SDR implementation of an OFDM transmitter andreceiver

Jaime Luque Quispe∗, Jackelyn Tume Ruiz∗, Luis G. MeloniFaculty of Electrical Engineering

State University of Campinas - UNICAMP{luque, jtumer, meloni}@decom.fee.unicamp.br

Abstract—The new level of connectivity among machines andpeople promised by the forthcoming 5G mobile communicationsystem will shape diverse application areas from healthcare andeducation to smart factories and autonomous driving. Theseapplications have a variety of different sets of requirements ondata rates, latency, and reliability which are expected to besupported by an enhanced version of the OFDM modulationused today by the physical layer of 4G LTE systems. Thedevelopment of a new OFDM-based air interface able to flexiblysupport that diversity of applications is of current interest. Onthe other hand, software defined radio is an appealing conceptto introduce flexibility in the radios as the functionality of thecommunication system relies on the software implemented ina programmable device. Aiming to analyze the factibility ofimplementing elements of new communication systems under anSDR approach, this work presents the prototyping of a SDROFDM communication link for video transmission using theopen-source tool GNU radio and the RF hardware interfaceUniversal Software Radio Peripheral. Preliminary results showsthat a SDR opens opportunities in both researching and teachingenabling the designing and testing of digital communicationsystems in a very practical way taking into account the realconditions of the channel and front-end impairments.

I. INTRODUCTIONOver the past years, the development of wireless com-

munications systems has been largely influenced by the in-creasing of the traffic demand. Emerging new applications oflow latency, higher connection density and ubiquitous gigabitconnectivity require more efficient and flexible modem andinformation processing algorithms thus motivating an intenseresearch and development work at industry and academia onthe physical layer (PHY) and medium access control (MAC)layer components of new communication systems, since theyaffects all the upper layers and overall system performancein terms of energy efficiency, spectral efficiency, achievablethroughput, quality of service (QoS), etc. On the other hand,the prototyping of a communication system with ongoingresearch needs to be flexible such that it can be able tosupport new algorithms and test different use cases. Nowadays,5G transmission and reception blocks are being formulatedtheoretically and analyzed by simulation, however there is alack of implementations that allow the proof of concepts andevaluation under real channel conditions, which deterioratesthe signal quality.

Software defined radio (SDR) is an appealing approachfor flexible prototyping of communication systems where

the functionality of the systems relies on the softwareimplemented in a programmable device: system on chip(SOC) field programmable gate array (FPGA), digital signalprocessor (DSP) or general purpose processor (GPP). Thus,the reconfiguration of the system is possible, even during theexecution time, by reloading a firmware or modifying thesoftware and recompiling it. Hence, SDR opens opportunitiesin both researching and teaching enabling the designing andtesting of digital communication systems in a very practicalway taking into account not only the above-mentioned realchannel conditions of the channel but also the front-endimpairments.

The objective of this work is to show that the SDR technol-ogy is ready to be employed in the designing and prototyp-ing of digital signal processing (DSP) modem algorithms tobuild reconfigurable communication systems. A reconfigurableplatform is of particular interest since building blocks canbe tailored for different applications, e.g. vehicle-to-vehicle(V2V) communications can require not only short framesto achieve low latency but also enlarged subcarrier spacing,narrowband internet of things (IoT) can require the use of asingle carrier waveform to achieve sufficient coverage withlow power, and custom cyclic prefix (CP) would be needed toachieve adequate service coverage in different application cellsizes. With this aim, this work presents the implementationof SDR communication link for video transmission using theGPP of a standard PC for the digital baseband functions andthe universal software radio peripheral (USRP) for the RFprocessing. The system uses the orthogonal frequency divisionmultiplexing (OFDM) multicarrier modulation scheme as anexample case since OFDM is widely used in many currentcommunication systems such as LTE and 802.11 and probablyalso in 5G new radio systems. As the software layer it wasused GNU radio which is a open-source set of libraries ofbaseband algorithms written in C++ and python.

