Interference and Capacity in Cellular Systems

Tallal Elshabrawy

*The wireless environment constitutes a shared mediumInterference is the major limiting factor in performance of wireless systems in generalTypes of Interference:Co-channel interferenceAdjacent channel interferenceInterference & System Capacity

Tallal Elshabrawy

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Frequency reuse = 7Co-channel interfering cells for cell allocated with channel group AFrequency reuse implies that several cells use the same set of channelsCo-channel Interference

Tallal Elshabrawy

*Co-channel Interference, SIR & System CapacityImproving SIR1 by increasing P1 would result in a decrease in SIR2Co-channel Interfering CellsRDRBS 1BS 2MT 1MT 2P1P2H12H11H22H21BS: Base StationMT: Mobile TerminalPx: Transmitter power by base station xHxy: Small-scale & Large-scale channel between base station x and mobile terminal ySIRy: Signal-to-Interference Ratio at mobile terminal yImproving BOTH SIR1 & SIR2 is possible by increasing the distance separation between BS1 and BS2

Tallal Elshabrawy

*Di(2R)j(2R)Q: Co-channel reuse RatioDistance Separation between Base Stations

Tallal Elshabrawy

*SIR ComputationsXRDDDDDDAssume interference from first tier (ring) of co-channel interferersDi: interfering distance from ith co-channel interferenceNB No. of co-channel interfering sites

Tallal Elshabrawy

*SIR ComputationsXRDD+RD+RDD-RD-RAssume interference from first tier (ring) of co-channel interferersWorst Case SIRDi: interfering distance from ith co-channel interferenceNB No. of co-channel interfering sites

Tallal Elshabrawy

*Improving SIR means increasing cluster size, which corresponds to a decrease in system capacityDecreasing the cell size does not affect the SIR as Q=D/R remains constant. A decrease in cell size corresponds to an increase in system capacitySIR & System Capacity

Tallal Elshabrawy

*ExampleN=7Q=4.6. Worst Case SIR = 49.56 (17 dB)To design cellular system with worst performance better than 18 dB, N=9 Capacity reduction = 7/9 In First Generation cellular systems, sufficient voice quality is achieved when SIR = 18 dB

Tallal Elshabrawy

*Adjacent Channel InterferenceAdjacent channel interference results from imperfect receiver which allows nearby frequencies to leak into the passbandAdjacent channel interference can be minimized through careful filtering and channel assignments

Tallal Elshabrawy

*Improving Coverage and Capacity in Cellular Systems: Cell SplittingSubdividing a congested cell into smaller cells, each with its own base station and a corresponding reduction in antenna height and transmitter power Cell splitting Increasing system capacity by increasing the number of clusters in a given areaDecreasing Transmitter PowerThe SIR is independent of transmitted power as long as it is the same for all base stationsThe SNR must be a above a minimum threshold controlled by PrWhy not make Transmitter Power as low as possible?

Tallal Elshabrawy

*Capacity of FDMA Cellular SystemsAssume interference from first tier (ring) of co-channel interferersCapacity: Maximum number of channels or users that could be supported while maintaining a certain SIR requirement

Capacity & Interference in 3G

Tallal Elshabrawy

*SINR in CDMA Systems: Base Station-to-Mobile (Forward Link)MT1MT2MTjMTKSingle cell with K active Mobile Terminalsd2djdKd1Effective SINR because of Codes Cross-Correlation (Pseudo-Random Codes)G is called the processing gain (Tsymbol/Tchip), reflects the decline in perceived interference due to spreading and de-spreadingThe term Noise reflects noise power in the spreaded bandwidthEffective SINR because of Codes Cross-Correlation (Orothgonal Codes)

Tallal Elshabrawy

*SINR in CDMA Systems: Mobile-to-Base Station (Reverse Link)MT1MT2MTjMTKSingle cell with K active Mobile Terminalsd2djdKd1Effective SINR because of Codes Cross-Correlation (Pseudo-Random Codes)G is called the processing gain (Tsymbol/Tchip), reflects the decline in perceived interference due to spreading and de-spreadingThe term Noise reflects noise power in the spreaded bandwidthEffective SINR because of Codes Cross-Correlation (Orothgonal Codes)

Tallal Elshabrawy

*The Near-Far EffectMT1MT2d1d2The Near-Far EffectSINR2|eff is much worse than SINR1|effThe strongest received mobile signal may capture the demodulator at the base stationThe Near-Far effect is the resultant of multiple users using the same transmit power level to communicate with the base station over the same frequency and in the same time within the same cell. Mobile-to-Base Station Communication (Uplink or Reverse Link)

Tallal Elshabrawy

*Strength-Based Power ControlTo address the Near-Far EffectControl the transmission power of each mobile terminal such that it is received at the base station at an equal level SMobile-to-Base Station Communication (Uplink or Reverse Link)Strength-Based Power ControlMT1MT2MTjMTKd2djdKd1

Tallal Elshabrawy

*Capacity of CDMA Cellular SystemsAssume interference from first tier (ring) of co-channel interferersAssume Forward Link and Ignore Noise

Tallal Elshabrawy

*Diversity in 2G/3G CDMA SystemsMulti-Path resistantRAKE Receiver can collect energy spread by the small-scale channelSuitable for bursty applicationsNo need for frequency planning (frequency reuse of one)Soft blocking and soft handoff

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