EECS 380: Wireless Communications Multiple Access

Transcription

1 EECS 380: Wireless Communications Multiple Access Michael L. Honig Northwestern University May 2013

2 Outline Finish diversity, error control coding Multiple Access techniques FDMA, TDMA CDMA (3G, b) OFDMA (4G, WiMax)

3 Diversity Idea: Obtain multiple independent copies of the received signal. Improves the chances that at least one is not faded. Macroscopic (space): copies of signal are received over distances spanning many wavelengths. Microscopic (space): copies of signal are received over distances spanning a fraction of a wavelength Different types

4 Macroscopic Diversity MSO Copies of signal are separated by many wavelengths.

5 Microscopic Space Diversity Antenna 2 s2 Antenna 1 s1 Want signals s1 and s2 to experience independent fading (why?). distance between antennas should be ½ wavelength. Ex: 900 MHz, λ = c/f 1/3 meter 2 GHz, λ 0.15 meter

6 Frequency Diversity channel gain signal power (wideband) coherence bandwidth B c Frequencies far outside the coherence bandwidth are affected differently by multipath. f 1 f 2 frequency Wideband signals exploit frequency diversity. Spreading power across many coherence bands reduces the chances of severe fading. Wideband signals are distorted by the channel fading (distortion causes intersymbol interference).

7 Time Diversity: Error Control Coding channel source bits errors noise, fading, interference introduces errors How can we improve reliability (control errors)? According to Shannon, we have to add redundancy: Add redundant bits to the source stream. Retransmit.

8 Path Diversity τ 1 τ 2 received signal adjust phase + Delay τ 2 - τ 1 adjust phase Called a RAKE receiver, since it rakes up (combines) the energy in the different paths. Can substantially increase the S/I. An important component of CDMA receivers. Each branch in the Rake is typically referred to as a finger.

9 Multiuser Diversity

10 Multiuser Diversity d 1 d 2 > d 1 Received power user 1 user 2 transmit to user 2 transmit to user 1 transmit to user 2 transmit to user 1 time The BST can choose to transmit to the user with the best channel. Exploits variations in signal strength across users.

11 Selection Diversity Antenna 2 s2 Antenna 1 s1 Received power antenna 1 antenna 2 select ant. 2 select ant. 1 select ant. 2 select ant. 1 time Choose the best signal (highest instantaneous SNR). Easy to implement (antenna switch).

12 Benefit of Selection Diversity (Example) Suppose that the signal on each antenna experiences independent Rayleigh fading. Determine the probability that the received signal is faded: Recall Rayleigh fading formula: Probability that the signal power is less than a x P 0 (average received power) = 1 e -a Hence the probability that the signals on both antennas are less than a x P 0 = (1 e -a ) 2 Without diversity, probability of a signal fade = 1 e -1 = 0.63 With 2-branch diversity, probability of a signal fade = = 0.39

13 Benefit of Selection Diversity (cont.) Suppose that there are N copies of the signal (e.g., N antennas, paths, coherence bands, etc.) Probability that the signal power is less than a x P 0 (average received power) = 1 e -a Hence the probability that all N signals are less than a x P 0 = (1 e -a ) N Without diversity, probability of a signal fade = 1 e -1 = 0.63 With 4-branch diversity, probability of a signal fade = = 0.16 Without diversity, Prob(signal is faded by more than 10 db) = 1 e With diversity this probability is (1 e -0.1 ) !

14 Coherent Combining S1 (ant. 1) S2 (ant. 2) adjust phase adjust phase + Coherent means that the phases of the two signals are estimated at the receiver and aligned. Performs better than selection combining (why?). Example: RAKE receiver Can weight the combined signals to maximize the received SNR. (How should the weights depend on the signal levels?)

15 Probability of Error with Fading add diversity Diversity can transform a fading channel back to a non-fading (additive noise) channel. Essential for mobile wireless communications.

