Chapter 9 Multiple Access Techniques for Wireless Communications

Transcription

1 Chapter 9 Multiple Access Techniques for Wireless Communications Yimin Zhang, Ph.D. Department of Electrical & Computer Engineering Villanova University Yimin Zhang, Villanova University 1

2 Outlines Duplexing Time Division Duplexing (TDD) Frequency Division Duplexing (FDD) Multiple Access FDMA TDMA Spread Spectrum Multiple Access - Frequency Hopped Multiple Access (FHMA) - Code Division Multiple Access (CDMA) Packet Radio ALOHA Carrier Sense Multiple Access (CSMA) Capacity Yimin Zhang, Villanova University 2

3 Brief Historical Overview Late 1940 s: Push to Talk FM Systems Half Duplex Mode Improved Mobile Telephone Service (ITMS) Fully Duplex, Auto-dial, Auto-trunking Phone Systems Techniques and Theory Developed in 1950 & 60 s AT&T Proposes Cellular System to FCC in FCC Authorizes US Advanced Mobile Phone System (AMPS) Deployed in Chicago 666 Duplex Channels (40 MHz in the 800MHz band) Late U.S. Digital Cellular (USDC) Implemented Code Division Multiple Access Developed by Qualcomm. New Personal Communication Service Licenses in the 1800/1900 MHz Band Auctioned Yimin Zhang, Villanova University 3

4 Duplex Methods of Radio Links Base Station Forward link Reverse link Mobile Station Yimin Zhang, Villanova University 4

5 Frequency Division Duplexing Frequency Division Duplexing (FDD) provides two distinct bands of frequencies for every user. Yimin Zhang, Villanova University 5

6 Time Division Duplexing Time Division Duplexing (TDD) uses time instead of frequency to provides both a forward and reverse links. Yimin Zhang, Villanova University 6

7 Trade-offs Between FDD and TDD Frequency Division Duplexing Geared toward Individual Channels for each User Frequency Separation must use Inexpensive Technology Time Division Duplexing Eliminates Need for Forward and Reverse Channels Time Latency Yimin Zhang, Villanova University 7

8 ECE 8708 Wireless Communications : Multiple Access Techniques Multiple Access Techniques Base Station Forward link Reverse link Mobile Station Mobile Station Mobile Station Yimin Zhang, Villanova University Mobile Station 8

9 Multiple Access Techniques Multiple Access FDMA TDMA Spread Spectrum Multiple Access - Frequency Hopped Multiple Access (FHMA) - Code Division Multiple Access (CDMA) Packet Radio ALOHA Carrier Sense Multiple Access (CSMA) Yimin Zhang, Villanova University 9

10 Multiple Access Techniques Term has its Origin in Satellite Communications System of Earth Stations and a Satellite Used to Mean Sharing a Communications Channel (of W Hz) among a Group of Users Signal Space of Time Bandwidth TW Where T k/r Signal Space D 2TW Yimin Zhang, Villanova University 10

11 Multiple Access Techniques Yimin Zhang, Villanova University 11

12 Frequency Division Multiple Access FDMA channel carries only one phone at a time. Bandwidth is relatively narrow (30 khz). Since FDMA is a continuous transmission scheme, fewer bits are needed for overhead purpose (e.g., sync) as compared to TDMA. Requires tight RF filtering. Yimin Zhang, Villanova University 12

13 Nonlinear Effects in FDMA Antenna at base station shared by channels Nonlinearities of power amps and combiners Results in signal spreading Generates intermodulation Causes adjacent channel interference and adjacent service interference Yimin Zhang, Villanova University 13

14 Yimin Zhang, Villanova University 14

15 Advanced Mobile Phone System (AMPS) First U.S. analog cellular system Based on FDMA/FDD NBFM modulates the carrier Total number of channels is given by: N B t 2 B B c guard where B t : total spectrum allocation B guard : guard band allocated at the edge of the spectrum band B c : channel bandwidth Yimin Zhang, Villanova University 15

16 Time Division Multiplex Access Yimin Zhang, Villanova University 16

17 Time Division Multiplex Access Data transmitted in a buffer-and-burst method Preamble Address and synchronization info for base station and subscriber identification Guard times Synchronization of receivers between a different slots and frames Yimin Zhang, Villanova University 17

