Allen, B

Beach, MA

Fletcher, PN

Hunukumbure, RMM

Karlsson, Peter

English

It is shown here that even for a small increase in the inter-element distance from the nominal half wavelength value, grating lobes could occur in smart antenna radiation patterns. This has practical implications when smart antennas are employed in Frequency Division Duplex (FDD) systems. This paper discusses the impact on the UMTS Terrestrial Radio Access (UTRA) FDD system. The presence of grating lobes is verified and the resultant performance degradation in an interference limited cell is quantified, using simulation results derived from actual field trial measurements taken within the UTRA ban

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                          Hunukumbure, R. M. M., Beach, M. A., Allen, B., Fletcher, P. N., &Karlsson, P. (2001). Smart antenna performance degradation due to gratinglobes in FDD systems. (pp. 5 p).Link to publication record in Explore Bristol ResearchPDF-documentUniversity of Bristol - Explore Bristol ResearchGeneral rightsThis document is made available in accordance with publisher policies. Please cite only the publishedversion using the reference above. Full terms of use are available:http://www.bristol.ac.uk/pure/about/ebr-terms.htmlTake down policyExplore Bristol Research is a digital archive and the intention is that deposited content should not beremoved. However, if you believe that this version of the work breaches copyright law please contactopen-access@bristol.ac.uk and include the following information in your message:• Your contact details• Bibliographic details for the item, including a URL• An outline of the nature of the complaintOn receipt of your message the Open Access Team will immediately investigate your claim, make aninitial judgement of the validity of the claim and, where appropriate, withdraw the item in questionfrom public view.Submission to 9th COST260 MCM, Gothenberg, Sweden.1Smart Antenna Performance Degradation due to Grating Lobesin FDD SystemsMythri Hunukumbure(1), Mark Beach(1), Ben Allen(1), Paul Fletcher(1) & Peter Karlsson(2)(1)Centre for Communications Research,University of Bristol, Quenen’s Building, Bristol BS8 1TR, UK.Tel: +44 (0) 117 928 8617,   Fax : +44 (0) 117 954 5206E-mail : Mythri.Hunukumbure@bris.ac.uk(2)Telia Research AB, Malmo, Sweden.ABSRACT :It is shown here that even for a small increase in the inter-element distance from the nominal halfwavelength value, grating lobes could occur in smart antenna radiation patterns. This has practicalimplications when smart antennas are employed in Frequency Division Duplex (FDD) systems. Thispaper discusses the impact on the UMTS Terrestrial Radio Access (UTRA) FDD system. The presence ofgrating lobes is verified and the resultant performance degradation in an interference limited cell isquantified, using simulation results derived from actual field trial measurements taken within the UTRAband.INTRODUCTION :Within the 3rd generation wireless systems, there will be a much greater demand for system capacity,mainly due to the high data rate and multimedia services they promise to offer. Smart antennas are likelyto play a significant role in meeting this need for capacity enhancement [1]. The radiation pattern of asmart antenna array is governed by the following factors -: radiation pattern of individual elements,number of elements, inter-element distance and amplitude/phase variations across the array.  Usually thefirst three factors remain constant and the radiated power is optimised by the latter.Due to the separation of up-link and down-link carrier frequencies in a FDD system, the inter-elementseparation as a function of wavelength will not remain constant. In certain cases, this can result in thegeneration of grating lobes within the same cell sector. If present, the grating lobes could captureadditional interfering users, bringing down the carrier to interference ratio (C/I). Another implicationwould be the capturing of more multi-paths from the wanted user. Hence the occurrence of grating lobeswould deteriorate the interference rejection capability of the smart antenna and also increase the multipathinterference at the receiver.This paper first analyses the conditions under which grating lobes would occur theoretically. Next anoverview of the field trails is presented, which provided the data for the simulations.  It is shown thatwithin the field trails set up grating lobes would occur for some of the transmitting locations in smartantenna beam-forming. Finally the impact of the grating lobes is analysed in a simulated UTRAenvironment.THEORETICAL ANALYSIS:A radiation pattern generated by a uniform linear antenna array will contain complete grating lobes, if thefollowing equation is satisfied for values other than n=0. [2][ ] πθθλπ .sinsin.. 