2-Antenna RF Front-End for Cognitive Radio

2-Antenna RF Front-End for Cognitive RadioJacek Liszewski, Holger Gabler, Wilhelm Keusgen, and Thomas Haustein

Fraunhofer HHI - Heinrich Hertz Institute, Einsteinufer 37, 10587 Berlin, Germany{jacek.liszewski,holger.gaebler, wilhelm.keusgen, thomas.haustein}@hhi.fraunhofer.de

Abstract—This document describes a prototype of ahardware module for cognitive radio applications. Thedeveloped remote radio unit (RRU) allows transmittingand receiving simultaneously using two antennas and20 MHz bandwidth in different radio frequency bands.The system consists of two boards: a baseband (BB)and a radio frequency (RF) board. In this paper themain focus lies on the functionality and design of theRF hardware.

Fig. 1. Photograph of the developed cognitive radio module.

I. INTRODUCTION

THE increasing traffic in mobile communicationnetworks demands better spectral and power ef-

ficiency. To boost the capacity, network nodes aredensified, which results in heterogeneous node de-ployment with macro, pico and femto cells operatingnext to each other and within its coverage area [1] [2].Therefore, scalability, emitted power and cost becomeincreasingly important and require a new type of radiofront-end hardware with preferably small form factorand good connectivity to the base band units (BBU).Here, digital signal processors, connected to severalRF units via optical fibre, offer high flexibility to ex-ploit transmission techniques like MIMO (MultipleInput Multiple Output) and dynamic spectrum access.Furthermore, energy efficiency per transmitted bit be-comes a more challenging task in cellular networks

if we consider the need for a 100-fold capacity in-crease over the next few years and the limited energyresources on our planet.

II. MOTIVATION AND STATE OF THE ART

SOLUTIONS

Current broadband radio system hardware architec-tures typically feature a functional separation betweena base band unit (BBU) and a radio front-end. Stan-dards like OBSAI (Open Base Station ArchitectureInitiative) or CPRI (Common Public Radio Interface)enable to use optical fibres to transmit digital signalswith high data rates. The architectures with sepa-rated radio front-ends, commonly known as remoteradio heads (RRH) or remote radio units (RRU), al-low bringing the radio front-end closer to the antenna,which results in significantly smaller cable-loss andreduced RF transmit power.

Actual trends in LTE (3GPP Long Term Evolu-tion) standardization cater to a wide range of techni-cal solutions satisfying the demands for more capac-ity and quality of service with significantly reducedcapital and operational costs. Among these spec-tral efficiency boosting techniques, multi-antenna ap-proaches are exploiting advantages from frequencyselective beam-forming for single base station siteupto coherent joint transmission from several sitesalso known as CoMP (Coordinated Multi-Point trans-mission and reception) [3]. Because full frequencyreuse is a prerequisite in cellular spectrum, consid-ered are advanced inter-cell interference coordina-tion and mitigation schemes, including coordinationin time, frequency and space domain, especially atcell overlaps. A further significant increase in pro-vided data rate per square-kilometer will come eitherfrom cell densification or additional spectrum.

In general, cell densification means deploying ad-ditional small cells within an existing macro-cellularnetwork providing hot-spot capacity boost where itis needed. Since these pico or micro cells will usebandwidths and number of antennas similar to macro-eNBs (evolved Node-B), aggregated data traffic will

7th Karlsruhe Workshop on Software Radios

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add up to the same amount of a few hundred Mbpsper node, where the required high capacity backhaul-ing can be provided using optical fibres. Combiningthese heterogeneous cell deployment concepts withideas of centralized RAN (Radio Access Network) orcloud-RAN architectures consequently will favor so-lutions separating base band unit and radio front-endunits using optical fibre interfaces.

Architectures with RRUs attract new research in-terest regarding deploying nodes of different classes(in terms of number of antennas, transmit power,operational bandwidths and frequency agility) andare promoted by companies like Alcatel-Lucent andNokia-Siemens-Networks under the term Light Radioor Liquid Radio.

Considering new techniques like carrier-aggregation, which require BBUs jointly operating inseveral frequency blocks (called component carriers(CC)) and which result in adjacent spectrum orfragmented spectrum usage, additional technicalchallenges have to be taken into account. This canbe realized by using fully integrated radio front-endswith large bandwidth supporting multiple CCs andgood carrier isolation. Alternatively, especially incase of distributed antenna and node deployments,solutions with several single-CC radio front-ends allconnected with the same BBU offers more flexibility.In return, the single-CC radio front-ends have to befrequency agile and able to be remotely synchronizedover the optical fibre link. A single solution fortime-division (TDD) and frequency-division duplex(FDD) operation allows a more flexible reuse of suchmodules in many use cases and in many countries.

III. HARDWARE DESIGN

One of the main requirements of the developedRRU was supporting CPRI with 2457.6 Mbit/s datarate, which is sufficient for duplex in-phase andquadrature (I/Q) data exchange between basebandand RF modules supporting 2-antenna operation with20 MHz bandwidth. A baseband reference clock canbe delivered from an internal oscillator, an externalclock source or a clock recovery mechanism from theCPRI protocol.

