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Wireless Communications

Wireless Communications. T.S. Rappaport Wireless Communications Principles and Practice,2 nd Edition. For mobility provided to the subscriber, a system should have the following parts. (1) Central Station: This contains the switching equipment and one RF transmitter and receiver.

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Wireless Communications

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  1. Wireless Communications T.S. Rappaport Wireless Communications Principles and Practice,2nd Edition

  2. For mobility provided to the subscriber, a system should have the following parts. • (1) Central Station: This contains the switching equipment and oneRF transmitter and receiver. • (2) Mobile telephones: This contains microphone, speaker , dialling facility, a radio transmitter and receiver. • Each mobile phones requires two frequencies. One for transmission from central station to mobile(Forward channel or downlink) and second for mobile to central station( Reverse channel or uplink).

  3. (1) Call from terrestrial network to mobile: Central station will check weather the called mobile is free. If it is free then it will signal the mobile on the downlink frequency and the mobile will reply on uplink frequency. After which the central station will complete the connection. • (2) Call from mobile: Mobile will signal the central station on the uplink.(This means the central station must scan all the mobiles phones continuously). If the called party is free then the central station will provide the connection.

  4. If the total 100000 subscribers in the system ,200000 radio channels is needed. • Maximum frequency of speech signal fmax= 4Khz and if we assume FM and frequency deviation is 12 Khz then, Bandwidth required = 2(12+4) =32 KHz. ( let us take 30KHz) That means 30 kHz per channel. For 200000 subscribers total BW= 30000 X 200000= 6000 MHz. Only 50 MHz allocated in 800-900. MHz. If demand assignment is used and out of 100000 customers only 5% using the system at a same time, then 0.05 X 6000 MHz = 300 MHz which is prohibitively large.

  5. Frequency reuse concept

  6. Frequency reuse • If 21 radio channels and 1000 clusters, then the effective number of channels are 21000. • Using demand assignment as many as 2 Lakhs customers can be served. • If TDMA is used 8*2 lakhs = 16 lakhs customer can be served.

  7. Cellular Concept • Developed by Bell Labs 1960’s-70’s. • Areas divided into cells. hexagon geometry cell shape. • a few hundred meters in some cities, 10s km at country side. • Each served by base station with lower power transmitter. • Each gets portion of total number of channels. • Neighboring cells assigned different groups of channels, interference minimized.

  8. Frequency Reuse • Adjacent cells assigned different frequencies to avoid interference or crosstalk • Objective is to reuse frequency in nearby cells • 10 to 50 frequencies assigned to each cell • transmission power controlled to limit power at that frequency escaping to adjacent cells • the issue is to determine how many cells must intervene between two cells using the same frequency

  9. Frequency Reuse • S = total number of channels available for use. • k = number of channels allocated to each cell. • N= cluster size • each cell allocated a group k channel. A cluster has N cells with unique and disjoint channel • groups, N typically 4,7,12 • Total number of duplex channels S = kN • Cluster repeated M times in a system • Total number of channels that can be used (capacity) C = MkN = MS

  10. Larger the value of M, Larger the capacity C. That means cluster size N has to be small. • This will reduce the distance between the cells having same set of frequencies, and the interference will increase. • Therefore there as to be compromise between the requirement of larger capacity and smaller interference.

  11. Effect of cluster size N • channels unique in same cluster, repeated over clusters • keep cell size same • large N : weaker interference, but lower capacity • small N: higher capacity, more interference need to maintain certain S/I level • frequency reuse factor: 1/N • each cell within a cluster assigned 1/N of the total available channels

  12. Design of cluster size • In order to connect without gaps between adjacent cells • N = i2 + ij + j2 where i and j are non-negative integers • Example i = 2, j = 1 • N = 22 + 2(1) + 12 = 4 + 2 + 1 = 7 • Next page example • move i cells along any chain or hexagon. • then turn 60 degrees counterclockwise and move j cells.

  13. Example • N=19 • (i=3, j=2)

  14. Major components of a mobile system • Mobile handsets. • Base stations. • Mobile Switching Center (MSC) • Communication between mobile handsets is with base stations over a standard air interface that specifies 4-different channels. • (a) Forward voice channels (from base station to mobile set) • (b) Reverse voice channels (from mobile set to base station) • (c) Forward control channels • (d) reverse control channels. • The last two are required for establishment of the call and other controls.

  15. Base Station: A fixed station in a mobile radio system used for radio communication with mobile stations. Base stations are located at the center or on the edge of a coverage region and consist of radio channels and transmitters and receiver antennas mounted on a tower. • Mobile Station: A station in the cellular radio service intended for use while in motion at unspecified locations. Mobile stations maybe handheld personal units(portables) or installed in vehicles(mobiles). • Subscriber: A user who pays subscription charges for using a mobile communication system. • Transceiver: A device capable of simultaneously transmitting and receiving radio signals.

  16. Mobile Switching Center (MSC): Switching center which coordinates the routing of calls in a large service area. In a cellular radio system, the MSC connects the cellular base stations and the mobiles to the PSTN. An MSC is also called a Mobile telephone Switching Office. • Control Channel: Radio channel used for transmission of call setup, call request, call initiation and other control purposes. • Forward Channel: Radio channel used for transmission of information from the base station to the mobile. • Reverse Channel: Radio channel used for transmission of information from the mobile to the base station. • Roamer: A mobile station which operates in a service area(market) other than that from which service has been subscribed. • Handoff: The process of transferring a mobile station from one channel or base station to another.