0 ndg ±=−         n=0,1,2 ……         ………………. (1) θg = angular position of the Grating lobe in azimuth plane         d  = inter-element distance θ0 = angular position of the main lobe in azimuth plane             λ = carrier wavelengthSubmission to 9th COST260 MCM, Gothenberg, Sweden.2Assuming a tri sectored cell, the maximum steering angle is ±60°.    i.e. θ0(max) = ±60°for n=1,      dgλ±=θ 8667.0sinin real space,   1sin ≤θ g ,           8667.1≤λd       or     λ*536.0≥d    ………..(2)Thus, if the inter-element spacing is greater than 0.536 times the wavelength, grating lobes could occurwithin –90°<θg<90° region when the main beam is steered towards θ0(max). Some of the energy in thegrating lobe can spill into the operating cell sector, even if its apex is outside the [-60° 60°] region.The UTRA-FDD spectrum allocations provide a nominal 190MHz separation between the up-link anddown-link carriers.  Analysing the frequency allocation in UK [3] for example, it is evident that if λ/2inter element spacing is employed on the up-link carrier, grating lobes will occur in down-link operation.Selecting the down-link carrier frequency for element spacing will reduce the electrical size of the arrayin the up-link, thus widening the main beam. Further there are practical design constraints in reducing theelement separation.THE FIELD TRAIL CAMPAIGN:The following results are derived from data gathered through an extensive field trial campaign conductedin Bristol. Employing a state of the art channel sounder (Medav RUSK BRI) the channel was illuminatedwith a 20MHz multi-tone signal centred at 1920MHz, which was within the allocated frequency bands forUTRA. The channel response was received by an eight element antenna array and processed by theMedav receiver. Data logging across all elements was carried out within the coherence time of thechannel. Further technical information on the field trails can be found in [4].During post processing, the signal bandwidth was windowed to 5MHz, to comply with UTRA standards.The 1925-1930MHz sub-band was selected, which implies a carrier frequency at 1927.5MHz. The arraywas originally designed to operate in the 1800MHz GSM band with the physical element separation fixedto 85mm. This translates to 0.546λ at this carrier frequency. As the threshold ratio obtained in equation(2) is exceeded here, grating lobes can be expected in the operating cell sector. Figure (1) presents theradiation patterns in beam steering with the 8 element array, and calibrated element radiation patterns.  a) Radiation patterns from the 8 element array           b) Individual element patterns (measured) Figure (1) : Radiation patterns of the Smart antennaFigure (1a) shows the presence of a significant spillage from a grating lobe in the operating sector, whenthe main beam is steered into -60°. Ideally the grating lobe should peak at 74.8°.  The figure also showsSubmission to 9th COST260 MCM, Gothenberg, Sweden.3how the grating lobe spillage diminishes, when the main beam is steered to -47°. Similar results areimplied at the opposite cell sector boundary. Amplitude weights derived from Dolph-Chebyshevtechnique have been incorporated, to achieve a 20dB side lobe reduction. Note that these radiationpatterns within the cell sector slightly deviate from the ideal, due to the roll off effects in the individualelement’s radiation patterns as shown in figure (1b).  Beyond the azimuth limits of the cell sector, theindividual radiation patterns roll off sharply, so the grating lobes would not have significant effects on theadjacent sectors.RESULTS :In order to validate the presence of grating lobes, the signal powers received from five field trial locationswere analysed. The transmitter was stationary during the 10s measurement periods. Smart antenna beamsteering was carried out towards the dominant directions of signal arrival (DoA). To capture the signalsfrom probable grating lobes, beam-forming was also carried out for the angles opposite to the dominantDoA’s. Table (1) presents the results, i.e. the average received power and the dynamic range1 of signalfluctuations for the above 2 cases.Main beam at ‘true’ DoA Main beam ‘opposite to’ DoAUserLocationSteeringangleAveragePower (dBm)DynamicRange (dB)SteeringangleAveragePower (dBm)DynamicRange (dB)A 60° -18.58 3.25 -60° -19.09 3.01B 56° -27.36 2.98 -56° -27.71 2.88C 24° -36.40 3.16 -24° -40.19 4.73D-60° -16.47 1.01 60° -16.77 0.84E-44° -29.64 3.14 44° -35.60 4.61      Table (1): Analysis of received signal power for different locationsDue to their DoA’s, locations A, B and D were susceptible in generating