IEEE COMMUNICATIONS LETTERS, VOL. 9, NO. 5, MAY 2005
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Measurements of the Performance Degradation of UMTS Receivers due to UWB Emissions Dajana Cassioli, Member, IEEE, Samuela Persia, Valerio Bernasconi, Member, IEEE, and Alberto Valent
Abstract— Results of experimental trials performed to assess the coexistence of the UMTS systems and the UWB emissions are presented. The performance degradation of the UMTS system vs. the in-channel UWB interference power has been measured in terms of the bit error rate and the noise rise. Index Terms— UWB signals, coexistence, UWB interference.
I. I NTRODUCTION
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ESIDES the great commercial and scientific interests that the ultra-wideband technology (UWB) has attracted in the last years, it has also given rise to unprecedented problems related to the coexistence with the existing radio systems. Indeed, one of the most advantageous features of UWB is the possibility of efficiently reusing the radio spectrum, since the UWB spread the signal energy over a very wide bandwidth [1]. Balancing the needs dictated by the coverage/capacity issues and the ones related to the coexistence, the power spectral density (PSD) of UWB signals must be maintained below the sensitivity level of most radio receivers operating in bands adjacent or overlapping the UWB frequencies. An intense regulation activity in the US has led to the First Report and Order issued by the Federal Communications Commission (FCC) in February 2002 [2] that establishes the limits of the emission power of UWB devices according to the frequency masks specific for each application. If in the US the regulation process has been finalized after more than 5 years of active work, in Europe it has started quite recently and at the moment the European regulation bodies are still working towards the definition of an even more restrictive ruling for the UWB operations. Of course the scenario of the radio communications in Europe is different from the US and gives rise to different problems, pushing towards new regulation solutions. One of the main concerns is the performance degradation that the UWB emissions may induce in the UMTS systems which are going to be launched today by most European providers. The coexistence of the UWB with the UMTS system has been investigated through theoretical analysis and simulations [3], [4], not yet through real measurements. In this letter the main results of experimental trials on an UMTS system in typical operating conditions, but in the presence of an UWB
Manuscript received September 30, 2004. The associate editor coordinating the review of this letter and approving it for publication was Prof. Gianluca Mazzini. This work has been done within ULTRAWAVES - Contract no IST2001-35189 of the Fp5 of the European Community. D. Cassioli and S. Persia are with RadioLabs, Rome, Italy (e-mail:
[email protected]). V. Bernasconi and A. Valent are with Telecom Italia Lab (TILAB), Turin, Italy. Digital Object Identifier 10.1109/LCOMM.2005.05032.
interfering signal, are presented. The performance degradation of the UMTS system has been measured in terms of the increase in the bit error rate (BER) vs. the UWB power level when the UMTS received power is fixed slightly over the minimum level needed to get a BER = 10−3 . Then, some measurements have been performed to evaluate the increase in the UMTS transmitted power needed to compensate for the effects of the UWB in-channel interference, in order to maintain the BER at 10−3 , i.e. the so called “noise rise.” II. M EASUREMENT S ETUP A coexistence test is performed by applying an interferer signal (the UWB signal) to an operating receiver (the UMTS phone) while monitoring its performance: any degradation in the receiver performance, beyond what is expected under normal operating conditions, can be attributed to the interference between the two systems. These kind of experiments are based on either conducted or radiated laboratory tests. In the former the UWB signal and the signal transmitted by the system under test (SUT) are combined together through a RF combiner, and injected into the SUT receiver. In the latter, instead, the UWB device and the SUT receiver are connected to their respective antennas, thus the signals propagate through a radio channel and are combined at the SUT receiver antenna. In both cases the key parameter is the UWB power detected by the SUT receiver. In real life the UWB and the SUT signals are radiated through a radio channel and combined at the SUT receiver. However, interference tests serving as a basis for accurate interference models should be independent from the antennas’ patterns, the measurement environments and propagation conditions. In this sense, the conducted tests are more reliable. Once that the experimental results from conducted tests are available, the subsequent interference models can be extended to account for the propagation effects and the antennas’ behaviors. Very similar results can be obtained through conducted and radiated tests performed in the anechoic room, where the signal propagation is predictable with a high accuracy [5]. This paper presents experimental results of UWB interference over an UMTS link, based on conducted tests. The trials have been performed in the UMTS laboratory developed at Telecom Italia Lab (TILAB) to test real UMTS devices that will be put in operation by Telecom Italia Mobile (TIM), the main Italian operator of mobile communications. A simplified block diagram of the measurement setup is shown in Fig. 1. A Universal Radio Communication Tester Rohde & Schwarz (R&S) CMU 200 simulates a UMTS base station (BS), i.e. it provides a simulated UMTS signal at a
c 2005 IEEE 1089-7798/05$20.00
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IEEE COMMUNICATIONS LETTERS, VOL. 9, NO. 5, MAY 2005
R&S CMU200
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Phone B − default UWB sequence Phone A − default UWB sequence Phone A − modulated UWB sequence
20 dB RF adder
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Block diagram of the measurement setup.
