power boosting and compensation during ota testing of a real 4g lte base station in reverberation...

IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. XX, NO.X, XXX 201X 1

Power Boosting and Compensation During OTATesting of a Real 4G LTE Base Station in

Reverberation ChamberDavide Micheli, Massimo Barazzetta, Franco Moglie,Senior Member, IEEE

and Valter Mariani Primiani,Member, IEEE

Abstract—A fully operational 4G LTE base station is tested ina reverberation chamber to analyze its performance in presenceof a multipath environment, typical of wireless and vehicularcommunications. Transmission quality parameters are measuredranging from the empty chamber situation (very rich multipathchannel) to a very high loading condition to mitigate multipath. Inthat way both outdoor and indoor propagation are accounted for.A large attenuation is inserted between the transmitter and theantenna to reduce the signal received by the user to real life valuesencountered in both outdoor and indoor environments. In thesescenarios, operators may choose to transmit a constant powerspectral density throughout all LTE spectrum, or to increase theenergy of control channels at the expense of data channels inorder to enforce transmission and provide better quality to theuser, especially in poor radio conditions. This is called “powerboosting”, and its effect is analyzed in present paper.

Index Terms—4G-LTE, BLER, CQI, indoor propagation,MIMO, MCS, multipath, power boosting, reverberation cham-bers, throughput, SINR.

I. I NTRODUCTION

REVERBERATION chambers (RC) are electrically largecavities where the electromagnetic field is statistically

uniform, isotropic, and depolarized [1], [2]. They are widelyused to carry out electromagnetic compatibility (EMC) com-pliance testing for both immunity and emission [3]. OtherEMC applications are in the field of shielding effectivenessmeasurement [4], material absorbing properties characteriza-tions [5], [6], the measurement of the diversity gain [7], andthe total efficiency [8], [9]. Recently, it was demonstratedtheability of this electromagnetic environment to replicate mul-tipath propagation [10] so making them able to test wirelesscommunication devices [11], [12]. This kind of test in RCrequires the tuning of some important parameters, such as theRician K-factor [13], the coherence bandwith and the RMSdelay spread [14]. In that way reliable throughput (TP) testcan be carry out on wireless local area network (WLAN)multiple-input and multiple-output (MIMO) systems [15]–[17]. Recently complete over-the-air (OTA) tests of MIMO[18], and long term evolution (LTE) terminals [19], [20] also

D. Micheli is with Telecom Italia, Via di Val Cannuta 250, 00166 Rome,Italy e-mail: [email protected]

M. Barazzetta is with Nokia Networks Italia, Via Roma 108, 20060 Cassinade’ Pecchi, Italy e-mail: [email protected]

F. Moglie and V. Mariani Primiani are with Dipartimento di Ingegneriadell’Informazione, Universita Politecnica delle Marche, via Brecce Bianche12, 60131 Ancona, Italy e-mail: [email protected]; [email protected]

Manuscript received Xxxxx XX, XXXX; revised Xxxxxxxx XX, XXXX.

including Doppler spread presence [21] have been carried outusing RC. The present work reports OTA test data carriedout on a real fourth generation (4G) LTE base station (BS)installed in the RC of the EMC laboratory at the UniversitaPolitecnica delle Marche. The test of a fully operating BSinside an RC is of interest for the manufacturer because theRC provides a very rich multipath environment able to stressthe BS functions and to check its adaptation capability. In[22] the Authors demonstrated the feasibility of using an RCto test a fully operational BS connected to the external mobileoperator network. During the OTA test it is possible to checkalot of BS parameters and traffic statistical counters to study theBS adaptation capability to the multipath propagation tunedinside the RC. For example, it is also possible to estimatethe BS downlink transmitted power [23] to take under controlthe electromagnetic emission of the BS in comparison withradiation protection limits [24]. In the previous paper [22] weperformed some tests varying the RC loading conditions, thestirrer rotating speed, and the BS TP. In [22], the power of theBS was fixed and the attenuation between the transmitter andthe antenna was put to 60 dB to generate a received powerat the terminal equal to approximately -80 dBm, a typicalvalue encountered in outdoor situations [25]. Also, RC loadingconditions were set to emulate outdoor propagation conditions.

In the present work we extend the analysis considering:

1) a higher path loss attenuation to reduce the user receivedsignal level to emulate indoor propagation situations;

2) the introduction of a larger RC loading to better emulateindoor propagation channel characteristics;

3) the introduction of a power compensation in order tomaintain the same mobile received power when RCloading conditions are varied, in order to highlight themultipath variation effects only;

4) the analysis of the power boosting function effects on thetransmission quality.

