EE360 – Lecture 5 Outline

1. EE360 – Lecture 5 Outline • Announcements: • Revised lecture 4 slides (minus typos) posted • Paper summary deadlines: 4/27, 5/23 • Project deadlines: Abstract 5/11, Progress report 6/6 • MAC Channels • Time Division and GSM • Direct Sequence Spread Spectrum • Frequency Hopping • Tradeoffs • User Capacity

2. Multiple Access Channels • Multiple users transmitting to a single receiver • Signals have different path gains (near-far problem) • Channel can be divided using TD, FD, or CD

3. ... Slot N Slot 2 Slot 1 Slot 3 Time Division Frames Frame (Tf) • In TDD half the slots are for upstream traffic and half for downstream traffic • Generic structure: not all frames used in all systems, and order may vary Information Message Preamble Guard Time Control, Signaling Synch Bits Guard Time Info. Bits (Training) CRC Header

4. Frame Details • Preamble contains address and sync information used by base and mobile • Guard times allow sync of receivers between different frames • Users are assigned a position in each frame (delay of Tf between bursts) • Superframes (frames of frames) may have additional control frames

5. Slot Structure • Header:guard (ramp) time for receiver synch. between slots • Synch:Used to establish bit synch (also for equalizer training) • Control:Used for handshaking, control, and supervisory messages • Info. Bits: Coded or uncoded information bits, may include pilot symbols/sequences for channel measurement and equalizer training. • Guard Time: Prevents overlap at base of slots arriving from different terminals.

6. Requirements • Equalizer requirements: adaptive equalizer must compensate for time-varying ISI. • Minimum N=t/Ts symbols for training. • For t=20msec and Rb=280 Kbps, N=6 minimum (GSM: N=26) • If Tf~Tc, need to retrain every frame (GSM: Tf=4.615 ms, Tc=1/fD=12.5ms for fD=80 Hz, retrains every frame). • Guard time requirements: must compensate for LOS propagation delay (R/c for R the cell radius) and delay spread t due to multipath (reverse link only). • No delay spread: Tg>R/c=3.3 msec for R=1Km. • Do not need guard time for LOS propagation delay if base station synchronizes to received (instead of transmitted) signal. • With delay spread t: Tg>R/c+t, but typically have a smaller guard time.

7. GSM Slots Tail 3b Data 57b Flag 1b Equal. Train 26b Flag 1b Data 57b Tail 3b Guard 8.25ms • Multiframe has 26 frames (each frame is 4.615ms), with 24 for data and 2 for control. Each call in progress assigned a control channel. • Slot time is 577ms • 26b equalizer training designed to handle delay spread up to 20 msec. (equalizer design not part of spec.) • Guard time less than maximum t. • Flag bits distinguish voice from data • Transmission rate approx. 270 Kb/s

8. Spread Spectrum MAC • Basic Features • signal spread by a code • synch. between pairs of users • compensation for near-far problem (in MAC channel) • compression and channel coding • Spreading Mechanisms • direct sequence multiplication • frequency hopping Note: spreading is 2nd modulation (after bits encoded into digital waveform, e.g. BPSK). DS spreading codes are inherently digital.

9. X X Sci(t) Sci(t) Direct Sequence Linear Modulation. (PSK,QAM) d(t) s(t) Linear Demod. Channel • Chip time Tc is N times the symbol time Ts. • Bandwidth of s(t) is N+1 times that of d(t). • Channel introduces noise, ISI, narrowband and multiple access interference. • Spreading has no effect on AWGN noise • ISI delayed by more than Tcreduced by code autocorrelation • narrowband interference reduced by spreading gain. • MAC interference reduced by code cross correlation. Synchronized SS Modulator SS Demodulator

10. BPSK Example d(t) Tb sci(t) Tc=Tb/10 s(t)

11. Narrowband Narrowband Filter Interference Original Data Signal Data Signal ISI Other with Spreading ISI Other SS Users SS Users Modulated Receiver Demodulator Data Input Filtering Spectral Properties 8C32810.117-Cimini-7/98

12. Code Properties Autocorrelation: Cross Correlation • Good codes have r(t)=d(t) and rij(t)=0 for all t. • r(t)=d(t) removes ISI • rij(t)=0 removes interference between users • Hard to get these properties simultaneously.

