Multiband Transceivers - [Chapter 3] Basic Concept of Comm. Systems

Multiband RF Transceiver System Chapter 3 Basic Concept of

Communication Systems

Department of Electronic EngineeringNational Taipei University of Technology

Outline

• Wireless Standards

• Multiple Access Method

• FDMA� OFDMA

� TDMA

� CDMA (DS-CDMA, FH-CDMA)

• Duplexing� FDD

� TDD

• Signal Quality: Probability of error, BER, and SNR

Department of Electronic Engineering, NTUT2/32

Introduction

VLF LF MF HF VHF UHF SHF EHF

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z

30 k

Hz

300

kHz

3 M

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30 M

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30 G

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Typical Transceiver

• Transmitter (TX)

• Receiver (RX)

DATA IN

CODING INTER-LEAVING

MODU-LATION

PULSESHAPINGFILTER

UP-CONVERSION

POWERAMPLIFIER

DUPLEXFILTER

DATA OUT

DUPLEXFILTER

LOW NOISEAMPLIFIER

DOWN-CONVERSION

CHANNELSELECTION

FILTER

DEMODU-LATION

DEINTER-LEAVING

DECODING


Wireless Systems (I)

• The wireless systems vary frombroadcast and televisiontransmission to cellular telephones and wireless local areanetworks (WLAN).

The coverage area or in cellular systems the cell size has reduced when personalmessaging and rising data rates have been adopted.

• The capacity must be shared to small units with variousmethods.

For example, in the Third Generation Partnership Project (3GPP), the system usesdirect sequence spread spectrum multiple access. Often, the system is calledUniversal Mobile Telecommunications System (UMTS), which has been actuallythe name for the European proposal.


Wireless Systems (II)

NMT450 GSMNADCIS-54

IS-95 PDC DECT CT-2 GPS

Signal Analog Digital Digital Digital Digital Digital Digital Digital

Application Cellular Cellular Cellular Cellular Cellular Cordless Cordless Positioning

Frequency RangesUplink/ Reverse(MHz)

453-457.5880-915,1720-1785,1850-1910

824-849824-849,1930-1990

810-826,1429-1453

1880-1900 864-868 -

Frequency RangesDownlink/ Forward (MHz)

463-467.5925-960,1805-1880,1930-1990(6

869-894869-894,1850-1910

940-9561477-1501

1880-1900 864-868 1575.42

Multiple Access FDMA TDMA TDMA DS-CDMA TDMA TDMA FDMA DS-CDMA

Duplexing FDD FDD FDD FDD FDD TDD TDD -

Modulation FM GMSK π/4-DQPSK QPSK π/4-DQPSK GPSK B-FSK

Carrier spacing (kHz) 25 200 30 1250 25 1762 100 -

Carrier Bit Rate (kb/s) - 270.833 48.6 1228.8 42 32 72 1000

User per carrier 1 8 3 < 63 3 12 1 -

Handset output power na 3.7mW-1W 2.2mW-6W ≤ 200mW na 250mW 1mW-10nW -

Speech Data Rate (kb/s) - 13 8 8-13 6.7 32 32 0.050


Wireless Systems (III)

WCDMA cdma2000

Area Europe, Japan USA

Channel Bandwidth (MHz)

(1.25), 5, 10, 20 1.25, 5, 10, 15, 20

Downlink RF channel structure

Direct Spread Direct Spread or multicarrier

Chip rate (Mcps) (1.024), 4.096, 8.192, 16.3841.2288, 3.6864, 7.3728, 11.0593, 14.7456

Spreading Modulation

Balanced QPSK (downlink)Dual channel QPSK (uplink)

Balanced QPSK (downlink)Dual channel QPSK (uplink)

Data ModulationQPSK (downlink)BPSK (uplink)

QPSK(downlink)BPSK(uplink)

Multirate Variable spreading and multicode Variable spreading and multicode

Spreading Factors 4-256 4-256


Multiple Access Methods

• The Multiple Access Method:

Defines how the information in a single traffic channel is organized with respect tothe other transmitted channels at the same band or elsewhere.

