Compatibility and Safety Volume for Electromagnetic Exposure Limits in Shared Sites for 2G and 3G Wireless Communications

Transcription

1 Compatibility and Safety Volume for Electromagnetic Exposure imits in Sared Sites for G and 3G Wireless Communications Rogelio Jiménez Jiménez*, Diego Ortega abajos*, Florentino Jiménez **, Rafael Herradón** * Telefónica Móviles España, S.A. jimenez_r@tsm.es y ortega_di@tsm.es ABSTRACT At tis moment, mobile communications operators are starting to develope tird generation network wit UMTS tecnology. In many cases, tis tecnology will be placed in facilities already used by oter systems, specially GSM network, so, problems suc electromagnetic compatibility and exposure limits regulations values sould be carefully studied before introducing te new system in old locations. I. INTRODUCTION A GSM base station can produce wrong operation in a sared UMTS base station, due eiter its carrier own emission (receiver blocking) or its band spurios emission outside. Te problem can be equally arise in te oter sense: UMTS base station can affect to GSM base station troug te same procedures. Non-linear caracteristics of sared devices in bot RF signals of GSM BTS and UMTS node B (as a diplexor or antenna) can produce intermodulation products wic can be into reception band of bot of tem and, terefore, communication interferences. We include in our study, te presence of anoter operator in te same area of coverage wit an adjacent cannel in UMTS band. Besides tat, some considerations and tools will be presented about electromagnetic emissions exposure limits compliment. Starting from te case of only location wit UMTS, comparisons will be done wit sared locations wit bot UMTS/DCS and UMTS/DCS/GSM. II. COMPATIBIITY BETWEEN SYSTEMS. Figure 1 sows a typical utility one GSM/DSC base station and one UMTS node B. Marked points as 1 and are used as reference for spurious, blocking and sensibility specifications; tey are transmission and reception antennas connectors. Also, an antenna and its cables ave been drawn for eac system. Isolation between antennas as been rated to 30 db, according to multiband antenna manufacturer specifications. In oter cases, it will be necessary to exactly calculate isolation value wic as great influence in electromagnetic compatibility among systems, as will be sow in next points. Figure 1. Sared UMTS and GSM. A. UMTS Node B Blocking Receiver According to UMTS nodes B specifications described in document 3G TS of 3GPP [1], maximum level of signal interference in order to not blocking te receiver is 15 dbm. For a GSM base station transmitting a 47 dbm carrier wit 30 db isolation between antennas, node B would be receiving tis carrier wit 17 dbm, 3 db above blocking limit. Te solution proposed to tis problem is to add a band pass filter in node B reception input in order to attenuate GSM/DCS transmission bands at least 3 db. B. GSM/DCS BTS Receiver Blocking Document GSM 11.1 of te ETSI [], set te maximum level of signal continuous interference outside reception band in order to not blocking te receiver, is 8 dbm for 900 MHz band and 0 dbm for 1800 MHz band. For a UMTS base station transmitting a 46 dbm level carrier wit 30 db isolation between antennas, GSM/DCS base station would receive 16 dbm, 8 db and 16 db above blocking level for 900 MHz band and1800 MHz band, respectively. It would be necessary a band pass filter in BTS reception imput to attenuate UMTS band transmission in 8 or 16 db, depends on te band. C. Transmision Spurious from Node B UMTS As standard UMTS as been developed after te GSM system, possible armful effects of te first envelope to te second ave been considered in it. In tis way, node B UMTS spurious emission levels for GSM reception band, eiter in 900 MHz or in 1800 MHz, are limited to 98 dbm (100 KHz bandwidt measure). Terefore, tey will not produce any problem.