16 The Multiple Access Problem How can multiple mobiles access (communicate with) the same base station? Frequency-Division (AMPS) Time-Division (GSM) Code-Division (3G, Bluetooth) Direct Sequence/Frequency-Hopped Orthogonal Frequency Division Multiple Access (OFDMA) Random Access (Wireless Data)

17 Duplexing (Two-way calls) Frequency-Division Duplex (FDD) Channel 1 Channel 2 Time-Division Duplex (TDD) Time slot (frame) 1 Time slot (frame) 2

18 Combinations FDMA/FDD (AMPS) TDMA/FDD (GSM) TDMA/TDD (IS-136 or 2G in the U.S.) CDMA/FDD (IS-95, CDMA2000) CDMA/TDD (3G/UMTS) Frequency-Hopped CDMA/TDD (Bluetooth) OFDMA/TDD and FDD (WiMax, 4G)

19 The Multiple Access Problem How can multiple mobiles access (communicate with) the same base station? Frequency-Division (AMPS) Time-Division (IS-136, GSM) Code-Division (IS-95, 3G) Direct Sequence/Frequency-Hopped Orthogonal Frequency Division Multiple Access (OFDMA) Random Access (Wireless Data)

20 uplink Cellular Spectrum (50 MHz) A* A B A* B* downlink AMPS (1G): 30 khz Channels 416 FDD Channels (requires 12.5 MHz): 395 FDD voice channels 21 FDD control channels

21 Properties of FDMA Can be analog or digital (AMPS is analog). Narrowband: channel contained within coherence bandwidth undergoes flat fading. Low capacity Best for circuit-switched (dedicated) connections. Requires guard channels for adjacent channel interference.

22 The Multiple Access Problem How can multiple mobiles access (communicate with) the same base station? Frequency-Division (AMPS) Time-Division (GSM) Code-Division (IS-95, 3G) Direct Sequence/Frequency-Hopped Orthogonal Frequency Division Multiple Access (OFDMA) Random Access (Wireless Data)

23 Time Division Multiple Access Channel f 1 Frame N time slots H: Frame Header H N H Channel f 2 H 1 2 Time slots... N H Channel f K H N H

24 Time Slot Frame H N time slots N H H: Frame Header Preamble and synch Time Slot Data to or from user K + control information Guard time

25 TDMA/Time-Division Duplex H H { { Uplink time slots Downlink time slots

26 Properties of TDMA Data transmission occurs in bursts. Must ensure small delays for speech. High peak to average power on reverse link. Can measure signal strength in idle time slots (e.g., for handoff). Can assign multiple time slots for higher data rates. Significant overhead/complexity for synchronization. Guard times needed between time slots for delay spread. May require an equalizer to mitigate intersymbol interference.

27 Global System for Mobile Communications (GSM) Originated in Europe Main objective: allow roaming across countries Incompatible with 1G systems More than an air-interface standard: specifies wireline interfaces/functions TDMA/FDMA, FDD Dynamic frequency assignment 200 khz channels kbps

28 GSM Frame Structure bits µs Frame TS TS TS TS TS TS TS TS ms T n : nth TCH frame S: Slow Associated Control Channel frame I: Idle frame T 0 T 1 T 2... T 10 T 11 T 12 S T 13 T 14 T 15 T 22 T 23 T 24 I/S 120 ms Speech Multiframe = 26 TDMA frames 200 khz FDD channels divided into 8 time slots per frame Total number of available channels = (12.5 MHz 2 X Guard Band)/200 khz 100 khz guard bands è 62 channels Total number of traffic channels = 8 X 62 = 496 Channel data rate = kbps Without overhead, data rate/user = 24.7 kbps

29 GSM Frame Structure bits µs Frame TS TS TS TS TS TS TS TS ms T n : nth TCH frame S: Slow Associated Control Channel frame I: Idle frame T 0 T 1 T 2... T 10 T 11 T 12 S T 13 T 14 T 15 T 22 T 23 T 24 I/S 120 ms Speech Multiframe = 26 TDMA frames 200 khz FDD channels divided into 8 time slots per frame Total number of available channels = (12.5 MHz 2 X Guard Band)/200 khz 100 khz guard bands è 62 channels Total number of traffic channels = 8 X 62 = 496 Channel data rate = kbps Without overhead, data rate/user = 24.7 kbps

