2008 International Conference on Electronic Design December 1-3,2008, Penang, Malaysia

Low Power FSK Detection at Low Probability Bit- errorsHamood shehab, Widad Ismail, Mandeep Singh

School of Electrical and Electronics Engineering, Universiti Sains Malaysia, Seri Ampangan,14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia

1 -Tel: +60174854339, Email:hash_30165@yahoo.com2- Tel: +6 0124252683, Email: eewidad@eng.usln.my3 -Tel: +6 04-5996024, Email: eemandeep@eng.usm.my

Abstract

For portable deep space communications applicationswhere low power is a key requirement At the receiver,non coherent FSK quadrature detectors is suitable thatenable to develop novel design and digitallyimplemented as VLS! circuits An importantconsideration in an FSK low power receiver is whetherto use coherent detection or non coherent detection.This question arises because the carrier phase, whileintentionally varied in the transmitter by themodulating waveform, can be altered by the channeland thus affects the accuracy ofdetection. When thereis perfect time synchronization between the transmitterand the receiver, coherent reception can beimplemented using coherent oscillators, matchedfilters, and amplitude detectors. When phase coherencecannot be assured, the demodulator and matchedfilterare followed by an envelope detector. At low powerlevels, in order to obtain a good quality signal, onemust use a certain level of signal power whilemaintaining an acceptable Bit Error Rate (BER). Thistradeoff is often measured as a function of energy perbit vs. noise power density (Eb/No) required at thereceiver's input. Higher level modulation schemes (M­ary keying), except M-ary FSK, decrease bandwidthoccupancy but increase the required received power,and hence trades power efficiency for bandwidthefficiency.

1. INTRODUCTION

In digital wireless communication systems the receiverpower is a very important issue and since thetransmitter power can be elevated, power is traded forreduced BER performance [1, 3, 16]. The modulatingsignal (e.g., the message) may be represented as a timesequence of symbols or pulses, where each symbol hasm finite states. Each symbol represent n bits ofinformation, where n = log2m bits/symbol. ManyDigital modulation techniques are used in modemwireless communication systems, and many more aresure to be introduced. Some of these techniques havesubtle differences between one another, and eachtechnique belongs to a family of related modulationmethods. For example, Frequency Shift Keying (FSK)

978-1-4244-2315-6/08/$25.00 ©2008 IEEE.

1

may be either coherently or non coherently detected;and may have two, four, eight or more possible levelsper symbol, depending on the manner in whichinformation is transmitted within a single symbol. Acoherent system requires carrier or phasesynchronization at the receiver end in order to detectthe signals whereas non coherent detection does notrequire any sort of synchronization. Consequently thedesign of non coherent systems is much simpler,making them more popular when the power is animportant issue .while non coherent systems have aBER performance of 3db less than the traditionalcoherent receiver at a BER of 10E-5. This FSKdetector is suitable for low power wirelesscommunications. M-ary frequency shift keying (FSK)is a power efficient modulation scheme whoseefficiency improves as the number of frequenciesemployed (M) increases at the expense of additionalcomplexity and smaller bandwidth efficiency [10, 13,15]. This scheme has been found advantageous in lowrate low power applications. The process of bringingthe bandpass signal down to baseband is denoted asdemodulation. The distinguish detection fromdemodulation by denoting detection as the process ofextracting the information from the basebanddemodulated signal. A receiver consists of ademodulator and detector .non coherent techniques(where the reference phase is unknown); can often beimplemented in demodulation or detection.

2. Factors That Influence the Choice ofDigital Modulation. an Overview

Digital modulation is the process by which digitalsymbols are transformed into wave forms that arecompatible with the characteristics of the channel [2, 4,14]. Several factors influence the choice of a digitalmodulation scheme. A desirable modulation schemeprovides low bit error rates at low received signal-to­noise ratios, performs well in multipath and fadingconditions, occupies a minimum bandwidth, and iseasy and cost effective to implement. The performanceof a modulation scheme is often measured in terms ofits power efficiency and bandwidth efficiency. Powerefficiency describes the ability of a modulationtechnique to preserve the fidelity of the digital message

Fig(2)Coherent BFSK receiver

Fig (1) Binary FSK Waveform

BITl

,. ~. .,lNFOKJdAIlON,WAVE,.,. ,.... ,...

