5
3G LTE Simulations Using Measured MIMO Channels Yngve Sel´ en and Henrik Asplund Ericsson Research Ericsson AB, Isafjordsgatan 14E, SE-164 80 Stockholm, Sweden. Email: <first name>.<last name>@ericsson.com Abstract—In this article we present downlink simulation re- sults for a realistic implementation of the LTE (Long Term Evolution) 3G standard. In contrast to previous studies, actual measured channels (as opposed to computer generated artificial channels) have been used in the simulation. The used 2×2 MIMO channels were measured using two realistic receiver mockups, one laptop and one handset, as well as a pair of reference dipole antennas. The results suggest that LTE is able in practice to support multi stream transmission with very high data rates, even for small hand held terminals. Also, the improvements of 2 × 2 MIMO over SISO transmission are clearly shown. I. I NTRODUCTION The use of mobile broadband has shown a tremendous increase recently: the portion of packet data traffic in the WCDMA networks surpassed that of voice in spring 2007 [1] and the increase in packet data traffic is expected to continue as the prices of HSPA subscriptions drop and as the coverage increases. This puts high pressure on operators to increase the capacities of their networks, and on the industry for enabling such an increase also in the long term future via more efficient and flexible communication standards. LTE (Long Term Evolution) [2] is one track of 3G evolution, which is currently being standardized within the Third Generation Partnership Project (3GPP). The technique is based on orthog- onal frequency division multiplexing (OFDM) in the downlink and single carrier frequency division multiple access (SC- FDMA) in the uplink. LTE offers several important benefits both for operators and end-users. Among the most important are [1], [2] High throughput: LTE allows for peak throughput rates above 200 Mbps, and LTE peak rates of up to 160 Mbps have already been demonstrated in experimental systems [3]. Low latency: The latency requirements are much tighter in LTE than in WCDMA, with radio access network round trip times below 10 ms. Flexibility: LTE supports both the frequency division duplex (FDD) and time division duplex (TDD) modes on the same base station platform [3]. Also, LTE supports a greater bandwidth flexibility, from below 5 MHz up to 20 MHz, than WCDMA. The goal of the present article is to demonstrate the link performance that can be expected from LTE, and to show that MIMO transmission with multiple data streams will indeed be a reality in the LTE networks, also for small handsets. To this end, we present simulations of LTE downlink transmission for a single user occupying 10 MHz of bandwidth in the 2.66 GHz band. 1 The novelty of the present simulation study is that actual measured 2×2 MIMO channels for some realistic antenna setups have been used. Also, the simulator resembles a realistic implementation which includes channel estimation errors, realistic link adaptation, error vector magnitude (EVM) etc. (this is further described in Section III). This allows stud- ies of how the expected link performance of an actual deployed system can be expected to vary with different settings. The results, presented in Section III-C, show that, under realistic conditions, LTE will be able to support very high data throughput as well as multi stream transmission. This conclusion holds also for user equipments (UEs) with small form factors, such as handsets and laptops. II. CHANNEL MEASUREMENTS The equipment used for channel measurements consists of a base station (BS) located on the roof of Ericsson’s headquarters in Kista, Stockholm, Sweden, and a UE mounted inside a measurement van. The BS transmits pilot symbols over a bandwidth of 20 MHz at a carrier frequency of 2.66 GHz. These are measured by the UE and are used to estimate the MIMO channel. The UE continuously logs the channel, at a rate high enough to sample the channel more than twice per wavelength, as the van drives along a pre-defined route; see, e.g., Figure 7 (the map covers an area of about 800 by 850 m). The route includes parts with line-of-sight as well as highly shadowed areas. The GPS positions of the UE are logged simultaneously with the channels, so that each channel measurement can be associated with a geographical position. This procedure is then repeated for different antenna setups on the BS and the UE sides. A. Antenna Configurations We present results for three different antenna setups. On the TX side (base station) a pair of cross polarized (±45 ) antennas with a common phase center was used. This enables a very compact antenna installation, see Figure 1. On the RX side the following receiver antenna setups were used: 1 This is in operating band VII evaluated for UTRA/FDD [4], and corre- sponds to a wavelength λ of about 11 cm.