30 GSM Frame Structure bits µs Frame TS TS TS TS TS TS TS TS ms T n : nth TCH frame S: Slow Associated Control Channel frame I: Idle frame T 0 T 1 T 2... T 10 T 11 T 12 S T 13 T 14 T 15 T 22 T 23 T 24 I/S 120 ms Speech Multiframe = 26 TDMA frames 200 khz FDD channels divided into 8 time slots per frame Total number of available channels = (12.5 MHz 2 X Guard Band)/200 khz 100 khz guard bands è 62 channels Total number of traffic channels = 8 X 62 = 496 Channel data rate = kbps Without overhead, data rate/user = 24.7 kbps

31 6.12 s GSM Time Slots Hyperframe = 2048 superframes lasts ~3 hrs 28 min 54 sec Superframe 120 ms Multiframe Frame Time slot ms µs Normal Burst Traffic Channel (TCH) 148 bits/time slot 114 coded information bits Frame efficiency 74% (total bits overhead bits)/(total bits)

32 GSM Capacity Total bandwidth = 12.5 MHz, 200 khz channels è 62 channels With cell cluster size N=3 (typical), capacity is (62/3) x 8 ~ 165 users/cell

33 The Multiple Access Problem How can multiple mobiles access (communicate with) the same base station? Use different frequencies (FDMA) Use different time slots (TDMA) Use different pulse shapes (CDMA)

34 Code Division Multiple Access Users transmit simultaneously over the same frequency band Performance limited by interference

35 Two-User Example User 1: T/2 T time bits: 1 s 1 (t) T 2T 3T 4T 5T User 2: T/2 T time s 2 (t) T 2T 3T 4T 5T received signal r(t)= s 1 (t)+s 2 (t) 2 How to recover each users bits? -2 T 2T 3T 4T 5T

36 Chip Sequence chips User 2: T/2 T time User 2 s chip sequence (1, -1) is called a signature. chip duration T c symbol duration T=2T c s 2 (t) bits: T 2T 3T 4T 5T Transmitted chips:

37 Chip Sequence chips User 1: User 1 s signature is (1, 1). T/2 T time chip duration T c symbol duration T=2T c s 1 (t) 1-1 bits: T 2T 3T 4T 5T Transmitted chips:

38 Two-User Example s 1 (t) -1 T 2T 3T 4T 5T Transmitted chips: s 2 (t) T 2T 3T 4T 5T Transmitted chips: r(t)= s 1 (t)+s 2 (t) 2-2 T 2T 3T 4T 5T Received chips:

39 Correlation Given two sequences: a 1, a 2, a 3,, a N b 1, b 2, b 3,, b N The correlation between the sequences is defined as: (a 1 x b 1 ) + (a 2 x b 2 ) + (a 3 x b 3 ) + + (a N x b N ) Examples: correlated with = correlated with = correlated with = = 14 If the correlation between two sequences is zero, they are said to be orthogonal.

40 Correlator Receiver r(t) Sample Chips Correlate with desired user s signature Bit Decision < 0 à 0 > 0 à 1 estimated bits

41 Why Does This Work? amplitude A 1 s 1 Correlate with User 1 s signature signature (1,1) 2A 1 A 2 s 2 Correlate with 0 User 1 s signature The user signatures are orthogonal. Now observe that: A 1 s 1 + A 2 s 2 Correlate with User 1 s signature 2A 1

42 Correlator, or Matched Filter Receiver A 1 s 1 + A 2 s 2 Correlate with User 1 s signature Bit Decision < 0 à 0 > 0 à 1 User 1 s bits Correlate with User 2 s signature Bit Decision < 0 à 0 > 0 à 1 User 2 s bits The correlator is matched to user 1 s signature s 1, and rejects s 2 (and vice versa).

43 Observations Users transmit simultaneously (not TDMA). Users overlap in frequency (not FDMA). Spectrum: User 1 signal bandwidth is roughly 1/T Spectrum: User 2 0 frequency signal bandwidth is roughly 1/T c = 2/T 0 frequency Bandwidth expansion (factor of 2) è spread spectrum signaling.