Power boosting is a condition where the transmission powerof some downlink control channels (for example, the refer-ence signals) is increased (boosted) against the power of thedata channels. It may improve signaling and retransmissionmechanisms at cell borders. Operators may choose this wayto enhance robustness of physical level signaling in LTE. Theside effect, is a non linearity in the spectrum, such that thepower spectral density is not anymore constant throughoutthe LTE band. Control channels power are boosted against


PC

PC

PC

Tx Antenna

StirrerStirrer

80 dB

80 dBLTE

BS

Notebook and

LTE device

Switch

Switch

Optical Link

LTENetwork

Fig. 1. Schematic view of the measurement setup. The notebook inside theRC is equipped by a USB data card and a software test tools. It is connectedvia optical link to a remote PC positioned outside the RC. The LTE basestation is connected by an optic fiber to the LTE network of Telecom Italia.

data channels, which in some circumstances may suffer. Thus,we extended the analysis to the transmission quality effects,pointing out if these are positive or negative.

II. EXPERIMENTAL SETUP

Fig. 1 reports a schematic block diagram of the adoptedmeasurement set-up. We employed the RC placed at EMCLaboratory of Universita Politecnica delle Marche. Chamberdimensions, performance and facilities are described in [22],[26]. The setup is similar to the one used in [22], where it hasbeen observed that BS performances are mainly affected byRC loading conditions rather than stirrer rotating speed.

Because of this, the new test session has been carried outsetting a fixed stirrer speed of 30 deg/s. The RC fundamentalmode resonates at about 45 MHz, therefore the RC is wellovermoded in correspondence of the BS operating frequency(1800 MHz 3GPP band 3, 10 MHz bandwidth). The RC is fedby the LTE BS (Nokia Networks Flexi Multiradio) through adouble polarized transmitting antenna. Another issue realizedduring previous test sessions was the difficulties to set signalstrength to a value lower than -80 dBm. This correspond toa total attenuation of 60 dB on each branch of the MIMOtransmission [22]. With such high signal in the RC, it wasnot possible to stress the system and pull it to the cell edgeequivalent condition, with the scope to observe how the linkadaptation algorithms behaved in these situations. In orderto overcome this limitation, new tests have been carried outinserting an 80 dB total attenuation along each transmitterbranch, with the purpose to achieve a realistic received powerlevel for the terminal positioned inside the RC (about -100 dBm). Another rationale in moving the “working point” to-100 dBm is to test the system in multipath propagation con-ditions when reference signal received power (RSRP) is low,since these conditions frequently happen in indoor premises[25], and to observe link adaptation behavior in this case. Theuser terminal consists into a notebook equipped by a universalserial bus (USB) data card category 3 [27]. The terms “datacard” and “user equipment” (UE) are used interchangeably. A

Fig. 2. Inner view of the RC in the “very high” load condition.

Fig. 3. Q-factor when the RC is very highly loaded.

laptop based drive test tool (Nemo Outdoor V7.0), was runningon the notebook to record the main quality parameters of theair interface. The laptop was connected via a pair of switchesand an optical link [17] to another personal computer (PC)positioned outside the RC, where the Nemo Outdoor sessionhas been remotely controlled. Conditions were varied startingfrom the empty RC by adding electromagnetic (EM) absorbingmaterial. Beside the empty RC condition and the high loadalready used in [22], here we have introduced a very highload condition by adding further absorbing material for a totalof 12 pyramidal absorbers plus 4 planar sheets as shown inFig. 2. Some absorbers were placed in vertical positions asthe same figure shows in order to emphasize their effect. Inthat way, it was possible to further reduce the RC Q-factor toabout 600 in the 1.8 GHz band, Fig. 3, and to increase thepower delay profile (PDP) decay, Fig. 4, in order to achievea RMS time delay spread (τRMS) around to 70.1 ns, similar tothat encountered in real life indoor environment. The chamberquality factor and the power delay profile were measuredinserting two log-periodic antennas inside the chamber andadopting a vector network analyzer to measure the scatteringparameters between the transmitting and receiving antennas[28]. The whole frequency range of Fig. 3 was investigatedsetting a frequency step of 31.5 kHz in order to avoid aliasingin the PDP computation window of 5µs, and performing acomplete stirrer rotation (360◦) for each frequency. Pertainingthe measurement uncertainty, the RC was verified accordingto the IEC standard. In particular, a standard deviation of thefield uniformity σ8 and σ24 of about 1.5 dB were obtainedabove 1 GHz [29] (3 dB required by the IEC). Moreover,we computed the stirrer statistical independent positionsac-


Fig. 4. PDP when the RC is very highly loaded.