18 Time Division Multiple Access Yimin Zhang, Villanova University 18

19 Time Division Multiple Access TDMA shares a single carrier frequency with several users, where each user makes use of nonoverlapping time slots. Data transmission in TDMA systems is not continuous, but in bursts. TDMA uses different time slots for transmission and reception, thus duplexers are not required. If FDD is used, TDMA/FDD systems intentionally induce several time slots of delay between the two links so that duplexers are not required in the mobile unit. Equalization is usually necessary, since the rates are generally high. High synchronization overhead is required in TDMA. Guard times are necessary to separate users; Guard times should be minimized. Yimin Zhang, Villanova University 19

20 Efficiency of TDMA Frame efficiency b 1 OH 100 b Number of overhead bits per frame η f T % b N b + N b + N b + OH r r t p t g N r b g N r : number of reference bursts per frame N t : number of traffic bursts per frame b r : # of overhead bits per reference burst b p : # of overhead bits per preamble in each slot b g : # of equivalent bits in each guard time interval Total number of bits per frame b T T f R T f : frame duration; R : channel bit rate Yimin Zhang, Villanova University 20

21 Number of Channels in TDMA System Number of channels: total number of slots multiplied by the channels available N m ( B tot B 2 B c guard ) m: maximum number of TDMA users supported on each radio channel Yimin Zhang, Villanova University 21

22 Example Yimin Zhang, Villanova University 22

23 Example Yimin Zhang, Villanova University 23

24 Example Yimin Zhang, Villanova University 24

25 Outlines Spread Spectrum Multiple Access - Frequency Hopped Multiple Access (FHMA) - Code Division Multiple Access (CDMA) Yimin Zhang, Villanova University 25

26 Frequency Hopping Typical frequency-hopping waveform pattern Yimin Zhang, Villanova University 26

27 Direct Spread Yimin Zhang, Villanova University 27

28 Wideband and Narrowband CDMA Yimin Zhang, Villanova University 28

29 Code Division Multiple Access Many users of a CDMA system share the same frequency band. Either TDD or FDD may be used. Unlike TDMA and FDMA, CDMA has a soft capacity limit. Multipath fading may be substantially reduced. Small-scale fading is mitigated if the spread spectrum bandwidth is greater than the coherence bandwidth of the channel. RAKE receiver can be used to achieve path diversity. Single frequency reuse can be used and soft handover can be performed to achieve diversity. Near-far problem exists and power control is required. Yimin Zhang, Villanova University 29

30 Summary of Multiple Access FDMA power TDMA time time time frequency power frequency CDMA power frequency Yimin Zhang, Villanova University 30

31 Space Division Multiple Access Adaptive antennas are used at the base station to direct the beam towards the desired user and form a null in the directions of other users. Yimin Zhang, Villanova University 31

32 Packet Radio Shared Multiple Access Medium M 5 Packet Radio access, many subscribers attempt to access a single channel in an uncoordinated (or minimally coordinated) manner. 1. Any transmission from any station can be heard by any other stations 2. If two or more stations transmit at the same time, collision occurs Yimin Zhang, Villanova University Figure

33 Packet Radio Protocols Three types of random accesses: ALOHA, slotted ALOHA, and CSMA-CD Throughput (Traffic occupancy) R λτ λ : mean arrival rate τ : packet duration To obtain a reasonable through put, the rate at which new packets are generated must lie within 0 < R <1. Normalized throughput T R Pr[ no collision ] λτ Pr[ no collision ] Yimin Zhang, Villanova University 33

34 Probability of Collision The probability that n packets are generated by the user population during a given packet duration interval is assumed to be Poisson distributed and is given by Pr( n) R e n! n R Letting n 0 results in probability of no collision Pr( 0) e R Yimin Zhang, Villanova University 34

35 Collisions Yimin Zhang, Villanova University Figure

36 ALOHA Basic idea: let users transmit whenever they have data to be sent. When collision occurs, wait a random time ( why? ) and retransmit again. Differences between regular errors &collision Regular errors only affect a single station Collision affects more than one The retransmission may collide again Even the first bit of a frame overlaps with the last bit of a frame almost finished, then two frames are totally destroyed. Yimin Zhang, Villanova University 36