44 Users and Bandwidth Expansion To guarantee orthogonal signatures (no interference), the length of the signatures must be the number of users. Example (4 users): signature: signature: User 1: T/4 3T/4 T/2 T time User 2: 3T/4 T/4 T/2 T time signature: signature: User 3: 3T/4 User 4: 3T/4 T/4 T/2 T time T/4 T/2 T time The chip rate is 4 times the symbol rate, hence the bandwidth expansion is a factor of 4.

45 Correlator Receiver (4 users) s 1 + s 2 + s 3 + s 4 Correlate with User 1 s signature Bit Decision < 0 à 0 > 0 à 1 User 1 s bits Correlate with User 2 s signature Bit Decision < 0 à 0 > 0 à 1 User 2 s bits Correlate with User 3 s signature Bit Decision < 0 à 0 > 0 à 1 User 3 s bits Correlate with User 4 s signature Bit Decision < 0 à 0 > 0 à 1 User 4 s bits

46 Processing Gain Processing Gain (PG) Bandwidth of Spread Spectrum Signal Symbol Rate The PG is essentially the bandwidth expansion factor, given by (1/T c )/(1/T) = T/T c (chips per symbol), which is the length of the signature. The signature (sequence of chip values) is also called a spreading code. The signature may be randomly generated, in which case it is called a pseudo-noise (PN) sequence. Direct-Sequence CDMA uses a spread spectrum signalling scheme in which the signal is spread by transmitting a sequence of chips at a rate faster than the symbol rate.

47 DS-CDMA Transmitter Source bits Spreader chips Modulator RF signal (generate chips) (e.g., QPSK) Ex: 100 source symbols, processing gain = 10 è 1000 chips Nyquist chip shape sin 2πf c t T c time Baseband signal X Passband (RF) signal

48 Orthogonality and Asynchronous Users s 1 (t) s 2 (t) T 2T 3T 4T 5T T 2T 3T 4T 5T time Asynchronous users can start transmissions at different times. Chips are misaligned è signatures are no longer orthogonal! Orthogonality among users requires: Synchronous transmissions No multipath

49 Correlator, or Matched Filter Receiver delay s 1 (t) + s 2 (t-τ) Correlate with User 1 s signature Correlate with User 2 s signature Bit Decision < 0 à 0 > 0 à 1 Bit Decision < 0 à 0 > 0 à 1 Signal 1 + multiple acess interference (MAI) From user 2 Signal 2 + multiple acess interference (MAI) From user 1 The multiple access interference adds to the background noise and can cause errors. For this reason, CDMA is said to be interference-limited. Because CDMA users are typically asynchronous, and because of multipath, it is difficult to maintain orthogonal signatures at the receiver. Consequently, in practice, the signatures at the transmitter are randomly generated.

50 Correlation and PG Example: PG=4 s 1 : s 2 : Correlation = -2 Energy in s 1 (or s 2 ) is (-1) 2 + (-1) = 4 Normalized correlation = correlation/energy = -2/4 = -1/2 Example: PG=10 Conclusion: On average, the correlation between signatures decreases as the signature length (PG) increases.

51 Correlation and Bandwidth s 2 (t) T 2T 3T 4T 5T s 2 (t) frequency T 2T 3T 4T 5T Increasing the PG increases bandwidth, but decreases the correlation between user signatures. 0 frequency A 2 s 2 Correlate with User 1 s signature correlation between s 1 and s 2 à multiple access interference Increasing the PG decreases multiple access interference. Bandwidth expansion therefore provides immunity to interference (all kinds: analog, multiple access, multipath, narrowband, etc).

52 Example IS-95 (2G CDMA) Total bandwidth = 1.25 MHz Data rate = 9.6 kbps PG 130 3G/CDMA2000 Total bandwidth = 1.25 MHz Data rate varies between kbps (voice) up to 2 Mbps (1X-DO) PG varies from 1 to 130

53 Properties of CDMA Robust with respect to interference No frequency assignments (eases frequency planning) Asynchronous High capacity with power control Power control needed to solve near-far problem. Wideband: benefits from frequency/path diversity. Benefits from voice inactivity and sectorization. No loss in trunking efficiency. Soft capacity: performance degrades gradually as more users are added. Soft handoff

54 Near-Far Problem SO THEN THE THIRD TIME I CALLED CUSTOMER SERVICE, I SAID &%$#%^

55 Near-Far Problem User 1 amplitude A 1 User 2 amplitude A 2 A 1 s 1 (t)+a 2 s 2 (t) Correlate with User 1 s signature A 1 +A 2 (correlation of s 1 and s 2 ) Bit Decision < 0 à 0 > 0 à 1 User 1 s bits Output of correlator receiver is signal + interference. As the interferer moves closer to the base station, the interference increases. In practice, power variations can be up to 80 db! Conclusion: User 1 s signal is overwhelmed by interference from user 2!