cording to the IEC procedure, obtaining a minimum of 60for the worst case where the chamber was very high loaded.More independent positions were obtained with a reducedchamber loading, of course. This value is much higher thanthat suggested by the IEC standard around 1.8 GHz andensures an uncertainty of about±1.5 dB on the mean powermeasured over the stirrer rotation with a confidence level of95% [30]. The above uncertainty contributions may affect themeasurement of the RSRP and of the RSRQ levels by thereceiver. We have not precise information about the residualuncertainty of the receiver system combined with the adoptedsoftware (Nemo Outdoor V7.0), but it is within what specifiedby the 3GPP standard [31, Chapt. 9.1]:±6 dB for the RSRPand ±3 dB for the RSRQ. So, the RC uncertainty is lessthan that required by the standard regarding received powermeasurements. Finally, measurements are averaged not onlyover the 60 (or more) independent stirrer positions but alsoover a 15 minutes interval, so further reducing the deviationof the mean values. On the other hand, we would like toremark that this uncertainty comes from systematic errorsthat are the same in each measurement we carried out. Moreprecisely, our measurements of the RSRP and RSRQ signalswere carried out with the same receiver and with the samechamber to compare different BS operating conditions. In thissense, we are more interested to relative variations of theBS parameters and not on absolute values only, even if theRC is also able to provide reliable measurements of absolutepower levels, and of the statistical counters of the BS asconfirmed in [23]. The time delay spread is a parameter thateffectively quantifies the environment multipath propagation,and ITU Recommendations specify this value for differentenvironments [32], [33]. In [22] we were looking for outdoorconditions (level around -80 dBm), and we set a medium RCload, that providedτRMS = 346 ns and a high RC load havingτRMS = 151.37 ns that are representative of an “Urban Macro”and an “Urban Micro” situation respectively [32] for a no lineof sight (NLOS) condition.

In the present paper, we add examples of indoor propagation(lower signal level, around -100 dBm), therefore we use theformer high load condition to simulate a commercial environ-ment and the very high load condition to simulate a residentialenvironment propagation. TheτRMS value ranges from 70 ns

to 150 ns passing from a residential to a commercial indoorenvironment at frequencies close to those used in the presentwork [33]. All above value are computed assuming a thresholdof 30 dB below the peak of the profile [10]. The empty RCcondition (τRMS = 1163 ns) is also considered in order to highlystress the BS by a very rich multipath environment (worstcase).

In general, the connection between reverberation chambersand real-world environments is not as simple as having asimilar RMS delay spread. This quantity may serve as a goodfirst order measure of comparison and other parameters suchas taps delay and Doppler spread should be considered. Theseare typically introduced by combining a channel emulatorto a reverberation chambers [34], [35] and adopting a LTEsignal generator to test LTE receivers. Small chambers areadopted to that purpose to strongly reduce the time delayspread and reproduce a certain channel model in order to maketest comparable to that obtained in an anechoic chamber forexample. Our purpose is a little bit different. Our goal is totest a real BS inside an hostile environment that could berepresentative of a larger variety of real world situations, wherechanges to the environment, including moving obstructions(e.g, machinery or people), moving the transmitter, movingthereceiver, or changing boundary conditions (opening a door),sometimes produce vastly different channels which are difficultto model by single sets of taps delay for example. To thatpurpose we adopt a chamber larger than that standardized forterminal tests, able to provide higher time delay spread andsetting very low power level inside the chamber to stress theBS operation and to check the quality of the service understrong variable conditions. The terminal is here used only toactivate the BS transmission and acquire its data. This test,even not related to a single precise actual channel model, isof great interest for the manufacturer of the BS and for mobilephone network operators like Telecom Italia. In fact, the realradio coverage and propagation conditions, in especial wayinindoor environments, are currently one of the major issues ingranting quality of voice and data packet services [36]. It isestimated that the great part of the user traffic is generatedinindoor environment where the radio condition are sometimesvery bad due to the building structure attenuation of signalstransmitted by the external BS to the indoor user equipmentslike phones, tablet, dongle [37]–[39]. The above considerationsled us to consider a large RC more adequate w.r.t. other kindsof testing techniques currently used for terminals only (multi-probe method or two stage method), even though a completecomparison is not currently available in literature for a realBS. Modeling of indoor environments could be quite difficultand computational time consuming and in this context the useof a loaded reverberation chamber help the LTE performanceanalysis in a scenario where multipath propagation is closestas possible to that of indoor environment.

III. D ESCRIPTION OF THEBS OPERATING CONDITIONS

AND ANALYZED PARAMETERS

Downlink tests were carried out varying the RC Q-factor(conditions “empty”, “high load”, “very high load”), and


Fig. 5. Downlink physical channel frame structure for one MIMO branch.Example of 50 physical resource blocks for 1 ms interval. Boosted andde-boosted elements are indicated together with an example ofbroadcastchannel positions. Orange resource elements refer to P-CFICH, P-HICH andPDCCH (P-CFICH is boosted). Red resource elements refer to referencesignals (boosted). White resource elements refer to data traffic (de-boosted).Black resource elements refer to discontinuous transmission. Violet resourceelements refer to broadcast channel (PBCH). Blue resource elements refer toprimary synchronization (PSS). Cyan resource elements referto secondarysynchronization (SSS).

