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8. ADVANCED DATA COMMUNICATIONS
8.1 M-ARY DATA COMMUNICATIONS SYSTEMS
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Figure 8-1 Modular and typical waveforms for QPSK.
Principles of Communications, 5/E by Rodger Ziemer and William Tranter Copyright © 2002 John Wiley & Sons. Inc. All rights reserved.
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Figure 8-2 Detection of QPSK.
Principles of Communications, 5/E by Rodger Ziemer and William Tranter Copyright © 2002 John Wiley & Sons. Inc. All rights reserved.
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Figure 8-3 Error probability for QPSK.
Principles of Communications, 5/E by Rodger Ziemer and William Tranter Copyright © 2002 John Wiley & Sons. Inc. All rights reserved.
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OQPSK System MSK Systems
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Figure 8-4 Block diagrams for parallel MSK modulator and demodulator
Figure 8-4 Block diagrams for parallel MSK modulator and demodulator. (a) Modulator. (b) Demodulator.
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Figure 8-5 (a) MSK type 1 modulation. (b) MSK type II modulation
Figure 8-5 (a) MSK type 1 modulation. (b) MSK type II modulation (Adapted from Figure 3.6 of Digital Communications by Satellite by V. K. Bhargava et al Copyright © 1981 John Wiley & Sons, Inc. Used with permission.)
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M-ary Data Transmission in terms of Signal Space
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Figure 8-7 Computation of signal-space coordinates.
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Figure 8-8 Signal space for QPSK.
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Figure 8-9 Representation of signal plus noise in signal space, showing N, the noise component that can cause the received data vector to land in R2.
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Example 8.1 QPSK in terms of signal Space
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M-ary Phase-Shift Keying
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Figure 8-10 (a) Signal space for M-ary PSK with M = 8
Figure (a) Signal space for M-ary PSK with M = 8. (b) Signal space for M-ary PSK showing two half planes that can be used to overbound PE.
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Quadrature-Amplitude-Shift Keying (QASK)
= Quadrature-Amplitude-Modulation (QAM)
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Figure 8-11 Signal-space & detector structure for 16-QAM
Figure 8-11 Signal-space & detector structure for 16-QAM. (a) Signal constellation and decision regions for 16-QAM. (b) Detector for 16-QAM. (Binary representations for signal points are Gray encoded).
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Coherent Frequency-Shift Keying
Nocoherent Frequency-Shift Keying
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Figure 8-12 Signal space showing decision regions for 3-ary coherent FSK.
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Figure 8-13 Receiver for noncoherent FSK.
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Bit Error Probability from Symbol Error Probability
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Figure 8-14 Bit error probability versus Eb/N0 for M-ary PSK and 16- QASK.
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Figure 8-15 Bit error probability versus Eb/N0 for coherent M-ary FSK.
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Example 8.2 Comparison of M-ary Communication System Example 8.3 8.2 Bandwith Efficiencies of Digital Modulation Formats
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Figure 8-16 Abbreviated list of binary data formats, Adapted from J. K
Figure Abbreviated list of binary data formats, Adapted from J. K. Holmes, Coherent Spread Spectrum Systems, New York: John Wiley, 1982.
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Figure Power spectral densities for NRZ, unipolar RZ, and split-phase data modulation data formats.
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Power spectral density of a synchronous data stream generated by a binary, zero mean, wide sense stationary sequence
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Figure 8-18 Fractional out-of-band power for BPSK, QPSK or OQPSK, and MSK.
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(QPSK,OQPSK,MSK) (BPSK) (MSK) (QPSK or OQPSK)
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8.3 Synchronization Network Synchronization Carrier Synchronization Bit Synchronization Word Synchronization Chip Synchronization : used in spread spectrum system Pseudo-Noise(PN) Sequences
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Figure Block diagram of a system for deriving a clock that is coherent with a random-bit stream. PLL : phase locked loop
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Figure 8-21 Generation of a 7-bit PN sequence. (a) Generation
Figure Generation of a 7-bit PN sequence. (a) Generation (b) Shift register contents.
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Figure 8-22 Correlation function of PN code.
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Figure 8-23 Synchronization by PN code
Figure Synchronization by PN code. (a) PN transmitter-receiver portion for synchronization. (b) Error signal at VCO.
