Chapter 2 Overview Part 1 (last week) Digital Transmission System Frequencies, Spectrum Allocation Radio Propagation and Radio Channels Part 2 (today) Modulation, Coding, Error Correction Part 3 (next week) Capacity limits Duplexing schemes Media Access Protocols - 1 -
Structure of Digital Transmission System Digital Source Sink - 2 -
Modulation Modification of the amplitude, phase or frequency of one or more sineshaped carriers Combination between amplitude and phase modulation also possible Analogue modulation schemes for transmission of continuous signals AM, FM, PM Digital modulation schemes for transmission of discrete signals Binary Phase Shift Keying: change between two different phases Quadrature Amplitude Modulation with 16 states (16QAM): 16 combinations of different amplitudes and phases - 3 -
Digital Modulation Schemes Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK): very simple low bandwidth requirements very susceptible to interference Frequency Shift Keying (FSK): needs larger bandwidth 1 0 1 1 0 1 t t Phase Shift Keying (PSK): more complex robust against interference 1 0 1 t - 4 -
Advanced Phase Shift Keying BPSK (Binary Phase Shift Keying): Q bit value 0: sine wave bit value 1: inverted sine wave very simple PSK 1 0 I low spectral efficiency robust, used e.g. in satellite systems 10 Q 11 QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol I symbol determines shift of sine wave needs less bandwidth compared to BPSK more complex A 00 01 Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PHS) t 11 10 00 01-5 -
Quadrature Amplitude Modulation Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation it is possible to code n bits using one symbol 2 n discrete levels, n=2 identical to QPSK bit error rate increases with n, but less errors compared to comparable PSK scheme Example: 16-QAM (4 bits = 1 symbol) Symbols 0011 and 0001 have the same phase φ, but different amplitude a. 0000 and 1000 have different phase, but same amplitude used in standard 9600 bit/s modems Q 0010 0011 a φ 0001 0000 I 1000-6 -
Hierarchical Modulation DVB-T modulates two separate data streams onto a single DVB-T stream High Priority (HP) embedded within a Low Priority (LP) stream Multi carrier system, about 2000 or 8000 carriers QPSK, 16 QAM, 64QAM Example: 64QAM good reception: resolve the entire 64QAM constellation poor reception, mobile reception: resolve only QPSK portion 6 bit per QAM symbol, 2 most significant determine QPSK HP service coded in QPSK (2 bit), LP uses remaining 4 bit 10 00 Q 000010 010101 I - 7 -
Frequency Bandwidth information source r channel information sink sender receiver For the transmission of symbols with rate r a frequency bandwidth of f r is necessary (with appropriate pulse shaping). - 8 -
Signal length vs. Bandwidth finite length signal e.g. rectangle ( bit ) short signal infinite frequency spectrum wide frequency spectrum pulse shaping long signal narrow frequency spectrum infinite length signal e.g. sine finite frequency spectrum - 9 -
Spread Spectrum Technology Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Solution: spread the narrow band signal into a broad band signal using a special code protection against narrow band interference power interference spread signal detection at receiver protection against narrowband interference Side effects: coexistence of several signals without dynamic coordination tap-proof Alternatives: Direct Sequence, Frequency Hopping f power signal spread interference f - 10 -
Effects of spreading and interference dp/df dp/df i) dp/df ii) f sender dp/df f user signal broadband interference narrowband interference dp/df iii) iv) v) f receiver f f - 11 -
Spreading and frequency selective fading channel quality 1 2 3 4 5 6 narrowband channels frequency narrow band signal guard space channel quality 2 1 2 2 2 2 spread spectrum channels spread spectrum frequency - 12 -
DSSS (Direct Sequence Spread Spectrum) (1) XOR of the signal with pseudo-random number (chipping sequence) many chips per bit (e.g., 128) result in higher bandwidth of the signal Advantages reduces frequency selective t b fading in cellular networks 0 1 base stations can use the same frequency range several base stations can detect and recover the signal soft handover Disadvantages precise power control necessary t c 0 1 1 0 1 0 1 0 1 1 0 1 0 1 0 1 1 0 1 0 1 1 0 0 1 0 1 0 t b : bit period t c : chip period user data XOR chipping sequence = resulting signal - 13 -
DSSS (Direct Sequence Spread Spectrum) (2) user data X spread spectrum signal modulator transmit signal chipping sequence radio carrier transmitter correlator received signal demodulator lowpass filtered signal X products integrator sampled sums decision data radio carrier chipping sequence receiver - 14 -
FHSS (Frequency Hopping Spread Spectrum) (1) Discrete changes of carrier frequency sequence of frequency changes determined via pseudo random number sequence Two versions Fast Hopping: several frequencies per user bit Slow Hopping: several user bits per frequency Advantages frequency selective fading and interference limited to short period simple implementation uses only small portion of spectrum at any time Disadvantages not as robust as DSSS simpler to detect - 15 -
FHSS (Frequency Hopping Spread Spectrum) (2) t b user data f 0 1 t d 0 1 1 t f 3 f 2 f 1 slow hopping (3 bits/hop) f t d t f 3 f 2 f 1 fast hopping (3 hops/bit) t t b : bit period t d : dwell time - 16 -
FHSS (Frequency Hopping Spread Spectrum) (3) user data modulator narrowband signal modulator spread transmit signal transmitter frequency synthesizer hopping sequence received signal demodulator narrowband signal demodulator data hopping sequence frequency synthesizer receiver - 17 -