activating/deactivating downlink power boosting [23], whichfavors the transmission of reference signals, penalizing thetransmission power of the downlink data channels. This setupdiffers from the setup used in [22], where power boosting wasalways enabled to check how reliable was the estimation ofpower emissions from the BS based on statistic counters, oncepower spectral density was not constant throughout the LTEspectrum [23]. This could be used to make control channelsmore reliable, especially for example when the user is movingand making handovers, or in interfered situations. A schemeof the downlink physical channel frame structure for oneMIMO branch is reported in Fig. 5 where the boosted resourceelements in the LTE frame are highlighted. Boosted channelsare reference signals (RS, explained in [22]), physical controlformat indicator channel (P-CFICH) and physical hybrid-ARQinformation channel (P-HICH). P-CFICH is used by the UEto decode the structure of PDCCH (physical downlink controlchannel) while P-HICH carry the physical acknowledgmentsof data sent in uplink. File transfer protocol (FTP) applicationwas used in all the times, and the client and the serverwere optimized in order to reach, with a single file transfer,

the maximum TP allowed by air interface. Another issuethat was observed in [22] was how RC load affected theRSRP received by the user, that depends also on the usageof boosting. In order to quantify the only effect of load,we verified the RSRP and reference signal received quality(RSRQ) values with empty and highly loaded RC – sameconditions used in [22] – but without power boosting. In bothcases (with / without boosting), higher loads correspond tolower values of RSRP because of the presence of absorbingmaterials. This way, it was difficult to understand if thedifferences in quality perceived by user in high load or emptyRC conditions were due to different multipath conditions, oralso to different RSRP values. In this new test session, thedifference in RSRP between the two different RC loadingconditions has been measured and compensated by settingan additional attenuation. This has the scope to isolate, andthus quantify, the real effect of multipath propagation overLTE performances since the two loading conditions are nowevaluated at same RSRP received by UE. Likewise, the BStransmission power has been increased for the very high loadcase compared to the high load case, in order to isolate theeffect of difference in multipath between these two cases. In alltests, following radio parameters – which are defined in [22]–have been used in order to quantify the quality perceived by theuser: reference signal received power (RSRP); reference signalreceived quality (RSRQ); signal to interference noise ratio(SINR); block error rate (BLER); channel quality indicators(CQI); modulation and coding schemes (MCS). They areanalyzed both in the cases of boosting on and off, applyingan offset of+6 dB on downlink control channels in the firstcase. These parameters are all reported as averaged valuesmeasured over a period of 15 minutes, during which an FTPdownload was running from a server to the user equipment.Test duration was set to 15 minutes because the BS collectstatistics in the same timeframe. These statistics are usedbythe operator for quality supervision. There were no powercontrol algorithms active on the reference signals, thus thepower received by UE was almost constant throughout a test,once the attenuation was fixed. The other parameters maydepend on channel occupation, but since the file transfer wasactive throughout the entire period, they were almost time-invariant. That is the reason why only averaged values areshown. Power (RSRP) and quality (RSRQ) of reference signalsare measured by the UE and recorded by Nemo outdoor, likeSINR and CQIs. CQIs (Channel Quality Indicators) are alsoa measure of the downlink quality performed by UE, andsignaled back to BS every few milliseconds. All of these arerecorded by the drive test tool, which averages them over fewseconds time window. BS will use CQIs in order to select thebest MCS which define – in each transmission time interval(TTI), which lasts 1 ms – the size of the transport block(TB) that best fit to the radio conditions in order to obtaina certain BLER target (set to 10%). The better are the radioconditions, the higher is the selected MCS and the bigger isthe amount of data transferred in each TB, but the lowest is thedegree of protection, and viceversa. More detailed informationcould be found in [25]. Requests for retransmission fromUE are also used to tune further the size of the transport


Fig. 6. Performance results with flat power spectral density,without anyboosting and attenuation 60 dB. RSRP and RSRQ are investigated.

block. BS takes decisions about how to transmit data everyfew milliseconds, and link adaptation algorithms are very fastand complicated to be recorded without an averaging process.Thus, there are three different time-scales in the average:a15 minutes, long-term average and a few seconds averageperformed by the drive test equipment, while the BS takesdecisions every one or few milliseconds. The TP reported inresults section is calculated at physical layer, thus it includesretransmissions. Retransmissions may occur if receiving enddoes not acknowledge properly a transport block, leadingthe BS to reduce MCS and TB size, improving protection.Boosting is used, among other things, to improve the decodingcapabilities of the downlink control channel, thus to makeit more reliable. But this power is stolen to data channels,and this may cause some side effects that this paper try toinvestigate.

IV. RESULTS

In [22], the perceived quality was measured running testsin empty and highly loaded chamber, always with powerboosting, setting different stirrer rotating speed. Thesetestshave been repeated now for the 30 deg/s speed only, withoutboosting, such that power spectral density (PSD) is constantthroughout the LTE spectrum. Fig. 6 reports the RSRP andRSRQ values, Fig. 7 reports the SINR and BLER values,Fig. 8 reports the CQI and MCS values, and Fig. 9 reportsthe TP for this case. As in the “boosting on” case, emptyRC conditions favor the persistence of energy inside the RC,but quality perceived by the user (which is quantified bythe RSRQ) is lower than in the high load case. This isreflected also on SINR and BLER indicators. Lower values ofRSRQ, SINR and BLER, cause lower CQI, and push the linkadaptation of the BS to select lower MCS to be allocated tothe user, which in turn reflect into lower TP. Moreover, powerof reference signals is decreased when boosting is switchedoff. Thus, RSRP and RSRQ are slightly lower with respectto the case with boosting on. Comparison between boostingon and off cases is reported in Table I. When RC is loaded(less multipath) and signal strength is good, there are noimpacts when giving more energy to the reference signals,since TP is quite close to the maximum achievable. Boostingprovides more energy to reference signals and physical control

Fig. 7. Performance results with flat power spectral density,without anyboosting and attenuation 60 dB. SINR and BLER are investigated.

