In our Software Defined Radio project we perform a feasibility study of a software defined radio for two communication standards: HiperLAN/2 and Bluetooth. In this paper the Matlab/Simulink implementation of the HiperLAN/2 demodulator for the demonstrator of the project is discussed, with special attention for the frequency offset corrector. This type of correction is necessary to prevent large bit error rates that are caused by inter-subcarrier interference. The method that is proposed in this paper uses preamble section A to estimate the frequency offset. Simulation results for an AWGN channel show that the method is capable of correcting frequency offsets up to the boundary defined in the standard [1]. It was observed that frequency offset correction using only preamble section A is sensitive to ¿for example¿ synchronization errors in case real-life analog front-end signals are used

C. H. Slump

F. W. Hoeksema

F.W. Hoeksema

Preamble Section A

R. Schiphorst

V. J. Arkesteijn

English

In our Software Defined Radio project we perform a feasibility study of a software defined radio for two communication standards: HiperLAN/2 and Bluetooth. In this paper the Matlab/Simulink implementation of the HiperLAN/2 demodulator for the demonstrator of the project is iscussed, with special attention for the frequency offset corrector. This type of correction is necessary to prevent large bit error rates that are caused by inter-subcarrier interference. The method that is proposed in this paper uses preamble section A to estimate the frequency offset. Simulation results for an AWGN channel show that the method is capable of correcting frequency offsets up to the boundary defined in the standard [1]. It was observed that frequency offset correction using only preamble section A is sensitive to –for example– synchronization errors in case real-life analog front-end signals are used

van Hoesel, L.F.W.

Hoeksema, F.W.

Schiphorst, Roelof

Arkesteijn, V.J.

Slump, Cornelis H.