The remainder of this work is structured as follows: A basicmodel of a OFDM communication system is presented insection II. Section III presents the SDR implementation ofthis model and presents some performance results. Section IVpresents our conclusions.

source-coded bit stream

Mapper subcarrierallocator

pilots

guard subcarriers

IFFTCP

insertion

packet formation

MUX

preamble generator

Fig. 1: Block diagram of an OFDM transmitter

II. SYSTEM MODEL

OFDM is a multicarrier modulation scheme based on thedivision of a serial data stream in a group of parallel datastreams which are transmitted on orthogonal subcarriers. Byproperly configurating the OFDM waveform, when it is sentthrough a time-dispersive channel or frequency selective fad-ing, the subcarriers can be affected only by a constant channelor frequency flat fading. In this kind of channels, the receivedsignal can be distorted by intersymbol interference (ISI),which is a crosstalk between signals arrived with differentdelays. This problem is addressed in OFDM by inserting acopy of the last part of the symbol, also known as cyclicprefix (CP), at the beginning to absorb channel delay spreads.The CP helps to preserve the synchronization and maintainthe subcarrier orthogonality, and it also introduces a cyclicconvolution between the transmitted symbol and the impulseresponse of channel thus enabling the use of one-tap equalizersto compensate the channel distortion. A CP-OFDM signal ofN subcarriers can be expressed in the time domain by

x[n] =

∞∑l=−∞

N−1∑k=0

Sk,lp[n− lNT ]ej2πk(n−lNT )/N (1)

where N , NCP and NT are the number of samples of thepayload, CP and entire OFDM symbol respectively, i.e., NT =NCP +N .A block diagram of a transmitter is shown in Fig.1, where astream of bits is converted into complex data symbols by theconstellation mapper and then placed on determined positionsof a N -order inverse fast Fourier transform (FFT) by thesubcarrier allocation block. The subcarrier allocator also placesthe pilots for channel estimation purposes and leaves unusedsome subcarrier positions (guard subcarriers) to contain theout-of-band emission (OOBE) from the active subcarriers.Then, groups of N frequency-domain samples are converted tothe time domain by the inverse FFT and then a CP is appendedat the beginning of each OFDM symbol. Optionally, these CP-OFDM symbols can be pulse shaped in the time-domain forspectral containment. For example, the power spectrum density(PSD) of 1 given by

PSDx = T

N−1∑k=0

E[|Sk,l|2

]∣∣∣∣sinc [T (f − k

TN

)]∣∣∣∣2, (2)

3 2 1 0 1 2 360

50

40

30

20

10

0

Fig. 2: Effect of pulse shaping on the reduction of the OOBEof a OFDM signal. The worst case β = 0 is included forcomparison.

can be better contained by using a raised cosine (RC) pulseshaping filter and in this case, assuming the frequency responseof the filter to be P (f), the PSD results in

PSDx(f) =1

T

N−1∑k=0

E[|Sk,l|2

]∣∣∣∣P (f − k

TN

)∣∣∣∣2. (3)

In Fig. 2 is shown the reduction of the OOBE of a 5-MHzLTE waveform achieved by the application of some values ofroll-off length β of the RC filter.

Additionally, in packet-based data transmissions, specialpreamble OFDM symbols can be prepended in the frontof every frame (one frame is composed of a number ofOFDM symbols). These symbols can be used for detection,time and frequency synchronization and initial channel stateestimation [gnuradio]. Only for a sake of illustration, thepreamble insertion is indicated at the end of the Fig. 1. Atthis point, the OFDM signal is ready for conversion to theRF domain and further transmission. These processes usuallyinvolve additional digital signal processing such as digital upconversion (DUC) and digital pre-correction of possible IQimbalance of the RF components.

