Transmitter Architectures and Signal Processing · Signal Processing Markku Renfors Laboratory of Electronics and Communications Engineering ... any odd-order) nonlinearity is the

Transmitter Architectures ELT-44007/TxArch/1M. Renfors, TUT/ELT/WICO 04.09.18

An Introduction toTransmitter Architectures and

Signal Processing

Markku Renfors

Laboratory of Electronics andCommunications Engineering

Tampere University of Technology, [email protected]


Topics

1. About co-operation of transmitters and receivers- Effects of duplexing method

2. Transmitter architectures

3. Power amplifier problems- Signal distortion- Spectral purity- Linearization methods

4. Power amplifier issues in the OFDM case


About the Co-Operation ofTransmitter and Receiver

Duplexing TechniquesUplink and downlink signals can be separated usingdifferent carrier frequencies or different time slots:· In Time Division Duplex (TDD), TX and RX use the same

frequency but different time slots· In Frequency Division Duplex (FDD), RX and TX are

working at the same time but use different carrierfrequencies.

The GSM system is based on FDD, but it has also a TDDelement.· In the basic system, the TX and RX are not working at the

same time· In multislot services (HSCSD, GPRS) with high data rate,

the RX and TX may need to operate at the same time.

3G (WCDMA) and 4G (LTE) systems are based on theFDD principle and the transmitter and receiver are usuallyoperating at the same time.· There are also TDD variants, but they are not widely

used.

WLAN systems are based on TDD.


Simultaneous Operation ofTransmitter and Receiver

There are two ways of sharing the antenna between thetransmitter and receiver:· Switching: Connecting the antenna to RX or TX as

needed.· Duplexer: Using a band-splitting filter to connect the RX

band to the receiver and transmitter to the TX band.

As a passive device, the duplexer attenuates the receivedsignal and thus increases the noise figure of the receiverconsiderably. Better noise figure can be achieved withswitching, even though passive filtering is usually neededalso in this case both in RX and TX.

If the TX and RX are not working at the same time, then· Either switching or duplexer can be used.· The same frequency synthesizer can be used for RX and

TX (if the settling is fast enough).· Less spurious frequencies, because TX is silent when RX

is working.

If the TX and RX are working at the same time, then· Duplexer has to be used.· The frequency synthesizer has to produce all the

frequencies needed by TX and RX at the same time.· Controlling the spurs gets more difficult; isolation between

TX and RX is a big concern.


Issues in the Transmitter Side

Main performance criteria

- Spectral purity- Signal distortion

Transmitter architecture

- Two-stage conversion (with IF) is a safe solution, but hasdifficulties in integration.

- I/Q imbalance is a problem in "low-IF" type ofarchitectures.

- Direct conversion has problems, like carrier leakage andI/Q imbalance (in less critical form as in low-IF). Alsoleakage from power amplifier output to LO may have badeffects.

- Direct Digital Synthesis (DDS) is a promising approach,but the DA-converter performance is still a problem atcarrier frequencies of mobile systems.

- DSP based enhancement of the signal quality can beused also on the transmitter side.


Power Amplifier (PA)

· For constant envelope modulations (FSK, GFSK, MSK,GMSK) nonlinear power amplifiers can be used and goodpower efficiency is achieved.- With exactly constant envelope signals, nonlinearities

produce only harmonics, but no other new spectralcomponents. (justification after a few pages.)

· In other cases (linear modulations, and especially CDMAand OFDM), linear power amplifier is needed.- Nonlinearity causes spectral regrowth and signal distortion.

- Here the PA output spectrum is shown with different inputbackoff (IBO) values (see below).

-60 -40 -20 0 20 40 60-120

-100

-80

-60

-40

-20

0

20

frequency in MHz

PS

D in

dB

Spectral Regrowth

windowed OFDM,IBO = 0 dB 6 dB 12 dB 18 dB 100 dB


Power Amplifier Characteristics

The figure illustrates a typical signal amplitude transfer characteristic of aPA.- The output signal amplitude (envelope), and correspondingly, the

output power is saturated to a certain maximum level.- The characteristic is fairly linear with small signal levels close to the

origin.- The ideal soft-limiting (or clipping) characteristic (red) has maximum

linear range within the saturation values. This models also an ideallylinearized PA.