Fig. 8. Performance results with flat power spectral density,without anyboosting and attenuation 60 dB. CQI and MCS are investigated.

Fig. 9. Performance results with flat power spectral density,without anyboosting and attenuation 60 dB. The TP is investigated.

TABLE IA COMPARISON BETWEEN CASE WITH POWER BOOSTING AND WITHOUT

POWER BOOSTING, 60 dBATTENUATION , STRONG SIGNAL INRC

Chamber RSRP RSRQ SINR TP

load (dBm) (dB) (dB) (Mbps)

Empty, boosting [22] -72.82 -7.11 26.67 57.22

High load, boosting [22] -79.03 -5.92 29.34 69.31

Empty, no boosting -76.32 -11.23 22.89 59.01

High load, no boosting -86.32 -10.36 26.64 69.13


Fig. 10. Performance results in presence of lower signal (attenuation 80 dB).Results for power boosting activated and not activated are shown. RSRP andRSRQ are investigated.

Fig. 11. Performance results in presence of lower signal (attenuation 80 dB).Results for power boosting activated and not activated are shown. SINR andBLER are investigated.

format indicator channel (P-CFICH) stealing some energyfrom the physical downlink shared channel (PDSCH). As aconsequence, when multipath increases (empty RC) and signalis still strong, there is a negative effect of boosting on TP,which degrades from 59 Mbps to 57 Mbps if boosting is active.Because of this, it make sense to explore what happens withlower signal strength.

A. Testing with lower signal strength in the RC

When additional 20 dB of attenuation is added on thetransmission branches, above mentioned parameters react asindicated in Fig. 10 for the RSRP and RSRQ values, inFig. 11 for the SINR and BLER values, in Fig. 12 for theCQI and MCS values, and in Fig. 13 for the TP. Boostingapplies on reference signals, thus both reference signal strengthand quality get lower if this is missing. At these conditionsof signal strength, the situation where boosting leads morebenefits is the one with high load in RC, where +6 dB inRSRP, RSRQ and SINR means +11 Mbps in TP. This issimilar to what we observed with stronger signal strengths(Table I) where boosting behavior is better in the high loadcase, while in the empty RC case the TP reduce from 59 to57 Mbps when this functionality is activated. Second thingto remark is the effect on SINR (but also on BLER, MCS,CQI) when signal strengths are low (-100 dBm) and load is

Fig. 12. Performance results in presence of lower signal (attenuation 80 dB).Results for power boosting activated and not activated are shown. CQI andMCS are investigated.

Fig. 13. Performance results in presence of lower signal (attenuation 80 dB).Results for power boosting activated and not activated are shown. The TP isinvestigated.

changed in chamber. Indeed ,when moving from highly loadedto empty RC, SINR improved by roughly 5 dB (Fig. 11).This is in contrast with what we observed with higher signalstrengths (-80 dBm, Fig. 7) where it decreased by roughly4 dB when changing load in the same way. The empty RCmaintains longer the energy inside, and when the signal is low,the negative effect of multipath does not impact that muchon SINR, BLER, MCS, CQI. The effect of empty chamberwhen signal strength is low (-100 dBm) is very well visiblecomparing Fig. 11, Fig. 12, and Fig. 13 (attenuation 80 dB)with Fig. 7, Fig. 8, and Fig. 9 (attenuation 60 dB). In thefirst case, CQIs – which are measured by the data card andreported to BS as a measure of quality – get better whenchamber is empty. As well, allocated coding schemes arelarger, 64 QAM is allocated more frequently and TP is higher.In the latter case, the trend is just opposite. What becomevisible when moving to very low signal strength is that thereare two effects to be considered when adding panels in RC:the first is the reduction of multipath [22], the second is areduction of the useful signal energy, that could be quantifiedwith RSRP, which is even more tangible if boosting is off,since the RSRP start to be quite weak (-107 dBm) and bothSINR and BLER degrade such that lower coding schemesmust be used in order to protect transmission. Moreover, if welook to Fig. 13, we have evidence about how boosting bring


benefits when chamber is loaded with panels in low signalcondition. In this case, some words should be spent aboutCQI. If boosting is on, less energy is given to PDSCH sincecontrol channels are boosted. Because of this, CQIs reportedby UE are slightly lower. On the other side, boosting allowbetter PDCCH decoding, which is crucial to schedule the UEproperly, such that this is able to send more frequently positiveacknowledgments for the received data. This have the effecttodecrease the BLER, increase the MCS and modulation order,allowing better TP. Downlink signal strength and quality aredescribed by RSRP and RSRQ, but quality is also measuredwith CQIs, where it is ranked with integer values from 1(worst) to 15 (best quality). In LTE, RSRP and RSRQ areused by the UE – for example – for mobility purposes, whileCQI are signaled by the UE back to the BS in order for thisto adapt to the actual channel conditions and choose the rightMCS. Moreover, BLER is an indication of the retransmissionrate, and when a retransmission is requested, the BS may reactselecting a more reliable transmission. Further details aboutlink adaptation is available at [40]. Looking at Fig. 12 andFig. 13 for the high load cases, comparing cases with boosting“on” and “off”, we see how these parameters work together:even if CQIs with boosting are lower, in these poor radioconditions TP is higher because control channel (PDCCH) isdecoded better, acknowledgments transmission is more reliableand BLER is lower than the case with boosting off.