University of Twente Research Information

FREQUENCY OFFSET CORRECTION IN A SOFTWAREDEFINED HIPERLAN/2 DEMODULATOR USINGPREAMBLE SECTION AL.F.W. van Hoesel, F.W. Hoeksema, R. Schiphorst, V.J. Arkesteijn, C.H. SlumpUniversity of Twente, Department of Electrical EngineeringP.O. box 217 – 7500 AE Enschede – The Netherlandsemail:{l.f.w.vanhoesel, f.w.hoeksema, r.schiphorst, v.j.arkesteijn,c.h.slump}@el.utwente.nlIn our Software Defined Radio project we perform a feasibility study of a soft-ware defined radio for two communication standards: HiperLAN/2 and Blue-tooth. In this paper the Matlab/Simulink implementation of the HiperLAN/2demodulator for the demonstrator of the project is discussed, with special at-tention for the frequency offset corrector. This type of correction is necessaryto prevent large bit error rates that are caused by inter-subcarrier interference.The method that is proposed in this paper uses preamble sectionA estimatethe frequency offset. Simulation results for an AWGN channel show that themethod is capable of correcting frequency offsets up to the boundary definedin the standard [1]. It was observed that frequency offset correction usingonly preamble sectionA is sensitive to –for example– synchronization errorsin case real-life analog front-end signals are used.1 INTRODUCTIONThe software defined radio (SDR) project [2] at the University of Twente is cur-rently investigating the feasibility of a software defined radio. This is done by de-veloping a demonstrator that is capable of demodulation of two communication stan-dards: HiperLAN/2, a high-speed wireless local area network (WLAN) standard andBluetooth, a low-cost and low-speed personal area network standard. Figure 1 showsthe configuration of the communication system in the SDR project (see [3]). For theproject ananalog front-endwas designed (see [4]).In this paper the Matlab/Simulink implementation of the HiperLAN/2 demodula-tor for the demonstrator of SDR project will be discussed and special attention willbe given to distortions in the received signal, that are caused by mismatched mixers intransmitter and receiver.Transmitter ChannelAnaloghardwareAnalog todigitalconversionDigitalchannelselectionDemodulatorReceiverFigure 1: Overview of the SDR projects system configuration2 HIPERLAN/2Orthogonal Frequency Division Multiplexing(OFDM) has been chosen as mod-ulation technique in HiperLAN/2, because it has a good performance on an indoorradio channel. OFDM is a special kind ofmulticarrier modulation. The modulationtechnique divides the high data-rate information into several parallel bit streams andeach of those bit streams modulates (in groups of 1,2,4 or 6 bits) a separatesubcarrierusing BPSK, QPSK, 16QAM or 64QAM. In this way the radio channel is divided intoseveral independent subchannels, that each carry data at a low rate.The physical layer of HiperLAN/2 transmits 52 subcarriers in parallel per radiochannel. Four of the 52 subcarriers are used to transmitpilot tones. These pilot tonesassist the coherent demodulation in the receiver. Table 1 gives an overview of theHiperLAN/2 OFDM parameters that will be used throughout this paper.The transmitted HiperLAN/2 signals(t) is given by [1] (the carrier frequency inthe transmitter is denoted withfs):s(t) =√2 · <{s̃(t) e2πfst}(1)In which:s̃(t) =∑ns̃n(t) (2)And:s̃n(t) =∑NST2l=−NST2Cl,n e2πl∆f (t−TCP−nTS) , nTS ≤ t < (n+ 1)TS0 , else(3)In equation 3,Cl,n denotes the complex symbol value that is transmitted on sub-carrierl during OFDM symboln. In practice a HiperLAN/2 transmitter will work withsampled versions of the baseband signals̃(t) of equation 3. This baseband signal caneasily be obtained by taking theInverse Fast Fourier Transform(IFFT) of the subcar-rier values in a certain OFDM symbol.Table 1: HiperLAN/2 OFDM parametersParameter ValueSampling rate fsample 20 MHz (, 1/T )Symbol interval TS 4.0µs (= 80 · T )Useful symbol part duration TU 3.2µs (= 64 · T )Cyclic prefix duration TCP 0.8µs (= 16 · T )Number of data carriers NSD 48Number of pilot carriers NSP 4Total number of carriers NST 52Subcarrier spacing ∆f 0.3125 MHz (= 1/TU )A train of OFDM symbols, containing data of higher protocol layers in Hiper-LAN/2, is proceeded by apreamble. The preamble is a sequence of known OFDMsymbols and can consist of three different so calledpreamble sections–namedA, BandC–, depending on the type of MAC frame that is transmitted. In [1] the preamblesections are discussed in detail.3 HIPERLAN/2 DEMODULATOR ARCHITECTUREThe HiperLAN/2 receiver not only has to convert the received signal to data bitsby performing the inverse functions of the transmitter, but it also has to try to undo dis-tortions caused by radio channel and hardware of transmitter and receiver. A literaturestudy