Multiple Access Techniques

Multiple Access Techniques Instructor: Prof. Dr. Noor M. Khan Department of Electrical Engineering, Faculty of Engineering, Mohammad Ali Jinnah University, Islamabad Campus, Islamabad, PAKISTAN Ph: +92 (51) 111-878787 Ext. 129 (Office), 186 (ARWiC Lab) Fax: +92 (51) 2822743 emails: noor@ieee.org, noormkhan@jinnah.edu.pk S 1 I BS T B 2 S I K S I T B T B Multiple Access Techniques 1

Cellular System MSC PSTN Multiple Access Techniques 2

Frequency Reuse Pattern G F Bandwidt h B A E C D f G F B A E G F C D B A E G F C D B A E C D Multiple Access Techniques 3

Handoff (Handover) BS3 BS2 BS1 Multiple Access Techniques 4

Single Cell Multiple Access Uplink M N Downlink BS M K M 1 M 2 Multiple Access Techniques 5

Multiple Access System A cell M 1 M 2 M K f Bandwidth BS M K M 1 M 2 Multiple Access Techniques 6

Multiple Access Techniques Many mobile users to share the available radio spectrum simultaneously Multiple Access Frequency division multiple access - FDMA Time division multiple access - TDMA Code division multiple access - CDMA Space division multiple access - SDMA Packet Radio Access Multiple Access Techniques 7

Duplex System Uplink Downlink BS Downlink = Forward Link Uplink = Reverse Link Multiple Access Techniques 8

Duplexing Need to talk and listen simultaneously Duplexing (uplink and downlink for the same user) Two types of duplexing schemes Frequency division duplexing - FDD Time division duplexing TDD (3G, 4G) For example: there could be FDMA/FDD (AMPS)or TDMA/FDD (GSM, IS-54) etc. Multiple Access Techniques 9

Channel Capacity C Amount of information (number of bits) transmitted/received C, with vanishingly small probability of error (Shannon 1948) TRANSMITTER B CHANNEL P N C RECEIVER P = B log 1 + N C 2 C Capacity B Bandwidth P Signal Power N Noise Power Multiple Access Techniques 10

Communication Channel Capacity Communication Channel Resources are: T - Time B - Bandwidth P P - Signal to Noise Ratio S = log 2 1 + 2 2 σ σ S I I = T B S T B Amount of information (number of bits) transmitted/received, with arbitrarily small probability of error (Shannon 1948) 1 + = B log 2 2 Multiple Access Techniques 11 C P σ

FDMA Each user is allocated a unique frequency band or channel The allocation is done on demand S I T B FDMA AMPS Multiple Access Techniques 12 S T B

FDMA Features One channel carries one phone circuit Idle speech periods results in wasted bandwidth Bandwidth of the channels are relatively narrow (for example: 30 khz) Synchronous transmission with low overheads Multiple Access Techniques 13

Example: Start of AMPS During this time the American cellular radio system, known as the advanced mobile phone system, or AMPS, was developed primarily by AT&T and Motorola, Inc. AMPS was based on 666 paired voice channels, spaced every 30 kilohertz in the 800-megahertz region. AMPS system employed an analog-frequency modulation, and was designed to support both mobile and portable subscriber units. Multiple Access Techniques 14

AMPS Frequencies Band Frequency Range (MHz ) Use A 824 to 835 and 845 to 846.5 Transmit from mobile 869 to 880 and 890 to 891.5 Receive at mobile B 835 to 845 and 846.5 to 849 Transmit from mobile 880 to 890 and 891.5 to 894 Receive at mobile 416 channels of 30 khz Multiple Access Techniques 15

FDMA Features Higher cell site system costs than TDMA High cost of band-pass filters and other RF circuitry. Tight RF filtering Mitigating adjacent channel interference Narrowband System; Very strong flat fading The number of channels that can be simultaneous supported N= (B t -2B guard )/B c Where, B t is total spectrum allocation; B guard is guard band and B c is channel bandwidth Synchronization not necessary Multiple Access Techniques 16

S TDMA Each user occupies a cyclically repeating time slot Asynchronous transmission I digital modulation must be used & there are overheads for synchronization. T B Multiple Access Techniques 17 S TDMA GSM T B

TDMA Frame Structure Multiple Access Techniques 18

GSM Frame Structure In GSM, a set of 8 TDMA slots is called a frame. Mobile terminals transmit in bursts of 577 microseconds. One burst fits into one time slot. There are two types of GSM frames: Traffic Frames Signaling Frames Multiple Access Techniques 19

