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2. 2 6. Multiple Access Techniques

3. 3 Multiplexing vs. Multiple Access Multiple Access Overview Frequency Division Duplexing Time Division Duplexing Multiple Access Techniques Contents

4. 4 Multiplexing refers to a set of techniques that enable the sharing of the usable channels among multiple users for the transfer of individual information streams. The user information streams are usually joined at a common access point to the available channel. Several signals from different channels share a channel with greater capacity, to improve bandwidth utilization. Multiple access is a form of multiplexing scheme where multiple users that are geographically dispersed gain access to the shared channel. In other words, the sharing is usually carried out with respect to a remote resource (not on the same location). Multiplexing vs. Multiple Access

5. 5 A satellite transponder/cellular base station (BS) differs from a conventional microwave line-of-sight repeater in that many earth stations/mobile handsets can access the satellite/BS from widely different locations at the same time. Such a capability is called multiple access. Ground Station Ground Station Satellite BTS Multiple Access Overview

6. 6 A multiple access system Multiple access permits the communication resources to be shared by a large number of users seeking to communicate with each other. It is desirable that the sharing of resources be accomplished without causing serious interference to each other. Channel Tx 1 Tx 2 Tx X Rx Multiple Access Overview (cont.)

7. 7 Desirable for mobile users to send and receive information simultaneously from the base station. PSTN: talk and listen at the same time – duplexing – generally required for wireless systems. Duplexing: frequency and time domains. Frequency division duplexing (FDD) – two distinct frequency bands for users – forward and reverse links. Duplex channel consists of forward and reverse simplex channels, separated by constant frequency gap. Reverse Channel Forward Channel frequency f c,R f c,,F Frequency separation Frequency separation should be carefully decided Frequency separation is constant Frequency Division Duplexing (FDD)

8. 8 Time division duplexing (TDD): multiple users share a single radio channel by taking turns in the time domain. Individual users are allowed to access channel in assigned time slots. Duplex channel consists of forward and reverse simplex time slots. If time separation between forward and reverse time slots is small – transmission and reception of data appears simultaneous to users. FRRRR 0 1 2 3 4 5 6 7 … …. Reverse Channel Forward Channel time TiTi Time separation T i+1 channel Slot number FFF Time Division Duplexing (TDD)

9. 9 Frequency allocation must be carefully coordinated within same system and also out-of-band users between the two bands. Frequency separation must be coordinated to permit use of inexpensive RF and oscillator technology. Time latency creates inherent sensitivities to propagation delays of individual users. Reverse Channel Forward Channel Time Time separation Reverse Channel Forward Channel Frequency separation Frequency Frequency Division Duplexing Time Division Duplexing FDD vs. TDD

10. 10 Four major access techniques: 1.Frequency division multiple access (FDMA) 2.Time division multiple access (TDMA) 3.Code division multiple access (CDMA) 4.Space division multiple access (SDMA) Two major multiple access systems: 1.Narrowband 2.Wideband Multiple Access Techniques

11. 11 large number of narrowband channels usually FDD Narrowband FDMA Narrowband TDMA FDMA/FDD FDMA/TDD TDMA/FDD TDMA/TDD Narrowband Systems

12. 12 f t user 1 user n forward channel reverse channel forward channel reverse channel... Logical Separation FDMA/FDD

13. 13 f t user 1 user n forward channelreverse channel forward channelreverse channel... Logical Separation FDMA/TDD

14. 14 f t user 1user n forward channel reverse channel forward channel reverse channel... Logical Separation TDMA/FDD

15. 15 f t user 1user n forward channel reverse channel forward channel reverse channel... Logical Separation TDMA/TDD

16. 16 large number of transmitters on one channel TDMA techniques CDMA techniques FDD or TDD multiplexing techniques TDMA/FDD TDMA/TDD CDMA/FDD CDMA/TDD Wideband Systems

17. 17 code f user 1 user n forward channelreverse channel forward channelreverse channel... Logical Separation CDMA/FDD

18. 18 code t user 1 user n forward channelreverse channel forward channelreverse channel... Logical Separation CDMA/TDD

19. 19 Advanced Mobile Phone System (AMPS) FDMA/FDD Global System for Mobile (GSM) TDMA/FDD US Digital Cellular (USDC) TDMA/FDD Digital European Cordless Telephone (DECT) FDMA/TDD US Narrowband Spread Spectrum (IS-95) CDMA/FDD Cellular System Multiple Access Techniques in Use Multiple Access Technique

20. 20 Assign individual frequency channels to individual users. Examples: AMPS Examples: CT2 & DECT Channel 1 Channel 2 Channel N Channel 2 Channel 1 Channel N Frequency Separation Forward Channel Reverse Channel FDD-FDMA Frequency Channel 1 Channel 2 Channel N Forward Channel Reverse Channel Frequency Amplitude Time TDD-FDMA Channel 1 Channel 2 Channel N Frequency Division Multiple Access (FDMA)