2 D. Transmission Spurious from BTS GSM/DCS GSM spurious specifications establiss 30 dbm limit (3 MHz measure bandwidt) for UMTS reception band. On te oter and, maximum level of spurious can arrive to node B is 107 dbm (-18 db relationsip C/I cocannel and sensibility node B 15 dbm, for a 3.84 MHz carrier). Under same conditions tat in previous sections, wit 30 db isolation between antennas, spurious will overcome in 47 db te admitted value. Te solution is to add a filter in transmission output. Tis filter will eliminate UMTS reception band, by adding 47 db attenuation. Previous calculations are done wit maximum value allowed by specifications. Actual measures by a commercial team, indicate tat te spurious level is below spectra noise analyzer's level, -85 dbm for a 100 khz bandwidt; making corrections for a 3 MHz bandwidt, tey are 70 dbm tat will need some filtrate, too. E. Intermodulation Products Transmission and reception bands of te systems under study are in te following table: Tx Band (MHz) Rx Band (MHz) GSM GSM UMTS Table 1. Bands and frequencies of analised systems It s easy to verify tat two transmission frequencies combinations don t produce second or tird order intermodulation products in te reception bands of any of tese systems. However, by using tree frequencies, it s possible tat intermodulation products could be in any reception band. In te following table some examples are included: RX F1 (MHz) F (MHz) F3 (MHz) INTERMODUATION (MHz) GSMUMTS-UMTS DCSUMTS-UMTS DCSDCS-DCS Table. Intermodulation between systems III. EXPOSURE IMITS It s fundamental to acieve calculations and measures in order to comply te uman exposure limits to electromagnetic fields. European organisms as CENEEC, American one s as ANSI/IEEE or FCC, as well as oter more international regulations as ICNIRP and OMS, are dedicating great efforts to regulate electromagnetic emissions due to cellular services. Reason is increasing social restlessness against tis type of emissions. Tis opposition depend on te countries. In general, a 3G network will need more places tan a G network. However, in typical urban areas it will be unavoidable not to use all te places already installed wit oter communication networks, specially G. Bot cases will be studied in deep. Recommendation K.5 from UIT-T ave been taken to set a limiting value reference. Safety references given by te ICNIRP are included in it, as table 3 sows: SYSTEM Density power limit, S (W/m) GSM GSM UMTS 10 Table 3. Exposure limits for different systems Tis value of power density can be teoretically calculated in any point, considering free space conditions, in te following way: Were N is te total number of trasmission antennas, EIRP k is te total EIRP of te k-antenna, d k is te distance between te considered point and te k-antenna, M k te reflection coefficient for te k-antenna and F k te radiation pattern from k-antenna, tis depends on its distance, relative azimut, eigt and electric plus mecanical slope tat can be assigned to it. Recommendation K.5 indicates tat an installation is complaint wit ICNIRP exposure limits if EIRP, for a 3G system, is lower tan: Were is te antenna radiation center eigt, α is te beam tilt and θ bw, te alf power beamwidt. We obtaine, for a typical eigt of 6 m and a tilt of 6º, an EIRP of 10,81 W. Tis value ardly will be reaced in an installation wit tese caracteristics. For tat reason, we will do anoter type of evaluations. A. Parameters Considerations In tis work, a trisectorial UMTS configuration implemented in a commercial Node B will be studied. Top output power is 46 dbm considering two carriers for eac sector, if tere was more carriers output power sould be divided. A commercial double band antenna will be cosen, wit 18 gain dbi for band, 65º of orizontal alf-power beamwidt and 6º of vertical alfpower beamwidt, crossed polarization and isolation between bands iger tan 30 db. Start point is a typical situation in 3G network development: a station on a building roof. Te target is (1) ()

3 calculate areas wit density power levels iger tan admitted limits. Tese areas are named as exclusion areas, or accessibility category 4 in K.5 (figure 3): antenna. Tis power profile can be obtained for several floor types. So, figure 4 represents power density versus distance to te antenna, for an EIRP of 1.59 W, wit te caracteristics of te antenna tat we are considering, a eigt of 6 m and a beam tilt of 6º. In tis figure, it as been considered a very dry floor, wit a average reflection coefficient of 1.5. a b S (W/m) Figure 3. Exclusion area T On te oter and, far field conditions sould be studied. Far field conditions start from a distance R given for R=1/ /λ. Were is te maximum antenna dimension. For UMTS system, a R = 7,8 m value is obtained. Tis value is too large for wireless communication considerations. Also, te