1 / 34

Course Summary

Course Summary. Overview/history of wireless communications (Ch. 1) Signal Propagation and Channel Models (Ch. 2 + 3) Fundamental Capacity Limits (Ch. 4) Modulation and Performance Metrics (Ch. 5) Impact of Channel on Performance (Ch. 6) Adaptive Modulation (Ch. 9) Diversity (Ch. 7)

aitana
Download Presentation

Course Summary

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Course Summary • Overview/history of wireless communications (Ch. 1) • Signal Propagation and Channel Models (Ch. 2 + 3) • Fundamental Capacity Limits (Ch. 4) • Modulation and Performance Metrics (Ch. 5) • Impact of Channel on Performance (Ch. 6) • Adaptive Modulation (Ch. 9) • Diversity (Ch. 7) • Spread Spectrum (Ch. 13) • Cellular Networks (Ch. 15)

  2. Future Wireless Networks:The Vision Ubiquitous Communication Among People and Devices Wireless Internet access Nth generation Cellular Wireless Ad Hoc Networks Sensor Networks Wireless Entertainment Smart Homes/Spaces Automated Highways All this and more… • Hard Delay/Energy Constraints • Hard Rate Requirements • +++

  3. “Mega-themes” of TTT4160-1 • The wireless vision poses great technical challenges • The wireless channel greatly impedes performance • Low fundamental capacity. • Channel is randomly time-varying • ISI and other interference must be compensated for • ... • Hard to provide performance guarantees (needed for multimedia!). • We can compensate for flat fading using diversity or adaptation. • (MIMO channels promise a great capacity increase.) • A plethora of ISI compensation techniques exist • Various tradeoffs in performance, complexity, and implementation.

  4. Design Challenges, cont’d • Wireless channels are a difficult and capacity-limited broadcast communications medium • Traffic patterns, user locations, and network conditions are constantly changing • Applications are heterogeneous - with hard constraints that must be met by the network(s) • Energy, delay, and rate constraints change design principles across all layers of the protocol stack (cross-layer design)

  5. d Pr/Pt d=vt Signal Propagation: Main effects • Path Loss • Shadowing • Multipath

  6. Statistical Multipath Model • Random # of multipath components, each with varying amplitude, phase, doppler, and delay • Narrowband channel (signal BW smaller than coherence BW): FLAT fading • Signal amplitude varies randomly (complex Gaussian). • Characterized by 2nd order statistics (Bessel function), average fade duration, etc. • Wideband channel: FREQUENCY-SELECTIVE • Characterized in general by channel scattering function (simplified: Bc BD)

  7. Modulation Considerations • We want: high rates, high spectral efficiency, high power efficiency, robustness to channel variations, cheap implementations... Trade-off required! • Linear Modulation (MPAM, MPSK, MQAM) • Information encoded in amplitude/phase • More spectrally efficient than nonlinear • Easier to adapt to channel conditions. • Issues: differential encoding, pulse shaping, bit mapping. • Nonlinear modulation (FSK) • Information encoded in frequency • More robust to channel and amplifier nonlinearities

  8. dmin Linear Modulation in AWGN • ML detection induces decision regions • Example: 8PSK • Ps (symbol error rate) depends on • # of nearest neighbors • Minimum distance dmin (depends on gs) • Approximate expression: • M is # of nearest neighbors; M relates dmin and average symbol energy.

  9. Ps Ts Linear Modulation in Fading • In fading gs - and therefore Ps - is random • Metrics: outage probability, average Ps , or combined outage and average. Ts Ps Outage Ps(target)

  10. Moment Generating Function (MGF) Approach • Simplifies average Ps calculation • Uses alternate Q function representation • Ps reduces to MGF of gs-distribution • Closed form, or simple numerical calculation for general fading distributions • In general: Fading greatly increases averagePs .

  11. Doppler Effects • High Doppler causes channel phase to decorrelate between symbols • Leads to an irreducible error floor for differential modulation • Increasing power does not reduce error • Error floor depends on BDTs product (higher the larger it is)

  12. ISI Effects • Delay spread exceeding one symbol time causes ISI (self-interference). • ISI leads to irreducible error floor • Increasing signal power increases ISI power • ISI requires that Ts>>Tm (Rs<<Bc) Tm 0

  13. Capacity of Flat Fading Channels • Three cases • Fading statistics known • Fade value known at receiver • Fade value known at receiver and transmitter • Optimal Adaptation • Vary rate and power relative to channel • Optimal power adaptation is water-filling • Exceeds AWGN channel capacity at low SNRs • Suboptimal techniques come close to capacity

  14. One of the M(g) Points log2 M(g) Bits To Channel M(g)-QAM Modulator Power: S(g) Point Selector Uncoded Data Bits Delay g(t) g(t) 16-QAM 4-QAM BSPK Variable-Rate Variable-Power MQAM Goal: Optimize S(g) and M(g) to maximize EM(g)

  15. gk g Optimal Adaptive Scheme • Power Water-Filling • Spectral Efficiency g Equals Shannon capacity with an effective power loss of K.