GSM Burst Structure A normal GSM burst contains: 3 tail bits allowing the transmitter to power up 57 coded data bits 1 bit stealing flag 26 bits training sequence for synch and adaptive equalizer 1 bit stealing flag 57 coded data bits 3 tail bits Guard time corresponding to 8.25 bits In GSM, a terminal has 28 microseconds of guard time to power up or switch off the RF signal. Multiple Access Techniques 20

TDMA Features Shares single carrier frequency with several users The number of time slots per frame depends on - modulation technique, available bandwidth, etc. Because of discontinuous transmission in TDMA, the handover process is much simpler A mobile can listen for other base stations during idle time slots Possibilities for Bandwidth on Demand Less sensitive to fading than FDMA Multiple Access Techniques 21

Efficiency The frame efficiency η f, is the percentage of bits per frame which contain transmitted data Number of overhead bits per frame is: b = N b + N b + N b + OH r r t p t g N N r - no of reference bursts per frame, N t - no of traffic bursts per frame, b p - no of OH bits per preamble in each slot and b g - no of equivalent bits each guard time b r - no of OH bits per reference burst r b g Multiple Access Techniques 22

Efficiency Total number information bits per frame b T = T f R T f -frame duration and R- channel bit rate Then η f 1 b = OH b T Multiple Access Techniques 23

Spread Spectrum Multiple Access (CDMA) Uses signals that have BW >> minimum required RF bandwidth Pseudo-noise sequence converts a narrow band signal to a wideband noise like signal Efficiency increases with the number of users Two main types of Spread Spectrum MAs Frequency hopped (FH) Direct sequence (CDMA) Multiple Access Techniques 24

Frequency Hopping MA Carrier frequencies of individual users are varied in pseudo randomly (using PN) Data is broken into uniformed sized bursts and transmitted on different carrier frequencies Instantaneous BW of a any one transmission is much smaller than the total spread BW Locally generated PN code is used to synchronize the receivers instantaneous frequency with the transmitters Multiple Access Techniques 25

FHMA At any given time FH signal occupies a single relatively narrow channel If the rate of change of carrier is greater then the data symbol rate FHMA system is called fast frequency hopping If the rate of change of carrier is smaller then the data symbol rate FHMA system is called slow frequency hopping FHMA provides higher level of Security Protection against signal fading Multiple Access Techniques 26

Frequency Hopped Multiple Access(FHMA) In FHMA the carrier frequencies of the individual users are varied in a pseudorandom fashion within a wideband channel. The instantaneous bandwidth of any one transmission burst is much smaller than the total spread bandwidth. At any given point in time, a frequency hopped signal only occupies a single, relatively narrow channel. M( ω ) = F{ m(t) } P( ω ) = F{ p(t) } ω ω B m B a Multiple Access Techniques 27

Code Division MA (CDMA) The signal is multiplied by a very large bandwidth spreading signal Spreading signal is a PN code sequence that has a chip rate much larger then the data rate of the message All users use the same carrier frequency S I T S T B CDMA IS-95 Multiple Access Techniques 28 B

Direct Sequence Spread Spectrum System Model m(t) Transmitter p(t) m(t) Message s(t) Spreading Code s(t (t) Channel n(t) p(t) Spread Signal n(t) AWGN Noise d(t) Receiver r(t) r(t) Received Signal d(t) Despread Signal s(t) r(t) = m(t)s(t) + d(t) = r(t)s(t) n(t) Multiple Access Techniques 29

DS SS Modulation in Time and Frequency TIME Domain Frequency Domain m(t) Message 1-1 1 1-1 s(t) Spreading Code p(t) Spread Signal Band-limited Spread Spectrum Signal Time Frequency Multiple Access Techniques 30

Properties of the Spreading Code, s(t) M( ω ) = F{ m(t) } ω 1) s(t) s(t) = 2) B S >> B m 1 B m S( ω ) = F{ s(t) } ω d(t) = r(t) s(t) d(t) = m(t)s(t)s(t) + n(t)s(t) d(t) = m(t) + n a (t) B S Multiple Access Techniques 31

Spectrum of SS Signal M( ω ) = F{ m(t) } ω p(t) = m(t) s(t) B m S( ω ) = P( ω ) = F { s(t) } F{ p(t) } B s B m B s ω ω P( ω) = M(ω) S(ω) since B S >> Bm Bp B S G = B B s m Multiple Access Techniques 32