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Page 1: 3G LTE Simulations Using Measured MIMO Channels

3G LTE Simulations Using Measured MIMOChannels

Yngve Selen and Henrik AsplundEricsson Research

Ericsson AB, Isafjordsgatan 14E, SE-164 80 Stockholm, Sweden.Email: <first name>.<last name>@ericsson.com

Abstract—In this article we present downlink simulation re-sults for a realistic implementation of the LTE (Long TermEvolution) 3G standard. In contrast to previous studies, actualmeasured channels (as opposed to computer generated artificialchannels) have been used in the simulation. The used 2×2 MIMOchannels were measured using two realistic receiver mockups,one laptop and one handset, as well as a pair of reference dipoleantennas. The results suggest that LTE is able in practice tosupport multi stream transmission with very high data rates,even for small hand held terminals. Also, the improvements of2 × 2 MIMO over SISO transmission are clearly shown.

I. INTRODUCTION

The use of mobile broadband has shown a tremendousincrease recently: the portion of packet data traffic in theWCDMA networks surpassed that of voice in spring 2007[1] and the increase in packet data traffic is expected tocontinue as the prices of HSPA subscriptions drop and as thecoverage increases. This puts high pressure on operators toincrease the capacities of their networks, and on the industryfor enabling such an increase also in the long term futurevia more efficient and flexible communication standards. LTE(Long Term Evolution) [2] is one track of 3G evolution, whichis currently being standardized within the Third GenerationPartnership Project (3GPP). The technique is based on orthog-onal frequency division multiplexing (OFDM) in the downlinkand single carrier frequency division multiple access (SC-FDMA) in the uplink. LTE offers several important benefitsboth for operators and end-users. Among the most importantare [1], [2]

• High throughput: LTE allows for peak throughput ratesabove 200 Mbps, and LTE peak rates of up to 160 Mbpshave already been demonstrated in experimental systems[3].

• Low latency: The latency requirements are much tighterin LTE than in WCDMA, with radio access networkround trip times below 10 ms.

• Flexibility: LTE supports both the frequency divisionduplex (FDD) and time division duplex (TDD) modes onthe same base station platform [3]. Also, LTE supportsa greater bandwidth flexibility, from below 5 MHz up to20 MHz, than WCDMA.

The goal of the present article is to demonstrate the linkperformance that can be expected from LTE, and to show thatMIMO transmission with multiple data streams will indeed be

a reality in the LTE networks, also for small handsets. To thisend, we present simulations of LTE downlink transmissionfor a single user occupying 10 MHz of bandwidth in the2.66 GHz band.1 The novelty of the present simulation studyis that actual measured 2×2 MIMO channels for some realisticantenna setups have been used. Also, the simulator resemblesa realistic implementation which includes channel estimationerrors, realistic link adaptation, error vector magnitude (EVM)etc. (this is further described in Section III). This allows stud-ies of how the expected link performance of an actual deployedsystem can be expected to vary with different settings.

The results, presented in Section III-C, show that, underrealistic conditions, LTE will be able to support very highdata throughput as well as multi stream transmission. Thisconclusion holds also for user equipments (UEs) with smallform factors, such as handsets and laptops.

II. CHANNEL MEASUREMENTS

The equipment used for channel measurements consistsof a base station (BS) located on the roof of Ericsson’sheadquarters in Kista, Stockholm, Sweden, and a UE mountedinside a measurement van. The BS transmits pilot symbolsover a bandwidth of 20 MHz at a carrier frequency of 2.66GHz. These are measured by the UE and are used to estimatethe MIMO channel. The UE continuously logs the channel,at a rate high enough to sample the channel more than twiceper wavelength, as the van drives along a pre-defined route;see, e.g., Figure 7 (the map covers an area of about 800 by850 m). The route includes parts with line-of-sight as wellas highly shadowed areas. The GPS positions of the UE arelogged simultaneously with the channels, so that each channelmeasurement can be associated with a geographical position.This procedure is then repeated for different antenna setupson the BS and the UE sides.

