, For the worst-case SIR calculation, let the mobile be placed at a corner of the cell. • Example: AMPS requires that SIR be greater than 18dB • N should be at least 6.49 for n=4. • Minimum cluster size is 7

  19. Worst case Co-channel Interference D+R D D+R R A D-R D D-R First tier of co-channel cells for a cluster size of N=7 Note: the marked distances are approximations 20

  20. Co-channel Interference Approximation of the SIR at point A Using the co-channel ratio Numerical example: If N=7, n = 4, then Q~4.6 and 21

  21. Cluster Size for a given S/I • Consider the downlink channel interference. Assume that the mobile is located at the edge of the cell. Consider only the interference from the first tier of co-channel cells (6 cells if N = 7). • We want the SIR to be greater than a given minimum SIRmin • Using the co-channel reuse ratio and because Q=D/R: 22

  22. Adjacent Channel Interference • Adjacent channel interference: interference from adjacent in frequency to the desired signal. • Imperfect receiver filters allow nearby frequencies to leak into the passband • Performance degrade seriously due to near-far effect.

  23. Adjacent channel interference can be minimized through careful filtering and channel assignment. • Keep the frequency separation between each channel in a given cell as large as possible • A channel separation greater than six channel bandwidth is needed to bring the adjacent channel interference to an acceptable level.

  24. 2.5.3 Power Control for Reducing Interference • Ensure each mobile transmits the smallest power necessary to maintain a good quality link on the reverse channel • long battery life • increase SIR • solve the near-far problem

  25. 2.6 Trunking and Grade of Service • Trunking allow a large number of users to share the relatively small number of channels in acell by providing access to each user, on demand, from a pool of available channels. • Exploit the statistical behavior of users so that a fixed number of channels or circuits may accommodate a large, random user community. • In a trunked radio system, each user is allocated a channel on a per call basis, and upon termination of thecall, the previously occupied channel is immediately returned to the pool of availablechannels. • In a trunked mobile radio system, when a particular user request service and all of the radio channels are already in use, the user is blocked, or denied access to the system. In some systems, a queue may be used to hold the requesting users until a channel becomes available.

  26. The grade of service (GOS) is a measure of the ability of a user to access a trunked system during the busiest hour. • It is the wireless designer’s job to estimate the maximum required capacity and to allocate the proper number of channels in order to meet the GOS. GOS is typically given as the likelihood that a call is blocked, or the likelihood of a call experiencing a delay greater than a certain queuing time.

  27. 2.6 Trunking and Grade of Service • Erlangs: One Erlangs represents the amount of traffic density carried by a channel that is completely occupied. • Ex: A radio channel that is occupied for 30 minutes during an hour carries 0.5 Erlangs of traffic. • Grade of Service (GOS): The likelihood that a call is blocked. • Each user generates a traffic intensity of Erlangs given by H: average duration of a call. : average number of call requests per unit time • For a system containing U users and an unspecified number of channels, the total offered traffic intensity A, is given by • For C channel trunking system, the traffic intensity, is given as

  28. Ex: The AMPS cellular system is designed for a GOS of 2% blocking. This implies that the channel allocations for cell sites are designed so that 2 out of 100 calls will be blocked due to channel occupancy during the busiest hour. • There are two types of trunked systems which are commonly used. • Blocked calls cleared. • Blocked calls delayed

  29. The derivation of Erlang B formula (also known as blocked calls cleared formula) is based on the M/M/m/ Queue assumption: • M/M/m/ Queue: Assume that blocked calls cleared trunking with several further assumptions. • Blocked calls cleared • Offers no queuing for call requests. • For every user who requests service, it is assumed there is no setup time and theuser is given immediate access to a channel if one is available. • if no channels are available, the requesting user is blocked without access and isfree to try again later. • Call arrive as determined by a Poisson distribution. • There are memoryless arrivals of requests, implying that all users, including blockedusers, may request a channel at any time. • There are an infinite number of users • The duration of the time that a user occupies a channel is exponentially distributed,so that longer calls are less likely to occur

  30. Erlang B Formula where C is the number of trunked channels offered by a trunked radio system and A is the total offered traffic (capacity of the trunked radio system).

  31. Improving Capacity in Cellular Systems • Methods for improving capacity in cellular systems • Cell Splitting: subdividing a congested cell into smaller cells. • Sectoring: directional antennas to control the interference and frequency reuse. • Coverage zone : Distributing the coverage of a cell and extends the cell boundary to hard-to-reach place.

  32. Cell Splitting • Cell splitting is the process of subdividing a congested cell into smaller cells (calledmicrocells), each with its own base station and a corresponding reduction in antennaheight and transmitter power. • Cell splitting increases the capacity of a cellular system since it increases the numberof times that channels are reused.

  33. Example: system of 32 cells with cell radius of 1.6km Total frequency bandwidth supporting 336 traffic channels Reuse factor (or cluster size) = 7 What geographic area is covered? Total number of supported channels? Solution: Cell area = 6.65km2 Covered area: 32*6.65=213km2 Channels/cell = 336/7=48 Total channel capacity: 32*48=1536 channels • Same question for a system of 128 cells with cell radius of 0.8km. As before: • - total frequency bandwidth supporting 336 traffic channels • reuse factor (or cluster size) = 7 • Solution: • Cell area: 1.66km2 • Covered area: 128*1.66=213km2 • Total channel capacity: 128*48=6144 45

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