Spread Spectrum System Before Spreading After Spreading After Despreading M( B m ω ) P - signal P power P(ω) ω B M( B m ω ) P N 0 = ω s(t) 2 =1 2 σ = N0Bm B s G = B Multiple Access Techniques 33 k B m B s T ω ω m P SNR b = Gσ P SNR o = σ 2 2

Hiding Signal in Noise P(ω) P AWGN N 0 = kt ω D(ω ) P Before despreading AWGN ω B m After despreading LPI Low Probability of Interception CDMA Spread Spectrum

Multiple Access S T S T S T B B B FDMA AMPS TDMA GSM CDMA IS-95 Multiple Access Techniques 35

Two CDMA Signals M 1 ( ω ) P ω M 2 ( ω ) P ω B m P 1 ( ω ) P 2 ( ω ) B m N 0 = k T ω M ~ 1( ω ) P Signal Interference B s AWGN N 0 = k T ω B m B s 2B s Multiple Access Techniques 36

Two Signals in Noise r(t) = m 1 (t)s 1 (t) + m 2 (t)s 2 (t) + n(t) d 1 (t) = r(t)s 1(t) d1 (t) = m 1(t)s 1(t) s 1(t) + m 2 (t)s 2 (t)s 1(t) + n(t)s 1(t) d1 (t) = m 1(t) + p 2 (t)s 1(t) + n(t)s 1(t) Signal Interference Noise We want to find output SNR o to evaluate BER Multiple Access Techniques 37

Output Signal to Noise Ratio Signal power - > P Noise power - > Effective interference power - > Total noise power - > 2 σ P G σ 2 + P G SNR o = σ 2 P + P G = SNRm SNR 1+ G m Multiple Access Techniques 38

k Signals in Noise r(t) = m 1 (t)a 1(t) + m 2(t)a2(t) + + m k(t)ak (t) + n(t) d1 (t) = r(t)a 1(t) d1 (t) = m 1(t)a 1(t)a 1(t) + p2(t)a 1(t) + + pk(t)a 1(t) + n(t)a 1(t) d1 (t) = m 1(t) + p2(t)a 1(t) + + pk (t)a 1(t) + n(t)a 1(t) Signal Interference Noise Again we want to find output SNR o ; -> BER CDMA Spread Spectrum

M 1 ( ω ) B m K SS Signals P ω P 1 (ω) ( M 2 B ω ) m ω P P 2 (ω) P k (ω) 2 k N 0 = M k ( ω ) B k T m P ω ω ~ M 1 ( ω ) P Signal Interference B a AWGN N 0 = k T ω B m CDMA Spread Spectrum B a 2B a

Output Signal to Noise Ratio Signal power - > P Noise power - > 2 σ (k -1)P Effective interference power - > G (k -1)P Total noise power - > σ 2 + G SNR o = σ 2 + P (k-1)p G = 1+ SNRm (k-1) SNR G m CDMA Spread Spectrum

CDMA Each user has its own PN code which is orthogonal (near-orthogonal) to all other PN codes To receive the receiver needs to know the code used by the transmitter The power of other users at a receiver determines the noise floor after signal demodulation Therefore it is necessary to control the power of each user in order to avoid excessive interference Multiple Access Techniques 42

CDMA Features Many users share the same frequency Increasing the number of users does not raise the noise floor in a linear manner Multi-path fading may be substantially reduced Channel data (chip) rates are very high Multiple Access Techniques 43

Space Division MA (SDMA) Controls the radiated energy for each user in space Servers different users using directional antennas The different areas may be served by the same frequency (TDMA or CDMA) or different frequencies (FDMA) Multiple Access Techniques 44

Smart Antenna System (SDMA) (1) N K M K 1 BS 2 M 1 M 2 Multiple Access Techniques 45

Smart Antenna System (SDMA) (2) S I 1 T B BS S 2 I K T B S N S Multiple Access Techniques 46 I T B I N times the original amount of information I Smart Antennas are reusing channel resources B,T,S T B

Orthogonal Frequency Division Multiple Access (OFDMA) A method of encoding digital data on multiple carrier frequencies A method of digital modulation in which a signal is split into several narrowband channels at different frequencies. A digital transmission technique that uses a large number of carriers spaced apart at slightly different frequencies Multiple Access Techniques 47