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8.4 Spread-Spectrum Communication System
- Features of Spread-Spectrum Communication System AJ (anti-jamming) LPI (low probability of intercept) Robust to Multipath CDMA applications Precise range calculation : GPS(global positioning system) car navigation -Main classification Direct Sequence : DS Spread Spectrum Frequency Hopping : FH Spread Spectrum Time Hopping : TH Spread Spectrum Hybrid …. : DS/FH, TH/FH, etc…
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Figure 8-24 DS(direct sequence) spread-spectrum communication system
Figure DS(direct sequence) spread-spectrum communication system. (a) Transmitter. (b) Receiver.
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Figure 8-25 Block diagram of a FH spread-spectrum communication system
Figure Block diagram of a FH spread-spectrum communication system. (a) Transmitter. (b) Receiver.
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Direct-Sequence Spread Spectrum
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Figure 8-26 PE versus SNR for DSSS with Gp = 30 dB for (a) JSR = 25 dB, (b) JSR = 20 dB, (c) JSR = 15 dB, and (d) JSR = 10 dB.
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Figure Bit error probability for CDMA using DSSS with the number of users as a parameter; 127 chips per bit assumed.
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Figure 8-28 Code acquisition circuits for (a) DSSS and (b) FHSS using serial search.
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Code Synchronization
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8.5 SATELLITE COMMUNICATIONS
Antenna Coverage
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Figure 8-29. (a) Satellite repeater link
Figure (a) Satellite repeater link. (b) Frequency-translation satellite communications relay. (c) Demod/remod satellite communications relay.
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Figure 8-30 Polar representation of a general antenna gain function.
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Example 8.4
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Earth Station and Transmission Methods
Figure Satellite ground station receiver/transmitter configuration The command transmitter and telemetry receivers are not shown.
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Figure 8-32 Illustration of multiple-access techniques. (a) FDMA
Figure Illustration of multiple-access techniques. (a) FDMA. (b) TDMA. (c) CDMA using frequency-hop modulation (numbers denote hopping sequences for channels 1, 2, and 3.
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Figure 8-33 Details of a TDMA frame format.
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Figure Signal and noise powers in the uplink and downlink portions of a bent-pipe satellite relay system.
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Link Analysis: Bent-Pipe Relay
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Example 8.5
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Figure Transition probability diagram for uplink and downlink errors on a demod/remod satellite relay. Figure Transition probability diagram for uplink and downlink errors on a demod/remod satellite relay.
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Link Analysis:OBP Digital Transponder
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Example 8.6
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Figure 8-36 Comparison of bent-pipe and OBP relay characteristics.
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Figure Hexagonal grid system representing cells in a cellular radio system; a reuse pattern of seven is illustrated.
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Link Budget Analysis
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From performance analysis to physical transceiver specs
Link budget analysis connects performance analysis to physical transceiver specs Transmit power Antenna directivities at Tx and Rx Quality of receiver circuitry (how much noise do we incur) Desired link range
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Taking stock of what we know
Given bit rate and signal constellation (including any error correction code that is used), we know the required modulation degrees of freedom per unit time (the symbol rate) This gives minimum Nyquist bandwidth Now factor in excess bandwidth Given the constellation and desired BER, can find Eb/N0 required, and hence the required SNR SNR=(energy/bit * bits/sec)/(noise PSD*bandwidth) Given receiver noise figure and bandwidth, can find noise power We can now specify the receiver sensitivity (min required received power): Receiver sensitivity (typically quoted in dBm)
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Friis’ Formula (free space propagation)
Carrier wavelength Range Transmit power antenna directivity Receive Antenna directivity (gain with respect to isotropic antenna) is often expressed in dB scale Friis’ Formula in dB scale
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Link Budget Equations More generally, the received power is given by
for free space propagation Add on a link margin (to compensate for unforseen or unmodeled performance losses) to get the link budget:
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Example: link budget for a WLAN
WLAN link at 5 GHz 20 MHz bandwidth, QPSK, excess bandwidth 33%, receiver noise figure 6 dB BIT RATE Symbol rate Bit rate REQUIRED SNR 10-6 BER for QPSK requires Eb/N0 of 10.2 dB (use BER formula ) ) In dB,
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Link budget for WLAN (contd.)
Receiver sensitivity (transmit power of 100 mW, or 20 dBm, and antenna directivities of 2 dBi at Tx and Rx) Find the max allowable path loss for the desired receiver sensitivity: Invert path loss formula for free space propagation Range is 107 meters Of the order of advertised WiFi ranges Max. allowable path loss Noise power Try variants: redo for other constellations; carrier frequency of 60 GHz with antenna directivities of 20 dBi.
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