Mobile Communications TCS 455

Transcription

1 Mobile Communication TCS 455 Dr. Prapun Sukompong Lecture 23 1 Office Hour: BKD Tueday 14:00-16:00 Thurday 9:30-11:30

2 Announcement Read Chapter 9: Section 1.2 from [Bahai, Multi-carrier Digital Communication: Theory And Application Of OFDM, 2002] Uploaded to the SIIT online lecture note ytem. Check the coure web ite for ome intereting reource and reference. 2

3 Where are we? 1. Baic communication ytem (review) 2. Cellular communication, Principle of cellular radio 3. Duplexing: TDD v FDD 4. Multiple acce cheme: FDMA, TDMA, CDMA 5. Application: Spread Spectrum Communication (DSSS) and GPS 6. Multi-carrier and OFDM ytem 7. Application: GSM, UMTS (W-CDMA) 8. Application: WiMAX and n 9. Mobile radio propagation and channel modelling, Diverity, Equalization, Channel coding 10.MIMO/SDMA Aignment 5% Cla Participation (10%) and Quizze (5%) 15% Midterm Examination 40% Final Examination (comprehenive) 40% 3 Final Exam: 9-12PM, March 9 BKD 3506

4 4 More reading..

5 Chapter 5 OFDM 5 Office Hour: BKD Tueday 14:00-16:00 Thurday 9:30-11:30

6 6 OFDM Let S 1, S 2,, S N be the information ymbol. The dicrete baeband OFDM modulated ymbol can be expreed a Some reference may ue different contant in the front Note that: N kt ( t) S k exp j, 0 t T N k 0 T 1 2 kt N 1 Sk 1 0, texp j T k 0 N T N 1 1 2kt 2kt Re ( t) ReSkco ImSkin N k 0 T T c k t Some reference may tart with different time interval, e.g. [-T /2, +T /2]

7 OFDM Application Wi-Fi: a and g verion DVB-T (the terretrial digital TV broadcat ytem ued in mot of the world outide North America) DMT (the tandard form of ADSL) WiMAX 7

8 8

9 Motivation Firt, we tudy the wirele channel. There are a couple of difficult problem in communication ytem over wirele channel. Alo want to achieve high data rate (throughput) 9

10 Chapter 5 OFDM Wirele Channel 10 Office Hour: BKD Tueday 14:00-16:00 Thurday 9:30-11:30

11 Single Carrier Tranmiion Baeband: N 1 t p t kt k0 Paband: x t Re k 1 t p t 0, T j2 fct t e 1, t 0, T 0, otherwie. 1.2 (a) (b) Time Time

12 Wirele Comm. and Multipath Fading r t x t h t n t x t n t v i0 t h t v i0 i i i i h1 t 0.5 t 0.2 t 0.2T 0.3 t 0.3T 0.1 t 0.5T h2 t 0.5 t 0.2 t 0.7T 0.3 t 1.5T 0.1 t 2.3T (b) (a) (b) Time Time Time

13 Frequency Domain The tranmitted ignal (envelope) 1 P(f) f H 1 (f) 1.5 Channel with weak multipath f H 2 (f) 1.5 Channel with trong multipath f 13

14 COST 207 Channel Model Baed on channel meaurement with a bandwidth of 8 10MHz in the 900MHz band ued for 2G ytem uch a GSM. 14 [Fazel and Kaier, 2008, Table 1-1]

15 3GPP LTE Channel Model 15 [Fazel and Kaier, 2008, Table 1-3]

16 Wirele Propagation [Bahai, 2002, Fig. 2.1] 16

17 Three tep toward OFDM 1. Solve Multipath Multicarrier modulation (FDM) 2. Gain Spectral Efficiency Orthogonality of the carrier 3. Achieve Efficient Implementation FFT and IFFT 17

18 Chapter 5 OFDM Multi-Carrier Tranmiion 18 Office Hour: BKD Tueday 14:00-16:00 Thurday 9:30-11:30

19 Multi-Carrier Tranmiion Convert a erial high rate data tream on to multiple parallel low rate ub-tream. Each ub-tream i modulated on it own ub-carrier. Since the ymbol rate on each ub-carrier i much le than the initial erial data ymbol rate, the effect of delay pread, i.e. ISI, ignificantly decreae, reducing the complexity of the equalizer. 19 [Fazel and Kaier, 2008, Fig 1-4]