13. ISI Rejection • Transmitted signal: s(t)=d(t)sci(t). • Channel:h(t)=d(t)+d(t-t). • Received signal: s(t)+s(t-t) • Received signal after despreading: • In the demodulator this signal is integrated over a symbol time, so the second term becomes d(t-t)r(t). • For r(t)=d(t), all ISI is rejected.

14. MAC Interference Rejection • Received signal from all users (no multipath): • Received signal after despreading • In the demodulator this signal is integrated over a symbol time, so the second term becomes • For rij(t)=0, all MAC interference is rejected.

15. Walsh-Hadamard Codes • For N chips/bit, can get N orthogonal codes • Bandwidth expansion factor is roughly N. • Roughly equivalent to TD or FD from a capacity standpoint • Multipath destroys code orthogonality. • Used in IS-95 MAC

16. Semi-Orthogonal Codes • Maximal length feedback shift register sequences have good properties • In a long sequence, equal # of 1s and 0s. • No DC component • A run of length r chips of the same sign will occur 2-rl times in l chips. • Transitions at chip rate occur often. • The autocorrelation is small except when t is approximately zero • ISI rejection. • The cross correlation between any two sequences is small (roughly rij=G-1/2 , where G=Bss/Bs) • Minimizes MAC interference rejection

17. Frequency Hopping d(t) FM Mod Nonlinear Modulation. (FSK,MSK) s(t) FM Demod Nonlinear Demod. Channel • Spreading codes used to generate a (slow or fast) “hopping” carrier frequency for d(t). • Channel BW determined by hopping range. • Need not be continuous. • Channel introduces ISI, narrowband, and MAC interference • Hopping has no effect on AWGN • No ISI if d(t) narrowband, but channel nulls affect certain hops. • Narrowband interference affects certain hops. • MAC users collide on some hops. Sci(t) VCO Sci(t) VCO FH Modulator FH Demodulator

18. Di(f-fc) Spectral Properties 1 3 2 4 1 4 2 3 Dj(f-fc)

19. Slow vs. Fast Hopping • Fast Hopping - hop on every symbol • NB interference, MAC interference, and channel nulls affect just one symbol. • Correct using coding • Slow Hopping - hop after several symbols • NB interference, MAC interference, and channel nulls affect many symbols. • Correct using coding and interleaving if # symbols is small. • Slow hopping used in cellular to average interference from other cells

20. FH vs. DS • Linear vs. Nonlinear • DS is a linear modulation (spectrally efficient) while FH is nonlinear • Wideband interference/jamming • Raises noise spectral density, affects both techniques equally. • Narrowband interference/jamming • DS: interfering signal spread over spread BW, power reduced by spreading gain in demodulator • FH: interference affects certain hops, compensate by coding (fast hopping) or coding and interleaving (slow hopping). • Tone interference • DS: tone is wideband, raises noise floor for duration of the tone. Compensate by coding (tone duration=symbol time) or coding and interleaving (tone duration>symbol time). Similar affect as NB interference in FH. • FH: Tone affects certain hops. Compensate by coding or coding and interleaving.

21. FH vs. DS • ISI Rejection • DS: ISI reduced by code autocorrelation. • FH: ISI mostly eliminated. • MAC interference • DS: MAC interference reduced by cross correlation of spreading codes. Each additional user raises noise floor. • Overall SNR reduced • FH: MAC interference affects certain hops. Each additional user causes more hops to be affected. • More bits likely to be received in error. • Overlay systems: high-power NB interferers • Similar impact as with regular interferers • DS: Noise floor raised significantly • FH: Hops colliding with interferers are lost • Can notch out interfering signals

22. Evolution of a Scientist turned Entrepreneur • “Spread spectrum communications - myths and realities,” A.J. Viterbi, IEEE Comm. Magazine, May ‘79 (Linkabit 5 years old - A TDMA company). • “When not to spread spectrum - a sequel,” A.J. Viterbi, IEEE Comm. Magazine, April 1985 (Linkabit sold to M/A-Com in 1982) • “Wireless digital communications: a view based on three lessons learned,” A.J. Viterbi, IEEE Comm. Magazine, Sept.’91. (Qualcomm CDMA adopted as standard).