56 Closed-Loop Power Control User 1 raise power lower power User 2 Base station gives explicit instructions to mobiles to raise/lower power. Needed to solve near-far problem (equalizes received powers). Introduced by Qualcomm in the late 80 s. Requires closed-loop feedback. BST controls powers through feedback channel. Why closed-loop?

57 Closed-Loop Power Control User 1 raise power lower power User 2 Base station gives explicit instructions to mobiles to raise/lower power. Needed to solve near-far problem (equalizes received powers). Introduced by Qualcomm in the late 80 s. Requires closed-loop feedback. Open-loop power control (no feedback) is inadequate due to frequency-selective fading.

58 Closed-Loop Power Control: Properties User 1 raise power lower power User 2 Crucial part of CDMA cellular systems (IS-95, 3G). Minimizes battery drain. Complicated (increases cost) Requires overhead: control bits in feedback channel to tell transmitter to lower/raise power. Cannot compensate for fast fading.

59 Properties of CDMA Robust with respect to interference No frequency assignments (eases RF planning). Asynchronous High capacity with power control. Power control needed to solve near-far problem. Wideband: benefits from frequency/path diversity. Benefits from voice inactivity and sectorization. No loss in trunking efficiency. Soft capacity: performance degrades gradually as more users are added. Soft handoff

60 Bandwidth and Multipath Resolution reflection (path 2) direct path (path 1) multipath components are resolvable signal pulse τ (delay spread) signal pulse T > τ τ T < τ Narrow bandwidth è low resolution Receiver cannot distinguish the two paths. T Wide bandwidth è high resolution Receiver can clearly distinguish two paths.

61 CDMA and Path Diversity CDMA uses wideband signals (chips are very narrow pulses), so that multipath is resolvable. A RAKE receiver collects ( rakes up ) the energy in the paths: power delay profile τ received signal delay τ adjust phase + received signal with combined multipath

62 Properties of CDMA Robust with respect to interference No frequency assignments (eases RF planning). Asynchronous High capacity with power control. Power control needed to solve near-far problem. Wideband: benefits from frequency/path diversity. Soft capacity: performance degrades gradually as more users are added. Benefits from voice inactivity and sectorization. No loss in trunking efficiency. Soft handoff

63 Performance depends on CDMA Capacity E b Energy per bit N 0 Interference + Noise power per unit bandwidth Let S= Transmitted power (per user), R= information rate (bits/sec), W= Bandwidth, K= Number of users E b = S/R (energy per second / bits per second) N 0 = (Number of interferers x S)/W = ((K-1) x S)/W Therefore E b /N 0 = (W/R)/(K-1) = (Processing Gain)/(K-1) For a target E b /N 0, the number of users that can be supported is K = (Processing Gain)/(E b /N 0 ) + 1

64 CDMA Capacity: Example For IS-95, want E b /N 0 7 db For 3G, want E b /N 0 3 to 5 db Suppose W=1.25 MHz (single-duplex), R= 14.4 kbps, target E b /N 0 = 7 db: K= 1 + [( )/( )]/ Compare with GSM, cluster size N=3: K= 8 (users/channel) (# of 200 khz channels) = / ( ) 16

65 Increasing CDMA Capacity Must reduce interference Antenna sectorization Interference reduced by 1/3 Trunking efficiency is not a major issue (no channels/time slots). other-cell interference Voice inactivity automatically increases the capacity relative to TDMA with dedicated time slots. CDMA has a soft capacity: each additional user marginally degrades performance for all users.