B. Power compensation

The difference in RSRP between the two loading conditionsof the RC (high and empty) has been measured and compen-sated by inserting an additional attenuation when the RC isempty, with the aim to analyze the effect of multipath only.The measured level differences was about 10 dB, that corre-sponds to the RC Q-factor variation from the two conditions(see Fig. 3 in [22]). Therefore, LTE performances in highlyloaded RC has been compared with performances in emptyRC with the same value of RSRP.

The recorded RSRP values for the condition “high load”was -107 dBm (without boosting), and the recorded RSRPvalues for the condition “empty RC” – after having introduced10 dB attenuation – was quite similar: -105 dBm (also withoutboosting).

Analyzing the differences between these two cases is anempiric way to quantify the effect of multipath, which is muchmore present when RC is empty. The results are shown inFig. 14 for the SINR and BLER values, in Fig. 15 for theCQI and MCS values, and in Fig. 16 for the TP. Results arein absence of power boosting. With power compensation,performances degrade when multipath is enhanced (emptyRC). For example, SINR reduces of 1.2 dB and TP reducesof 3.4 Mbps, while MCS is forced to more robust indexpassing from 14 to 12. The usage of a more protected MCScaused lower BLER (Fig. 14), but according to tables in [40],TB size reduced from12 960 to 9 912 bits per TTI, and TPbecomes lower. The variation of all parameters is reportedin Table II, where “CW0” and “CW1” refer to the differentcode words of the MIMO communication. Table II also reports

Fig. 14. Comparison between LTE performances in highly loadedand emptyRC, with the same RSRP to quantify the effect of multipath only.SINR andBLER are investigated.

Fig. 15. Comparison between LTE performances in highly loadedand emptyRC, with the same RSRP to quantify the effect of multipath only.CQI andMCS are investigated.

Fig. 16. Comparison between LTE performances in highly loadedand emptyRC, with the same RSRP to quantify the effect of multipath only.The TP isinvestigated.

the same analysis, when boosting function is activated. Thetable shows the degradation when we take away all panelsfrom the chamber, maintaining same RSRP level. That is whynegative values are shown. Table show that effect of multipath– at same RSRP – is slightly higher if boosting is on. TheLTE receiver sample the signal with a rate that depends onthe bandwidth. For 10 MHz, this is equal to 65 ns [41].The empty chamber presents a much larger decay time [22],and addition of multipath may cause stronger degradation,


TABLE IILTE PERFORMANCE VARIATIONS DUE TO MULTIPATH ENHANCEMENT

WHEN THE RC PASS FROM A HIGH LOAD CONDITION TO A EMPTY

CONDITION WITH THE SAME RSRPLEVEL . VARIATIONS ARE REPORTED

FOR BOTH “ POWER BOOSTING ON” AND “ POWER BOOSTING OFF”OPERATING CONDITIONS

∆ ∆ ∆ ∆ ∆

SINR CQI MCS CW0 MCS CW1 TP

(dB) (Mbps)

No boosting -1.2 -0.9 -2.7 -3.4 -3.4

Boosting -1.8 -0.8 -4.6 -3.9 -7.0

specially in cases – like the boosting – where some energyis stolen from the data channels. This is consistent with whatwe observed in Table I where, with empty RC, it caused littleTP degradation. But empty RC is a really severe environment,thus we repeated almost same exercise increasing the amountof absorbing panels.

C. Testing with very high load in RC, low signal strength

The higher the load in RC, the lower the RSRP signalstrength. On the other hand, the larger the amount of absorbingmaterial, the lower the multipath effect on signal propagation.In a new setup, the RC has been loaded as reported inFig. 2 with the purpose to approach as much as possiblethe propagation of an indoor residential environment [33].Results are compared with empty and high load RC conditions.The addition of new panels, generating a “very high load”condition, turns into a situation where the quality perceivedby the user is even worse than what we got for the “highload” case. In order to compensate this, we increased thetransmission power of the BS of the difference between theRSRP recorded in the very high load case versus the RSRPrecorded in the high load case (4.5 dB). With this action, wewould like to measure the benefit in removing the negativeeffect of multipath, when additional absorber sheets are addedinside the RC, checking high load versus very high loadconditions at the same value of RSRP. The recorded RSRPvalues for the condition “high load” were -107 dBm (withoutboosting) and -102 dBm (with boosting). The recorded RSRPvalues for the condition “very high load” – after introducing+4.5 dB of additional transmission power – were also -107dBm (without boosting) and -101 dBm (with boosting). In thefollowing Figures, these two comparable situations are drawnwith checkered bars. Results without boosting are shown inFig. 17 for the RSRP and RSRQ values, in Fig. 18 for theSINR and BLER values, in Fig. 19 for the CQI and MCSvalues, and in Fig. 20 for the TP, where also data withoutpower compensation (with very high load) are also reportedfor sake of comparison.