  17. How a cellular telephone call is made • When the cellular phone is turn ON and is not engaged in a call it first scans the group of forward control channels to determine the one with the strongest signal and then monitors that control channel until the signal drops bellow the usable level. • At this point it again scans the control channel in search of the strongest base station

  18. Timing sequence of call from landline to mobile • MSC receives call from landline network, sends the requested subscriber telephone number(Mobile Identification number MIN) to all base stations. • All the base stations broadcast the MIN over FCCs. The mobile set receives the message and checks the MIN. If it is its own then • It sends acknowledgement over RCC and also sends the Electronic Serial Number(ESN). Base station passes this information to MSC. • MSC verifies that the mobile has valid MIN,ESN pair. • MSC requests BS to move mobile to unused voice channels • BS sends message to mobile on FCC to move to specified voice channel. Mobile moves to specified channel.BS sends the message for ringing the phone. • MSC connects the mobile with calling party. • Mobile begins voice transmission on RVC and reception on FVC. • Base station begins voice transmission on RVC and reception on FVC.

  19. Mobile originated call • Mobile sends a call initiation request along with MIN and the number of the called party on RCC. Base station receives this data and relays it to MSC • MSC verifies MIN,ESN pair. Makes connection to the called party • MSC instructs BS to move mobile to a pair of voice channels • BS tells mobile to use a set of voice channels • Mobile starts conversation on voice channels

  20. Channel Assignment Strategies • Fixed Channel Assignments • Each cell is allocated a predetermined set of voice channels. • If all the channels in that cell are occupied, the call is blocked, and the subscriber does not receive service. • Variation includes a borrowing strategy: a cell is allowed to borrow channels from a neighboring cell if all its own channels are occupied. • This is supervised by the Mobile Switch Center: Connects cells to wide area network; Manages call setup; Handles mobility

  21. Channel Assignment Strategies • Dynamic Channel Assignments • Voice channels are not allocated to different cells permanently. • Each time a call request is made, the serving base station requests a channel from the MSC. • The switch then allocates a channel to the requested call based on a decision algorithm taking into account different factors: frequency re-use of candidate channel and cost factors. • Dynamic channel assignment is more complex (real time), but reduces likelihood of blocking

  22. Cell splitting • It may happen that the traffic density in a particular area increases so much that the available channels in a cell are not to take care of the traffic. • In that case the cell is split into a number of small cells called microcells. • Each of the microcell has its own base station. • These base stations have smaller antenna heights and smaller power. • Capacity increases because of additional number of channels per unit area.

  23. Cell spliting

  24. Interference and System Capacity • major limiting factor in performance of cellular radio systems • sources of interference: • other mobiles in same cell • a call in progress in a neighboring cell • other base stations operating in the same frequency band • Non-cellular system leaking energy into the cellular frequency band • effect of interference: • voice channel: cross talk • control channel: missed or blocked calls • two main types: • co-channel interference • adjacent channel interference

  25. Co-Channel Interference • cells that use the same set of frequencies are called co-channel cells. • Interference between the cells is called co-channel interference. • Co-channel reuse ratio: Q = D/R=sqrt(3N) • R: radius of cell • D: distance between nearest co-channel cells • Small Q  small cluster size N  large capacity • large Q  good transmission quality • tradeoff must be made in actual cellular design

  26. Co-channel Reuse Ratio

  27. Sectoring • The co channel interference in the system can be improved by a technique called sectoring. • In this each cell is divided into a number of sectors. • Number of sectors used are typically 3 or 6.

  28. Adjacent Channel Interference • Interference resulting from signals where are adjacent in frequency to the desired signal. • Due to imperfect receiver filters that allow nearby frequencies to leak into pass band. • Can be minimized by careful filtering and assignments, and by keeping frequency separation between channel in a given cell as large as possible, the adjacent channel interference may be reduced considerably.

  29. Handover/handoff • Reasons for handover • Moving out of range • Load balancing • Cell, BSC (base station controller), MSC (mobile switching center) • Handover scenarios • Intra-cell handover (e.g., change frequency due to narrowband interference) • Inter-cell, intra-BSC handover (e.g., movement across cells) • Inter-BSC, intra-MSC handover (e.g., movement across BSC) • Inter MSC handover (e.g., movement across MSC)

  30. Handoffs • Important task in any cellular radio system • must be performed successfully, infrequently, and imperceptible to users. • identify a new base station • channel allocation in new base station • high priority than initiation request (block new calls rather than drop existing calls)

  31. =handoff threshold - Minimum acceptable signal to maintain the call •  too small: • Insufficient time to complete handoff before call is lost • More call losses •  too large: • Too many handoffs • Burden for MSC

  32. Styles of Handoff • Network Controlled Handoff (NCHO) • in first generation cellular system, each base station constantly monitors signal strength from mobiles in its cell • based on the measures, MSC decides if handoff necessary • mobile plays passive role in process • burden on MSC • Mobile Assisted Handoff (MAHO) • present in second generation systems • mobile measures received power from surrounding base stations and report to serving base station • handoff initiated when power received from a neighboring cell exceeds current value by a certain level or for a certain period of time • faster since measurements made by mobiles, MSC don’t need monitor signal strength

  33. Prioritizing handoff • Dropping a call is more annoying than line busy • Guard channel concept • Reserve some channels for handoffs • Waste of bandwidth • But can be dynamically predicted • Queuing of handoff requests • There is a gap between time for handoff and time to drop. • Better tradeoff between dropping call probability and network traffic. • Reduce the burden for handoff • Cell dragging • Umbrella cell

  34. Umbrella Cell

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