Interference

ECE-T512 – Wireless Systems Interference/Noise

Interference/Noise • Interference is the major limiting factor in cellular radio performance – particularly urban networks – Voice channels – Cross-talk and diminished call quality – Control channels – Missed/blocked calls

• Co-channel interference (CCI) is introduced when same carrier frequency is used in different cells. • Adjacent channel interference (ACI) occurs when neighboring cells use spectrally adjacent carrier frequencies that “spill-over” into the band of interest – occurs due to imperfect receive filters / image frequency rejection

Interference/Noise
= Carrier to noise ratio  = Carrier to interference ratio Thresholds must be exceeded to prevent outage: – Thermal noise outage = ON = P(
<
TH) – CCI outage = OI = P( < TH)

Interference/Noise Thresholds
TH and TH depend on: • Modulation/coding scheme • Receiver structure • Link quality measure • Propagation environment • Mobile velocity

Outage Probability O = Outage Probability = P(
<
TH or  < TH) General trends: • Fast mobile users  Path loss/Shadowing dominant in outage probability calculations • Slow mobile users  Multipath Fading dominant in outage probability calculations • Lightly loaded systems  Noise dominant • Highly loaded systems  Interference dominant

Interference and Cellular System Capacity Co-channel cells - cells in a given coverage area that use the same set of frequencies --- interference between signals from these cells is CCI Thermal noise can be overcome by increasing carrier power -- SNR increased CCI cannot be combated in this way - increasing carrier power also increases interference power seen by cochannel cells Reducing CCI can be accomplished through physical separation of co-channel cells, specifically, by increasing: Q = D/R = sqrt(3N) – N = cluster size – D = co-channel re-use distance – R = cell radius