most important ting is to be out of near field reactive part. To acieve tat, it will be enoug to admit a distance equal to te maximum antenna dimension, R = m. Tis distance will be considered as starting point of te near field Tis approac is valid if: 0.1 λ 50 λ. Tis expression is correct in tis case. B. UMTS Base Station on Not Sared Sites EIRP will be calculated according to te transmission caracteristics of te previous commercial antenna, incluiding cables and connectors losses: Parameter Transmitted power Cable and connectors losses Gain antenna EIRP Value 46 dbm 3 db 18 dbi 61 db Densidad de potencia (W/m) Distancia a la antena (m) Figure 4. Power density versus distance to antenna) Te results sown in figures clearly indicates tat tese power density values are very far from reference limits, 10 W/m for UMTS. IV. SAFETY VOUME. We can continue in tis way calculating safety distances, but we will not be reflecting spacial variation of te antenna radiation pattern and its influence in accesibility area. In order to include tese spacial variations, we can calculate a safety volume [6] tat wraps te limit surface of te antenna pattern beam, like it s represented in figure 5. If te system is being used as voice service and, terefore, power as no limit, EIRP will be equal to 159 W, troug te main propagation direction, of course. Accessibility distance, considering te possible reflection in te floor (M), gives: Were S is te value of power density. S as a limit value of 10 W/m for UMTS, wit a typical average reflection coefficient of 1.5 for a dry roof. We obtain a distance of D=3.88 m, a quite low value. However, wen an overprotection of 6 db is used, te result is D = 7.75 m. According to recommendation K.5 of UIT-T, we ave calculated exposure power versus distance to (3) Figure 5. Safety volume. imit surface is defined as te minimum surface of a especific installation wic complies wit exposure limits. Tis surface will ave a complicated form, in te practice it s better to calculate te rectangular parallelepiped tat wraps it tangently. Tis polyedron is defined by te following magnitudes: m1, m,, v1 and v, as it s sowed in te figure 5. Teir calculation is direct starting from te antenna radiation pattern, only it is necessary to numeric, or grapically, determine angles θ H, θ A, θ V1 and θ V in wat te projection on te

4 axis corresponding of radiation pattern is maximum (figures 6 and 7). Numeric expressions for tese magnitudes are: (θ) antenna gain. (5) (6) (7) (8) (9) Wit D acc as reference distance and G m1,c = m1, (1) m1, If we suppose previous caracteristics for UMTS system and a quite ig power for DCS, for example a 56 dbd ERP, for two carriers in te same antenna connector, te result is a EIRP of 1,306 W. Te antenna, as we ave already indicated, as very similar caracteristics for bot systems. Wit tese data, we obtain te following values (in meters): m1c =4.64, mc = -0.38, c =3.57, v1c = - vc =0.38 If te installation is on a structure over te roof we can conclude tat limit surface doesn't go into te exclusion area. It s not easy to define power density profiles, in tis case and in te following one, because power density limits are different for different systems. But wit te rectangular parallelepiped you can get a good approac to consider contributions from every systems. B. Safety Volume wit UMTS/DCS Systems in Presence of GSM 900 System. Figures 6. Definition of safety volume, orizontal pattern Figures 7. Definition of safety volume, vertical pattern Calculating in tis way safety volumes, we will be able to define in a simple way accessibility areas, altoug in many aspects it is overestimated. If we use specific values we are using before, te following results are obtained (in meters): m1 =3.17, m = -0.18, =.43, v1 = 0.18, v = It is necessary to add to v1 y v alf of antenna lengt in order to build a rectangular parallelepiped tat wraps te limit surface beams correctly. Tis is a typical case of an urban area site. For eac sector, we ave an GSM antenna and a UMTS/DCS dual band antenna. We ave defined 0,50 m distance between antennas. Tis situation is sowed in figure 8, considering only one sector. It would be te case of simultaneous exposure to multiple sources. Recommendations indicate tat you must comply wit following expressions to comply wit reference limits: 1MHz 300GHz E i E i 1 i= 100 khz c i> 1MHz El, i 1MHz 300GH H z j H j 1 j= 100 khz d j> 1MHz Hl, j However, tese expressions don't produce practical results in engineering calculations. Solution would be to calculate rectangular parallelepiped dimensions wat wraps limit surface. A. Safety Volume wit UMTS/DCS Systems wit Dual Band Antenna. Te magnitudes tat define tis parallelepiped can be redefined adding bot systems contributions, in tis way: v1,c = v1, (10) v1, c = (11) Figure 8. GSM and DCS/UMTS.