The new platform consists of two parts: baseband(BB) interface board and radio frequency (RF) board(fig. 2). The whole module fits into an AMC (Ad-vanced Mezzanine Card) slot. The front panel of thedeveloped module contains an optical SFP connectorand SMA connectors for reference clocks and anten-nas.

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Fig. 2. Radio system architecture with separated remote radiounit (RRU) consisting of baseband (BB) interface board and radiofrequency (RF) board.

A. Baseband Board

The baseband board is based on a Xilinx Spartan-6 field programmable gate array (FPGA). It con-tains two integrated sets of digital-to-analog convert-ers (DAC) and analog-to-digital converters (ADC) tosupport two transmit and two receive signal paths(fig. 3). The main benefit of the Spartan-6 FPGAis low cost and integrated high-speed GTP serialtransceivers which support the CPRI protocol at highdata rates. The CPRI data format used in this designrequires that I and Q samples are 15-bit and one an-tenna stream has a sample rate of 30.72 MSPS. Fur-thermore, the BB board contains a phase-locked loop(PLL) chip for the reference clock synthesis and dis-tribution. The PLL can be driven by a CPRI clockfrom the FPGA or by an external clock reference.

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Fig. 3. Block schematic of the BB card.

The FPGA includes the CPRI protocol implemen-tation, interfaces of control input/output lines and set-tings for different programmable components on BBand HF boards. The applied Spartan-6 also offersresources for additional signal processing (e.g. digi-tal baseband filtering, predistortion) and gain control.The FPGA can be programmed using a PC with aJTAG adapter. The BB board is equipped with a flashmemory which enables saving FPGA programs andloads it to the FPGA after board resetting. A light-ing LED signalizes the end of FPGA programming.The module requires 12 V DC power supply from theAMC slot or an external supply connected with a ca-ble. For typical applications the supply current equalsapp. 1.5 A. The module is equipped with a 2-A fuse.


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Fig. 4. Block schematic of the designed 2-antenna RF front-end.

To work properly, most of the integrated circuits (ICs)on both boards have to be programmed using a se-rial peripheral interface (SPI). Some default settingscan be saved in flash memory and loaded automati-cally during board initialization. Additionally, the in-put/output lines for SPI programming are routed to anextra connector which enables communication with aPC via LPT port.

B. RF Board

The RF card and the BB card communicate via twointerface connectors. The interface includes differen-tial analog baseband (I/Q) signals, power supply, con-trol input/output lines and a reference clock signal.The RF card (fig. 4) includes two radio transceiverswith antenna connectors. Two coherent signal pathsallow a MIMO operation with a single module. Dif-ferent independently configurable local oscillators areprovided for I/Q modulators and demodulators. Alloscillators are locked to a common reference clock,which can be generated on the board with a crystal os-cillator, or delivered from BB board, or from externalsource. The clock signal from the crystal on the RFboard can be also used as reference for the BB board.The design of the printed circuit board enables usingtwo different quartz oscillators (TCXO or OCXO),depending on phase noise requirements for the RFsignal synthesis. To change the source of the refer-ence clock, a resistor bridge has to be re-soldered andthe oscillator has to be switched off using a jumper.

The TX and RX signal paths include low-pass LC-filters for signal reconstruction and anti-aliasing, re-

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Fig. 5. Baseband filter characteristics for RF signal bandwidthsof 20 MHz and 40 MHz.

spectively. The maximal sampling rate of the 14-bitDACs and the 12-bit ADCs equals 124 MSPS and 64MSPS, respectively. It allows a baseband signal in-terpolation, which lowers the baseband filter require-ments. For the TX and RX Butterworth filters of order5 and 7 has been calculated. Fig. 5 show the proposedfilter characteristics for 20 and 40 MHz signal band-widths. The constructed filters consist of discrete-value capacitors and inductors with some tolerances,which can influence the calculated characteristics.

To minimize static imbalance between I and Q sig-nal components, the gain of each DAC output canbe tuned. The TX impairments can be tested usingknown signals generated in FPGA, e.g. from signalmemory, or from an implemented numerically con-trolled oscillator. Using an adapter board (fig. 6), thetransmit baseband signals can be viewed on an oscil-loscope or looped back to the ADCs. The DC part ofthe baseband signal, interfering with the LO leakagein the modulator, causes unwanted peaks in frequencydomain. To minimize these effects, the DC offsets ofeach I and Q signals can be modified using tunablevoltage from auxiliary DACs (AUX DACs).

Programmable 1-dB-step attenuators are intendedfor power control in the TX front-ends. The transmit-ted signals reach a maximal power of about 20 dBmat power amplifier output.

The RX front-end is equipped with a low noise am-plifier (LNA) at its first stage. Moreover, a 3rd orderLC bandpass filter is implemented as next stage (notdepicted in fig. 4).

