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International Journal of Computer Applications (0975 – 8887)Volume 126 – No.15, September 2015Frequency and Timing Offset Analysis in OFDM usingGNU RadioG. YamunaT. PadmavathiVarghese ThattilM.Tech studentECE DepartmentCVR College of EngineeringSenior Assistant professorECE DepartmentCVR College of EngineeringProf & Head of ECEDepartmentCVR College of EngineeringABSTRACTTo enhance the performance and reduce the Inter SymbolInterference (ISI) at enhanced data rates in Multiplexing (OFDM) is being used. The performance ofOFDM is degraded due to frequency and timing offsets whichincrease the Bit Error Rate (BER) in the wirelesscommunication. Frequency offsets are caused due todifference between transmitter / receiver oscillators andDoppler shift. Timing offsets are due to symbol timing andsampling clock drift. This is estimated and compensated in thereceiver.In this paper packet based data transmission test bed has beenimplemented for OFDM to analyze frequency and timedeviation. The effect of frequency offset and timing offset inpacket reception over different modulation techniques likeBPSK, QPSK is evaluated. By incorporating the noisevoltage, normalized frequency and sample timing offset thechannel model is simulated in the noisy environment andeffect of these parameters considered to maximize thethroughput.The OFDM packet based data transmission with offsetcorrection is simulated by using GNU Radio which is free &open-source software tool that provides signal processingblocks with a facility of modifying codes in these blocks.KeywordsOFDM, BER, Frequency offset, Timing offset, GNU Radio,SDR.1. INTRODUCTIONThe scope of wireless communication is increased to a greatextent because of the use of multicarrier modulationtechniques, among which Orthogonal Frequency DivisionMultiplexing (OFDM) is one of the most popular techniques.OFDM is a modulation technique that is being used for manyof the latest wireless and telecommunications standards. Itallows many users to transmit in an allocated band, bysubdividing the available bandwidth into many narrowbandwidth carriers [1]. Each user is allocated several carriersto transmit their data. The transmission is in such a way thatthe carriers used are orthogonal to one another [2], thusallowing them to be packed together much closer thanstandard frequency division multiplexing (FDM). This leadsto OFDM providing a high spectral efficiency. Additionallythe advantage of OFDM is, it reduces fading effect andincreases the data rate in transmission [3].Inter Symbol Interference (ISI). Doppler shift in receivedfrequencies destroys the subcarrier’s orthogonality andproduces Inter Carrier Interference (ICI) [5] in the system.These ISI and ICI increases the packet losses due to which theperformance of an OFDM system to be degraded.The basic model of OFDM communication system withchannel model for the transmission of data symbols to thereceiver shown in Fig 1.Fig 1: Block diagram of conventional OFDM systemThis channel model produces LOS (Line of sight)communication and also various reflections due to whichmultipath effects come to the picture. To minimize themultipath effects and noise introduced by the channel, channelestimation is used. Generally, the received signal obtainedthrough a multipath fading channel contains the channelimpulse response and additive white Gaussian noise which isgiven by,0 n N-1R(n) H(n)C(n) W(n)(1)Where C(n) is transmitted complex data symbol which ismodulated by a BPSK or QAM modulation, H (n) ischannel coefficient in frequency domain and W (n) isadditive white Gaussian noise.The channel coefficient H (n) isH n r 1j 0 hjnej2πTf Dj N (λ τj)(2)In equation (1.2), λ is the delay spread index, h j is thecomplex impulse response of the jth path, fDj is the jth path’sDoppler frequency shift, τj is the jth path delay timenormalized by the sampling time, and r is the total number ofthe propagation path. After removing the guard interval fromR(n) the received samples are obtained and are demodulatedto extract the pilot signals. The transmitted data samples C(n)When the signal exists beyond Guard Interval (GI) it leads to13

can be recovered by simply dividing the received signal by thechannel response,Y n R(n)H(n)(3)1.1 Effect of Offsets and ChannelEstimationThe performance degradation in OFDM is due to frequencyoffset and Timing offset. Frequency offset is defined as thedifference between the nominal frequency and actual outputfrequency. It occur due to a difference in the frequencies ofthe local oscillators in the transmitter and receiver, gives riseto a shift in the frequency domain which is also referred to asfrequency offset [4, 5]. This offset can also be caused due tothe Doppler shift in the channel. Timing offset is occurringdue to different rates between the sample clocks of thetransmitter and receiver. It also occurs due to changes insymbol timing. The demodulation of a signal with theseoffsets can cause large BER and may degrade the performanceof a symbol synchronizer. It is therefore important to estimatethe frequency offset and minimize its impact. These offsetscan be estimated and corrected by using channel estimatorfollowed by equalizer.Channel estimation is the process of analyzing / characterizingthe effect of the physical medium on the transmitted data.International Journal of Computer Applications (0975 – 8887)Volume 126 – No.15, September 2015delay spread 25% of total subcarriers are allocated for cyclicprefix (CP). Along with CP, preamble is used for detectionand synchronization and initial channel state information(CSI).After modulation the entire