How To Understand The Theory Of Time Division Duplexing

Transcription

1 Multiple Access Techniques Dr. Francis LAU Dr. Francis CM Lau, Associate Professor, EIE, PolyU Content Introduction Frequency Division Multiple Access Time Division Multiple Access Code Division Multiple Access Dr. Francis CM Lau, Associate Professor, EIE, PolyU 2

2 multiple access Introduction techniques allowing users to share simultaneously a finite amount of radio spectrum duplexing two-way communications to occur simultaneously Dr. Francis CM Lau, Associate Professor, EIE, PolyU 3 Introduction frequency division duplexing (FDD) frequency separation between each forward and reverse channel is constant throughout the system, regardless of the particular channel being used Dr. Francis CM Lau, Associate Professor, EIE, PolyU 4 2

3 Introduction time division duplexing (TDD) Dr. Francis CM Lau, Associate Professor, EIE, PolyU 5 Introduction narrowband systems signal bandwidth is comparable to the coherence bandwidth of the channel radio spectrum divided into a large number of narrowband channels usually operated using FDD frequency separation as large as possible to minimize interference TDD also possible Dr. Francis CM Lau, Associate Professor, EIE, PolyU 6 3

4 Introduction wideband systems signal bandwidth is much larger than the coherence bandwidth of the channel what type of fading occurs? TDMA allocates time slots to many users on the channel and allows only one user to access the channel at any one time CDMA allows all users to access the channel at the same time work with both FDD and TDD Dr. Francis CM Lau, Associate Professor, EIE, PolyU 7 Introduction Dr. Francis CM Lau, Associate Professor, EIE, PolyU 8 4

5 Frequency Division Multiple Access (FDMA) Dr. Francis CM Lau, Associate Professor, EIE, PolyU 9 FDMA each user is assigned a unique frequency band or channel no other user can share the same channel during the period of the call in FDD systems, a channel consists of a frequency pair is assigned one frequency for forward channel one frequency for reverse channel Dr. Francis CM Lau, Associate Professor, EIE, PolyU 0 5

6 FDMA each FDMA channel has a relatively narrow bandwidth because only one user is being supported symbol time is large compared to the average delay spread what type of fading occurs? lower complexity and lower data rate compared with TDMA fewer bits needed for overhead compared with TDMA Dr. Francis CM Lau, Associate Professor, EIE, PolyU nonlinear effects FDMA many channels share the same antenna at the base station power amplifiers or power combiners are nonlinear when operating at or near saturation for maximum power efficiency nonlinearities cause intermodulation (IM) which can interfere with other channels Dr. Francis CM Lau, Associate Professor, EIE, PolyU 2 6

7 Example 9. Find the intermodulation frequencies generated if a base station transmits two carrier frequencies at 930MHz and 932MHz that are amplified by a saturated clipping amplifier. If the mobile radio band is allocated from 920MHz to 940MHz, designate the IM frequencies that lies inside and outside the band. Dr. Francis CM Lau, Associate Professor, EIE, PolyU 3 Solution 9. Intermodulation distortion products occurs at frequencies mf +nf 2 for all integer values of m and n. Some of the possible IM frequencies that are produced by a nonlinear device are (2n+)f 2nf 2, (2n+2)f (2n+)f 2, (2n+)f 2 2nf, (2n+2)f 2 (2n+)f required signals Dr. Francis CM Lau, Associate Professor, EIE, PolyU 4 7

8 Time Division Multiple Access (TDMA) Dr. Francis CM Lau, Associate Professor, EIE, PolyU 5 TDMA divide the radio spectrum into time slots only one user is allowed to either transmit or receive in each time slot N time slots comprise a frame data transmitted in a buffer-and-burst method noncontinuous transmission Dr. Francis CM Lau, Associate Professor, EIE, PolyU 6 8

9 TDMA Dr. Francis CM Lau, Associate Professor, EIE, PolyU 7 TDMA mobile assisted handoff (MAHO) can be performed by a subscriber by listening on an idle slot in the TDMA frame possible to allocate different number of time slots per frame to different users (e.g. GPRS) higher transmission rate gives rise to a signal bandwidth larger than the coherence bandwidth of the channel What type of fading occurs? larger overheads compared with FDMA Dr. Francis CM Lau, Associate Professor, EIE, PolyU 8 9

10 Efficiency TDMA frame efficiency: percentage of bits per frame that contain transmitted data information rate/transmission rate Dr. Francis CM Lau, Associate Professor, EIE, PolyU 9 Example 9.3 Consider GSM, which is a TDMA/FDD system that uses 25MHz for the forward link, which is broken into radio channels of 200kHz. If 8 speech channels are supported on a single radio channel, and if no guard band is assumed, find the number of simultaneously users that can be accommodated in GSM. Dr. Francis CM Lau, Associate Professor, EIE, PolyU 20 0

11 Solution 9.3 number of simultaneously users that can be accommodated in GSM N = (25MHz/200kHz) x 8 = 000 Dr. Francis CM Lau, Associate Professor, EIE, PolyU 2 Example 9.4 If GSM uses a frame structure where each frame consists of eight time slots, and each time slot contains bits, and data are transmitted at kbps in the channel, find (a) the time duration of a bit, (b) the time duration of a slot, (c) the time duration of a frame, (d) how long must a user occupying a single time slot wait between two successive transmissions. Dr. Francis CM Lau, Associate Professor, EIE, PolyU 22