66 Properties of CDMA Robust with respect to interference No frequency assignments (eases RF planning). Asynchronous High capacity with power control. Power control needed to solve near-far problem. Wideband: benefits from frequency/path diversity. Soft capacity: performance degrades gradually as more users are added. Benefits from voice inactivity and sectorization. No loss in trunking efficiency. Soft handoff

67 Interference and CDMA Capacity If interference is reduced by a factor 1/g, then the number of interferers can be increased by g (N 0 is replaced by g x N 0 ): K = 1+ ( 1/ W / R g)(e / N b 0 ) If W/R is large, then reducing interference by 1/g (approximately) increases the capacity by a factor of g. Previous example: voice activity of 1/3 combined with 120 o sectors increases capacity by a factor of 9!

68 Refining the Capacity Estimate Capacity for previous example is Have not accounted for: Other-cell interference Approximately 1/3 to 1/2 of total interference power K à 1/(1+1/2) K 108 Multipath / fading Some multipath is combined by the Rake receiver, the rest is interference Power control inaccuracy Precise capacity predictions become difficult, best to rely on field trials

69 Properties of CDMA Robust with respect to interference No frequency assignments (eases RF planning). Asynchronous High capacity with power control. Power control needed to solve near-far problem. Wideband: benefits from frequency/path diversity. Benefits from voice inactivity and sectorization. No loss in trunking efficiency. Soft capacity: performance degrades gradually as more users are added. Soft handoff

70 Soft Handoff (CDMA) Make before break BEFORE DURING AFTER MSC MSC MSC BSC BSC BSC BSC BSC BSC Hard Handoff (TDMA) MSC MSC MSC BSC BSC BSC BSC BSC BSC

71 Applications of Spread-Spectrum Military (preceded cellular applications) Cellular Wireless LANs (overlay)

72 Military Spread Spectrum Can hide a signal by spreading it out in the frequency domain. spread 0 frequency noise level Requires a very large PG (several ). Enemy must know spreading code (the key containing 100 s of bits) to demodulate too complicated for simple search. Spread spectrum signals have the LPI/LPD property: low probability of intercept / low probability of detect. Spread spectrum used for covertness, not multiple access. 0 frequency

73 Applications of Spread-Spectrum Military (preceded cellular applications) Cellular Wireless LANs (overlay)

74 CDMA vs. TDMA (early 1990s) TDMA Proven technology Large investment in research, development CDMA Earlier military applications Near-far problem Enticing (exaggerated?) performance claims

75 2G CDMA: IS-95 or cdmaone Introduced by Qualcomm (San Diego) Direct-Sequence Spread Spectrum signaling FDD Wideband channels (1.25 MHz) Tight, closed-loop power control Sophisticated error control coding Multipath combining to exploit path diversity Noncoherent detection Soft handoff High capacity Air-interface only: uses IS-41

76 TDMA vs. CDMA: Performance Critera Capacity: Users per Hz per km 2 Channel conditions System assumptions Perfect power control? Modulation and coding? Complexity Flexibility Integrated services (voice/data) Multimedia Variable rate/qos Power control (CDMA) Synchronization (TDMA) Equalization Frequency assignment

77 cdma2000 3G Air Interfaces Wideband (W)-CDMA Also referred to as multicarrier CDMA 1X Radio Transmission Technology (RTT): 1.25 MHz bandwidth (1 carrier) Supports 307 kbps instantaneous data rate in packet mode Expected throughput up to 144 kbps 1xEV (Evolutionary): High Data Rate standard introduced by Qualcomm 1xEV-DO: data only, 1xEV-DV: data and voice Radio channels assigned to single users (not CDMA!) 2.4 Mbps possible, expected throughputs are a few hundred kbps 1xEV-DV has twice as many voice channels as IS-95B Also referred to as Universal Mobile Telecommunications System (UMTS) European proposal to ITU (1998) Backwards compatibility with 2G GSM and IS-136 air interfaces Network and frame structure of GSM ``Always on packet-based data service Supports packet data rates up to 2 Mbps Requires minimum 5 MHz bandwidth, FDD, coherent demodulation 6 times spectral efficiency of GSM