Multicarrier Modulation (MCM) Idea Use of a number of carriers simultaneously for one signal/user in order to reduce ISI Approach If c symbols/s are to be transmitted, distribution over n subcarriers, each with a code rate of c/n symbols/s Results in n carriers with lower speed and less problems of ISI. Frequency selective fading leads to attenuation of single carriers only Typical guard time between symbols It is also possible to send identical symbols over several carriers - 18 -
MCM OFDM OFDM is a special implementation of MCM with orthogonal subcarriers + very efficient FFT based algorithms for modulation/demodulation Orthogonal subcarriers: T 0 sin (2 π f k t+ϕ k )sin(2 π f j t+ϕ j )=0 with f k f i =n/t, n=1,2, Symbol rate 1/T for each subcarrier is selected in a way, that they equal the next carrier with separation Δf, thus subcarriers are orthogonal irrespective of φ. - 19 -
OFDM Implementation MC Modulator generates k independent QAM subcarriers, each with a symbolrate 1/T. Simplification: use of ifft (Inverse Fast Fourier Transform) for modulation and FFT for demodulation - 20 -
Structure of an OFDM communication system Receiver - 21 -
Application of OFDM ADSL and VDSL broadband access via telephone lines All recent WLAN standards, e.g. IEEE 802.11a, g, n, ad Digital audio broadcasting systems: EUREKA 147, Digital Radio Mondiale, HD Radio, T-DMB and ISDB-TSB Terrestrial digital TV systems DVB-T, DVB-H, T-DMB and ISDB-T. IEEE 802.16 or WiMax Wireless MAN standard Flash-OFDM cellular system. Ultra wideband (UWB) systems Power line communication (PLC) - 22 - Source Wikipedia, TZI Nov. FB 061 Kommunikationsnetze
OFDM Key Features Advantages Equalizer less complex than in single-carrier systems, in particular for high sampling rates Robust to inter-symbol interference due to guard interval and low data rates per carrier Robust to fading due to easy estimation of channel attenuation Adaptable to different channel conditions (e.g. slowly varying, fast fading, high diversity) by suitable selection of subcarrier number N Efficient implementation by FFT High spectral efficiency Allows Single Frequency Networks Disadvantages Sensitive to Doppler shift Sensitive to Frequency synchronization problems High peak to average power ratio - requires the use of linear power amplifiers http://www.supelec.fr/d2ri/flexibleradio/cours/ofdmtutorial.pdf http://www.radio-electronics.com/info/rf-technology-design/ofdm/ofdma-cdma.php - 23 -
OFDM in IEEE 802.11a/g OFDM with 52 used subcarriers (64 in total) 48 data + 4 pilots 312.5 khz spacing pilot 312.5 khz -26-21 -7-1 1 7 21 26 channel center frequency subcarrier number From: Schiller, Mobilkommunikation - 24 -
Error recovery Forward Error Correction (FEC) Added redundancy to correct transmission errors at the receiver channel coding errors after decoding occur if error-correction capability of the code is passed No feedback channel required Channel condition affects the quality of data transmission Varying reliability, constant bit throughput Automatic Repeat Request (ARQ) Small amout of redundancy is added to detect transmission errors Retransmission of data in case of detected error Feedback channel required Channel condition affects throughput constant reliability, but varying throughput Hybrid FEC/ARQ
Structure of Digital Transmission System Digital Source Sink - 26 -
Principle of Channel Coding u k channel encoder x n u = [u 0, u 1,..., u k-1 ] x = [x 0, x 1,..., x n-1 ] Encoder: device that maps information word u onto code word c by adding redundancy code rate R c =k /n Systematic encoder: codeword x contains information word u Non-systematic encoder: codeword x does not explicitly contain information word u - 27 -
Visualizing Distance Properties with Code Cube - 28 -
Error Correcting Codes (1) Linear Block Codes k-digit information word is transformed into n-digit codeword Convolutional Codes m-bit word is transformed into n-bit word, code rate m/n transformation is a function of last k information symbols, where k is the constraint length of the code. Codes symbols are calculated by modulo 2 additions of memory counters - 29 -
Error Correcting Codes (2) Example: (2,1,3)-convolutional code with generators g 1 =7 8 and g 2 =5 8 Code is non-systematic and non-recursive (NSC-Code) R c =1/2, L c =3 m = 2-30 -
Properties of Convolutional Codes Only a small number of simple codes is of practical interest not constructed by algrebraic methods but by computer (advantage of simple mathematical description) Easy processing of soft-decision input, compute soft-decision output (for block codes, only hard-decision decoding) Systematic and non-systematic encoders (mostly non-systematic codes are of practical interest) - 31 -
Interleaving - 32 -
Concatenated Codes Parallel concatenated codes (turbo codes) Serial concatenated codes - 33 -
ARQ Return Channel is required ARQ protocols Stop and Wait Waits until positive ack. received or timer expires Go back N Continous transmission of data, if NACK received, continues N steps back Selective Repeat Hybrid ARQ David/Benkner: Digitale Mobilfunksysteme. Teubner - 34 -
ARQ: General Architecture source sink ARQ control discrete channel ARQ control Feedback channel ACK/NAK
ARQ: General Architecture source sink ARQ control discrete channel ARQ control Feedback channel ACK/NAK
ARQ: Stop-and-Wait - 37 -
ARQ: Go-Back-N - 38 -
ARQ: Selective Repeat Strict implementation not possible because infinite buffer needed limit buffer size and stop transmission once buffer is full Highest performance of all presented approaches - 39 -
ARQ Pros & Cons Advantages Simple protocol Quasi adaptive scheme, adapts to channel properties and can therefore be very efficient Disadvantages Difficult to guarantee constant end-to-end delay and constant net (user) bit rate If channel quality is very low, retransmission is not efficient enough (will be retransmitted and have error again) - 40 -
Hybrid FEC/ARQ Systems - 41 -