21. 21 Only one circuit per channel at a time. If idle, cannot be used by other users. Wasted resource. With assigned channel, BS and MS transmit and receive simultaneously. Channel is normally narrow bandwidth (30 kHz for AMPS). FDMA is usually implemented in narrowband systems. Symbol time is large compared to average delay spread. ISI is low and little or no equalization is needed. Transceiver complexity is lower compared to TDMA system. Fewer overhead bits (for synchronization, framing, etc.). Higher cell site system cost – more high cost bandpass filters. FDMA Features

22. 22 Use of costly duplexers at transmitter and receiver. Tight RF filtering to minimize adjacent channel interference. Intermodulation (IM) problem: channels share same antenna – signal through power amplifier and power combiners – nonlinearities cause signal spread in frequency. AMPS: single channel of two duplex channels (separated by 45 MHz) used for every call. Channel vacated after call completion or handoff. Analog narrowband frequency modulation (NBFM) adopted. Number of channels simultaneously supported in FDMA system: FDMA Features (cont.)

23. 23 Division of spectrum into time slots – each time slot for a user to transmit or receive. Channel: a time slot that reoccurs every frame where N time slots make up a frame. Buffer-and-burst transmission method – non-continuous. Digital data and digital modulation. Channel 1 Channel 2 Channel N Forward Channel Reverse Channel Time Amplitude Frequency TDD-FDMA Channel N Channel 2 Channel 1 Time Division Multiple Access (TDMA)

24. 24 TDMA / FDD: different carriers for uplink and downlink. Time slots for uplink and downlink are separated in time to avoid the need for duplexer. Slot 1Slot 2Slot 3 … Slot N Preamble Information Message Trail Bits One TDMA Frame Trail Bits Sync. Bits Information Data Guard Bits The frame is cyclically repeated over time. TDMA Features

25. 25 Single carrier shared by several users. Non-continuous transmission – low battery consumption – MS off most of the time. Handoff is easier due to available idle time for MS to listen to other BSs – mobile assisted handoff is possible. Different time slots for transmission and reception – no duplexer. With FDD, only switch is needed – between Trx and Rx. Adaptive equalization needed due to high transmission rates. Desirable to minimize guard time but may cause bandwidth expansion – interference to adjacent channel. High synchronization overhead and additional guard slots. TDMA Features (cont.)

26. 26 Possible to allocate different numbers of time slots to different users – bandwidth on demand. Frame efficiency (η f ): percentage of transmitted data containing information as compared to that of overhead for access scheme. Effective efficiency for end user is lower. where b OH is the number of overhead bits per frame and b T is the total number of bits per frame. Number of channels: where m is maximum number of user per channel. TDMA Features (cont.)

27. 27 Controls radiated energy for each user in space using spot beam antennas base station tracks user when moving Cover areas with same frequency: – TDMA or CDMA systems Cover areas with same frequency: –FDMA systems Primitive applications are “Sectorized antennas” Adaptive antennas simultaneously steer energy in the direction of many users at once User A User B User C User D User A User C User E User B User D Space-Division Multiple Access (SDMA)

28. 28 Uses transmission bandwidth several orders of the minimum required RF bandwidth. Pseudo-noise (PN) sequence used to convert a narrowband signal to wideband noise-like signal. Provide immunity to multipath interference and robust multiple access capability. Bandwidth efficient for multiple user environment. Two types: 1.Frequency hopped multiple access (FHMA) 2.Direct sequence multiple access (DS) or code division multiple access (CDMA) Spread Spectrum Multiple Access (SSMA)

29. 29 Data is sent by hopping transmitter carrier to seemingly random channels, only known to desired receiver. On each channel, small bursts of data are sent using conventional narrowband modulation before transmitter hops again. Processing gain, PG = B ss / B where B ss and B are total hopping bandwidth and instantaneous bandwidth respectively. Fast frequency hopping: more than one frequency hop during each transmitted symbol. Slow frequency hopping: one or more symbols are transmitted in the time interval between frequency hops. Frequency Hopped Spread Spectrum (FH-SS)

30. 30 Periodic change of transmission frequency. A sequence of modulated data bursts with time-varying, pseudorandom carrier frequencies – hopset. Hopping occurs over a frequency band that includes a number of channels. Channel: spectral region with central frequency in the hopset and a bandwidth containing most of the power in a narrowband modulation burst (e.g. FSK) having the corresponding carrier frequency. Frequency of channel used in hopset is called instantaneous bandwidth. Bandwidth over which hopping occurs is called total bandwidth. Frequency Hopped Spread Spectrum (FH-SS) (cont.)

31. 31 Amplitude Time Frequency PN=1 PN=2 PN=3 PN=5 PN=6 PN=7 PN=8 PN=4 Hopping period Channel bandwidth Hopping by PN sequence FHMA

32. 32 If the rate of change of the carrier frequency is greater than symbol rate – fast frequency hopping system. Fast frequency hopping is a frequency diversity technique for FHMA. If the rate of change of the carrier frequency is lower than symbol rate – slow frequency hopping system. FHMA normally adopts energy efficient contact envelope modulation – linearity of devices, e.g., amplifier is not an issue. Security – high when large number of hopping channels are used. Immunity to fading is high – error coding and interleaving can be applied. Example: Bluetooth and HomeRF. FHMA (cont.)