grating lobes at the other end ofthe cell sector. The received signal parameters from these locations show a close similarity for the twoopposing steering angles. Evidently the grating lobe components capture the signals from dominantDoA’s when the main beam is at the opposing angles. The slight reductions in the average power and thedynamic range reflect the slightly lower gain and beam width of the grating lobe spillage. The DoA’s forlocations C and E are within the [-47° 47] range, so no grating lobe components will appear within thecell sector, in beam-forming. When the main beam is at the ‘wrong’ angle, the average received power forlocations C and E, is considerably reduced as compared to locations A, B and D.  For locations C and E,the main beam may still capture secondary multipath components and the side lobes will capture thedominant rays. The corresponding increase in the dynamic range for these locations suggests enhancedmultipath activity.1Monte-Carlo simulations were conducted to investigate the capacity constraints the grating lobes couldgenerate, in the presence of intra-cell interference. Data and control channels were modelled according tothe UTRA-FDD physical layer specification [5], assuming a spreading factor of 32 for all users.  Thisspreading factor implies a very high date (120kb/s) for the users. Perfect knowledge of the channels wasassumed and power control was introduced at the receiver to eliminate ‘Near-Far’ effects. Channelresponses at the receiver were gathered after beam forming towards the DoA of the single wanted user. Afixed level of additive white Gaussian noise was added to the channels to give a signal to noise ratio of10dB. Figure (2) shows the results of bit error rate vs. the number of interfering users, for 2 locations ofthe wanted user.                                                          1 Defined as the ratio of maximum to minimum instantaneous received powerSubmission to 9th COST260 MCM, Gothenberg, Sweden.4    Figure (2) : BER vs Interference for two user locationsAs discussed above, beam-steering towards location D causes a grating lobe within the cell sector. Theincreased BER for location D suggests that the grating lobe captures significant levels of additionalinterference. On average, the BER for location D has increased by 0.068, which accounts for a 93%degradation from values obtained for location E. It should be noted that if an error correction codingscheme was incorporated, both BER curves would improve by a factor of 10-2. The same set of users(uniformly spread across the cell) was considered in both cases, except for the swapping of the wanteduser. Hence this performance degradation can be attributed to the formation of grating lobes.CONCLUSIONS:It is shown that grating lobes can occur in FDD systems and they would degrade the spatial filteringcapability of a smart antenna receiver. The effects of grating lobes are presented in the form of a BERdegradation, in an interference limited cell. In UTRA FDD systems, the occurrence of grating lobes willresult in a significant capacity reduction.REFERENCES:1) M. Beach, C. Simmonds, P. Howard, P. Darwood, ‘’Smart Antennas : an enabling technology for thewireless revolution’’ Conference  proceedings, AP 2000,  Davos, Switzerland.2) M.I. Skolnik, ‘Introduction to Radar systems’ (1980,second edition) Published by McGraw Hill.Chapter 8, pp. 283.3) S. Deghan, D. Lister, R. Owen, P. Jones , “W-CDMA Capacity and planning issues”IEE Electronics and Communications Engineering Journal,  June 2000, pp101-1184) M. Beach, B. Allen, P. Karlsson, “Correlation of Power Azimuth Spectrum for varying FrequencyDivision Duplex spacings”, Conference proceedings EPMCC 2001, Vienna, Austria.5) 3GPP Technical specification : 3G TS 25.213. V3 1.1(Under the official web site of the 3rd Generation Partnership Project – http://www.3GPP.org)ACKNOWLEDGEMENTS:The authors wish to gratefully acknowledge the funding support under HEFCE JREI’98 which enabledthe procurement of Medav channel sounder and Allgon systems AB, for providing the antenna array.They also thank Darren McNamara (affiliated to CCR, university of Bristol) for his helpful comments.Submission to 9th COST260 MCM, Gothenberg, Sweden.5

Introduction to Radar systems’

Smart Antennas : an enabling technology for the wireless revolution’’ Conference proceedings, AP

W-CDMA Capacity and planning issues” IEE Electronics and Communications Engineering Journal,

Smart antenna performance degradation due to grating lobes in FDD systems

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Smart antenna performance degradation due to grating lobes in FDD systems

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