The demodulator circuit includes amplifiers for au-tomatic gain control (AGC). The circuit is designed


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Fig. 6. Photographs of the module components and adapter;from above: BB board, BB board with measurement adapter forbaseband and control signals, RF board.

for open loop gain control using AUX DACs andAUX ADCs. This solution enables implementing ofdifferent gain control algorithms in FPGA.

Table 1 shows parameters of the applied com-ponents on the RF board. The maximum transmitpower and low power at receiver input determinethe required isolation between TX and RX signalpaths. TX, RX and oscillator component groups areplaced in separate shielding cans (fig. 1) to minimizecrosstalk. The design enables a good flexibility re-garding frequency ranges and RF signal bandwidths.Determining bandwidth limitations are caused by BBand RF filters.

To make the RF module more flexible regard-ing customization for certain operational frequencybands, the duplex components, i.e. band-pass fil-ters, diplexers for frequency domain duplex (FDD)or switches for time domain duplex (TDD), will beplaced on extra plug-on modules. Currently, modulesfor LTE band 7 and ISM band are developed. Us-ing available surface acoustic wave components al-lows small size and low costs of the duplex modules.

IV. MEASURED PERFORMANCE

Fig. 7 shows measured spectra of output signalsgenerated with the module prototype. To enable us-

TABLE IRF BOARD PARAMETERS

IP1-dB OP1-dB fgL fgU BW/dBm /dBm /MHz /MHz /MHz

TX - 20 800 3700 700RX -50 - 800 2700 130LO - - 25 3000 -

ing different test sequences, an arbitrary waveformgenerator has been implemented on the FPGA. Themeasurement results present the capability of trans-mitting broadband signals with variable power usinga noise sequence with a bandwidth of 20 MHz anda peak-to-average power ratio (PAPR) of 9.5 dB. Atthe 2.65 GHz carrier frequency, the maximal average(RMS) in-band signal power reached about 5 dBm.The measurement with the band-limited noise sig-nal also demonstrates an unwanted out-of-band emis-sion, which will be minimized by an additional hard-ware optimization and appropriate algorithms imple-mented on FPGA.

The developed prototype has been also tested com-municating with a LTE-Advanced user equipment(UE), which is available at Heinrich Hertz Institute[4]. An OFDM signal (fig. 8) has been generated us-ing the cognitive radio module and send out via anantenna. The UE was able to receive the signal withlow bit error rate. The successful OFDM transmis-sion has additionally proved a good performance ofthe RF board regarding frequency stability and phasenoise.

V. CONCLUSIONS AND OUTLOOK

The hardware module for cognitive radio applica-tions has been developed and tested. The functional-ity and configuration of the baseband BB board andthe RF board have been described. Measurement ex-amples validate the successful design.

In the next steps a flexible duplex module with aswitching network is planned. The module will allowdynamic switching between frequency and time du-plex in different frequency bands. The system will befully suitable for multi-band and multi-standard cog-nitive radio applications.

To improve the performance of the RF front-end, real-time digital pre-distortion and I/Q imbal-ance compensation algorithms will be implementedon FPGA [5].


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Fig. 7. Measured spectra of a generated noise signal with a 20MHz bandwidth and different output power.

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Fig. 8. Measured spectrum of an OFDM signal generated us-ing developed boards. The signal is compatible with HHI LTE-Adwanced transceivers.

ACKNOWLEDGMENTS

The authors would like to thank Andreas Kortke,Marcin Ostopinko, Joseph Bohm, Dennis Wieruchand Thomas Wirth for their support.

REFERENCES

[1] M. Farber, “Densification in mobile networks and the poten-tial evolution paths of the base station,” in Wireless Com-munication Systems (ISWCS), 2011 8th International Sym-posium on, nov. 2011, pp. 181 –185.

[2] W. Mulder and T. Wirth, “Are we ready for the femtolution?”in E-Letters, IEEE COMSOC Multimedia Communications,Sep. 2010.

[3] V. Jungnickel, L. Thiele, T. Wirth, T. Haustein, S. Schif-fermuller, A. Forck, S. Wahls, S. Jaeckel, S. Schubert,H. Gabler, C. Juchems, F. Luhn, R. Zavrtak, H. Droste,G. Kadel, W. Kreher, J. Mueller, W. Stoermer, and G. Wan-nemacher, “Coordinated Multipoint Trials in the Downlink,”in GLOBECOM Workshops, 2009 IEEE, 30 2009-dec. 42009, pp. 1 –7.

[4] T. Wirth, L. Thiele, T. Haustein, O. Braz, and J. Stefanik,“LTE Amplify and Forward Relaying for Indoor CoverageExtension,” in Vehicular Technology Conference Fall (VTC2010-Fall), 2010 IEEE 72nd, sept. 2010, pp. 1 –5.

[5] J. Liszewski, B. Schubert, W. Keusgen, and A. Kortke,“Low-Complexity FPGA Implementation of Volterra Predis-torters for Power Amplifiers,” in IEEE Topical Conferenceon Power Amplifiers for Wireless and Radio Applications(PAWR), Phoenix, USA, Jan. 2011, pp. 41–44.


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2-Antenna RF Front-End for Cognitive Radio

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