78 Verizon 2 ATT/Cingular Sprint; Clearwire 3 T-Mobile NexTel 1 Service Providers and Technologies Cellular & PCS (850 & 1900 MHz) Cellular (850 & 1900 MHz) PCS (1900 MHz) PCS (1900 MHz) Public service band (800 MHz) U. S. Cellular Cellular & PCS 1 Merged with Sprint. 2 Plans to deploy LTE. (850 & 1900 MHz) 3 Rolled out WiMax in Baltimore, Portland. CDMA 2000; Kbps 1 x EV-DO up to 2.5 Mbps GSM/GPRS/EDGE UMTS/HSPA CDMA2000; 1 x EV-DO GSM/GPRS/EDGE iden (TDMA) & WiDEN 4 up to 512 kbps Kbps up to 2.5 Mbps Kbps kbps near 100 kpbs 1 x EV-DO up to 2.5 Mbps 4 Wideband version of iden.

79 Applications of Spread-Spectrum Military (preceded cellular applications) Cellular Wireless LANs (overlay)

80 Spread Spectrum Overlay FCC requirements on spectrum sharing in the unlicensed (ISM) bands: Listen before talk Transmit power is proportional to the square root of the bandwidth. spread spectrum signal hospital monitor telemetry frequency Spread spectrum signaling is robust with respect to a narrowband interferer. To a narrowband signal, a spread spectrum signal appears as low-level background noise.

81 Variable-Rate CDMA

82 To increase the data rate we can: Variable-Rate CDMA Increase the number of signatures per user More signatures à more power, more interference Reduce the number of chips per bit Decreases immunity to interference (must increase power) Increase the number of bits per symbol QPSK à 8-PSK à 16 QAM requires more power How is voice capacity affected by the presence of high-rate data users?

83 Frequency-Hopped CDMA Idea: Hop from channel to channel during each transmission. f 5 frequency f 4 f 3 User 1: blue f 2 f 1 time slots time

84 Frequency-Hopped CDMA Idea: Hop from channel to channel during each transmission. frequency f 5 f 4 f 3 collision bits are lost User 1: blue User 2: red f 2 f 1 time slots time

85 Hop Rate Can make synchronous users orthogonal by assigning hopping patterns that avoid collisions. Fast hopping generally means that the hopping period is less than a single symbol period. Slow hopping means the hopping period spans a few symbols. The hopping rate should be faster than the fade rate (why?).

86 Hop Rate Can make synchronous users orthogonal by assigning hopping patterns that avoid collisions. Fast hopping generally means that the hopping period is less than a single symbol period. Slow hopping means the hopping period spans a few symbols. The hopping rate should be faster than the fade rate so that the channel is stationary within each hop.

87 Properties of FH-CDMA Exploits frequency diversity (can hop in/out of fades) Can avoid narrowband interference (hop around) No near-far problem (Can operate without power control) Low Probability of Detect/Intercept Spread spectrum technique can overlay Cost of frequency synthesizer increases with hop rate Must use error correction to compensate for erasures due to fading and collisions. Applications Military (army) Part of original standard Enhancement to GSM Bluetooth

88 Inventor of Frequency-Hopping Hedi Lamar, the famous actress of the 1930 s has one of the first U.S. patents on frequency hopping with co-author and composer George Antheil.

89 Bluetooth: A Global Specification for Wireless Connectivity Wireless Personal Area Network (WPAN). Provides wireless voice and data over short-range radio links via low-cost, lowpower radios ( wireless cable). Initiated by a consortium of companies (IBM, Ericsson, Nokia, Intel) Standard has been developed (IEEE ).

90 Bluetooth Specifications Allows small portable devices to communicate together in an ad-hoc piconet (up to eight connected devices). Frequency-hopped spread-spectrum in the 2.4 GHz UNII band. Interferes with b/g/n 1600 hops/sec over 79 channels (1 MHz channels) Range set at 10m. Gross data rate of 1 Mbps (TDD). 64 kbps voice channels Second generation (Bluetooth 2.0+) supports rates up to 3 Mbps. Competes with Wireless USB.