At the receiver side, two important process are neededbefore data demodulation, namely the synchronization, whichdetects the beginning of the OFDM symbols and estimatesthe frequency offset, and the channel estimation, which servesto compensate the distortions inserted by the channel. Afterthese processes and the discarding of the CP, the time-domainsamples are converted into the frequency domain and fromthem the payload data symbols are extracted and then equal-ized using the channel response previously estimated. Theequalized data symbols (complex numbers) are then convertedto bits and passed to a forward error correction (FEC) blockto recover the original information.

A. Synchronization

Synchronization is a critical stage of the reception process.A poor synchronization accuracy leads to severe degrada-tion of link performance thus making impossible to recover

LO

ADC

Timing Estimation

Frequency Estimation NCO

DFTsync.sync.y(t) r[n]

m

Fig. 3: Block diagram of the time and frequency OFDMsynchronization stages

transmitted information. The synchronization aims to correctany timing and frequency deviations of the received signalrespect to the transmitted one. It also includes the samplingclock synchronization. Time synchronization ensures that thereceiver samples only useful OFDM payload samples thusavoiding ISI from adjacent OFDM symbols while frequencysynchronization removes the frequency errors (e.g. due toDoppler effect or receiver oscillator mismatching) to guaranteethe orthogonality of the subcarriers and thus avoiding ICI. Ablock diagram of a synchronizer is shown in Fig.3.

By assuming that the transmit carrier frequency is fc andthe received signal y(t) is down-converted by a non-matchedlocal oscillator of frequency fLO, a carrier frequency error(CFO) is produced whose value is fo = fc − fLO. Then, thesampled signal r[n] can be expressed as

r[n] = y[n]ej2πfonTs , (4)

where Ts is the sampling frequency and n is the sampleindex. ∆m is the deviation in samples of the received signalrespect to an ideal signal whose beginning is aligned withthe start of sampling. ∆m and fLO are estimated in thetiming and frequency estimation blocks respectively. Givenan estimated frequency offset of f̂o, a numerically controlledoscillator (NCO) rotates the signal by exp(j2πf̂onTs). Then,the data subcarriers can be recovered by means of a FFT forsubsequent processing. In order to track online the input signaland compensate the errors as soon as possible, the synchro-nizer needs to respond immediately [DIEGO BARRAGAN]which requires fast signal processing algorithms. They canbe classified in non data-aided and data-aided synchronizationalgorithms whose difference is that the first scheme exploitsthe redundancy of the CP to perform the time and frequencyoffset estimation while the second one utilizes time andfrequency resources as preamble and pilot subcarriers.

B. Channel estimation and equalization

As mentioned above, the radio channel distorts the trans-mitted signal varying its amplitude and phase, thereby theresponse of the channel needs to be estimated to compensate(equalize) the errors introduced in the OFDM frame.

There are different techniques to perform channel estimationwhich can be classified into blind channel estimation anddata-aided estimation techniques. The first scheme does notrequire dedicated time slots and reference subcarriers (trainingsymbols or pilot subcarriers) for channel tracking, so thespectrum efficiency can be preserved. Similar to data-aidedfrequency estimation, channel estimation can employ referenceinformation multiplexed with the useful payload. In case of

knownreference symbols

(.)-1

RX pilotsamples

channelestimate

LPF

filteredchannelestimate channel

compensation

Fig. 4: Channel estimation and equalization processes

pilot subcarriers, they usually use low order modulations (e.g.BPSK or QPSK), for easy recovering. The main advantageof this scheme is its simplicity, but unlike blind estimationtechniques, it loses spectrum efficiency. The estimation andcompensation processes using this approach are representedin Fig. 4.

III. SDR COMMUNICATION SYSTEMPROTOTYPING

This section details about an implementation of the systempresented above using USRP RF interface cards and the open-source tool GNU Radio through its graphical interface GNUradio companion (GRC).

A. Hardware employed

There were utilized the following modules:• Two standard PC with Linux operating systems and GNU

radio installed. One PC acts as the transmitter and theother as the receiver.