33. 33 Narrowband message signal multiplied by a very large bandwidth signal called spreading signal. Spreading signal: PN code sequence with chip rate much higher than data bit rate. All users transmit on same carrier frequency simultaneously and independently but with different PN codewords which highly orthogonal to each other. Intended receiver correlates received signal in time with the codeword used by the originating transmitter – despreading of intended signal. Other signals for other receivers remain spread and appears noise-like to the intended receiver. CDMA

34. 34 a code is associated to every channel several channels can overlap on the same bandwidth Power Time Frequency FDMA Power Time Frequency TDMA Power Time Frequency CDMA CDMA (cont.)

35. 35 unmatched PN-sequence Channel Time Demodulator Time correlate RX(intended) Time Modulator Data Symbol PN-sequence * TRX Matched PN-sequence Time RX(others) Demodulator Time correlate Signal still spread Recover data symbol CDMA (cont.)

36. 36 Reverse Channel Time Frequency Channel 1 Channel N Forward Channel Amplitude Channel 3 Channel 1 Channel 2 Channel N FDD CDMA

37. 37 User N‘s signal despread while other remain TRX ( user 1 to user N) modulator Channel demodulator Correlate user N User N RX Spreading CDMA

38. 38 Many users share same frequency – TDD or FDD possible. Soft capacity limit – increase in number of users – increase in noise floor – performance degradation – no absolute limit to number of users. Multipath fading can be substantially reduced due to wideband signal and the inherent frequency diversity. Very small chip duration used – RAKE receiver can be used to improve performance via multipath combination. Soft handoff – MSC monitors a user from more than one base stations – choose the best signal without hard handoff. Self-jamming: spreading codes not exactly orthogonal. Near-far problem. CDMA (cont.)

39. 39 Spread spectrum technique is a method in which a signal is transmitted on a bandwidth considerably greater than the minimum required bandwidth. It can accommodate many users under the same bandwidth without significant interference among users. In multiple-user, multiple access interference (MAI) environment, spread spectrum systems become very bandwidth efficient. Spread Spectrum Modulation Techniques

40. 40 Spread spectrum signals are pseudorandom and have noise- like properties when compared to the digital information data. Spreading waveform is controlled by a pseudo-noise (PN) sequence. PN code appears random but can be reproduced in a deterministic manner at intended receivers. Cross-correlated at receiver with a correct PN code will despread spread spectrum signal and restore the modulated message in the same narrowband as the original data. Cross-correlated with undesired user results in very small amount of wideband noise at receiver output. Spread Spectrum Modulation Techniques (cont.)

41. 41 1)Inherent interference rejection capability – up to a certain number of users, interference between spread spectrum using same frequency is negligible. 2)Can recover spread spectrum signal when jammed by narrowband interferer – affect only small portion of signal. 3)Frequency planning is not critical or is unnecessary. 4)Resistance to multipath fading. Wideband signals are frequency selective. Spread spectrum signals have uniform energy over large bandwidth – only a small portion of spectrum affected. 5)Time domain perspective: time delayed versions have poor correlation with intended PN sequence and thus appear as uncorrelated users. Multipath contributions become invisible. 6)Property (4) and (5) can be exploited in a RAKE receiver: anticipates multipath propagation delays and thus combination of information from multipath components provides stronger version of signal. Spread Spectrum Modulation: Advantages

42. 42 Frequency Division Multiplexing (FDM) is a technique that divides the channel into multiple orthogonal subcarriers: –Input data stream is divided into several substreams of a lower data rate (increased symbol duration). –each substream is modulated and simultaneously transmitted on a separate subchannel with carrier orthogonal to each other OFDM is more spectral efficient as compared to FDM (allows more transmission channels). OFDM is less susceptible to narrowband interference and multipath... Frequency Space between channels (guard band) to avoid interference FDM OFDM Orthogonal Frequency Division Multiplexing (OFDM)

43. 43 Conventional modulation – Single Carrier OFDM – Multi Carrier Modulation Separate parallel modulations Serial-to-parallel conversion 2B –f c –B–f c +B –f c –f c –B–f c +B –f c 101010101.. –f cN –f c1 –f c2 –f c1 –f cN –f c2 10.. 101010101.. 2B Orthogonal frequency division multiplexing (OFDM) (cont.)

44. 44 TransmitterReceiver OFDM

45. 45 References Bernard Sklar, “Digital Communications : Fundamentals and Applications”, Prentice Hall, 2nd Edition, 2001. Digital Modulation in Communications Systems — An Introduction, Keysight Application Note 1298, Literature Number 5965-7160E. Cotter W. Sayre, “Complete Wireless Design”, McGraw Hill, 2nd Edition 2008. Behrouz A. Forouzan, “Data Communications and Networking”, 4th Edition, McGraw Hill, 2007.