We discover this way that SINR with very high load andpower compensation is 10.7 dB, against 10.1 dB of thehigh load case, without power compensation, thus a 0.6 dBimprovement on the SINR averaged over 15 minutes period.This difference could be considered well within typical RCuncertainties. In our case, the comparison was done withoutchanging anything in the chamber and varying the attenuation

Fig. 17. Comparison between LTE performances without power boosting inempty, highly loaded RC, and very highly loaded RC. The very high load isreplicated with positive power compensation to quantify theeffect of multipathonly. RSRP and RSRQ are investigated.

Fig. 18. Comparison between LTE performances without power boosting inempty, highly loaded RC, and very highly loaded RC. The very high load isreplicated with positive power compensation to quantify theeffect of multipathonly. SINR and BLER are investigated.

Fig. 19. Comparison between LTE performances without power boosting inempty, highly loaded RC, and very highly loaded RC. The very high load isreplicated with positive power compensation to quantify theeffect of multipathonly. CQI and MCS are investigated.

on the external transmitter only. This procedure leads to areduction of the uncertainty [28].

The BS select higher MCS in the power-compensated case.As we also observed before, this means the selection of aless protected coding scheme and the growth of the BLER.This BLER increment is acceptable since it is still lower


Fig. 20. Comparison between LTE performances without power boosting inempty, highly loaded RC, and very highly loaded RC. The very high load isreplicated with positive power compensation to quantify theeffect of multipathonly. The TP is investigated.

Fig. 21. Comparison between LTE performances with power boostingactivated in empty, highly loaded RC, and very highly loaded RC. The veryhigh load is replicated with positive power compensation to quantify the effectof multipath only. RSRP and RSRQ are investigated.

Fig. 22. Comparison between LTE performances with power boostingactivated in empty, highly loaded RC, and very highly loaded RC. The veryhigh load is replicated with positive power compensation to quantify the effectof multipath only. SINR and BLER are investigated.

than the BLER target. When the power boosting function isactivated, the same comparisons are reported in Fig. 21 forthe RSRP and RSRQ values, in Fig. 22 for the SINR andBLER values, in Fig. 23 for the CQI and MCS values, and inFig. 24 for the TP. Since the TP is an important figure ofmerit for the OTA MIMO testing, we summarize in Table III

Fig. 23. Comparison between LTE performances with power boostingactivated in empty, highly loaded RC, and very highly loaded RC. The veryhigh load is replicated with positive power compensation to quantify the effectof multipath only. CQI and MCS are investigated.

Fig. 24. Comparison between LTE performances with power boostingactivated in empty, highly loaded RC, and very highly loaded RC. The veryhigh load is replicated with positive power compensation to quantify the effectof multipath only. The TP is investigated.

TABLE IIISUMMARY OF THE MEASURED TP FOR ALL THE TESTING CONDITIONS

Test Description Boosting On Boosting Off

TP std TP std

Empty 36.776 8.771 36.988 7.312

High Load 23.967 5.309 12.462 4.487

Empty, Att. + 10 dB - - 9.021 2.220

Very High Load 14.041 1.799 10.080 2.829

Very High Load, Power Up 26.568 5.765 13.911 4.003

all measured averaged values (15 minutes), including theirstandard deviation (std).

Relevant indicators are reported in Table IV, where vari-ations between the high and very high load with powercompensation are shown, for both boosting on and off. Thepositive compensation is slightly higher when boosting on,confirming the better behavior of the feature when multipathreduce. Also TP shows an improvement of 2.6 Mbps whenboosting is on, and 1.4 Mbps if boosting is off. Thesecorrespond to an improvement of roughly 11% in both cases,when moving from the high load to the very high load (withpower compensation) condition. With so low RSRP values, theimprovement that come from multipath reduction, expressed


TABLE IVLTE PERFORMANCE VARIATIONS DUE TO MULTIPATH REDUCTION WHEN

THE RC PASS FROM A HIGH LOAD CONDITION TO A VERY HIGH LOAD

CONDITION AND POWER COMPENSATION. VARIATIONS ARE REPORTED

FOR BOTH “ POWER BOOSTING ON” AND “ POWER BOOSTING OFF”OPERATING CONDITIONS

∆ ∆ ∆ ∆ ∆

SINR CQI MCS CW0 MCS CW1 TP

(dB) (Mbps)

No boosting 0.6 0.429 1.378 0.795 1.449

Boosting 1.1 0.527 1.008 1.144 2.601

in percentage, is similar in both boosting on and off cases.