• The multiple access gives a frame for the radio design, and ithas a strong influence on the choice of the radio architectureand on the specification of the analog receiver.

• The multiple access can be done in the frequency, time or codedomain.


Frequency Division Multiple Access (FDMA)

• FDMA:

The band is divided only to narrow frequency slots, and every transmitter receiverpair has its own designated band.

• It is not either practical to keep the whole cellular band in onesystemas a single frequency slot, and hence the FDMAistypically at the background of other access methods.

FDMA TDMA

DS-CDMA FH-CDMA

time

frequency


Orthogonal Frequency-Division Multiple Access

• Orthogonal Frequency-Division Multiple-Access (OFDMA):

It is an advanced form of FDMA, for example, used in 4G systems. It is a multi-userversion of the popular orthogonal frequency-division multiplexing (OFDM) digitalmodulation scheme.

• OFDM can combat multipath interference with morerobustness and less complexity, and OFDMAcan furtherimproves OFDMrobustness to fading and interference. Highersensitivity to frequency offsets and phase noise.


Time Division Multiple Access (TDMA)

• TDMA:

Each freq. channel is divided into time-slots, and every nth slot is reserved for asingle traffic channel. (In GSM, a 200 kHz radio channel consists of 8 time-slots.)

• In TDMA systems, the synchronization of different mobileTXs at a single carrier is required to prevent the trafficchannels fromoverlapping in time.

FDMA TDMA

DS-CDMA FH-CDMA

time

frequency


Code Division Multiple Access (CDMA)

• CDMA:

Systems are based on the pseudorandom sequences of orthogonal codes. The codecan be either a digital bit stream (direct sequence spread spectrum, DS-SS), or afrequency pattern (frequency hopped spread spectrum, FH-SS). Also, time-hopping(TH-SS) is a possible coding method.

• The orthogonality between the codes means that they haveideally no correlation with each other.

FDMA TDMA

DS-CDMA FH-CDMA

time

frequency


Direct Sequence CDMA (DS-CDMA)

• DS-CDMA:

The transmitted data is multiplied with the spreading code, which is at a higherdata rate than the information. The scrambling of the data spreads the informationover a much wider band than necessary.

• Spreading Factor (or Processing Gain, PG):

where Bt and Bi are the transmission and information bandwidths, respectively.

• Despreading Process:In the receiver, the radio channel is multiplied with the same code, and alluncorrelated information, i.e. noise and other code channels, are scrambledagain. Therefore, the PG describes the improvement of the SNR as

tp

i

BG

B=

corrp

ch

SNRG

SNR=


Principle of the DS-CDMA

Spectrum of modulation data

Scrambledspectrum

Other code channels

Received radio channel

Spectrum after despreading

Transmitting data

Pseudorandom code

Received data

Radio Channel

TRANSMITTER RECEIVER

Pseudorandom code


Bit and Chip

• The units of the information and the spreading sequence arecalled abit and achip, respectively.

• The chip rate is typically fixed, which defines a certainbandwidth of the radio channel.

By changing the bit rate, the processing gain varies but the signal at the radioband remains unchanged if we consider only the spectral behavior. Hence, noreconfiguration of the hardware is required when variable data rates aretransmitted.

The information at a high data rate, due to the smaller processing gain,requires a larger power for the transmission.


Frequency Hopping CDMA (FH-CDMA)

• FH-CDMA:

The information spreading is performed with pseudorandom jumps of thetransmission frequency inside the total system band.

(From http://www.wirelesscommunication.nl)

• In FH systems, thebandwidth of a radiochannel can benarrower than in thedirect sequence, butthe jumping betweenthe frequencies setstrict requirements forthe frequency synthesis.


Power

Frequency

Time Desired signal hopsfrom one frequencyto another

Duplexing

• In all two-way communications with only one antenna, thetransmitter and receiver must be isolated fromeach other.Otherwise, the Tx output power would saturate the Rx.

� Frequency division duplexing (FDD):The transmission and reception are accomplished at different frequencies witha duplex filter (typically used in cellular systems).