¥\ANNMWNM·BFSK'!lAVE

BIrO

4. Coherent binary FSK detectorThe coherent receiver needs to be in phasesynchronization with the transmitter. It consists of twocorrelators with a common input and which aresupplied with two locally generated reference signals

cp l(t) and cp 2(t) as shown in figure(2) [12, 6] . Thecorrelator outputs are then subtracted one from anotherand the resulting difference .d. is compared with athreshold of zero volts. If d>O, the receiver

decides in favor of 1 else in favor of O. The and cP 2(t)are extracted by applying the received signal to a pairtransmitted signal consists of both the frequencies f1

and f2. The reference signals cp l(t) of narrow bandfilters, one tuned to f1 and the other tuned f2. Hencethe reference signals are in phase coherence with thetransmitted signals.

at low power levels. In a digital communicationsystem, in order to increase noise immunity, it isnecessary to increase the signal power. However, theamount by which the signal power should be increasedto obtain a certain level of fidelity (i.e., an acceptablebit error probability) depends on the particular type ofmodulation employed. The power efficiency(sometimes called energy efficiency) of a digitalmodulation scheme is a measure of how favorable thistradeoff between fidelity and signal power is made, andis often expressed as the ratio of the signal energy perbit to noise power spectral density (Eb/N0) required atthe input of the receiver for a certain probability oferror (10 -5) [8, 9]. In general, increasing the data rateimplies decreasing the pulse width of a digital symbol,which increases the bandwidth of the signal. Thus,there is an unavoidable relationship between data rateand bandwidth occupancy. However, some modulationschemes perform better than others in making thistradeoff. Bandwidth efficiency reflects how efficientlythe allocated bandwidth is utilized and is defined as theratio of the throughput data rate per Hertz in a givenbandwidth. If R is the data rate in bits per second, andB is the bandwidth occupied by the modulated radiofrequency signal, then bandwidth efficiency isexpressed as T)B =RIB bps/Hz The characteristics ofthe modulation technique will determine theperformance of the system architecture in thedepicted environment. In design of a digitalcommunication system, very often there is a tradeoffbetween bandwidth efficiency and power efficiency. Inother words Power efficiency describes the ability ofthe selected modulation to maintain desired signalfidelity.

3. FSK DETECTIONFSK detection is the process of extracting theinformation symbols from a modulated carrier wave.The binary FSK waveform displayed in Figure(l)thetop waveform represents the information symbols andthe bottom waveform represents the modulated carriersignal [13, 7]. The detection process is mainlycategorized into two types namely; coherent detectionand non coherent detection. In the case of coherentdetection, the receiver exploits the knowledge of thecarrier.s phase in order to detect the symbols whereasin the case of non coherent detection, the receiver doesnot utilize any phase reference information from thecarrier. Coherent based receivers are usually complexto design but have the best BER performances. Also,coherent designs consume a lot ofpower. Non coherentreceivers have simple design, consume less power buthave reduced BER performances.

2

5. Non Coherent binary FSK detectorQuadrature receiver

The non coherent receiver no needs to be in phasesynchronization with the transmitter. Figure (3) showsthe block diagram of a non coherent binary FSK

Fig (3) Quadrature receiver for non coherentdetection

detector. Since phase information of the carrier isabsent, the detector is implemented as an energydetector. The receiver consists of two channels namely;the in phase (I) channel and the. Quadrature phase (Q)channel. The two branches of the 1 channel areconfigured to detect the signal with frequency wi; the

reference signals being V2lT cos (Wit) for the'!,

branch and V2lT sin (Wit) for the .Q. branch. Asimilar arrangement exists for the detection of signalwith frequency w2 as can be seen from the twobranches of the Q channel. The block following theproduct integrators perform a squaring operation toprevent the appearance of any negative values. Adifference of the energies of the channels I and Q iscalculated and fed to the decision stage which decidesin favor of wi if the difference is greater than zero elsein favor of w2. The quadrature non coherent detectorrequires twice as many branches as the coherentdetector and is not very power efficient [6, 7, 11, 12].