known power level. The main advantage given by the use of an instrument such as a UMTS simulator is the high reproducibility of the measurement scenarios. The tester is able to simulate the physical channels of a WCDMA cell and to report the BER and the block erasure rate (BLER), measured at the receiver of the mobile terminal. The UMTS signal1 transmitted from the R&S CMU 200, before being injected into the receiver of the UMTS phone, is combined through a RF adder with the UWB signal generated by the ULTRAWAVES development platform (DVP-Tx), the UWB Generator in Fig. 1. A 20 dB attenuator is placed at the RF connector of the UMTS phone, in order to protect the instruments against a too high input power. A step attenuator (0-110 dB) is used to vary the UWB power starting from the maximum power emitted by the DVP-Tx (∼ 10 dBm). The ULTRAWAVES platform generates a direct sequence (DS) signal of sub-nanoseconds pulses which modulates a carrier at a frequency of 4.78 GHz. The duration of a single pulse is 400 ps. The default sequence is a m-sequence of 1023 chips. The coexistence experiments have been performed using either the default sequence or another sequence that emulates the transmission of a stream of 330 bits, each spread by an msequence codeword of 31 chips. In the following this latter is referred to as the “modulated sequence.” Both sequences are periodically transmitted with a period equal to the sequence duration. The BER degradation due to the UWB interference over the downlink channel has been measured on the 12.2 kbps Reference Measurement Channel (RMC). The CMU 200 instrument measures the BER by comparing the received data (with errors) with the previously transmitted data (without errors). The UMTS terminal under test works in loop mode operation, i.e. the UMTS phone relays the downlink signal back to the CMU 200 through the uplink channel, with a power level high enough that the measured errors could be surely attributed to the downlink transmission. It is reasonable to assume a scenario where the downlink channel is more sensitive to the UWB interference, since the UMTS terminal will be in the near proximity of UWB devices more likely than the UMTS BS. III. E XPERIMENTAL T RIALS To assess the coexistence of UMTS and UWB, the impact of a single UWB transmitter over a single UMTS receiver has been considered, performing two kinds of measurement: measurements of the BER produced by the insertion of the 1 In these experiments we consider the downlink and uplink channels assigned to TIM, i.e. UARFCN 10638 for the downlink (around 2127.6 MHz) and UARFCN 9688 for the uplink (around 1937.6 MHz).
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BER of the UMTS vs. the in-channel UWB interference power.
UWB interferer signal, when the UMTS system is working at a constant very low power level; measurements of the noise rise, defined as the power level increase needed by the UMTS downlink channel to keep the BER below a given threshold (e.g. 10−3 ) in the presence of an UWB transmission. The measurements have been carried out on two different UMTS phones, varying the UWB power level, the cell traffic load and the kind of UWB transmitted signal. The cell traffic load has been simulated by the CMU 200 varying a parameter called Orthogonal Channel Noise Simulator (OCNS) power, which emulates the user signals transmitted on code channels orthogonal to the considered downlink data channel. An accurate calibration of the measurement setup has been performed before the test campaign. The UWB signal power at the input of the RF adder was measured, setting to 0 dB the step attenuator. The PSD level of the considered UWB signals in the UMTS downlink channel was -124.6 dBm/Hz for the default sequence and -124.0 dBm/Hz for the modulated sequence. Assuming a flat PSD for the UWB signals over the UMTS transmission band, and considering a filter bandwidth of 4.4 MHz, the UWB signal power that falls in a single UMTS downlink channel has been calculated to be -58.2 dBm for the default sequence and -58.1 dBm for the modulated sequence. The attenuation introduced by each segment of the test bench has been measured, i.e., referring to the labels in Fig. 1, LAB = 30.1 dB and LCB = 30.3 dB. The reference power for the physical channels of the UMTS downlink is given by the power level of the pilot channel. The power of the other physical channels is expressed by the excess power with respect to this reference power. In our setup, the power settings on the CMU 200 were: user data channel DPDCH, -7 dB; P-SCH, -2 dB; S-SCH, -2 dB; PCCPCH, -2 dB; S-CCPCH, -2 dB; PICH, -5 dB; AICH, -5 dB; DPCCH/DPDCH ratio, 0 dB. The traffic load was emulated by setting OCNS=0 dB for a low cell load, OCNS = 3 dB for a medium cell load and OCNS = 9 dB for the maximum cell load. The UMTS standard defines the sensitivity level of a receiver as the minimum received power on the user data channel (DPDCH) that ensures a BER less than 10−3 . The sensitivity of the two considered UMTS phones has been
CASSIOLI et al.: MEASUREMENTS OF THE PERFORMANCE DEGRADATION OF UMTS RECEIVERS DUE TO UWB EMISSIONS
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Noise rise (dB)
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Phone A the noise rise vs. the UWB interference power is shown in Fig. 3 for low, medium, and maximum cell load. Beyond the quasi-linear increase of the noise rise with the in-channel UWB power, the curves show that a key role is played by the choice of the transmitted UWB sequence. For the default sequence, the noise rise increases in average of 0.5 dB for an increase of 1 dB of UWB interference power, and this behavior is independent of the cell load. The modulated UWB signal, instead, is found to be less harmful for the UMTS receiver, since the noise rise increases more slowly than the previous case.
