Midterm Review Midterm only covers material from lectures and HWs - PowerPoint Presentation

1. Midterm ReviewMidterm only covers material from lectures and HWsOverview of Wireless Systems:Nothing from Chapter 1 is on the MT Signal Propagation and Channel ModelsChapter 2.1-2.4, 2.6-2.10Modulation and Performance MetricsChapter 3.1,3.2.1-3.2.2, 3.3Fundamental Capacity LimitsChapter 4Impact of Channel on Performance Chapter 6 Diversity TechniquesChapter 7.1,7.2.1-7.2.2,7.2.4,7.3.1,7.4.1

2. Future Wireless NetworksWireless Internet accessNth generation CellularWireless Ad Hoc NetworksSensor Networks Wireless EntertainmentSmart Homes/SpacesAutomated HighwaysAll this and more…Ubiquitous Communication Among People and DevicesHard Delay/Energy ConstraintsHard Rate RequirementsNot on MT

3. Design ChallengesWireless channels are a difficult and capacity-limited broadcast communications mediumTraffic patterns, user locations, and network conditions are constantly changingApplications are heterogeneous with hard constraints that must be met by the networkEnergy, delay, and rate constraints change design principles across all layers of the protocol stackNot on MT

4. Current/Futre Wireless SystemsCurrent Systems4G Cellular Systems802.11b/a/n/ac Wireless LANsSatellite SystemsPaging SystemsBluetoothZigbee radiosEmerging Systems (Can cover in bonus lecture)Ad hoc/mesh wireless networksCognitive radio networksWireless sensor networksEnergy-harvesting radiosDistributed control networksCommunications/SP in Health, Bio-medicine, and NeuroscienceNot on MT

5. Signal PropagationPath LossFree space, 2-path,…Simplified modelShadowingdB value is GaussianFind path loss exponent and shadow STD by curve fittingMultipathRay tracingStatistical modeldPr/Ptd=vt

6. Outage Probabilityand Cell Coverage AreaPath loss: circular cellsPath loss+shadowing: amoeba cellsTradeoff between coverage and interferenceOutage probabilityProbability received power below given minimumCell coverage area% of cell locations at desired powerIncreases as shadowing variance decreasesLarge % indicates interference to other cells

7. Statistical Multipath ModelRandom # of multipath components, each with varying amplitude, phase, doppler, and delayLeads to time-varying channel impulse responseNarrowband channelNo signal distortion, just a complex amplitude gainSignal amplitude varies randomly (Rayleigh, Ricean, Nakagami).2nd order statistics (Bessel function), Average fade duration

8. Wideband ChannelsIndividual multipath components resolvableTrue when time difference between components exceeds signal bandwidth Scattering functionYields delay spread/coherence BW (st~1/Bc)Yields Doppler spread/coherence time (Bd~1/Tc)tWidebandtrs(t,r)=FDt[Ac(t,Dt)]Delay Power SpectrumDoppler Power SpectrumfAc(f)0Bc

9. Capacity of Flat Fading ChannelsChannel CapacityMaximum data rate that can be transmitted over a channel with arbitrarily small errorCapacity of AWGN Channel: Blog2[1+g] bpsg=Pr/(N0B) is the receiver SNRCapacity of Flat-Fading ChannelsNothing known: capacity typically zeroFading Statistics Known (few results)Fading Known at RX (average capacity)

10. Capacity in Flat-Fading: g known at TX/RXOptimal Rate and Power AdaptationThe instantaneous power/rate only depend on p(g) through g01gg0gWaterfilling

11. Channel InversionFading inverted to maintain constant SNRSimplifies design (fixed rate)Greatly reduces capacityCapacity is zero in Rayleigh fadingTruncated inversionInvert channel above cutoff fade depthConstant SNR (fixed rate) above cutoffCutoff greatly increases capacityClose to optimal

12. Frequency Selective Fading ChannelsFor time-invariant channels, capacity achieved by water-filling in frequencyCapacity of time-varying channel unknownApproximate by dividing into subbandsEach subband has width BcIndependent fading in each subbandCapacity is the sum of subband capacitiesBcfP1/|H(f)|2

13. Linear Modulation in AWGN: MPSK and MQAMML detection induces decision regionsExample: 8PSKPs depends on# of nearest neighborsMinimum distance dmin (depends on gs)Approximate expression dmin

14. Linear Modulation in FadingIn fading gs and therefore Ps randomMetrics: outage, average Ps , combined outage and average.PsPs(target)OutagePsTsTs

15. Moment Generating Function ApproachSimplifies average Ps calculationUses alternate Q function representationPs reduces to MGF of gs distributionClosed form or simple numerical calculation for general fading distributionsFading greatly increases average Ps .

16. Doppler EffectsHigh doppler causes channel phase to decorrelate between symbolsLeads to an irreducible error floor for differential modulationIncreasing power does not reduce errorError floor depends on fDTb as

17. Delay spread exceeding a symbol time causes ISI (self interference).ISI leads to irreducible error floor: Increasing signal power increases ISI powerISI imposes data rate constraint: Ts>>Tm (Rs<<Bc) Delay Spread (ISI) Effects0Tm13452TsDelay Tm

18. DiversitySend bits over independent fading pathsCombine paths to mitigate fading effects.Independent fading pathsSpace, time, frequency, polarization diversity.Combining techniquesSelection combining (SC)Maximal ratio combining (MRC)Can have diversity at TX or RXIn TX diversity, weights constrained by TX power

19. Selection CombiningSelects the path with the highest gainCombiner SNR is the maximum of the branch SNRs.CDF easy to obtain (Pip(gi<gthr)), pdf found by differentiating the CDFPout obtained from CDF. Average Ps typically found numericallyDiminishing returns with number of antennas.Can get up to about 20 dB of gain.

20. MRC and its PerformanceWith MRC, gS=gi for branch SNRs giOptimal technique to maximize output SNRYields 20-40 dB performance gainsDistribution of gS hard to obtainStandard average BER calculationHard to obtain in closed formIntegral often divergesMGF Approach:TX diversity gain with CSI same as RX diversitysss

21. Main PointsWireless channels introduce path-loss, shadowing and multipath fadingShadowing introduced outageFlat-fading causes large power fluctuationsISI causes self-interferencePerformance of digital communications in wireless channels randomCharacterized by outage probability and average probability of error in flat-fadingCharacterized by irreducible error floors in ISI/DopplerNeed mechanisms to compensate for multipathDiversity compensates for effects of flat fading.