SC - Single carrier systems One carrier carries data stream

Transcription

1 Digital modulation SC - Single carrier systems One carrier carries data stream MC - Multi-carrier systems Many carriers are used for data transmission. Data stream is divided into sub-streams and each of these sub-streams is transmitted using different carrier.

2 Multicarriermodulation f 1 R b /N Modulator 1 Data R b =1/T b Parallel / Serial R b /N R b /N f 2 Modulator 2 f N Modulator N s(t) Multicarrier transmission

3 Radio channel Signal propagation in radio channel Multipath propagation Signal attenuation Signal reflection Signal diffraction Signal refraction Signal fading Doppler effect

4 Radio channel Multipath propagation Received power Delay 1 2

5 Multicarriermodulation Received signal is a sum of signals which arrives to the receiver on different paths. Copies of the original signal reaches the receiver with different power level, different delays and are shifted in phase. It causes overlapping (interference) of transmitted signal elements. Interference level depends on length (duration) of channel response and transmission speed. Inter-Symbol Intereference (ISI) reduction is possible by employment of multicarrier transmission - OFDM (Orthogonal Frequency Division Multiplexing).

6 Multicarrier modulation- OFDM Orthogonal Frequency Division Multiplexing Type of multicarrier (multitone) transmission) Available bandwidth of the transmission channel is divided into many (N) narrowbands (subchannels). Data is transmitted parallelly in selected subchannels Subchannels carriers are orthogonal (gap between carriers is f=1/t m, where T m is a duration of modulated element) Signal generation and reception are realized using Fourier transform algorithms (IFFT in transmitter and FFT in receiver)

7 Multicarrier modulation- OFDM Channel with multipath propagation, max. channel response duration max =224 s System with single carrier transmission speed Rb =1/T b = 7.4 Mbit/s. ISI (Inter-Symbol Interference) level : max /T b = 1600 Multicarrier system (OFDM) Data stream with speed R b is divided into N parallel sub-streams, each of speed R mc = 1/T mc = R b /N. ISI is reduced to level: max /T mc = max /(T b N) For DVB-T (number of carriers is N=8192): max /T mc =0.2

8 Multicarriermodulation N carriers data frequency B f0 carrier B T=1/f 0 time symbol OFDM

9 Multicarrier modulation- OFDM serial to parallel conversion symbol mapping parallel to serial conversion guard interval insertion guard interval removal serial to parallel conversion symbols to bit streams conversion parallel to serial conversion

10 Multicarrier modulation- OFDM OFDM advantages Reduction of signal distortions caused by InterSymbol Interference(ISI) Employment of slow bitrate parallel transmission instead of high bitrate single stream transmission cause extension of modulated element duration to the value related to channel response length. High spectral efficiency High flexibility enabling system optimization from point of maximal transmission speed. It is realised by proper allocation of power and modulation format in frequency subchannels.

11 Multicarrier modulation- OFDM OFDM Disadvantages Susceptibility for signal fading (loss) Precise synchronization reguired Special training sequences and pilot signals are used. Cannot be used in nonlinear channels where constant envelope signals are required OFDM signals characterize high amplitude changes.

12 Multicarrier modulation- OFDM DMT is a type of OFDM modulation and is used in DSL (Digital Subscriber Loops) systems. DMT use 224 carrier in downlink direction (downstream) and 32 carriers in uplink direction (upstream), the gap between adjacent carriers is kHz 61

13 Multicarrier modulation- OFDM Applications: Digital TV DVB-T (Digital Video Broadcasting for Terrestrial) Digital radio DAB (Digital Audio Broadcasting) High speed data transmission on wired subscriber loops ADSL (Asymmetric Digital Subscriber Loops) VDSL (Very High Speed Digital Subscriber Loops) Wireless Local Area Networks (Wi-Fi) (IEEE g,n) WiMax systems (802.16) Cellular telephone network LTE (Long Term Evolution

14 Multiple Access Multiple Access Techniques

15 Multiple Access Fixed Assignment Protocols: FDMA (SCPC) TDMA CDMA Demand Assignment Protocols: DAMA-TDMA DAMA- FDMA reservation ALOHA Random Access Protocols: ALOHA S-ALOHA SREJ-ALOHA

16 Multiple Access Desirable Features High efficiency in terms of the throughput Low access delay Stability Efficient starting up of new stations Low complexity