� Time division duplexing (TDD):The transmission and reception are at the same band but they do not overlap intime with a switch (DECT in an example).

• In GSM, both FDD and TDD are used simultaneously.However, a duplex-filter is typically used in mobile terminalsinstead of a switch, due to the better immunity against signalsfrom other mobiles.


Digital Modulation

• Reasons: Capacity, Accuracy, Noise, andDistortion

• The modulation can transmit one or more bits at the same time,and thesimultaneous bits form a symbol.

I and Q bits are distinguished with a 90° angle at RF. If the symbols are organizedin the way that only I- or Q-bit can change at a time, the constellation nevercrosses the origin and amplitude is almost constant (decrease the spectralefficiency).

Iin

RF out

Qin

( )cos RFtω

( )sin RFtωconstant envelope variable envelope

amplitudephase

Q

I

Q

I


Binary Phase Shift Keying (BPSK)

• BPSK:

The symbol has only 1-bit, which rotates carrier phase by 180° when input changes.

• The constellation always crosses the origin when the datachanges.

This means a large AM-component in the modulation and sharp transitions, which

produce a wide spectrum relative to the bit rate).

NRZ data

Pulse shaping filter

Carrier

RF out

Q

I


Binary Phase Shift Keying (BPSK)

• The power spectral density (PSD) of the NRZ baseband signal

where A is the signal amplitude andTb is the bit period. The bit periodTb isinversely proportional to the bit ratefb.

• At the carrier frequency the PSDis

wherefRF is the carrier frequency.

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2

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RF b

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0

−10

−20

−30

−40

−501.99 G 2.00 G 2.01 G

Frequency (Hz)

PS

D (

dB

)

Quadrature Phase Shift Keying (QPSK)

• QPSK:2-bits are modulated to the carrier

simultaneously.

• QPSKspectrumis condensedto 1/2 of the BPSKwhen thesame amount of data bits istransmitted.

• The envelope is not constant,but the AM-componentcompared to the BPSKismuch smaller (90° phase shiftsproduce less amplitude distortion than

180° transitions).

Iin

RF out

Qin

( )cos RFtω

( )sin RFtω


0

−10

−20

−30

−40

−501.99 G 2.00 G 2.01 G

Frequency (Hz)P

SD

(d

B)

QPSKBPSK

Probability of an Error

• The performance of the modulation is evaluated with theprobability of an error in the detection.


0.1

0.01

1E-3

1E-4

1E-5

1E-60 2 4 6 8 10 12

Eb/N0 (dB)P

e

Pe of QPSK signal• The required energy per bit (Eb)

versus the noise PSD(N0) iscompared to the probability of abit error Pe. The characteristiccurves can be plotted fordifferent modulations based onthe statistical analysis.

Bit Error Rate (BER)

• In wireless applications, the required BER forspeechtransmission is typically, i.e.10−−−−3, one error in one thousandtransmitted bits. In that case, theEb/N0 must be about 6.7 dBfor the QPSK.

• The required BER for thedata transmission is in the order of10−−−−6666 in wireless connections.

• The BER is actually defined after decoding of the transmission.The channel coding has effect on the requiredEb/N0.

For example, in WCDMA systems, different coding methods are used forspeech and data transmissions.


Signal-to-Noise Ratio (SNR)

• The signal-to-noise ratio (SNR), instead of theEb/N0, is apractical measure in circuit design. The relation is given in

whereS/N is the SNR,Bn is the effective noise bandwidth of the receiver, andfb isthe bit rate.

• Theoretically,Eb/N0 is equal to SNR whenBn is the same asfb.Although Bn is often somehow wider than fb in theimplementation, it is an appropriate estimate for the handcalculations, which can be confirmed with more accuratesystemsimulations.

0

b n

b

E BS

N N f= ⋅


Pulse-shaping Filtering

• Pulse shaping:

Before the modulator, the bits are filtered to smooth the bit transitions to limit themodulation bandwidth at passband.

• Without the pulse filtering, thespectrumof the modulation hasthe shape of a sinc-function withrelatively high side lobes.