parallel data converted into serialform and then transmitted through the AWGN channel model.In the testbed normalized frequency and sample timing offsetsare incorporated.In this testbed data aided channel estimation technique isused. It can be implemented in couple of ways namely blocktype and comb type. The block type channel estimation isdone through training symbols (sending known informationover one or more whole OFDM symbols without anydata), and in comb type pilots aided channel estimation(sending known information together with the data). Blocktype suits more for slow-fading channels and comb type isused for fast fading channels.At the receiver side preamble is used to detect the framestarting time and offset which occur due to multipath by usingSchmidl & Cox technique [10]. It uses two training symbolswith special pattern. With the help of first symbol it produce atrigger signal to find the frame starting time. The time domainpreamble proposed by Schmidl & Cox technique isPreamble p [PN PN ]2The channel estimation techniques are of two types, Blindchannel estimation techniques and Data-aided channelestimation techniques. The first technique is used to estimatethe channel state information (CSI) without the knowledge ofthe transmitted signal and the second estimation technique isto estimate the channel response by the known informationadded to the transmitted signals.Where, PN/2 represents samples of length N/2. The timingmetric of Schmidl & Cox is given asM d Reconfigurability is a key feature with all software-definedradio systems [9]. Instead of using different radios designedfor specific but different purposes, a single general-purposeradio can be used as the radio front-end, and the signalprocessing software (here GNU Radio) handles the processingspecific to the radio application.2. OFDM IMPLEMENTATIONThe implementation of OFDM communication system includetransmitter, receiver and channel model. The functionality ofthe OFDM communication system is verified with suitableSDR blocks shown in Fig 2.In the transmitter the data or message signal is generated withthe help of Galois linear feedback shift register (GLFSR)source. From GLFSR source Pseudo random sequence (PRC)data is encapsulated and is modified in to packet by includingpreamble and header. This packet is transmitted with the 64orthogonal subcarriers. These are modulated using amodulation schemes like BPSK, QPSK etc. In this test bedBPSK modulation is used to transmit header because of itslow Bit Error Rate (BER) and QPSK is used for payload.Among 64 subcarriers, 48 subcarriers are used for data, 4subcarriers are used to carry pilot symbols. To avoid ISI andP(d)2(5)(R d )2Where,1.2 GNU Radio CompanionGNU Radio is a free software development toolkit [6, 7] .Itprovides signal processing blocks to implement softwaredefined radios (SDR) [8, 9] and signal processing systems. Itcan be used with external RF hardware to create SDRs orwithout hardware in a simulation environment. It is widelyused in academic and commercial environments to supportboth wireless communications research and real-world radiosystems [7].(4)2P d R d 𝑁 12𝑘 0 𝑟 𝑁 12𝑘 0𝑁𝑑 𝑘 . 𝑟(𝑑 𝑘 )2𝑁𝑟(𝑑 𝑘 )2(6)2(7)The starting point of symbol is given by the maximum oftiming metric M (d). The timing metric of Schmidl & Cox hasa large plateau and has its peak for entire interval of CP.Frequency offset can also be calculated and corrected with theSchmidl & Cox technique [10]. The trigger signal along withreceived data is send to header and payload Demultiplexer(HPD) block. The HPD block copies the header and preambleinformation to its first output. The header data is demodulatedto identify the packet number and payload length andfeedback to the HPD block. Based on this information HPDcopies the payload to its second output and it alsodemodulated. From the preambles and pilot sequences thechannel estimation calculates the frequency offset with theSchmidl & Cox technique. The calculated frequency Offsetsis corrected in the serial equalizer block.Serializer plucks the data symbols from the occupiedcarriers and outputs them as a stream of complex scalars.Constellation decoder decodes constellation's points from acomplex space to (unpacked) bits based on the map ofthe constellation object. Payload demodulation follows thesame procedure as header demodulation except channelestimation block.14

International Journal of Computer Applications (0975 – 8887)Volume 126 – No.15, September 2015Fig 2: Block diagram of OFDM system3. EXPERIMENTAL RESULTSThe required parameters and specifications for OFDMtransmitter are defined in Table 1. The IFFT/FFT size is 64and cyclic prefix is ¼th of the IFFT/FFT size (FFT size 64and CP length 16 (1/4th of 64)). The cyclic prefix reduces theproblem caused due to delay spread and maintains continuityof the signal which ensures orthogonal reception of receivedsignal subcarriers.Table 1: Experimental ParametersParametersSpecificationsFFT Size64Occupied tones52Cyclic prefix16Sampling Rate10MModulation techniqueBPSK, QPSKSub carrier spacing156.25 KHzSystem ChannelBandwidth8 MHzFig 3: Frequency domain format of input signalIn this testbed 64 orthogonal carriers are used to modulate thedata. The resultant time domain representation of OFDMsignal is shown in Fig 4.The GLFSR source generates the input data with polynomialdegree of 2, with peak to peak amplitude of 1 to -1 which isshown in Fig 3.Fig 4: Time domain representation of OFDM signalThe functionality of OFDM Transmission and reception isverified by observing the demodulated signal at the output ofreceiver.The frequency domain representation of both signalsare displayed in Fig 5.15