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Cellular System Capacity

Cellular System Capacity. Maximum number of users a cellular system can support in any cell. Can be defined for any system. Typically assumes symmetric data rates, cells, propagation, and mobility. Depends on the user specifications and radio design data rate, BER, modulation, coding, etc.

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Cellular System Capacity

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  1. Cellular System Capacity • Maximum number of users a cellular system can support in any cell. • Can be defined for any system. • Typically assumes symmetric data rates, cells, propagation, and mobility. • Depends on the user specifications and radio design • data rate, BER, modulation, coding, etc.

  2. Which link dictates capacity? • Reverse link • Noncoherent reception • Independent fading of all users • Requires power control • Forward link • Coherent demodulation using pilot carrier. • Synchronous combining of multipath. • Conclusion: reverse link is the limiting factor in capacity. • Claim: other cell interference will tend to equalize performance in each direction.

  3. CDMA Cellular Capacity • Single-Cell System • Similar to MAC user capacity • G=W/R is processing gain (W is bandwidth, R is data rate) • h is interference plus noise • Assumes power control • Performance improvement through sectorization and voice activity 8C32810.44-Cimini-7/98

  4. Sectorization • Base station omni antenna is divided into M sectors. • Users in other sectors do not cause interference. • Number of users per sector is Ns=N/M (reduces interference by M). • Requires handoff between sectors at the base station

  5. Voice Activity • Suppress signal when voice user not active. • Voice activity a=.35-.4 (reduces interference by 60-65%). • Requires resynchronization for every talk spurt. • Higher probability of dropping users.

  6. New Capacity (per cell) • Capacity increased proportional to the number of sectors and inversely proportional to the voice activity (M/a typically around 8). • Claim: Thus, CDMA is competitive with FD/TD for a single-cell • Does not include impact of sectorization on out-of-cell interference.

  7. Multicell System • Codes reused in every cell. • No power control in forward link • Interference from adjacent cells can be very strong. • Power control in reverse link • All users within a cell have same received signal strength • Interference from other cells have variable power • Fast fading (interference and signal) neglected (S/I statistics). • The interferer’s transmit power depends on distance to his base station. • Received power at desired base depends on distance to base, propagation, and the interferer’s transmit power.

  8. Reverse Link Interference • Total path loss: propagation (d-4 falloff) and log-normal shadowing. • x is Gaussian, 8 dB STD. • Instantaneous interference power • rm is distance to interferer’s base • r0 is distance to desired base • xmis shadowing to interferer’s base • x0is shadowing to desired base • S is received power with power control • Power less than 1 since otherwise would handoff to desired base

  9. Average interference power • Ais the cell area. • r is the user density (r=2Ns/Sqrt[3]) • g is voice activity term (equals 1 w.p. a, 0 w.p. 1-a) • Must be integrated against distribution of m, r0, rm, x0, xm • Simplify distribution of m by assuming minimum distance. • r0, rm uniformly distributed. • Claim: I Gaussian since it’s a functional of a 2D white random process

  10. Mean and Variance • Numerical integration leads to E(I/S)=.247Ns • Second Moment: • Assumes autocorrelation of shadowing is a delta function. • Numerical integration leads to Var(I/S)=.078Ns • Calculations assume 8dB STD. • Total interference distribution ci binomial r.v. with probability a

  11. Capacity Calculation • Calculate probability Eb/N0 below target (BER exceeds target) based on Ns and these statistics.. • Compute outage probability as a function of Ns. • Assumes target Eb/N0 =5.. • d=30 • Similar calculation for uplink

  12. An Alternate Approach • Simulation approach • Includes three rings of interfering cells • Capacity for TDMA and CDMA compared • Similar assumptions about voice activity and sectorization • TDMA assumes FH with dynamic channel allocation

  13. Capacity degradation • Voice activity changed from .375 to .5, -30% change • Path-loss changed from 4 to 3, -20% change • Multipath fading added, -45% change • Handoff margin changed from 0 to 6 dB, -40% change • Power control error changed from 0 to 1 dB, -35% change

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