37 ALOHA The probability that n packets are generated by the user population during a duration of 2 packet time intervals Pr( n) n (2 R ) e n! 2 R Probability of no collision Pr( 0) e 2 R Normalized throughput T R Pr[ no collision ] R e 2 R Yimin Zhang, Villanova University 37

38 ALOHA 1.peak value at R0.5 with T for any given T, there are two values of R, corresponding to two modes: occasional collision mode with R T and frequent collision mode with R >> T Yimin Zhang, Villanova University Figure

39 Slotted ALOHA Yimin Zhang, Villanova University Figure

40 Slotted ALOHA Synchronize the transmissions of stations All stations keep track of transmission time slots and are allowed to initiate transmissions only at the beginning of a time slot. The probability that n packets are generated by the user population during a given packet duration interval is assumed to be Poisson distributed and is given by n R R e Pr( n) n! Probability of no collision Pr( 0) e R Normalized throughput T R Pr[ no collision ] R e R Yimin Zhang, Villanova University 40

41 ALOHA vs. Slotted ALOHA Peak value at R1 with T0.368 for slotted ALOHA, double compared with ALOHA. Yimin Zhang, Villanova University Figure

42 CSMA (Carrier Sensing Multiple Access) Problem with ALOHAs: low throughput because the collision wastes transmission bandwidth. Solution: avoid transmission that are certain to cause collision, that is CSMA. Any station listens to the medium, if there is some transmission going on the medium, it will postpone its transmission. Yimin Zhang, Villanova University 42

43 CSMA (Carrier Sensing Multiple Access) Suppose t prop is propagation delay from one extreme end to the other extreme end of the medium. When transmission is going on, a station can listen to the medium and detect it. After t prop, A s transmission will arrive the other end; every station will hear it and refrain from the transmission. Vulnerable period t prop in CSMA, compared to τ and 2τ in ALOHAs. Station A begins transmission at t0 A sense sense Station A captures channel at tt prop A sense sense Yimin Zhang, Villanova University Figure

44 Three different CSMA schemes Based on how to do when medium is busy: 1-persistent CSMA Non-persistent CSMA p-persistent CSMA CSMA/CD Yimin Zhang, Villanova University 44

45 1-persistent CSMA sense channel when want to transmit a packet, if channel is busy, then sense continuously, until the channel is idle, at this time, transmit the frame immediately. If more than one station are sensing, then they will begin transmission the same time when channel becomes idle, so collision. At this time, each station executes a backoff algorithm to wait for a random time, and then re-sense the channel again. Problem with 1-persistent CSMA is high collision rate. Yimin Zhang, Villanova University 45

46 Non-persistent CSMA Sense channel when want to transmit a packet, if channel is idle, then transmit the packet immediately. If busy, run backoff algorithm immediately to wait a random time and then re-sense the channel again. This is popular for wireless LAN applications, where the packet transmission intervals is much greater than the propagation delay to the farthermost user. Problem with non-persistent CSMA is that when the channel becomes idle from busy, there may be no one of waiting stations beginning the transmission, thus waste channel bandwidth. Yimin Zhang, Villanova University 46

47 p-persistent CSMA Sense channel when want to transmit a packet, if channel is busy, then persist sensing the channel until the channel becomes idle. If the channel is idle, transmit the packet with probability of p, and wait, with probability of 1-p, additional propagation delay t prop and then resense again. Yimin Zhang, Villanova University 47

48 CSMA-CD When the transmitting station detects a collision, it stops its transmission immediately (Not transmit the entire frame which is already in collision). The time for transmitting station to detect a collision is 2t prop. In detail: when a station wants to transmit a packet, it senses channel, if it is busy, use one of above three algorithms (i.e., 1- persistent, non-persistent, and p-persistent schemes). The transmitter senses the channel during transmission. If a collision occurred and was sensed, transmitter stops its left transmission of the current frame; moreover, a short jamming signal is transmitted to ensure other stations that a collision has occurred and backoff algorithm is used to schedule a future re-sensing time. Yimin Zhang, Villanova University 48

49 CSMA-CD The reaction time in CSMA-CD is 2t prop A begins to transmit at t0 A A A A detects collision at t 2 t prop -δ B B B It takes 2 t prop to find out if channel has been captured B begins to transmit at t t prop -δ; B detects collision at t t prop Yimin Zhang, Villanova University Figure