V. CONCLUSION

The analysis reported in [22] and in the present paperdemonstrated how RC could be used to test LTE performancesand stress the system varying signal strength, stirrer speedand RC load. In [22], we have shown how performanceswere sensible to RC load variations, but there was the needto increase the attenuation and re-verify the system at lowerRSRP values. In present paper, lower signal strengths havebeen experienced by adding +20 dB attenuation to reach userreceived levels typically encountered in indoor environments.To emulate the propagation characteristics of indoor residentialsituations, we consider also a very high loaded RC to reducethe τRMS down to 70 ns.

Contrary to what happened with higher signal levels in [22],we experienced better TP in empty RC (36 Mbps) rather thanin high load condition (12 Mbps, 23 Mbps, depending on theusage of boosting). This is due to the weaker contributionof multipath when signal is low, while on the other hand,additional absorber sheets caused both a reduction on theuseful signal and on multipath. To separate these two con-tributions and to evaluate the effect of multipath only, powercompensation has been introduced, measuring a relative TPdegradation when moving from the high load to the emptyRC conditions (3.4 Mbps without, 7.0 Mbps with boosting)and a TP enhancement when moving from the high load tothe very high load condition (1.4 Mbps without, 2.6 Mbpswith boosting). These numbers could be interpreted as theonly effect of multipath on TP when varying the RC Q-factorand moving from totally reflecting walls to more realisticconditions.

Finally, we started from an initial condition where the usageof boosting in [22] caused a light TP degradation with emptyRC and strong signals (from 59 Mbps to 57 Mbps). On thecontrary, with lower signal strengths (i.e. approaching residen-tial indoor environments), an improvement was achieved withhigh load and very high load conditions. This improvementwas from 12 Mbps to 23 Mbps in high load conditions, from10 Mbps to 13 Mbps in very high load conditions, and from13 Mbps to 27 Mbps in very high load conditions with powercompensations. The usability of this enhancement is provento be efficient in indoor environments, at least if a single useris present, with only one dominant cell. In scenarios whereseveral cells cover concurrently in the same place, a change

in the reference signal power may modify the dominance areaof each cell, and even a TP degradation may occur at cellborder. Thus, this functionality should be further studiedinmulti-users and multi-cells scenarios.

ACKNOWLEDGMENT

This research was supported by Telecom Italia under thecontract number ODA7010087992.

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Davide Micheli was born in Ancona, Italy, in 1967.He received the University degree in electronicsengineering from the University of Ancona (nowUniversita Politecnica delle Marche), Ancona, Italy,in 2001, and the University degree in astronauticengineering and Ph.D. degree in aerospace engi-neering from the “Sapienza” University of Rome,Rome, Italy, in 2007 and 2011, respectively. Heis currently with the Telecom Italia Laboratory, asa Researcher with Mobile Telecommunications andNeural Networks and collaborates with “Sapienza”

University of Rome, where his research activities are related to EM elds andcomposite materials interaction. His current research is focused on electricconductive polymers and radar-absorbing structures modeling.

Massimo Barazzetta ws born in Como, Italy, in1972. He received the University degree in telecom-munication engineering from the Politecnico di Mi-lano, Italy, in 1997. From 1999, he is with NokiaNetworks, acting as network planning engineer onsecond, third and fourth generation systems.

Franco Moglie (M. ’91 - SM ’12) was born inAncona, Italy, in 1961. He received the “Dottore In-gegnere” degree in electronics engineering from theUniversity of Ancona (now Universita Politecnicadelle Marche), Ancona, Italy, in 1986, and the Ph.D.degree in electronics engineering and electromagnet-ics from the University of Bari, Bari, Italy, in 1992.Since 1986, he has been a Tenured Researcher withthe Dipartimento di Elettronica ed Automatica, Uni-versita Politecnica delle Marche, where, since 2011,he has been with the Department of Information

Engineering. His current research interests include EM numerical techniquesand power applications of EM field. In particular, his research activity isin the field of the application of reverberation chambers for compliancetesting and for metrology applications. Dr. Moglie is a member of theIEEE Electromagnetic Compatibility Society and the Italian ElectromagneticsSociety.


Valter Mariani Primiani (M. ’93) was born inRome, Italy in 1961. He received the degree of“Dottore Ingegnere” in Electronics Engineering in1990 from the University of Ancona (now UniversitaPolitecnica delle Marche) Ancona, Italy. Now he isAssociate Professor in electromagnetic compatibility(EMC) with the Universita Politecnica delle Marche,Ancona, Italy. He is member of the Departmentof Information Engineering, Universita Politecnicadelle Marche, where he is responsible for the EMCLaboratory. His research interests concerns the pre-

diction of digital printed circuit board (PCB) radiation, the radiation fromapertures, the electrostatic discharge (ESD) coupling effects modeling, andthe analysis of emission and immunity test methods. More recently, he hasextended his research activity in the field of the application of reverberationchambers for compliance testing and for metrology applications. Prof. MarianiPrimiani is a member of the IEEE EMC Society, IEEE Instrumentation andMeasurement (IM) Society, and the Italian Electromagnetics Society.

power boosting and compensation during ota testing of a real 4g lte base station in reverberation...

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