23. Myths and Realities • Myth 1: Redundancy in error correction codes spreads signal bandwidth and thereby reduces processing gain • Reality: Effective processing gain increased by coding by considering symbol rate and energy • Reality today: coded modulation more efficient even without symbol argument. But tradeoffs between coding and spreading an open issue. • Myth 2: Error correction codes only good against uniform interference • Reality: Not true when coding combined with spread spectrum, since SS averages interference. • Reality today: Unchanged. • Myth 3: Interleaving destroys memory which can be used to correct errors, hence interleaving is bad • Reality: Memory preserved by soft-decisions even with an interleaver • Reality today: Unchanged, but interleavers may require excessive delays for some applications.

24. Myth 4: Direct sequence twice as efficient as frequency hopping • Myth=Reality. Argument is that DS is coherent and that accounts for 3dB difference. Analysis shows that higher level signaling alphabets does not help FH performance with partial band jammer. • Reality today: A true efficiency tradeoff of FH versus DS has not been done under more general assumptions. FH typically used to average interference. Appealing when continuous spreading BW not available.

25. When not to Spread Spectrum - A Sequel • Conclusion 1: When power is limited, don’t contribute to the noise by having users jam one another. • Conclusion 2: Network control is a small price to pay for the efficiency afforded by TDMA or FDMA • Power control is a big control requirement. • Conclusion 3: Interference from adjacent cells affects the efficiency of TDMA or FDMA less severely than in CDMA. • Conclusion 4: Treating bandwidth as an inexpensive commodity and processing as an expensive commodity is bucking current technology trends. • Caveat: Application was small earth terminals for commercial satellits.

26. Three Lessons Learned • Never discard information prematurely • Compression can be separated from channel transmission with no loss of optimality • Gaussian noise is worst case. Optimal signal in presence of Gaussian noise has Gaussian distribution. So self-interference should be designed as Gaussian.

27. Realities • Never discard information prematurely • Use soft-decisions and sequence detectors, if complexity okay. • Compression can be separated from channel transmission • For time-invariant single-user channels only. • Self-interference should be designed as Gaussian • Based on Viterbi’s argument, this represents a saddle (not optimal) point. • If the self-interference is not treated as interference, then Gaussian signaling is suboptimal (by Shannon theory).

28. MAC Capacity • User Capacity • How many users can be accommodated in the channel given performance specs. • Assumes identical users and white noise model for interference • Shannon Capacity Region • Upper bound on rate vector that all users can achieve simultaneously • No complexity or delay constraints. • Optimal signaling and reception (unless constraints are added) • Asymptotically small error probabilty. • Signals from other users not treated as interference

29. User Capacity • Applicable to CDMA, since TDMA and FDMA have fixed capacity (# of channels). • S/(N+I(M)) determined based on the total number of users M and the system model. • Can be deterministic or random (fading). • Interference I(M) modeled as AWGN • Based on the modulation, coding, channel model, etc., we find the probability of bit error Pe=f[S/(N+I(M))] • For a given performance Pewe invert the above expression to get the maximum possible M. • Often set N=0 to simplify inversion, implies an interference-limited system.

30. Probability of Error • Coherent BPSK: for m users, and a spreading gain G: • m is typically random. For L total users each with probability p of active transmission and voice activity factor a: Note that Pe is concave in m

31. Pe Approximation • By concavity of Peand Jensen’s inequality: Use RHS as approximation for Pe ``Spread spectrum for mobile communications”, Pickholtz, Milstein, Schilling

32. Effective Energy/Symbol • M is average number of active users. • r is the code rate • K is the out-of-cell interference ratio (equals zero for a purely MAC channel) • a is the voice activity factor • N is the number of chips per symbol • Factor of 2/3 assumes rectangular pulses, will decrease for other shapes. • Assumes no ISI, flat-fading, or diversity gain.

33. Required Es/N0 • Target Pe • Invert target Pe to get required Es/N0 • Example: DPSK Often cannot get greqd in closed form: Must use numerical techniques or obtain from BER curve.

34. User Capacity • Total number of users the MAC channel can support: • A rougher approximation Channel coding and interference mitigation increase user capacity

35. G 2 4 5 6 8 SS vs. Narrowband • BPSK at a BER of 10-4 requires Es/N07.4dB • Consider a two-user (M=2) DSSS system: S/I (dB) 3 6 7 7.8 9 7.4 Two-user DSSS system requires spreading gain of 5-6 to get desired BER, TD system could fit 5-6 users in this bandwidth Argument for DSSS based on frequency reuse and soft capacity