6. Non Coherent binary FSK envelopedetector

The non-coherent detection is used the envelopedetector and p and pass filters. An envelope detectorconsists of a rectifier and a low pass filter. Thedetectors are matched to the signal envelopes ratherthan the signals themselves and a decision as towhether a one or a zero was transmitted is made on thebasis of which two envelope detectors have the largest

3

amplitude. Although the envelope detector has asimpler design as compared to the quadrature detector,the fact that quadrature detectors can be implementeddigitally as VLSI circuits and also the use of filters andrectifiers in the envelope detectors increase their powerrequirement and cost than the quadrature [5, 12].

7. Non-coherent FSK and DopplerDependency:

Doppler shift is the change in frequency proportionalto the speed and direction of the moving object .It istrue that PSK is more bandwidth efficient, while thepower usage is more of an important issue thanbandwidth conservation in the system architecture.Non-coherent FSK is more power efficient than PSK.Non-coherent FSK handles the Doppler effects moreefficiently than PSK because of the differences insynchronization demands. N-FSK only has to track thecarrier frequency. PSK has to track the incoming phase[2,4, 7].

8. Simulations and ResultsThe Performance of probability of non coherent FSKsystem is simulated using MATLAB that M-aryfrequency shift keying (FSK) is a power efficientmodulation scheme whose efficiency improves as thenumber of frequencies employed (M) increases at theexpense of additional complexity and smallerbandwidth efficiency. M-ARY N-FSK is more powerefficient because as M increases, less power is requiredat the receiver per bit. This is because FSK signalspace is less crowded figure (4) and thus does notrequire a lot of power to distinguish each symbol. Thatis, FSK signals are more orthogonal which implies thatthese signals are "independent" of each other.

Fig(4) Signal-space diagram for binary FSK system

[1] Theodore S. Rappaport, .Wireless Communications:Principles and Practice., Prentice Hall, Second Edition.

[2] 1. G. Proakis, M. Saheli, .Communication SystemsEngineering., Prentice Hall, Second Edition.

[3] Fuqin Xiong, .Digital Modulation Techniques., ArtechHouse Publishers, 2002.

[4] J. G. Proakis, M. Saheli, .Digital Communication.,Prentice Hall, Third Edition.

[5] Alister Burr, .Modulation and Coding for WirelessCommunications., Prentice Hall, 2001.

[6] E. Grayver and B. Daneshrad, .A low power all digitalFSK receiver for space applications., IEEE TransCommunications, Volume: 49

[7] E. Grayver and B. Daneshrad, .VLSI implementation ofa 100-IlW multirate FSK receiver., IEEE J. Solid-StateCircuits, Volume: 36 Issue: 11, Nov. 2001, Page(s): 1821-1828.

[8] Bernard Sklar, Digital Communication, Fundamentalsand Applications. Englewood Cliffs, NJ: Prentice-Hall,1988.

[9] Martin. S. Roden, Digital Communication SystemDesign. Englewood Cliffs, NJ: Prentice-Hall, 1988.

References

[10] H.-C. Liu, J. S. Min and H. Samueli, .A low-powerbaseband receiver IC for frequencyh te Circuits,Volume: 31, Marchl996, Page(s) 384.394.opped spreadSpectrum communications., IEEE 1. Solid-Sta

[11] J. Min, A. Rofougaran, H. Samueli and A. Abidi,.An All-CMOS Architecture for a Low-PowerFrequency-Hopped 900 MHz Spread Spectrum

Transceiver,. Proceedings ofIEEE Custom IntegratedCircuits Conference,pp. 379-382, May 1994.