- Practical PA’s show increasing nonlinearity with increasing signalamplitude. There are various models for this nonlinearity (e.g., Rapp’smodel and Saleh’s model), which can be used in simulations.

More complete models include also the effect on signal phase:- AM-AM conversion: output amplitude versus input amplitude- AM-PM conversion: output phase versus input amplitudeThese memoryless models are not sufficient, e.g., in wideband caseswhere the transfer characteristics are different at different frequencies ofthe transmission channel.- Various PA models with memory exist in the literature.

-3 -2 -1 0 1 2 3-3

-2

-1

0

1

2

3Input-Output Relationship

Input Voltage

Out

put V

olta

ge


Effects of Nonlinear PAWith exactly constant envelope signals, nonlinearities produceonly harmonics, but no other new spectral components.This somewhat surprising result can be justified as follows(thanks to Mikko Valkama for suggesting the proof). Forsimplicity, let us consider a third-order polynomial model for thenonlinearity:

2 3( ) ( ) ( ) ( )y t x t ax t bx t= + +Assume also a generic bandpass model for the input signal:

( )( ) ( )cos ( )cx t A t t tw f= +Then it follows:

( ) ( ) ( )( ) ( )

( ) ( )

2 2 3 3

2 2

3 3

( ) ( )cos ( ) ( )cos ( ) ( )cos ( )

( )cos ( ) ( ) / 2 ( )cos 2 2 ( )3 1( )cos ( ) ( )cos 3 3 ( )4 4

c c c

c c

c c

y t A t t t aA t t t bA t t t

A t t t aA t aA t t t

bA t t t bA t t t

w f w f w f

w f w f

w f w f

= + + + + +

= + + + +

+ + + +

Only the spectral components around the carrier frequency are ofinterest. The spectral components at baseband, around 2fc and at3fc are easily removed by the RF bandpass filter, which is anywaypresent at the PA output. So the interesting part is:

( ) ( )33( ) ( )cos ( ) ( )cos ( )4c cy t A t t t bA t t tw f w f= + + +

We can see that the second-order (and more generally any even-order) nonlinearity doesn’t produce spectral components aroundthe carrier frequency, whereas third-order (and any odd-order)nonlinearity produces. The bandwidth of this term is three timesthe bandwidth of the baseband information signal.

However, for constant envelope modulation, A(t) is constant, andthe additional spectral component produced by the third-order (orin fact, any odd-order) nonlinearity is the same as the modulatedsignal spectrum, just weighted by a constant factor.


Back-OffBack-off measures the headroom between the averagetransmitted signal power and the maximum (saturated)output power of the PA.

The back-off may be calculated from:- the average signal power and saturated signal power at

the output (output back-off), or,- the average input signal power level and input power level

corresponding to the saturation in the linearized model(input back-off).

Increasing the back-off of the amplifier means:- The signal is contained better in the linear range, and

thus the effects of nonlinearities are reduced.- Power efficiency is reduced.


PA Linearization Methods

- Increasing back-off

- Various PAPR mitigation techniques, some of themgeneric, some system specific: Here the idea is to shapethe waveform in such a way that the maximum values areminimized.

- Predistortion: approximating the inverse function of thenonlinearity before the PA

- Cartesian feedback: feedback from the PA output tolinearize the characteristic.

- Linear amplification with nonlinear components (LINC):Any modulated waveform can be expressed as a sum oftwo constant envelope signals, which can be generatedusing two nonlinear PAs.

- Polar loop amplifier, Power VCO: Generating themodulated carrier signal directly using a VCO running atthe RF frequency and controlling the amplitude andphase with baseband signals.


The PAPR Problem in the OFDM Case - 1

Most of the recent and upcoming high data rate wirelesscommunication systems are based on OFDM (orthogonalfrequency division multiplexing).OFDM is a multicarrier modulation method which is based ofparallel transmission of multiple data symbol streams in differentsubcarriers.During an OFDM symbol, the transmitted waveform is the sum ofa high number (about 50 …6500 in current system specs) ofsinusoids, the phases and amplitudes of which correspond to thetransmitted data symbols (typically from a QAM constellation).The worst case peak envelope value is

{ }max mN A×

where N is the number of used subcarriersand {Am} is the symbol alphabet.