50 Packet Radio: Comparison 1. When a is small, i.e, t prop << τ, the CSMA-CD is best and all CSMAs are better than ALOHAs. 2. When a is approaching 1, CSMAs become worse than ALOHA. 3. ALOHAs are not sensitive to a because they do not depend on reaction time CSMA/CD 1-P CSMA Non-P CSMA Τ max 0.6 Slotted Aloha a Aloha t prop /τ Maximum achievable throughput of random access schemes Yimin Zhang, Villanova University Figure

51 Capacity of Cellular Systems Channel capacity for a radio system is defined as the maximum number of channels or users that can be provided in a fixed frequency band spectrum efficiency of wireless system. For a cellular system, the radio capacity is defined as m Bt B N c B t : Total allocated spectrum for the system B C : Channel bandwidth N : Number of cells in frequency reuse pattern Yimin Zhang, Villanova University 51

52 Capacity of Cellular Systems Carrier to Interference ratio C I D 0 6 D n n For maximum interference D 0 R D R D n C I min Q C 6 I min 1 / n Yimin Zhang, Villanova University 52

53 Yimin Zhang, Villanova University 53 ECE 8708 Wireless Communications : Multiple Access Techniques Because Q 3N Capacity of Cellular Systems n n c t c t c t I C B B Q B B N B B m / 2 min 2 / The radio capacity becomes In particular, when n4 min 3 2 I C B B m c t

54 Capacity of Cellular Systems m B c 2 3 Bt C I min Reducing (C/I) min can increase the capacity. In order to provide the same voice quality, (C/I) min may be lower in a digital systems when compared to an analog system. Typically, the minimum required C/I is about 12 db for narrowband digital systems, and 18 db for narrowband FM systems. Yimin Zhang, Villanova University 54

55 Yimin Zhang, Villanova University 55 ECE 8708 Wireless Communications : Multiple Access Techniques Capacity of Cellular Systems eq c t I C B B m 3 2 ' min 3 2 I C B B m c t 2 min ' c c eq B B I C I C Trade-off between channel bandwidth and power Reduce bandwidth to half need to increase C/I by a factor of 4.

56 Example 8dB Yimin Zhang, Villanova University 56

57 Yimin Zhang, Villanova University 57 ECE 8708 Wireless Communications : Multiple Access Techniques Capacity of Cellular Systems 2 min ' ' ' ' c c c c c c eq B B I R E I R E I C I C In digital cellular systems, Because I R E I R E I C c c b b c B c R 3 ' ' c c c c B B E E Reduce bandwidth to half need to increase bit energy by a factor of 8.

58 Capacity Comparison: TDMA vs. FDMA In FDMA, B t is divided into M channels, each with bandwidth B c. m B c 2 3 Bt C I B 2 3 B C I t min M min 3 t 2 M C I min C E b R b I I 0 B c For TDMA with multiple time slots and occupies the same spectrum, C ' E b Rb ' I I B ' 0 c ' Yimin Zhang, Villanova University 58

59 Example Yimin Zhang, Villanova University 59

60 Capacity of Digital Cellular CDMA Denote N as the number of users. SNR in RF With noise considered SNR S 1 ( N 1) S N 1 SNR in receiver after despreading E b S / R N ( N 1) S / W W / R 1 0 N E b N W / R ( N 1) + ( η / ) 0 S Number of users that can access the system N 1 + W E b / / R N 0 ( η / S ) Yimin Zhang, Villanova University 60

61 Capacity of Digital Cellular CDMA Capacity of FDMA and TDMA system is bandwidth limited. Capacity of CDMA system is interference limited. The link performance of CDMA increases as the number of users decreases. To increase the CDMA capacity - use sectoring: lower interference and noise (N 0 N 0 ) - switch off during periods with no voice activities (factor α) When the number of user is large N α W E b / / R N 0 ' Yimin Zhang, Villanova University 61

62 Example Yimin Zhang, Villanova University 62

63 Outlines Duplexing Time Division Duplexing (TDD) Frequency Division Duplexing (FDD) Multiple Access FDMA TDMA Spread Spectrum Multiple Access - Frequency Hopped Multiple Access (FHMA) - Code Division Multiple Access (CDMA) Packet Radio ALOHA Carrier Sense Multiple Access (CSMA) Capacity Yimin Zhang, Villanova University 63