[12] G. Schultes, A.L. Scholtz, E. Bonek and P. Veith,.A New Incoherent Direct Conversion Receiver,.Proceedings of the 40th IEEE Vehicular TechnologyConference, pp. 668-674, May. 1990.

[13] P. Kreuzgruber, .A class of binary FSK directconversion receivers,. Proceedings of the 44th IEEEVehicular Technology Conference, pp. 457-461, Jun.1994.

[14] J.G. Proakis, Digital Communications, 2nd Ed.New York: McGraw-Hill, 1989.±160

[15] A. D. Proudfoot, .Simple Multifrequency Tonedetector,Electronics Letters, vol. 8, pp. 524-525, Oct. 19,1972.

[16] H-C. Liu, 1. Min and H. Samueli, .A Low-PowerBaseband Receiver IC for Frequency-Hopped SpreadSpectrum Applications,. to appear in Proceedings ofIEEE Custom Integrated Circuits Conference,SantaClara,CA, May 1995.

[16] E. Bonek, G. Schultes, P. Kreuzgruber, W.Simburger, P. Weger, T. C. Leslie, 1. Popp, H Knappand N.Rohringer, .Personal CommunicationsTransceiver Architectures for Monolithic Integration,.IEEE International Symposium on Personal, Indoor,and Mobile Radio Communications, The Hague, TheNetherlands, Sep. 1994, pp. 363-368

9. ConclusionsThe most important advantage of non coherentdetections is that it is simple and thus requires a simplereceiver and low power consumption, as opposed tocoherent detection where the receiver is relativelycomplex and more consumption of power. ButCoherent demodulation leads to better probability inthe presence of AWGN as shown in figure (5). The nonCoherent binary FSK detector for Quadrature receiveris presented, evaluated and compared. ThePerformance of non coherent FSK system is simulatedusing MATLAB with 2,4and 8 level FSK system asshown as in figure (6) when increase the M- ary getbetter probability of BER error. The proposed nonCoherent ,FSK. For Quadrature receiver is well-suitedfor low- power to reduce hardware complexity.

'0" H~~~~:~~~~~!~:~ ~ ~ ~ !~H H~~H11~i~~~i~'~ffffirfffififf~~~t10'~Jn:~~~ ~:~ ~;'~~~i~l~flrffi~~ ~~ ~ ~·~U jr~~~!; ~~] ~~~~~:~ ~!~~ ~ ~::~ ;;1f 10') .. - -~ - - .. - - _.~ - - - - ~- - ..... ~: "r.' - ".:" - • -- - - •• - - '0 - -I. - - - - - -- - - •• ~ - •. - - - - ..

f 'o·r;~;~~!~!~~~~~~~~~~!~~~i~;;~~~~~I;~;~~;~~~!1~;~;~;~!;;~1! to',H~ ~~ ~~,~! ~~~, ~~ ~~!~ ~ ~~~~,~ H~~"~lm~l~ ~~~ ~~ ~ ~r~,~ ~ ~~~ ~~1

1O~r~ ~i~~! !!; ~!. ~~~~ ~ ~ 1~:~.~~ ~ !~r~~~; ~ ;:~;~l~ ~~.~~;~~ ~~~;n;! ~l~~ ~ ~ ~~;;1

Fig(5) Comparison of BER for coherent andnoncoherent 8- FSK

Fig(6) Symbol error probability of noncoherentlydemodulated M-ary FSK signals.

10'; :: ~:::::: :::::::: ::::=::: :::::::: :::::::: . _ •...•

10 L..-_-L-.._--L.-_-J.-_--L.I----I.----I'-----l-...L--_...l.L-_..J

·2 0 2 4 6 8 10 12 1.-~"'IR n",r hit (rim-'".... 1'"" .... _ •• \W~i

4