20 Frequency Diviion Multiplexing To facilitate eparation of the ignal at the receiver, the carrier frequencie were paced ufficiently far apart o that the ignal pectra did not overlap. Empty pectral region between the ignal aured that they could be eparated with readily realizable filter. The reulting pectral efficiency wa therefore quite low. 20

21 FDM: Better or Wore? Comparion with a ingle higher rate erial cheme The parallel ytem, if built traightforwardly a everal tranmitter and receiver, will certainly be more cotly to implement. Each of the parallel ubchannel can carry a low ignalling rate, proportional to it bandwidth. The um of thee ignalling rate i le than can be carried by a ingle erial channel of that combined bandwidth becaue of the unued guard pace between the parallel ub-carrier. The ingle channel will be far more uceptible to inter-ymbol interference. Thi i becaue of the hort duration of it ignal element and the higher ditortion produced by it wider frequency band, a compared with the long duration ignal element and narrow bandwidth in ub-channel in the parallel ytem. 21

22 FDM (3) Before the development of equalization, the parallel technique wa the preferred mean of achieving high rate over a diperive channel, in pite of it high cot and relative bandwidth inefficiency. 22

23 OFDM OFDM = Orthogonal frequency diviion multiplexing One of multi-carrier modulation (MCM) technique Parallel data tranmiion (of many equential tream) A broadband i divided into many narrow ub-channel Frequency diviion multiplexing (FDM) High pectral efficiency The ub-channel are made orthogonal to each other over the OFDM ymbol duration T. Spacing i carefully elected. Allow the ub-channel to overlap in the frequency domain. Allow ub-carrier to be paced a cloe a theoretically poible. 23

24 24 Orthogonality Two vector/function are orthogonal if their inner product i zero. The ymbol a b a b a b a b Vector: 1 1 n *, k k 0 k 1 a n b n Time-domain: *, 0 a b a t b t dt Frequency domain: * A, B A f B f df 0 i ued to denote orthogonality. Example: in t t 2 k 1 and co 2 k 2 on 0, T T T e t j2 n T Example: Complex conjugate t 3 and 5 t t on 1,1 9 on 0, T

25 Orthogonality in Communication CDMA TDMA FDMA 1 1 t S c t S f S C f k k k k k0 k0 1 1 t S c t kt S f C f S e k k k0 k0 where c(t) i time-limited to [0,T]. Thi i a pecial cae of CDMA with c t ct kt 1 S f S C f k f k 0 k where C(f) i frequency-limited to [0,f]. Thi i a pecial cae of CDMA with C f C f kf k k where j2 fkt The c k are non-overlapping in time domain. c k c k The C k are non-overlapping in freq. domain.

26 OFDM: Orthogonality T * 2k1t 2 2 k exp exp T 0 T k t ck tc t dt j j dt 1 2 T 0 2 k1 k2 t T, k k exp j dt T 0, k k When k k, 1 2 When k k, 1 2 T * k 1 1 k 2 0 c t c t dt dt T T 2 k k t c t c t dt j k * k exp j2 k1 k2 T 0 T 11 0 j2 k k 1 2 T

27 Frequency Spectrum N kt ( t) S k 1 0, texp j T k 0 N T 1 1 ct 0, T N N 2 kt k ck t c t exp j Ck f C f C f kf T T j2 f 2 1 t C f T e in ct f c k t 1 t T in c T f T T, 2 2 T f 1 T Thi i the term that make the technique FDM. 27 N1 N1 ( t) S c t S( f ) S C f k k k k k0 k0 N 1 1 j2 f kf N k 0 S k 2 e T in c T T f kf

28 Subcarrier Spacing f 1 T OFDM FDM Spectrum Overlap in OFDM 28

29 Normalized Power Denity Spectrum Flatter when have more ub-carrier 29 [Fazel and Kaier, 2008, Fig 1-5]

30 Time-Domain Signal Real and Imaginary component of an OFDM ymbol i the uperpoition of everal harmonic modulated by data ymbol [Bahai, 2002, Fig 1.7] 30 N kt ( t) S k exp j, 0 t T N k 0 T N 1 1 2kt 2kt Re ( t) ReSkco ImSkin N k 0 T T

31 Summary So, we have a cheme which achieve Large ymbol duration (T ) and hence le multipath problem Good pectral efficiency One more problem: There are o many carrier! 31