The pulse shaping filter removes the sidelobes, and it has also effect on the AM-termand on the intersymbol interference (ISI).

0

−20

−40

−60

−80Am

plit

ud

e (d

Bc)

−1000 5 10 2015

Frequency (MHz)


Envelope Varying Due to Pulse Shaping

• Except of improving the spectral efficiency, the filter smoothout sharp transitions in the time domain, which increasesamplitude variations of the signal envelope.

• Spectrumregrowth:The limiting amplifiers, which are desireddue to their better efficiency as poweramplifiers, tend to reduce the amplitudevariations and hence destroy the bandlimitation. Therefore a linear poweramplifier is typically needed in the systemswith variable envelopes.

0

−20

−40

−60

−80Am

plit

ud

e (d

Bc)

−1000 5 10 2015

Frequency (MHz)

Constant envelope (PSK before shaping)

180� 180�90�

Time-varying envelope (PSK after shaping)

Smooth the sharp transition


Increase PSK Spectral Efficiency

• The spectral efficiency of a PSKmodulation can be increasedwhen the unit circle is divided into more than four possiblesections.

• The angle between constellation points reduces, and hence abetter phase accuracy is required.

8-PSK and higher order multiple phase shift keying (MPSK) modulations requiretheoretically larger Eb/N0 for a certain Pe than BPSK and QPSK.


Frequency Shift Keying (FSK)

• Instead of abrupt changes between fixed phase angles, the bitscan be coded as linear phase shifts.

The linear change in the phase means a constant frequency. The methods basedon that principle are called digital frequency modulations.

• In FSK, the phase shift is smooth and the amplitude does notchange (constant envelope modulation).

FSK is the digital counterpart of the analog FM. The difference is that the input ofthe modulator is binary data instead of analog information.


Minimum Shift Keying (MSK)

• The MSK modulations do not have narrowfrequency spurs atthe spectrumor rapid changes in phase.

The phase shift is linear and advances by 90° if the bit is 1. When the bit is 0, thephase returns back by the same amount.

• The 90° shift is organized by delaying theQ-data ½ symbolperiod compared toI-data.

Hence, the I- and Q-bits are never changing simultaneously.

• To limit the MSK spectrumwithout destroying the good time-response, aGaussian filter can be adopted for pulse shaping.

For example, due to the compromising properties, the Gaussian-filtered MinimumShift Keying (GMSK) is used in GSM.


Quadrature Amplitude Modulation (QAM)

• The modulations withmore than 2-bits per symbol can be usedto increase the spectral efficiency.

• In PSK and MSK modulations, the phase differences at theunit circle become smaller, which increases the susceptibilityto errors due to noise and distortion.

• A hybrid of phase and amplitudemodulations is an alternativemethod to increase the spectralefficiency.

The 16-QAM constellation carries 4-bits in asymbol. The transmission power of the higher-order QAM modulations is larger. Hence, thespectral efficiency trades off with the power.

Q

I


• Typically, the pulse shaping filter is digital, which requires an analog post-filtering after the D/A conversion to remove the digital replicas at clockharmonics.

• Alternatively, the upconversion of the transmitted signalcan be performedin several steps or the signal can be translated before the D/A conversiondirectly to some intermediate frequency (by using DDS).

A Simple DS-CDMA Transmitter

Data in

Pulse shapingfilter

Serialto

Parallel

C2

Up-converter

PA Band filter

90°

LO

C1


Summary

• In this chapter, some basics on communication systems wereintroduced, including multiple access, duplexing, digitalmodulation, bit error rate, and signal-to-noise ratio, .etc.

• The multiple access defines howthe information in a singletraffic channel is organized with respect to the othertransmitted channels at the same band or elsewhere.

• In all two-way communications with only one antenna, thetransmitter and receiver need to be isolated fromeach otherwith time- or frequency-division duplexing.

• The required BER defines the minimumEb/N0, and hence theminimumSNR.

• The spectral efficiency can be increased by using the M-PSKor QAM modulation.