low traffic load − default UWB sequence medium traffic load − default UWB sequence maximum traffic load − default UWB sequence low traffic load − modulated UWB sequence maximum traffic load − modulated UWB sequence
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The noise rise in the Phone A vs. the in-channel UWB power.
measured using the same measurement setup used for the interference trials, turning off the UWB interferer signal. The sensitivity value represents the operational threshold of a UMTS phone without interference, i.e. only due to the thermal noise, and represents the reference power for the performed measurements. The sensitivity on the RMC 12.2 kbps channel was measured as -117 dBm for the Phone A and -115.5 dBm for the Phone B. The BER measurements have been conducted by setting the power of the UMTS downlink channel slightly over the sensitivity level and varying the UWB power by means of the step attenuator. The UMTS power control has been defused. Fig. 2 shows the BER of the downlink data channel received by Phone B and Phone A vs. the UWB signal power (integrated over the UMTS channel of 4.4 MHz bandwidth)2 . The Phone A breaches the BER threshold of 10−3 when the average in-channel UWB power exceeds −109 dBm, when the default DS-UWB signal is transmitted, and −95 dBm when the modulated UWB signal is transmitted. The calls are dropped when the average inchannel UWB power exceeds −90 dBm and −84 dBm for the default and modulated UWB sequences, respectively. Thus, the modulated UWB signal is found impacting less than the default sequence. The performance of the Phone B has been tested only with the default UWB sequence. The BER=10−3 limit is breached when the average in-channel UWB power exceeds −108 dBm, while at −90 dBm the calls are dropped. Another figure of merit of the UMTS system performance in the presence of UWB interference is the increase of the power of the downlink channel required to maintain a BER less than 10−3 , while the interfering signal level is increasing. These measurements have been performed by incrementing the UWB signal power by 1 dB steps and consequently increasing the UMTS downlink received power in order to maintain the BER less than 10−3 , starting from the received power corresponding to the UMTS receiver sensitivity. The increase of the UMTS received power with respect to the receiver sensitivity is commonly referred to as the noise rise. For the 2 The high variability of BER values below 10−3 is related to the number of observed samples; indeed a very large number of samples is necessary to measure low values of BER, at the expense of the measurement time. For our purposes it is sufficient to verify that the BER remains below the 10−3 threshold.
IV. C ONCLUSIONS Some experimental trials on the coexistence of the UMTS system with a UWB device based on DS spread spectrum modulation have been performed. In the considered scenario, i.e. with the choice of periodic UWB signals and UMTS frequencies used for the test campaign, the maximum UWB power level in the UMTS channel tolerable by a UMTS receiver is found to be in the range around -110 / -95 dBm: beyond this value the BER of the UMTS receiver increases over 10−3 , which is the BER threshold specified for the conformance testing by the UMTS technical specifications. The emission mask issued by the FCC for UWB indoor communications gives a total maximum UWB allowed power of -45 dBm in the bandwidth of a single UMTS RF channel. According to the ruling draft issued by the European Telecommunications Standards Institute (ETSI) [6], the total maximum UWB power allowed in a single UMTS RF channel for indoor communications is about -60 dBm. These limits regulate the UWB effective isotropic radiated power (EIRP). Since the signal attenuation as given by the free-space theory is 20 log10 (4πdf /c) 40 dB for a distance d = 1 m and a frequency f = 2 GHz, the maximum allowed UWB signal power in the UMTS channel at 1 m is -85 dBm for the FCC ruling and -100 dBm for the ETSI regulation. Thus, the more restrictive limits imposed by the ETSI regulation are likely able to protect the UMTS system against the UWB interference, while the FCC ruling may fail. ACKNOWLEDGMENT The authors would like to thank F. Pattini, M. Migliorelli, R. Failli and F. Vatalaro for making this joint work possible, and D. Porcino and F. Mazzenga for helpful discussions. R EFERENCES [1] M. Z. Win and R. A. Scholtz, “Ultra -wide bandwidth time-hopping spread-spectrum impulse radio for wireless multiple -access communications,” IEEE Trans. Commun., vol. 48, pp. 679-691, Apr. 2000. [2] Federal Communications Commission, “First Report and Order 02-48,” 2002. [3] M.Hamalainen et al., “On the UWB system coexistence with GSM900 UMTS/WCDMA, and GPS,” IEEE J. Select. Areas Commun., vol. 20, pp. 1712-1721, Dec. 2002 [4] R. Giuliano, F. Mazzenga, and F. Vatalaro, “On the interference between UMTS and UWB systems,” in IEEE UWBST, pp. 339-343, Nov. 2003. [5] J. R. Hoffman et al., “Measurement to determine potential interference to GPS receivers from UWB transmission systems,” in NTIA Tech. Rep. 01-384. [6] CEPT-ECC WGPT SE24, “Interim report on UWB compatibility,” in Doc. ECC/SE(04)072rev1.