17 Multiple Access FDMA TDMA CDMA Advantages: - network timing not required Disadvantages: - intermodulation noise reduces the usable output power, hence there is a loss of capacity relative to single carrier capacity - uplink control power required - the frequency allocation may be difficult to modify Advantages: - uplink power control not needed - no mutual interference between accesses - digital circuitry Disadvantages: - network control (timing) required - stations transmits high bit-rate bursts requiring large peak power Advantages: - antijamming capabilities - network timing not required Disadvantages: - wide bandwidth per user required - precision code synchronization neeed

18 Multiple Access Fixed Assignment FDMA frequency frequency TDMA time time The Frequency or Time resource is shared between stations according to a scheme which does not vary with time

19 Multiple Access Demand Assignment FDMA TDMA frequency pool of frequency bands frequency pool of time slots time time The Frequency or Time resource is shared between stations according to the demand from individual stations

20 Multiple Access FDMA Frequency Division Multiple Access Available Channel Bandwidth f f1 f2 f3 f4

21 Multiple Access TDMA Time Division Multiple Access Preamble Data Reference Burst. e1 e2 en-1 en t Frame

22 Multiple Access carrier power T F TDMA speed R TDMA carrier power time T P R TDMA = R FDMA TP FDMA speed R FDMA T F time Example: R b =64kbit/s N=50 terminals FDMA R FDMA =64kbit/s total throughput: 50x64=3.2Mbit/s TDMA every terminal transmitting with speed R TDMA =3.2Mbit/s 17dB power increase high terminal cost

23 Spread Spectrum Spread Spectrum technique Transmitter spreads baseband signal from bandwidth B to W W/B - spreading factor ( ) Receiver despreads only signal with proper address Received signals with other addresses and jammer are spread by the receiver and act as a noise Addresses are periodic sequences that either modulate the carrier directly (DIRECT SEQUENCE SYSTEMS) or change the frequency state of the carrier (FREQUENCY HOPPING SYSTEMS) CDMA is based on spread spectrum transmission

24 Spread Spectrum Power spectral density Data signal Data signal after spreading frequency f 0 W B Data (information signal) R b =1/T b Spreading sequence (code) R c =1/T c Modulation gain (spreading factor) W B = R c R b

25 Spread Spectrum Direct Sequence System satellite data R b =1/T b d(t) X X X X c(t) Code generator R c =1/T c cos2 f c t R c >> R b cos2 f c t Code sync LPF c(t) Code generator 1 T b T (..)dt b 0 data d(t) T c T b code c(t) d(t) c(t) data received c(t)

26 Spread Spectrum undesired signals satellite data 1 SPREADING 2 W B 3 DESPREADING received data 4 1 psd (power spectral density) 3 psd jammer Signal + noise + other users signals. 2 psd B W frequency 4 psd f c frequency f c frequency frequency

27 Spread Spectrum Advantages: interference protection and antijamming capabilities (immunity increases with larger modulation gain) Many users can share the same frequency band Signal unavailable without spreading code knowledge CDMA do no require network synchronization Disadvantages: Wide bandwidth required Precision synchronization of spreading code required

28 Spread Spectrum CDMA receiver X X recovered data signal KOD SYNC c(t) CODE GENERATOR 2cos2 f c t CARRIER RECOVERY Coherent demodulation implies recovery of the transmitted carrier at the receiver side. Despreading is performed prior to demodulation in order to reduce interference from other than desired carriers and improve carrier recovery performance

29 Spread Spectrum Clock f = 1/T c T c T c T c CDMA - code generation Shift register n stages code rate: R c =1/T c period: 2 n -1 chips Example: Code Correlation Function -1/(2 n -1) R code ( ) T c Code Power Spectral Density 2 n -1 chips 0 -R c R c f=r c /(2 n -1) frequency T c T c T c + n=3 Shift register status period output code sequence

30 Spread Spectrum Frequency hopping (FH) Generated frequency depends on spreading code and data and is changed every T c Bit duration T b Slow SFH (T c T b, hop for a few data elements ) Fast FFH (T c < T b, many shops per data element) Immunity to interference increasing with number of used frequencies

31 Spread Spectrum Spreading sequences (codes) Maximal length pseudorandom sequences good auto and cross-correlation properties small number of sequences Gold and Kasami codes. Generated from maximal length pseudorandom sequences, Similar correlation properties, more sequences to use. Walsh and Hadamard codes

32 CDMA transmitter receiver User 1 noise cos2πf c t User 2 cos2πf c t User n cos2πf c t

33 CDMA System capacity assumption: noise - other users signals K - number of users (stations) R b - data bit rate R c - code bit rate C - carrier power (E b /N 0 ) W - required value of (E b /N 0 ) for specified BER E b = C R b N 0 = (K-1)C R c E b N 0 W = R c 1 R b (K-1) K = max R c 1 R b (E b N 0 ) W / + 1