Even though this worst case peak value occurs with extremelylow probability, OFDM signals have anyway quite high peak-to-average power ratio (PAPR). This makes the power amplifierdesign a challenging problem in OFDM systems.- Generally, PAPR is defined as the ratio of peak instantaneous power

measured over a specific interval (OFDM symbol duration, CDMAsymbol duration, or a block of single-carrier symbols) to the averagesignal power.

- For a constant envelope signal, the PAPR is 1 (0 dB).



The complementary cumulative distribution function(CCDF) gives the probability that PAPR χμ of a randomlygenerated N-carrier OFDM symbol exceeds the PAPRthreshold χ0.

A typical CCDF with 128 subcarriers:

For example, about 1 % of the envelope values are 9.7 dB,or more, above the average power.



Inband distortion- Noise like distortion in the signal band that depends on

the number of subcarriers, the PA characteristics, andthe used back-off => BER degradation.

- For high number of subcarriers the inband noise can beapproximated as Gaussian, so it is easy to predict theeffect on the system performance.

- Typical inband distortion effect, with 16 QAM subcarriermodulation and 64 subcarriers, as a function of theback-off (the figure includes also an FBMC case, but it isof no concern here):

4 5 6 7 8 9 10 11 12 13 14

10-4

10-3

10-2

10-1

100

Input back-off (dB)

BER, Oversampling 4, 2 Amplis, FB 4-PAM vs. OFDM 16-QAM

BE

R

2M=64 subchannels64 carriers


The PAPR Problem in the OFDM Case - 4Out-of-band distortion induces out of band radiation called asspectral regrowth. This is important to take into account whendesigning the transmitter. The power radiation masks the systemRF specs have to be followed.Spectra regrowth effect in OFDM with different values of back-offusing the soft-limiting model:

It is clear that the impact of non-linearity is far more critial for thespectral regrowth than for the inband distortion.Two important metrics for the transmitter non-idealities are:- ACPR (adjacent channel power ratio): Power appearing at the adjacent

channel in relation to the useful signal power. (Also adjacent channelloss ratio, ACLR; in dB units, only the sign changes.)

- EVM (error vector magnitude): Signal error power in relation to theuseful signal power; can be expressed in % or dB units. As can beseen from the figure, most of the error power is in the inband region.So EVM is primarily a metric for the inband distortion.

-60 -40 -20 0 20 40 60-120

-100

-80

-60

-40

-20

0

20

frequency in MHz

PS

D in

dB

Spectral Regrowth

windowed OFDM,IBO = 0 dB 6 dB 12 dB 18 dB 100 dB


A Basic PAPR Mitigation Method:Clipping with Windowing

- Basic clipping/soft-limiting method introduces abruptchanges to the envelope of the modulated signal,causing spectral regrowth.

- These effects can be smoothened by multiplying theorginal signal with a suitable window function.

- The basic windowing method was introduced in [5] andan enhanced method in [6].

clipping threshold

t

envelope clipped

windowed


References1. A. Loke, F. Ali, "Direct conversion radio for digital mobile phones -

Design issues, status, and trends," IEEE Trans. Microwave Theoryand Techniques, vol 50, no. 11, pp. 2422-2435, Nov. 2002.

2. P.-I. Mak, S.-P U, and R. P. Martins, “Transceiver architectureselection: Review, State-of-the-Art Survey and Case Study”, IEEECircuits and Systems Magazine, vol. 7, 2nd quarter 2007, pp. 6-25.

3. L. Larson et al., "Device and circuit approaches for improvedwireless communications transmitters," IEEE PersonalCommunications, pp.18-23, Oct. 1999.

4. Jose Tellado, Multicarrier Modulation with Low PAR: Applications toDSL and Wireless. Springer, 2000.

5. M.Pauli and H.-P.Kuchenbecker, "On the reduction of the out-of-band radiation of OFDM-signals, "IEEE Int. Conf. onCommunications, 1998, pp.1304-1308.

6. Olli Väänänen, Digital Modulators with Crest Factor ReductionTechniques. Doctoral Dissertation, Helsinki Univ. of Technology,2006.

Documents

Transmitter Architectures and Signal Processing · Signal Processing Markku Renfors Laboratory of Electronics and Communications Engineering ... any odd-order) nonlinearity is the