OFDM: Operation Total bandwidth available within a communications system is divided into smaller non-overlapping frequency sub-bands Usually a separate data signal is associated to each frequency sub-band Passband filter at receiver extracts requested frequency sub-band / data signal Multiple Access Techniques 48

OFDMA Each terminal occupies a subset of subcarriers Subset is called an OFDMA traffic channel Each traffic channel is assigned exclusively to one user at any time Multiple Access Techniques 49

OFDM System It is a special kind of FDM The spacing between carriers are such that they are orthogonal to one another Therefore, there is no need of guard band between carriers. Multiple Access Techniques 50

OFDM: Operation Multiple Access Techniques 51

OFDM: Operation Multiple Access Techniques 52

OFDM: System Model Multiple Access Techniques 53

OFDM: System Model Multiple Access Techniques 54

OFDM: System Model Multiple Access Techniques 55

OFDM: An Example Let s we have following information bits 1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, Just convert the serial bits to parallel bits C1 C2 C3 C4 1 1-1 -1 1 1 1-1 1-1 -1-1 -1 1-1 -1-1 1 1-1 -1-1 1 1 Multiple Access Techniques 56

OFDM: An Example Modulate each column with corresponding sub-carrier using BPSK Modulated signal for C1 Modulated signal for C2 Modulated signal for C3 Modulated signal for C4 Multiple Access Techniques 57

OFDM: An Example Final OFDM Signal = Sum of all signal V ( t) N 1 = n= 0 I n ( t)sin(2 π nt) Generated OFDM signal, V(t) Multiple Access Techniques 58

OFDMA s Encouraging Features Can easily adapt to severe channel conditions without complex time-domain equalization. Robust against narrow-band co-channel interference. Robust against ISI and fading caused by multipath propagation. High spectral efficiency as compared to conventional modulation schemes, spread spectrum, etc. Efficient implementation using Fast Fourier Transform (FFT). Low sensitivity to time synchronization errors. Tuned sub-channel receiver filters are not required (unlike conventional FDM). Multiple Access Techniques 59

OFDMA s Discouraging Features Sensitive to Doppler shift. Sensitive to frequency synchronization problems. High peak-to-average-power ratio (PAPR), requiring linear transmitter circuitry, which suffers from poor power efficiency. Loss of efficiency caused by cyclic prefix/guard interval. Multiple Access Techniques 60

Packet Radio Many subscribers attempt to access a single channel in an uncoordinated or minimally coordinated manner Transmission is done by using bursts of data Collisions from simultaneous transmission of multiple transmitters are detected by the BS - ACK or NACK Multiple Access Techniques 61

Packet Radio ACK/NACK provides perfect feedback, even though traffic delay due to collisions may be high Very easy to implement but has low spectral efficiency and may induce delays Multiple Access Techniques 62

Packet Radio Subscribers use a contention technique to transmit on a common channel Eg. ALOHA The performance of packet radio schemes can be evaluated by the throughput (T) - the average no of messages successfully transmitted per unit time and the average message delay (D) Multiple Access Techniques 63

Packet Radio Protocols Multiple Access Techniques 64

Packet Radio Protocols Assume that the system uses fixed packet sizes and fixed channel data rates Packets are generated by users randomly and packet transmissions occur with a Poisoon distribution having a mean arrival rate of λ pps If τ is the packet duration, in seconds, the traffic occupancy R= λ τ R - normalized channel traffic Multiple Access Techniques 65

Throughput Pr( n) = R n R V e n! V RV = λτ V Multiple Access Techniques 66

Throughput Pr( n ) = R n V e n! R V Pr(0) = e λτ V Multiple Access Techniques 67

Pure ALOHA User grabs a channel as soon as a message is ready for transmission After transmission, the user waits for an ACK/NACK (acknowledgement) In the case of collisions, the user waits for a random period of time and re-transmits The vulnerable period is double the packet length Multiple Access Techniques 68

Pure ALOHA 2R n (2 R ) e Pr( n ) = at n = 0 n! Multiple Access Techniques 69

Slotted ALOHA Time is divided into equal time slots of length greater than the packet duration τ Subscribers transmit messages only at the beginning of a new time slot The vulnerable period is only one packet duration The probability that no other packet will be generated during the vulnerable period is e -R. Then the throughput of the slotted ALOHA is: T = Re -R Multiple Access Techniques 70

ALOHA Throughput Multiple Access Techniques 71