Communications:More Issues Out There

ECE 4600Dr. Bradley J. Bazuin

Communications System• Analog or Digital Communications

– Information Message Type (Analog or Digital)– Modulation Type (Discrete or Continuous)

• Terminology– Message, Symbol, Signal, Noise

Information Message Format Encoding/

Encryption Modulation RF Mixing Transmitter

Reformat dencoding/dencryption Demodulation RF Mixing Receiver

Antenna

AntennaInformation Message

RF Signal

Noise

Interference

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Common Frequencies• AM Radio: 535-1705 kHz• FM Radio: 88-108 MHz• ISM: 433.05-434.79 and 902-928 MHz• ISM: 2.4-2.5 GHz (wireless Ethernet & Bluetooth)• ISM: 5.725-5.875 GHz (wireless Ethernet)• Cell : 824-849 and 869-894 MHz• PCS: 1850-1910 and 1930-1990 MHz• AWS: 1710-1755 and 2110-2155 MHz• BRS/EBS: 2.496–2.690 GHz• More Frequencies

– https://en.wikipedia.org/wiki/Cellular_frequencies– https://en.wikipedia.org/wiki/ISM_band

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Digital Signal ModulationBasic forms

• Amplitude Shift Keying (M-ASK)• Phase Shift Keying (M-PSK)• Frequency Shift Keying (M-FSK)

– Continuous Phase FSK (CP-FSK)– Gaussian Minimum Shift Keying

(GMSK)• Quadrature Amplitude Modulation

(QAM)

Advanced Forms

• Trellis-Code Modulation (TCM)– Include error correction with the

modulation. Reed-Solomon codes and Viterbi decode.

• Orthogonal Frequency Division Modulation (OFDM)

• Direct Sequence Spread Spectrum (DSSS)– Code Division Multiple Access

(CDMA)• Frequency Hopping (FH)

– fast or slow rates

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Digital Comm. System

Bernard Sklar, Digital Communications, Fundamentals and Applications, Prentice Hall PTR, Second Edition, 2001.

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Signal Processing Functions

Bernard Sklar, Digital Communications, Fundamentals and Applications, Prentice Hall PTR, Second Edition, 2001.

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Trellis-Coded Modulation (TCM)• Developing combined modulation and coding schemes• Use a redundant nonbinary modulation in combination with a

finite-state machine based encoding process.– FSM could be similar to convolutional encoding– A multi-level/phase modulation scheme

• The concept, when performing MATLAB simulations of encoded bit streams using MPSK or QAM symbols, is there an optimal combination? – if you know the symbols being used, could one convolutional code

leading to an appropriate trellis decoding perform better than another? Yes!

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TCM Encoding• Ungerboeck, G., "Channel coding with

multilevel/phase signals," Information Theory, IEEE Transactions on, vol.28, no.1, pp.55,67, Jan 1982.

• Initial paper describing trellis coded, soft decision encoding and modulation technique for communications.

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Orthogonal Frequency Division Multiplexing (OFDM)

• Multiplexing in both phase and frequency domains– Without the costly hardware of conventional FDM– Can be implemented using IFFT/FFT hardware

• OFDM form of Multicarrier (MC) modulation– Carriers are mutual orthogonal

• We parse up a given message into separate components to OFDM them onto a channel

a symbol is transmitted at a lower rate increased immunity to multipath may not have to employ equalization

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Applications• IEEE 802.11 (Wi-Fi) and IEEE-802.16 (WiMax)• Modems, DSLs• 4G Cellular Phones

– LTE and LTE Advanced Cellular Telephone

• OFDM system of parsing symbols onto separate frequencies and phases can be extended to multiple access (MA) applications.