• Two USRP boards B210, which are equipped with digital-to-analog converters (DACs) and analog-to-digital con-verters (ADCs), 2x2 MIMO antennas, reference clocksand RF chip transceivers with continuous frequency cov-erage from 70 MHz to 6 GHz and 56MHz of bandwidth.They also have 3.0 USB interface for communicatingwith the host PCs. This implementation used 2 GHz asthe carrier frequency.

B. Description

The GRC schematic containing the building blocks of theOFDM transmitter is shown in Fig. 5 and the receiver in Fig.6.

In the OFDM transmitter, it can be seen that there aredefined variables like sampling rate, Sync word 1, Sync word2, occupied carriers, pilot symbols, etc, which will be used inthe transmiter and receiver OFDM. The information comesof one TS video, indicated in the File source block, thisinformation is sent at Stream to Tagged Stream block thatwill add tags at evenly spaced intervals, the output informationpasses through an error corrector CRC32 to then be dividedinto Headers bits and Payload bits, which will be multiplexed.Was used BPSK and QPSK modulations respectively. Theoutput information will be sent to OFDM Carrier Allocatorblock, here is indicated the occupied carried and the CyclicPrefixed is added.

The information will be sent using a USRP Block, whereare configured the basic parameters for the transission, likesample rate, frequency, gain, etc. Was used a 2GHz frequencyand 2MHz sample rate.

Fig. 5: Block diagram of the transmissor system

Fig. 6: Block diagram of the receptor system

In the OFDM receiver, we can visualize four importantparts. First, the USRP source and the synchronizer, was useda Schmidl & Cox OFDM synchronizer to realize the frameoffset correction, immediately the samples are ducted into theHeader/Payload Demox block and a trigger signal is sent toindicate the begining of the frame. Then, is performed thechannel estimation into the header, the FFT shifts OFDM sym-bols to the frequency domain and the output information is sentto the Channel Estimation block. This block carrying out thechannel estimation and the frequency offset of the preamblesamples OFDM. The firts two symbols are of syncronizationwhich are used to estimated the equalization coefficient, thenthis symbols are discarded and the rest of symbols are sent outof the block. When the stream of OFDM symbols is sent atthe OFDM Frame Equalizer, it realize the equalization in oneor two dimensions for one OFDM frame, besides correct thecoarse frequency offset of the carrier. The OFDM Serializerselect the data symbols of the OFDM symbols and emits themas a sequency of complex values, which will be demodulated.The bits of the header then are interpreted by the PacketHeader Parser block, the results of the this interpretation isfeedback to the demuxer for lets it know the length of thepayload and the tagged stream. The payload demodulationdon’t need to do the estimation channel because the channelstate information is on the payload tagged stream. Finally, the

information receiver is saved in a output File, we obtained theTS video sended in the transmission.

Fig. 7: Physic implementation of transmission and receptionsystem

In the Fig. 7 can be see the hardware implemented torealize the proof of concept, booth computers have instaled thegraphics interface of the GNU Radio and the are connectedto their corresponding USRP. The left computer is doing theinformation transmission, in this case the information is aTS video, in the other hand, the right computer is doing thesignal reception and can be seen that a good quality video wasreceived as well as the OFDM spectrum.

IV. CONCLUSION

Increasingly, the SDR systems are introducing into radiocommunications world, proporcing scalables solutions easy toadapt to sundry kind of escenaries.

In the present work was implemented a example casesimple of undertand in orden to serve like introduction forthe construccion of more sophisticated platforms solutionsaimed at development of solutions oriented to 4G, LTE orIoT systems.

This work provided a comprensive literature review of oneof several SDR solutions: GNU Radio platform and USRPhardware. Based on the OFDM concepts was test out thatSDR allows desing flexible and scalables systems, capable toadapt to sundry communications sistems and new applications.

Also, was review the most important concepts to considerin the blocks used in the desing of the diagram blocks ofthe solution such as channel model, synchronism, channelestimation which are directly related to implementation ofOFDM system in hardware.

The realized example case, where was implemented a videotransmission and reception system using OFDM modulationsystem, has been successfully, the design was did in GNURadio and the USRP was used like a RF interface.

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