A. Antenna Configurations

We present results for three different antenna setups. Onthe TX side (base station) a pair of cross polarized (±45◦)antennas with a common phase center was used. This enablesa very compact antenna installation, see Figure 1. On the RXside the following receiver antenna setups were used:

1This is in operating band VII evaluated for UTRA/FDD [4], and corre-sponds to a wavelength λ of about 11 cm.

Page 2: 3G LTE Simulations Using Measured MIMO Channels

Fig. 1. The base station, with the used dual-polarized TX antenna markedin the figure.

Fig. 2. Configuration A: Reference antennas on the roof of the measurementvan. The used RX antennas have been marked in the figure.

A) Reference antennas: One electric dipole (horizontal po-larization) and one magnetic dipole (vertical polariza-tion) placed on the roof of the measurement van with anantenna distance of about 1 m (corresponding to about9λ). See Figure 2.

B) Handset mockup: A custom modified Sony EricssonK800i handset with two internal receiver antennas. SeeFigure 3(a). The antenna separation is about 9 cm, whichis slightly below the carrier wavelength λ = 11 cm.The handset was placed inside the measurement vanduring the channel measurement. This gave an additionalpathloss of about 10 dB, as compared to the referenceantenna placement on the roof.

(a) Configura-tion B: Handsetmockup.

(b) Configuration C: Lap-top mockup.

Fig. 3. Realistic mockups with the two used RX antennas marked.

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Fig. 4. Example of a measured power-delay profile showing the very highdynamic range of the measurements.

C) Laptop mockup: A Hewlett-Packard laptop with RX an-tennas on the top of the screen (built in) and on the sidesof the screen (added, custom built). See Figure 3(b). Thetwo RX antennas used (one on the top and one on theside of the screen) are marked in the figure. The antennaspacing is about 15 cm, which corresponds to about1.4λ. The laptop was placed inside the measurement vanduring the channel measurement.

Both mockups were positioned centrally in the measurementvan with no disturbing objects, such as persons or seats,in the near vicinity of the antennas. Note that the aboveconfigurations have 2 TX and 2 RX antennas, i.e., we study2 × 2 MIMO transmission.

B. Channel Characteristics

The measured channels were evaluated to determine themeasurement SNR. As can be seen in Figure 4 the dynamicrange in the measured impulse responses is very large. A highSNR in the measurements is very important when later uti-lizing the measured channels in simulations, as measurementnoise can potentially change the correlation properties of aMIMO channel.

For simulation purposes it is also important to sample thechannel adequately in time and frequency. An example of thetime-frequency behavior of the channel for one pair of transmitand receive antennas is shown in Figure 5. As the speedof the van varied due to traffic the resolution of the spatialsampling was also different during different segments of themeasurement routes. At all times the speed was sufficientlylow to ensure Nyquist sampling, e.g., at least two channelsamples per traveled wavelength.

One of the key properties for characterizing MIMO per-formance is the correlation of the fast fading experienced fordifferent selections of transmit and/or receive antennas. Figure6 shows the estimated receive antenna correlation for the threeconfigurations used at the terminal. In addition, the estimatedtransmit correlation between the dual-polarized antennas at theBS is also plotted. The correlations were estimated using the

Page 3: 3G LTE Simulations Using Measured MIMO Channels

Fig. 5. Example of the fast fading in time and frequency over a 20 MHzbandwidth and during a 2 s long segment.

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Fig. 6. Cumulative distribution functions for the correlation betweenreceive antennas for the three different UE antenna configurations. Also, thecorrelation between the two transmit antennas is shown.

measured fast fading coefficients over blocks of 20 MHz × 1 s,where the one second segment corresponds to between 0and 10 m due to the varying speed of the van. No effortwas made to separate shadowing from fast fading as it wasassumed that the influence on the correlation estimates wouldbe minor. As is evident from the figure, the correlationsbetween the antennas are low for all the configurations, bothat the transmitter and at the receiver. However, the referenceconfiguration shows a tendency of having somewhat highercorrelation values in the upper tail of the distribution. This isprobably a result of the scatterer-free mounting on the vehicleroof which may lead to higher correlation in the case of line-of-sight, in contrast to the mockups that were placed inside thevan where more polarization cross-scattering can be expected.