91 The Multiple Access Problem How can multiple mobiles access (communicate with) the same base station? Frequency-Division (AMPS) Time-Division (IS-136, GSM) Code-Division (IS-95, 3G) Direct Sequence/Frequency-Hopped Orthogonal Frequency Division Multiple Access (OFDMA) (WiMax, LTE) Random Access (Wireless Data)

92 Orthogonal Frequency Division Multiplexing (OFDM) substream 1 Modulate Carrier f 1 source bits Split into M substreams substream 2 substream M Modulate Carrier f 2 + OFDM Signal Modulate Carrier f M

93 OFDM Spectrum Total available bandwidth Power Data spectrum for a single carrier f 1 ß 0 f 2 f 5 f 6 f 3 f 4 subchannels frequency M subcarriers, or subchannels, or tones Orthogonal subcarriers è no cross-channel interference.

94 OFDM vs OFDMA OFDM is a modulation technique for a particular user. OFDMA is a multiple access scheme (allows many users to access a single receiver). Can OFDM be combined other multiple access techniques?

95 OFDM vs OFDMA OFDM is a modulation technique for a particular user. OFDMA is a multiple access scheme (allows many users to access a single receiver). Can OFDM be combined other multiple access techniques? Yes, e.g., FDMA and TDMA. OFDMA is different

96 OFDM vs OFDMA OFDM with FDMA OFDM users are assigned adjacent frequency bands. Frequency diversity is determined by (BW of signal)/(coherence BW) Overall User 1 User 2 User 3 User 4 OFDMA User subcarrier assignments are permuted across the entire available frequency band. So what?? Overall User 1 User 2 User 3 User 4

97 The Multiple Access Problem How can multiple mobiles access (communicate with) the same base station? Frequency-Division (AMPS) Time-Division (IS-136, GSM) Code-Division (IS-95, 3G) Direct Sequence/Frequency-Hopped Orthogonal Frequency Division Multiple Access (OFDMA) (WiMax, LTE) Random Access (Wireless Data)

98 Orthogonal Frequency Division Multiplexing (OFDM) substream 1 Modulate Carrier f 1 source bits Split into M substreams substream 2 substream M Modulate Carrier f 2 + OFDM Signal Modulate Carrier f M

99 OFDM Spectrum Total available bandwidth Power Data spectrum for a single carrier f 1 ß 0 f 2 f 5 f 6 f 3 f 4 subchannels frequency M subcarriers, or subchannels, or tones Orthogonal subcarriers è no cross-channel interference.

100 OFDM vs OFDMA OFDM is a modulation technique for a particular user. OFDMA is a multiple access scheme (allows many users to access a single receiver). Can OFDM be combined other multiple access techniques?

101 OFDM vs OFDMA OFDM is a modulation technique for a particular user. OFDMA is a multiple access scheme (allows many users to access a single receiver). Can OFDM be combined other multiple access techniques? Yes, e.g., FDMA and TDMA. OFDMA is different

102 OFDM vs OFDMA OFDM with FDMA OFDM users are assigned adjacent frequency bands. Frequency diversity is determined by (BW of signal)/(coherence BW) Overall User 1 User 2 User 3 User 4 OFDMA User subcarrier assignments are permuted across the entire available frequency band. So what?? Overall User 1 User 2 User 3 User 4

103 OFDM vs OFDMA OFDM (with FDMA) OFDM users are assigned adjacent frequency bands. Frequency diversity is determined by (BW of signal)/(coherence BW) OFDMA User subcarrier assignments are permuted across the entire available frequency band. Each sub-carrier may experience independent fading. Frequency diversity is determined by the number of sub-carriers. Also provides interference diversity. Overall User 1 User 2 User 3 User 4 Overall User 1 User 2 User 3 User 4

104 OFDM/TDMA and OFDMA OFDM/TDMA: t TDMA Each color represents a different user, which is assigned particular time slots. subchannels m TDMA\OFDMA Different sub-carriers can be assigned to different users. t N time slot Each user can be assigned a time/frequency slice. Requires a time/frequency scheduler.

105 WiMax OFDMA Frame Structure (TDD example) (downlink) (uplink)

106 Adaptive Rate Control channel gain large channel gain è higher data rate small channel gain è lower data rate f 1 f 2 frequency How can we control the rate per subchannel? Change the modulation format (e.g., choose from QPSK/16-QAM/64 QAM) Change the code rate (i.e., change the number of redundant bits) Requires feedback from receiver to transmitter