5 Complete magnitudes are given not only by te energy contribution but also for te geometric caracteristics. In tis case, we will ave te following expressions: ( GSM) m1( UMTS/DCS) ( GSM) m( UMTS/DCS) ( GSM) v1( UMTS/DCS) ( D, D ) = (13) m1c m1 mc = m (14) v1c = v1 (15) Max GSM UMTS/DCS v1ce = v1c (16) vc = v ( GSM) v ( UMTS/DCS) (17) Max( D GSM, D UMTS/DCS ) vce = vc (18) ( GSM) ( UMTS/DCS) c = (19) = S (0) ce ce We will independently calculate values for GSM system and for UMTS/DCS dual system. If we assume tat GSM system works wit an antenna of.40 m, wit 16.4 db gain in te main direction, 65º gap in orizontal plane and 6º in vertical one. We will also assume as worst case a 4 carriers transmition. We obtaine in tis case a,61 W EIRP. We are considering a typical separation between antennas of S=0,50 m, oter transmission conditions for UMTS/DCS antennas are tose previously considered. Wit tese data, we can define te safety volume. Tis parallelepiped include te wole system wit te following dimensions (in meters): m1c = 8.3, mc = -0.78, ce = 6.8, v1ce =- vce =1.98 In tis case, te more pesimistic one, we obtain a rater big polyedron. It would be necessary to consider specific conditions for eac installation and, if it s necessary, to establis an exclusion area for te public. C. Summary of Results. Empiric Measurements. Table 4 sows te most caracteristic parameters from safety volume (lengt in main direction propagation and widt) for several EIRP values of te different systems analyzed wit te defined values. UMTS(W) DCS(W) GSM(W) m1(m) (m) 159 X X X X X X Table 4. Summary of values from safety volume. We ave carried out measurements wit an electric field probe in sites over buildings roof wit one or several mobile systems working wit different powers. Measured values indicate tat safety volumes, teoretically calculated, are a simple and correct way to define an exclusion area. V. CONCUSIONS In cases of coexisting G and 3G systems in te same place, it will be necessary to keep in mind installation conditions (diplexers, splitters and antennas). It s necessary to evaluate isolation between tem, too. Intermodulation products effect will require an detailed study of specific place (installation, existent operators and frequencies used in te carriers). Solutions will consist on te filters introduction tat eliminate te band to reject. Tese filters will ave minimum attenuation in te required band. Te problem more difficult to solve it will be te one due to adjacent cannel between operators. Electromagnetic emissions analysis of different systems located in te same site can be complex. We can work wit a safety volume around te radiant system. Tis volume will include te exclusion area to te abitual permanency of people. REFERENCES [1] UMTS; UTRA (BS) FDD (3GPP TS version Release 1999), ETSI TS V3.5.0, December 000. [] Digital cellular telecommunications system (Pase ); Base Station System (BSS) equipment specification; Part 1: Radio aspect (GSM 11.1 version 4.9.), I-ETS ,Fort edition, July [3] Radio Equipment and Systems (RES); Electromagnetic Compatibility (EMC) for European digital cellular telecommunications system; Part : Base station radio and ancillary equipment), ETS , November 1994 [4] Electromagnetic compatibility and Radio spectrum Matters (ERM); General Elec. Compatibility (EMC) for radio communications equiment. EN v1.1.1, June [5] UMTS; Base station Electromagnetic Compatibility (EMC) (3GPP TS version Release 1999), ETSI TS v3.3.0, December 000. [6] Colegio Oficial de Ingenieros de Telecomunicación, El COIT y las emisiones radioeléctricas. [7] Guidance on complying wit limits for uman exposure to Electromagnetic Fields. ITU-T. Recommendation K.5.(/000 [8] Recommended practice for measurements and computations wit respect to uman exposure to Radiofrecuency Electromagnetics Fields, 3 KHz to 300 GHz.IEEE Draft.1998