III. SIMULATIONS

A. Stored Channels

Link simulations are typically done using computer-generated, stochastically modeled channels. Most channel

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Fig. 8. Cumulative distribution functions (CDFs) of the channel SNRs indB. The SNR limiting EVM at 4% is also marked in the figure.

models have various levels of simplification which, whilebeneficial from a complexity point of view, often result incertain properties not being captured by the model. Amongthose are primarily the variability and dynamics that occurunder real-life conditions.

By the above reasoning, link simulations using stored chan-nel data can be a good complement to traditional simulations,especially as a tool for validating that the design choices madeduring development of a system are sufficiently robust againstreal-life channel conditions.

Simulations using stored channel data can be quite chal-lenging to perform due to the large data sets required to storethe channels over km-long routes. The approach used hereinwas to replace the stochastic channel generation module in thelink simulator with a module that read a segment of channelimpulse responses from file, interpolated the channel datato symbol-rate and scaled the output, and finally shortenedthe impulse responses by discarding delay samples containingonly noise. One benefit of using playback of measured chan-

Page 4: 3G LTE Simulations Using Measured MIMO Channels

nels is that the user speed can be artificially varied to anydesired value. In the simulations presented below a pedestrianspeed (up to 1 m/s) has been used.

B. Simulation Setup

We simulated downlink transmission to a single user inan interference free environment over numerous one secondsegments along the measurement route, each represented by adot on the throughput maps in Figures 9-11. This was donefor each of the antenna settings described in Section II-A.The UE utilized 10 MHz bandwidth in the 2.66 GHz band.Link adaptation was enabled, whereby the UE could selectbetween 15 combinations of code rate (from 0.1 up to 0.93)and modulation type (QPSK, 16QAM and 64QAM), and alsoselect the number of simultaneous data streams (one or twofor 2 × 2 MIMO). The link adaptation was based on realisticestimation of the current signal to interference plus noise ratio(SINR) and also the channel estimation algorithm was realistic.An EVM of 4% (limiting the SNR to a maximum of 28.0 dB)was assumed at the transmitter. The simulator used a linearMMSE receiver and an LTE turbo codec with hybrid ARQallowing 3 retransmissions. After all control signaling and allpilot symbols have been accounted for, the maximum possiblethroughput available to the user was about 67 Mbps for theselected settings.

White Gaussian noise, with constant variance over the mea-surement route, was added to the received signal. This additivenoise dominated over the channel measurement noise for allpresented results. The noise level was the same for all antennaconfigurations, and so was the transmit power. Their respectivevalues were set in the simulator such that the received SNRbecame as shown in Figure 7 for Configuration C (laptop).For Configuration A (reference antennas) the SNR was about10 dB higher, and for Configuration B (handset) it was about5 dB lower; see Figure 8. These differences were due tolower pathloss for Configuration A (the RX antennas wereplaced outside the measurement van) and different antennaefficiencies.

C. Simulation Results

In Figure 9 the throughput values for Configuration A(reference antennas) are shown. The throughput was oftenhigh, also at locations where the UE did not have direct lineof sight to the BS (the throughput varied between 10 and67 Mbps, with a mean value of 48 Mbps). For 85% of allsimulation points, the mean number of data streams was above1.5. This can to a great extent be explained by the relativelyhigh SNR; above 20 dB in most locations (see Figure 7). Forthe other antenna configurations, the SNR was significantlylower due to the fact that the RX antennas were located insidethe measurement van and not outside on the roof.

In Figure 10, the throughput values for Configuration B(handset) are shown. They vary between 1 and 63 Mbps witha mean value of 26 Mbps. Furthermore, the mean number ofparallel data streams was above 1.5 for as many as 68% of

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Fig. 10. Throughput map for Configuration B (handset). The mean through-put is 26 Mbps.

all simulation points, i.e., two data streams were selected in amajority of the cases.