  16. Practical Adaptation Constraints • Constellation restriction • Constant power restriction • Constellation updates. • Estimation error. • Estimation delay. • Lead to practical adaptive modulation schemes (Ch. 9)

  17. Diversity • Send bits over independent fading paths • Combine paths to mitigate fading effects. • Independent fading paths - how to create? • Space, time, frequency, polarization diversity. • Combining techniques • Selection combining (SC) • Equal gain combining (EGC) • Maximal ratio combining (MRC) • ...

  18. Diversity Performance • Maximal Ratio Combining (MRC) • Optimal technique (maximizes output SNR) • Combiner SNR is the sum of the branch SNRs. • Distribution of SNR hard to obtain. • Can use MGF approach for simplified analysis. • Exhibits 10-40 dB gains in Rayleigh fading. • Selection Combining (SC) • Combiner SNR is the maximum of the branch SNRs. • Diminishing returns with # of antennas. • CDF easy to obtain, pdf found by differentiating. • Can get up to about 20 dB of gain.

  19. Spread Spectrum • Signal occupies channel bandwidth much larger than actual signal bandwidth • Two main types: • Direct Sequence Spread Spectrum (DSSS) • Frequency Hopping Spread Spectrum • Focus on DSSS here • Basis for CDMA

  20. Tc Direct Sequence Spread Spectrum (DSSS) • Bit sequence modulated by chip sequence • Spreads bandwidth by large factor (K) • Despread by multiplying by sc(t) again (sc(t)=1) • Mitigates ISI and narrowband interference • ISI mitigation a function of code autocorrelation • Must synchronize to incoming signal S(f) s(t) sc(t) Sc(f) S(f)*Sc(f) 1/Tb 1/Tc Tb=KTc 2

  21. x x x RAKE Receiver • Multibranch receiver • Branches synchronized to different MP components • These components can be coherently combined • Use SC, MRC, or EGC Demod sc(t) y(t) ^ dk Diversity Combiner Demod sc(t-iTc) Demod sc(t-NTc)

  22. CDMA: Multiple Access SS • Interference between users mitigated by code cross correlation • In downlink, signal and interference have same received power • In uplink, “close” users drown out “far” users (near-far problem) a a

  23. Code Space Code Space Code Space Time Time Time Frequency Frequency Frequency Bandwidth Sharing in general • FDMA • TDMA • CDMA • (Hybrid Schemes) 7C29822.033-Cimini-9/97

  24. Multiuser Detection • In all CDMA systems and cellular systems in general, users interfere with each other. • In most of these systems the interference is treated as noise. • Systems become interference-limited • Often uses complex mechanisms to minimize impact of interference (power control, smart antennas, etc.) • Multiuser detection exploits the fact that the structure of the interference is known • Interference can be detected and subtracted out • Must however have a good estimate of the interference ...!

  25. BASE STATION Cellular System Design • Frequencies, timeslots, or codes reused at spatially-separate locations • Efficient system design is interference-limited • Base stations perform centralized control functions • Call setup, handoff, routing, adaptive schemes, etc.

  26. Design Issues • Reuse distance • Cell size • Channel assignment strategy • Interference management • Power adaptation • Smart antennas • Multiuser detection • Dynamic resource allocation 8C32810.44-Cimini-7/98

  27. BASE STATION Dynamic Resource AllocationAllocate resources as user and network conditions change • Resources: • Channels • Bandwidth • Power • Rate • Base stations • Access • Optimization criteria • Minimize blocking (voice only systems) • Maximize number of users • Maximize “revenue” • Subject to some minimum performance for each user

  28. Higher LayerNetworking Issues NETWORK ISSUES • Architecture • Mobility Management • Identification/authentication • Routing • Handoff • Control • Reliability and Quality-of-Service 8C32810.53-Cimini-7/98

  29. A final return to QoS... Wireless Internet access Nth generation Cellular Wireless Ad Hoc Networks Sensor Networks Wireless Entertainment Smart Homes/Spaces Automated Highways All this and more… Applications have hard delay constraints, rate requirements, and energy constraints that must be met These requirements are collectively called QoS

  30. Challenges to meeting QoS • No single layer in the protocol stack can guarantee QoS: cross-layer design needed • It is impossible to guarantee that hard constraints are always met • Average constraints aren’t necessarily good metrics (e.g. in very slow fading, non-ergodic conditions).

  31. Cross-layer Design (or “IET meets ITEM”) • Application • Network • Access • Link • Hardware Delay Constraints Rate Requirements Energy Constraints Mobility Optimize and adapt across design layers Provide robustness to uncertainty Schedule dedicated resources

  32. The Exam: Practical stuff • Time: Saturday, June 2nd, 09.00 - 13.00 • Tools/aids allowed: Calculator only • List/sheet containing important/relevant formulas will be provided as part of the exam • Mostly: Expect same “style” of questions as in exercises

  33. Exam preparations • For exercises, and solutions to exercises: Consult course web page. • For questions to exercises: Consult the teaching assistant, Changmian Wang (Sébastien de la Kethulle has graduated and has a new job) • For questions to book: Consult Changmian Wang or Geir Øien (in that order ;-) ). • For questions to lecture notes: Consult Geir Øien or Changmian Wang (in that order...).

  34. Course curriculum • All curriculum can be found in course textbook, ”Wireless Communications” by Andrea Goldsmith • See list of chapters/sections in separate handout (can also be found on web page) • In general ”lectures and exercises define the curriculum” • Details not covered either in lectures or exercises will not be emphasized at exam!

More Related