ICT 5305 Mobile Communications. Lecture - 3 April Dr. Hossen Asiful Mustafa

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1 ICT 5305 Mobile Communications Lecture - 3 April 2016 Dr. Hossen Asiul Mustaa

2 Advanced Phase Shit Keying Q BPSK (Binary Phase Shit Keying): bit value 0: sine wave bit value 1: inverted sine wave very simple PSK low spectral eiciency robust, used e.g. in satellite systems 10 1 Q 0 I 11 QPSK (Quadrature Phase Shit Keying): 2 bits coded as one symbol symbol determines shit o sine wave needs less bandwidth compared to BPSK more complex A 00 I 01 Oten also transmission o relative, not absolute phase shit DQPSK - Dierential QPSK (IS-136, PHS) t

3 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 schemes Example: 16-QAM (4 bits = 1 symbol) Symbols 0011 and 0001 have the same phase φ, but dierent amplitude a and 1000 have dierent phase, but same amplitude. Q a φ I 1000

4 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 (the newer DVB-T2 can additionally use 256QAM) Q Example: 64QAM good reception: resolve the entire 64QAM constellation poor reception, mobile reception: 10 resolve only QPSK portion I 6 bit per QAM symbol, 2 most signiicant determine QPSK HP service coded in QPSK (2 bit), 00 LP uses remaining 4 bit

5 Spread spectrum technology Problem o radio transmission: requency dependent ading can wipe out narrow band s or duration o the intererence Solution: spread the narrow band into a broad band using a special code protection against narrow band intererence power intererence spread power detection at receiver spread intererence Side eects: coexistence o several s without dynamic coordination tap-proo Alternatives: Direct Sequence, Frequency Hopping

6 Eects o spreading and intererence dp/d dp/d i) ii) sender user broadband intererence narrowband intererence dp/d dp/d dp/d iii) iv) v) receiver

7 Spreading and requency selective ading channel quality narrowband channels requency narrow band guard space channel quality spread spectrum channels spread spectrum requency

8 DSSS (Direct Sequence Spread Spectrum) I XOR o the with pseudo-random number (chipping sequence) many chips per bit (e.g., 128) result in higher bandwidth o the Advantages reduces requency selective ading in cellular networks base stations can use the same requency range several base stations can detect and recover the sot handover Disadvantages precise power control necessary t b 0 1 t c t b : bit period t c : chip period user data XOR chipping sequence = resulting

9 DSSS (Direct Sequence Spread Spectrum) II user data X spread spectrum modulator transmit chipping sequence radio carrier transmitter correlator received demodulator lowpass iltered X products integrator sampled sums decision data radio carrier chipping sequence receiver

10 FHSS (Frequency Hopping Spread Spectrum) I Discrete changes o carrier requency sequence o requency changes determined via pseudo random number sequence Two versions Fast Hopping: several requencies per user bit Slow Hopping: several user bits per requency Advantages requency selective ading and intererence limited to short period simple implementation uses only small portion o spectrum at any time Disadvantages not as robust as DSSS simpler to detect

11 FHSS (Frequency Hopping Spread Spectrum) II t b user data 0 1 t d t d t b : bit period t t t t d : dwell time slow hopping (3 bits/hop) ast hopping (3 hops/bit)

12 FHSS (Frequency Hopping Spread Spectrum) III user data modulator narrowband modulator spread transmit transmitter requency synthesizer hopping sequence narrowband received demodulator demodulator data hopping sequence requency synthesizer receiver

13 Sotware Deined Radio Basic idea (ideal world) Full lexibility wrt modulation, carrier requency, coding Simply download a new radio! Transmitter: digital processor plus very ast D/A-converter Receiver: very ast A/D-converter plus digital processor Real world Problems due to intererence, high accuracy/high data rate, low-noise ampliiers needed, ilters etc. Examples Joint Tactical Radio System GNU Radio, Universal Sotware Radio Peripheral, Application Signal Processor D/A Converter Application Signal Processor A/D Converter

14 Cell structure Implements space division multiplex base station covers a certain transmission area (cell) Mobile stations communicate only via the base station Advantages o cell structures higher capacity, higher number o users less transmission power needed more robust, decentralized base station deals with intererence, transmission area etc. locally Problems ixed network needed or the base stations handover (changing rom one cell to another) necessary intererence with other cells Cell sizes rom some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less or higher requencies

15 Frequency planning I Frequency reuse only with a certain distance between the base stations Standard model using 7 requencies: Fixed requency assignment: certain requencies are assigned to a certain cell problem: dierent traic load in dierent cells Dynamic requency assignment: base station chooses requencies depending on the requencies already used in neighbor cells more capacity in cells with more traic assignment can also be based on intererence measurements

16 Frequency planning II 3 cell cluster cell cluster h h 2 h h 2 1 h 1 3 h 3 g 1 g 2 g 3 g 1 g 2 g 3 g 1 g 2 g 3 3 cell cluster with 3 sector antennas

17 Cell breathing CDM systems: cell size depends on current load Additional traic appears as noise to other users I the noise level is too high users drop out o cells

Mobile & Wireless Networking. Lecture 2: Wireless Transmission (2/2)

Mobile & Wireless Networking. Lecture 2: Wireless Transmission (2/2) 192620010 Mobile & Wireless Networking Lecture 2: Wireless Transmission (2/2) [Schiller, Section 2.6 & 2.7] [Reader Part 1: OFDM: An architecture for the fourth generation] Geert Heijenk Outline of Lecture