In Figure 11 we show the throughput values for Configu-ration C (laptop). They vary between 3 and 66 Mbps, with amean value of 35 Mbps. For 85% of all simulation points, themean number of streams was above 1.5.

We also show, in Figure 12, cumulative distribution func-tions (CDFs) of the obtained throughput values. This presen-tation makes it easier to directly compare the performancesfor the various receiver antenna configurations, and it is easyto appreciate the higher performance of the reference antennasfrom this figure. Also, the laptop mockup was able to performbetter than the handset mockup. However, these differencesstem, to a large extent, from differences in the SNR due todifferent pathloss and RX antenna gains. A SISO case withreference antennas, corresponding to Configuration A, has alsobeen included in the plot. Here, the same transmit power asfor the 2 × 2 MIMO case was used on a single transmitter

Page 5: 3G LTE Simulations Using Measured MIMO Channels

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Fig. 11. Throughput map for Configuration C (laptop). The mean throughputis 35 Mbps.

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Fig. 12. CDF for the throughput measurements. A SISO case correspondingto Configuration A has also been included.

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Fig. 13. CDF of the throughput measurements when the TX power has beenvaried such that all configurations experience the same SNR as in Figure 7.

antenna, and the vertically polarized antenna on the roof of themeasurement van was used at the receiver side (the leftmostantenna in Figure 2). For the SISO case, only one data streamcan be transmitted, and it is clear that 2 × 2 MIMO, withits multi stream capacity, gives a significant improvement ofthe performance. Note that the SISO curve has been obtainedusing the reference antennas. Therefore, its SNR is higher thanthat of the configurations B and C, which explains why it hasfewer throughput measurements at the lower end.

In Figure 13 the TX power has been scaled differentlyfor the different antenna setups, so that they all experienceapproximately the same SNR (that which is shown in Figure7).2 The performance differences are now much smaller. Thisresult indicates that the antenna correlations are sufficientlylow for all the studied configurations, and that the antennaefficiency is what distinguishes the different antenna setupswhen it comes to their throughput and multi-stream potential.

IV. CONCLUSIONS

We have performed simulations of a realistic 2 × 2 MIMOdownlink transmission for a single LTE UE in an interferencefree environment. The simulations used actual measured chan-nel data for different receiver antenna configurations, includingrealistic mockups (a handset and a laptop). The results showthat all configurations are able to support high data throughputas well as multi-stream transmission. The main performancedifferences were also shown to depend on the experiencedSNR (which is different due to different path loss and differentRX antenna efficiencies) while the antenna correlations weresufficiently low for all configurations.

The simulation study strengthens the belief that multi streamtransmission will be a realistic method for increasing the datathroughput in LTE systems, also for small hand held devices.

ACKNOWLEDGMENTS

The authors would like to thank Thomas Bolin at SonyEricsson Mobile Communications AB in Lund, Sweden, forsupplying the modified handset (antenna configuration B), andAnders Derneryd at Ericsson Research in Gothenburg, Swe-den, for supplying the laptop mockup (antenna configurationC). Also, thanks goes to Johan Furuskog at Ericsson Research,Kista, Stockholm, Sweden, for performing the channel mea-surements.

REFERENCES

[1] Ericsson AB, “Long term evolution (LTE): an introduction.” online:http://www.ericsson.com/technology/whitepapers/-lte_overview.pdf, October 2007. White Paper.

[2] E. Dahlman, S. Parkvall, J. Skold, and P. Beming, 3G Evolution: HSPAand LTE for Mobile Broadband. Oxford, UK: Academic Press, 2007.

[3] Ericsson AB, “Ericsson first to demonstrate LTE in both FDDand TDD modes on the same base station platform.” online:http://www.ericsson.com/ericsson/press/releases/-20080130-1186619.shtml, January 2008. Press release.

[4] 3GPP, “Universal mobile telecommunications system (UMTS) base sta-tion (BS) radio transmission and reception (FDD),” January 2008. 3GPPTS 25.104 version 8.1.0 Release 8.

2In the previously shown results, the TX power was the same for allconfigurations, whereas the SNR varied due to different path loss and antennagains for the different configurations.