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OFDM TransmitterCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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OFDM Transmitter Using IDFT

Figure 14.5-2

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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OFDM Modulator

fred haris, Orthogonal Frequency Division Multiplexing OFDM, Vehicular Technology Conference - 2004

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OFDM Demodulator

fred haris, Orthogonal Frequency Division Multiplexing OFDM, Vehicular Technology Conference - 2004

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OFDM is a Block Process

fred haris, Orthogonal Frequency Division Multiplexing OFDM, Vehicular Technology Conference - 2004

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Spread Spectrum Systems• There are two principal types of spread

spectrum systems:– Frequency Hopping (FH-SS)

• Blue tooth, GSM Cell Phones, Radios– Direct Sequence Spread Spectrum (DSS)

• GPS, CDMA Cell Phones

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DSS Signaling• Binary Symbols {+1, -1} multiplied by a

chipping code consisting of a pseudo-random binary symbol sequence.

– The symbol modulates a lengthy chip sequence by +/- 1.

– But multiplying by the chip sequence again the original signal returns.

tctxtxDSS

tctxtxtctxtx DSSDSS

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Autocorrelation• If we transmit the DSS signal, there is a time delay

due to distance. Thus, we must time align the received DSS with the chip sequence in order to receive it.Tc

A2

Tc-Tc

NTc-NTc

R() = A2 (1 - | | / Tc) for | | Tc= - A2 /N elsewhere

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Figure 15.1-1

DSS Transmitter System

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DSS Bandwidth• The bandwidth is based on the chip rate

repC

c TT

R

repCCc T

fTfTfG 1sinc 2

T

Rx

TfTfGx sinc

tctcEtxtxEtctctxtxEtctxtctxERDSS

Crep

CDSS TT

TT

R

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Bandwidth Expansion Factor• For DS spreading of the signal, the bandwidth has

gone from 1/T to 1/Tc

b

C

C

bBWex W

WTTg

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Correlation Receiver• Use the PN sequence as an optimal “filter” or

correlator sequence

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Receiver Noise & SNR• The noise power is filtered by the

correlation receiver– Mix by PN sequence– Integrate for the Bit Period, Tb

TTnn BBNR sinc0

Tnn B

fNfS 0

x

RD WN

SSNR

0

0

2NEQP b

e

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Figure 15.1-2

DSS Inherent Anti-Jam

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GPS Signal Characteristics• Two Principal Frequencies

– L1 Band at 1575.42 MHz with C/A and P(Y) codes– L2 Band at 1227.60 MHz with P(Y) code– L5 Band at 1176.45 MHz with “new” C/A code

• Direct Sequence Spread Spectrum Communications– Data Message at 50 bps consisting of 1500 bit pages (30 sec.)– C/A-code spreads the data using 1023-bit Gold codes at a chipping

rate of 1.023 Mcps (C/A – coarse-acquisition code)– P(Y)-code spreads the data using a code that Does not repeat at a

chipping rate of 1.0.23 Mcps (P – precision code)• Code Transmission

– The C/A- and P(Y)-codes are transmitted in quadrature on L1– The P(Y)-code is transmitted on L2

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GPS Receiver Characteristics• Receive up to 12 satellites simultaneously• User Minimum received power

– L1 C/A-Code:–130 dBm– L1 P-Code: –133 dBm– L2 P-Code: –136 dBm– kTB (20 MHz): –101 dBm

-10 -5 0 5 10-160

-150

-140

-130

-120

-110

-100

dBm

/ H

z

Frequancy (MHz)

Thermal Noise PowerC/A Code P(Y) Code

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GPS User Position Computation• GPS uses the triangulation of signals from

the satellites to determine locations on earth. • GPS satellites know their location in space

and receivers can determine their distance from a satellite by using the travel time of a radio message from the satellite to the receiver.

• After calculating its relative position to at least 4 satellites, a GPS receiver can calculate its position using triangulation.

• They also have a database (or almanac) of the current and expected positions for all of the satellites that is frequently updated from earth.

Earth

Satellite

r

u

s

SV to User Distancer = || s – u ||r = c x t

Measured Pseudorange = r + c x tu

Unknownsr(x, y, z), tUse 4 SVs to solve

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Figure 15.2-1

(a) Transmit

(b) Receive

FH-SS System.

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Figure 15.2-2

Output frequency versus data input for slow hop FH-SS system

Waterfall Displays: allow frequencies in time to be observed

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