12 Solution 9.4 If GSM uses a frame structure where each frame consists of eight time slots, and each time slot contains bits, and data are transmitted at kbps in the channel, find (a) the time duration of a bit T b = / kbps = µs (b) the time duration of a slot T s = T b = 0.577ms (c) the time duration of a frame T f = 8 T s = 4.65 ms (d) a user needs to wait one frame duration, i.e., 4.65 ms, between two successive transmissions Dr. Francis CM Lau, Associate Professor, EIE, PolyU 23 Example 9.5 If a normal GSM time slot consists of six trailing bits, 8.25 guard bits, 26 training bits, and two traffic bursts of 58 bits of data, find the frame efficiency Dr. Francis CM Lau, Associate Professor, EIE, PolyU 24 2

13 Solution 9.5 If a normal GSM time slot consists of six trailing bits, 8.25 guard bits, 26 training bits, and two traffic bursts of 58 bits of data, find the frame efficiency no. of data bits per time slot = 2 x 58 = 6 equivalent no. of bits per time slot = 2 x = frame efficiency = 6/56.25 = 74.24% Dr. Francis CM Lau, Associate Professor, EIE, PolyU 25 Code Division Multiple Access (CDMA) Dr. Francis CM Lau, Associate Professor, EIE, PolyU 26 3

14 CDMA Binary data Phase modulator 2P cos[ ω0t + θ ( t)] d 2Pc( t)cos[ ω 0t +θd ( t)] 2P cosω0t BPSK DS-SS transmitter BPSK spreading accomplished by multiplying s d (t) by a function c(t)= ± representing the spreading waveform c(t) transmitted signal Dr. Francis CM Lau, Associate Professor, EIE, PolyU 27 CDMA Binary data Phase modulator 2P cos[ ω0t + θ ( t)] d 2Pc( t)cos[ ω 0t +θd ( t)] 2P cosω0t c(t) Dr. Francis CM Lau, Associate Professor, EIE, PolyU 28 4

15 BPSK DS-SS Power spectral density of data-modulated carrier 2 2 S d ( f ) = PT sinc [( f f 0 ) T + sinc [( f + f 0 ) T 2 (two-sided psd of a BPSK carrier) { } Dr. Francis CM Lau, Associate Professor, EIE, PolyU 29 BPSK DS-SS psd of data- and spreading code-modulated carrier spreading code chip s t (t) is also a BPSK carrier with T replaced by T c T c = T/3 bandwidth of the transmitted signal spread by a factor of 3 level of the psd reduced by a factor of 3 Dr. Francis CM Lau, Associate Professor, EIE, PolyU 30 5

16 distortionless channel BPSK DS-SS interference and/or Gaussian noise receiver's best estimate of the transmission delay 2Pc( t Td ) cos[ ω0t + θd ( t Td ) + φ ] + interference Bandpass filter Data phase demodulator transmission delay Estimated data signal component c ( t Tˆ ) Despreading mixer d Pc( t T ) c( t Tˆ )cos[ ω t + θ ( t T ) + φ ] 2 d d 0 d d BPSK DS-SS receiver despreading: re-modulation or correlation of the received signal with the delayed spreading waveform Dr. Francis CM Lau, Associate Professor, EIE, PolyU 3 Spreading Codes pseudorandom (PN) codes m-sequence Gold codes Walsh Codes Dr. Francis CM Lau, Associate Professor, EIE, PolyU 32 6

17 7 Dr. Francis CM Lau, Associate Professor, EIE, PolyU 33 Hadamard matrix M n n x n matrix n = even integer elements are one row of the matrix contains all ones other rows contain n/2 no. of + and n/2 no. of any row differs from the other row in exactly n/2 positions ± Dr. Francis CM Lau, Associate Professor, EIE, PolyU 34 Hadamard matrix M n = 2 M = n n n n n M M M M M 2 = = ; M 4 M 4

18 Walsh-Hadamard Codes rows of the Hadamard matrix used as code words code mutually orthogonal M 4 = code 2 code 3 code 4 e.g. row and 2,.+.( )+.+.( ) = 0 breaks down in the presence of multipath Dr. Francis CM Lau, Associate Professor, EIE, PolyU 35 CDMA narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal all users use the same carrier frequency and may transmit simultaneously, TDD or FDD may be used unlike FDMA or TDMA, CDMA has a soft capacity limit increasing the no. of users raises the noise level, more errors occur Dr. Francis CM Lau, Associate Professor, EIE, PolyU 36 8

19 CDMA near-far problem mitigated using power control spread spectrum bandwidth much greater than the coherence bandwidth what type of fading? frequency reuse factor in CDMA cellular system is all cells use the same spectrum soft handoff Dr. Francis CM Lau, Associate Professor, EIE, PolyU 37 Summary multiple access techniques frequency division multiple access time division multiple access code division multiple access Dr. Francis CM Lau, Associate Professor, EIE, PolyU 38 9

20 Reading Summary Rappaport T. S., Wireless Communications: Principles and Practice, Prentice Hall PTR, Sections , Problems Rappaport T. S., Wireless Communications: Principles and Practice, Prentice Hall PTR, Problems , Dr. Francis CM Lau, Associate Professor, EIE, PolyU 39 20