showed that the following distortions occur in the system depicted in figure 1: 1)Channel effects (noise, interference and delay spread); 2) differences in mix frequen-cies in transmitter and receiver (the so calledfrequency offset); 3) phase offset in themixers (causes a so calledcommon phase offsetto all subcarriers); 4) sample time dif-ferences between receiver and transmitter; 5) distortions caused by the digital channelselection filter in the band of interest; 6) phase noise; 7) non-linearity of amplifiers; 8)quantization noise in the analog to digital converter and 9) computational noise.1The first five distortions can be corrected in the demodulator. The other distor-tions are not correctable against reasonable costs (see [3] and [7]). The demodulatorarchitecture, that is used in the SDR project, is depicted in figure 2. This architectureis implemented in Matlab/Simulink. In this paperfrequency offset correctionusingthe preamble sectionA will be discussed in detail. Other implementation aspects andalgorithms are described in [3].1 See [3], [5], [6], [7], [8], [9] and [10]ControlSynchronizationSerial toparallelPrefix removalFreq. offsetcorrectionInverse OFDMCommon phaseoffset correctionChannelequalizationDemappingReceiverparametersOutput bitsInputsignalFigure 2: Demodulator part of the receiver; the FFT function is located in the ”InverseOFDM”-block4 EFFECTS OF FREQUENCY OFFSETIn this section we will determine the influence of a frequency offset introduced bya mixer mismatch in transmitter and receiver on the performance of the HiperLAN/2system. Assume that there is a differencef∆ between the carrier frequencyfs in thetransmitter and the carrier frequency infr the receiver , then:2fr = fs + f∆ (4)For simplicity we will assume that the channel’s transfer function does not distort thereceived signal and that no noise is present in the system. In this case, the receivedbandpass signal is given with:r(t) =√2 · <{r̃(t)e2πfrt} (5)Thus:r̃(t) = s̃(t) e−2πf∆t (6)From the equation above it can be concluded that a frequency offset can be described atbaseband level as a time varyingrotationof the transmitted complex baseband samples.At this point we assume that the subcarrier values are retrieved from the sampledversion of the baseband signalr̃(t) by applying a Discrete Fourier Transformation2 Note that this frequency difference isnot time-dependent. Any time-dependent changes will be repre-sented byphase noise.(DFT). We will also assume that the frequency offsetf∆ is smaller than the subcarrierspacing∆f . A received subcarrier valuêCl is given by:Ĉl =1NN−1∑i=0r̃[i] e−2πi lN (7)With N = 64, the number of samples in the useful data part of an OFDM symboland r̃[i] is the sampled received baseband signal at time instancest = i/fsample. Ifequation 6 is substituted in the equation above, this results in:Ĉl =1NN−1∑i=0s̃[i] e−2π f∆fsampleie−2πlNi (8)A single loaded subcarrier valueCγ gives rise to the transmitted complex basebandsignal (see also equation 3,n = 0):s̃[i] = Cγ e2π γNi∣∣∣i=0 ... N−1(9)When equation 9 is substituted in equation 8 and the relationfsample = N ∆f is used,the result is:Ĉl =1NN−1∑i=0Cγ e2π iNγ−l− f∆∆f(10)In case no frequency offset is present (f∆ = 0), the received subcarrier value equalsthe transmitted subcarrier valuêCl = Cγ only in casel = γ.From equation 10 can be concluded that a frequency offset causesinter- ubcarrierinterference(loss of orthogonality). If a frequency offset is present of, for example,f∆ = 1/10 ∆f = 31.250 kHz, andCγ = 1, the received subcarrier valuêCl=γ will beslightly less than the transmitted subcarrier value and some of its energy will flow toneighboring subcarriers (see figure 3a and b).The HiperLAN/2 standard [1] defines that:fc − fsfc< 0.002 % (11)With fc the nominal carrier frequency. If we assume an equal demand forfr comparedto fc, f∆ may be up to250 kHz. A quick calculation using equation 10 shows thatat this frequency offset, almost all power of a transmitted subcarrier is found in theneighboring subcarrier in the receiver; large bit errors rates will occur. It follows that−30 −20 −10 0 10 20 3000.20.40.60.81Subcarrier|C|−30 −20 −10 0 10 20 3000.20.40.60.81Subcarrier|C|Inter−subcarrier interference a) b) (a) no frequency offset present and (b)a frequency offset of ∆ = 1/10 ∆f =31.250 kHz0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 100.10.20.30.40.50.60.70.80.91Frequency offset f∆ / ∆fPower(c) Influence of frequency offset onneighboring subcarrier in direction of thefrequency offsetFigure 3: Effects of frequency offsetthe HiperLAN/2 demodulator should try to compensate for frequency offsets.In figure 3c the (normalized) power that spills into a neighboring subcarrier in thedirection of the frequency offset is plotted (see also [9], chapter 21).5 IMPLEMENTATION OF FREQUENCY OFFSET CORRECTIONThe preamble sections are –just like regular OFDM symbols– distorted by a fre-quency offset. Since the sections containknownsubcarrier values, the data can be usedto measure effects of a frequency offset. We will denote the measured frequency offsetby f ′∆. This measurement should be as close as possible tof∆.There are two approaches to this problem; the frequency offset can be determinedin the frequency domain and in the time domain.3 The frequency domain method isdiscussed in [3] and uses the results of figure 3c. In our demodulator we choose toimplement the time domain method.We define a vector~vpreambleA that contains the known sample values of preamblesectionA. Consider the following definition for the angleΘ between two complexnumbers (a, b ∈ C anda, b 6= 0):Θ(a, b) , arctan( <(a)=(a))− arctan( <(b)=(b))(12)3 Note that the demodulator exists of two parts: a time domain part and a frequency domain part, since anFFT is used to demodulate the subcarriers (see figure 2).We will determine the rotation between theknownsample value in the preamble sectionand thereceivedinput sample. Subsequently the slope of theregression lineof therotation will be calculated. The regression slopeΘ̇ is defined as (see [3]):Θ̇(~vpreambleA , r̃) =N ·N−1∑i=0iΘ(~vpreambleA [i], r̃[i]) −N−1∑i=0i ·N−1∑i=0Θ(~vpreambleA [i], r̃[i])N ·N−1∑i=0i2 −(N−1∑i=0i)2(13)With N = 64 is the number of samples that we use, andr̃ a vector of64 receivedbaseband samples.The result of equation 13 is the estimatedrotation per sample. This value caneasily be used to calculatef ′∆:f ′∆ =Θ̇(~vpreambleA , r̃)2πfsample (14)Note that other distortions can have influence on the frequency offset estimation wecalculate (see the list in section 3).A positive value off ′∆ implies that it is estimated thatfr < fs (see equation 4).All input samples, that follow preamble sectionA, are corrected with:r̃′(t) , r̃(t) e2πf ′∆t = s̃(t) e2π(f ′∆−f∆)t (15)before further processing.6 SIMULATION RESULTSTo determine the influence of additive white Gaussian noise (AWGN) on the fre-quency offset estimation and correction, a frequency offset will be introduced to theinput of the receiver by multiplying the output of the transmitter with a complex powerof e (as in equation 6). The model configuration used for the experiment in this sectionis shown in figure 4.4In figures 5a and b the BER curves of the experiment discussed above, are depictedfor the subcarrier modulation technique 16QAM. Per simulation 10,000 OFDM sym-4 The ”ramp” block of Matlab/Simulink is used to generate the time dependent argument for the complexexponent ofe.Zero-OrderHoldBERPERRampz-212							  exp(jx)ComplexExponentialFigure 4: Matlab model for frequency offset simulationsbols were transmitted. The frequency offset in the simulation experiment is changedfrom f∆ = ∆f/5 to f∆ = ∆f in steps of∆f/5.From figure 5a, which shows the BER results with the frequency offset correctiondisabled, can be concluded that frequency offsets up tof∆ = 62.5 kHz do not needto be corrected. For that frequency offset, the inter-subcarrier interference will causeabout5% of the power of a neighboring subcarrier to spill into a subcarrier (see fig-ure 3c), apparently without consequences for the BER. Other frequency offsets causeBERs of≈ 0.5.Figure 5b shows the results when the frequency offset correction is enabled. Thefrequency offset corrector can correct all tested frequency offsets, except forf∆ = ∆f .Note that the maximum allowed frequency offset in the HiperLAN/2 system is250 kHz.The frequency offset results stay within1.5 dB of their theoretical expected value.In figure 6 the standard deviation of the frequency offset∆ minus the estimatedfrequency offsetf ′∆ is plotted. As can be seen from figure 5b, the system was not ableto correct the frequency offset forf∆ = ∆f = 312.5 kHz. In this case the frequencyoffset results in a2π phase shift in 64 samples (the useful data part). As the standarddeviation in this case is comparable to the standard deviations depicted in the othercurves, it must be concluded that other moments of the distribution (the mean comesto mind) are responsible for the large BERs.In interpreting the curves forf∆ = 62.5 kHz andf∆ = 250 kHz one would expecta similarity in shape because of symmetry considerations. In the figure we do not seethis. We think that this is caused by the implementation of equation 12: in case therotation per useful data part is near2π, even small noise disturbances may cause amodulo− 2π operation, influencing the regression.The last two curves, forf∆ = 125 kHz andf∆ = 187.5 kHz, show a similarshape, as can be expected from symmetry considerations. The modulo-problem doesnot seem to occur. In thef∆ = 187.5 kHz case, the rotation per sample is larger thanin the f∆ = 125 kHz case and is hence estimated more accurately, for a given noisepower.7 CONCLUSION AND DISCUSSIONFrom the analysis in section 4 and from the simulation results in figure 5a it can beconcluded that a frequency offset corrector is necessary in a HiperLAN/2 demodulator.In this paper a method for estimation of the frequency offset is proposed that uses onlypreamble sectionA. Simulation results of an AWGN environment showed that thismethod is capable of correcting frequency offsets up to 250 kHz, the frequency offsetboundary that follows from the HiperLAN/2 physical layer definition in [1].An analog front-end has been designed for our SDR demonstrator (see [4]). Pre-liminary measurement results with this analog front-end have shown that the frequencyoffset corrector in the demodulator does not always function correctly. We believe thisto be caused by the implementation of equation 12 in the estimator and fact that pream-ble sectionA is the first section of a burst and hence not all other distortions are yetestimated and corrected, because they are estimated using other preamble sections.Further research has to be done into the influence of other signal distortions on ourmethod of frequency offset correction and to the divisions of preamble sections amongthe various other corrections necessary in a HiperLAN/2 demodulator.REFERENCES[1] ETSI. Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Physical(PHY) layer. 2001. ETSI TS 101 475 V1.2.2 (2001-02).[2] http://nt5.el.utwente.nl/sdr .[3] L.F.W. van Hoesel. Design and implementation of a software defined Hiper-LAN/2 physical layer model for simulation purposes. Master of Science Thesis,Universiteit Twente, EL SAS 032 N 02, 2002.[4] V.J. Arkesteijn, E.A.M. Klumperink, and B. Nauta.An Analogue Front-End Test-Bed for Software Defined Radio. MMSA, 2002.[5] S. Armour, M. Butler, and A. Doufexi et al.A Comparison of the HiperLAN/2and IEEE 802.11a Wireless LAN Standards. IEEE Communications Magazine,pages 172–180, May 2002.[6] J. Medbo and P. Schramm.ETSI EP BRAN3ERI085B; Channel Models forHiperLAN/2 in Different Indoor Scenarios. March 1998.[7] S.A. Fechtel, G. Fock, H. Meyr, and M. Speth.Optimum receiver design forwireless broad-band systems using OFDM - Part I. IEEE Transactions on Com-munications, Vol 47 No. 11 p. 1668-1677, 1999.[8] R. van Nee. OFDM for wireless multimedia communications. Artech HousePublishers England, 2000.[9] L. Hanzo, T. Keller, and W. Web.Single- and Multi-carrier Quadrature Am-plitude Modulation; Principles and Applications for Personal Communications,WLANs and Broadcasting. Wiley, U.S.A., 2000.[10] L.W. Couch.Digital and analog communication systems. Prentice Hall, U.S.A.,fifth edition, 1997.2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 2210−410−310−210−1100Eb/N0BERTheoretical 16QAMf∆=62.5 KHzf∆=125.0 KHzf∆=187.5 KHzf∆=250.0 KHzf∆=312.5 KHz(a) The frequency offset correction isnot active2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 2210−410−310−210−1100Eb/N0BERTheoretical 16QAMf∆=62.5 KHzf∆=125.0 KHzf∆=187.5 KHzf∆=250.0 KHzf∆=312.5 KHz(b) The frequency offset correction is activeFigure 5: Frequency offset simulation results2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 2200.511.522.533.54x 104Eb/N0stdev(f ∆−f"∆)f∆=62.5 KHzf∆=125.0 KHzf∆=187.5 KHzf∆=250.0 KHzf∆=312.5 KHzFigure 6: Standard deviation between the frequency offset∆ and the estimated fre-quency offsetf ′∆

Frequency Offset Correction in a Software Defined HiperLAN/2 Demodulator Using Preamble Section A

this paper the Matlab/Simulink implementation of the HiperLAN/2  demodulator for the demonstrator of the project is discussed, with special attention  for the frequency offset corrector. This type of correction is necessary  to prevent large bit error rates that are caused by inter-subcarrier interferenc

CiteSeerX

An Analogue Front-End TestBed for Software Deﬁned Radio.

Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Physical (PHY) layer.

Design and implementation of a software deﬁned HiperLAN/2 physical layer model for simulation purposes.

Digital and analog communication systems.

ETSI EP BRAN3ERI085B; Channel Models for HiperLAN/2 in Different Indoor Scenarios.

OFDM for wireless multimedia communications.

Single- and Multi-carrier Quadrature Amplitude Modulation; Principles and Applications for Personal Communications, WLANs and Broadcasting.

https://ris.utwente.nl/ws/files/5374812/mmsa2002b.pdf

Frequency Offset Correction in a Software Defined HiperLAN/2 Demodulator using Preamble Section A

Frequency Offset Correction in a Software Defined HiperLAN/2 Demodulator using Preamble Section A

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