Interference analysis on UMTS-2100 co-existence with GSM-1900

Loureiro, A., Gallegos, D., Caldwell, G. Nokia Technology Institute (INdT)

Brasilia, Brazil Email: alexandre.loureiro@indt.org.br; david.gallegos@indt.org.br; george.caldwell@indt.org.br

Abstract—A possible mobile spectrum allocation for mobile operators that offer 2G/3G services is WCDMA 2100MHz system co-existing with a GSM 1900MHz system in the same coverage area. In this scenario inter-band interference exists at mobile subscriber (MS)/user equipment (UE) level and network/base transceiver station (BTS/Node B) level, and the objective of this paper is to observe this behavior. For that purpose, measurements were taken in a laboratory configuration where variables were adjusted in order to explore main RF environment characteristics in real life such as: the received signal level at GSM phone, the distance between WCDMA/GSM phones, the guard band between systems and the transmission output power of UEs. The results show that the interference levels can decrease the quality and the capacity of networks using this spectrum allocation.

Keywords: WCDMA, GSM and Interference analysis

I. INTRODUCTION Spectrum allocation for mobile operators in some countries

is presenting challenges. Spectrum is an expensive, scarce resource and with the increase on the number of subscribers, additional bands are needed. In addition to the original UMTS band (2100MHz), known as “Core band”, 3GPP specified extra bands to be adopted as WCDMA bands [1], which aims on supporting the different requirements that markets around the world are facing. One possible spectrum scenario that can become a potential issue is the one which has GSM 1900MHz and WCDMA 2100MHz sharing the same coverage region.

This scenario of interference despite being allowed by the UMTS specification [4] is not usual to be adopted to cover the same region in the major part of countries. Exceptions can be found in some emergent markets in South America and Asia. The WCDMA 2100MHz is common used in countries that follow the European mobile spectrum specification where uplink is from 1920MHz to 1980MHz and downlink is from 2110MHz to 2170MHz. The GSM 1900MHz follows the USA specification where uplink is from 1850MHz to 1910MHz and downlink is from 1930MHz to 1990MHz. The scope of this paper is to understand the potential interference risks created by this atypical system co-existence of WCDMA 2100MHZ with GSM 1900MHz in the same coverage region. In the user equipment/mobile station (UE/MS) side, for example, the interference can occur from WCDMA UE uplink transmission into GSM MS downlink reception because the WCDMA 2100

UL band is near to the GSM 1900 DL band as shown in Figure 1. The WCDMA UE transmission can interfere in the GSM MS reception and the interference increases depending the distance between the terminals.

Figure 1. WCDMA UL interfering GSM DL

The inter-band interference occurs due RF imperfections in the transmitter or receiver path like non-linearity of power amplifiers, filters, antennas, combiners, connectors etc. [2]. These imperfections generate out of band spurious emissions (OOBE) [7] or inter-modulation distortion (IMD) and the total interference is known as adjacent power leakage [6]. The adjacent channel leakage ratio (ACLR) indicates the ratio between the MS transmission power and the related leakage power in the adjacent carriers. Some other parameters can be analyzed like: adjacent channel power ratio (ACPR) [8], receiver desensitization, adjacent channel selective and blocking (ACS) [9]. The RF interference considerations about coexistence scenarios have been derived from either regulatory requirements or coexistence studies performed by standardization entities [6].

A reliable way to measure the interference and quality in a laboratory is by using radio frequency performance measurements. The interference is measured considering the carrier versus interference signal noise ratio (C/I) and the receiver quality (RxQual) from GSM mobiles.

This paper evaluates the potential interference caused by one WCDMA User Equipment transmission (uplink) at one

978-1-4244-8327-3/11/$26.00 ©2011 IEEE

GSM Mobile Station reception (downlink), as well as the interference caused by GSM Base Station on WCDMA Node B. This paper is organized as follows. Section II describes the scenario used to test and the parameters needed to do the interference analyses. Section III presents the results and discusses the interference analyses on the co-existence scenario. Finally, in Section IV, the conclusions are summarized.

II. SCENARIO The scenario presented in this paper is based on real

networks environments deployed by operators. The tests were performed in a laboratory equipped with a full 2G/3G network platform composed by 1 BTS (Base Transmitter Station), 1 BSC (Base Station Controller), 1 MSC (Mobile Switching Center), 1 RNC (Radio Network Controller), 1 NodeB (WCDMA BTS) and 1 MGW (Media Gateway). The main characteristics of this scenario are: low traffic loading, limited mobility and indoor coverage. The common parameters for all tested scenarios are summarized in Table I. A baseline test case for each technology GSM/WCDMA was carried out separately without inter-band interference, and was used as a reference to compare their performance when inter-band interference is applied. A drive-test tool was used to collect RXQUAL and C/I from the GSM MS while Net monitor software was used in the WCDMA UE phone to collect output power [3]. Figure 2 shows the test setup and the source of interference affecting the DL and UL path.

Some parameters were adjusted in order to explore main RF environment characteristics in real networks. Each of the parameters has a specific way to impact the overall performance. An explanation of what these parameters represent or simulate in real networks is described as follows.

The Received Signal Level at GSM phone (RXLEV GSM) varies from -40 dBm to -110 dBm, by 5 dB steps or lower. It simulates the different locations and DL coverage for GSM phone in relation to GSM BTS on a live network. These signal levels were implemented by adjusting the BTS transmission attenuation.

The Distance between WCDMA/GSM phones (d) considered the following values: 50 cm, 1m, 2m, 4m and 8m. It represents the distribution of different phone users on a live network. It is related to the level of interference from WCDMA phone on GSM phone due to path loss calculations.

TABLE I. COMMON PARAMETERS FOR ALL SCENARIOS

Parameter GSM WCDMA

DL Band 1975-1980 MHz 2155-2165 MHz

UL Band 1895-1900 MHz 1965-1975 MHz

Base Station / NodeB Output Power +37 dBm ( 5 W ) 20 W (Pilot

CPICH 2 W)

Maximum MS/UE Output Power +30 dBm ( 1 W ) +21 dBm

Voice Call Channel Mode EFR AMR FS12.2

BTS / NodeB Antenna Horizontal Pattern Omnidirectional Omnidirectional

BTS / NodeB Antenna Gain 2 dBi 2 dBi

BTS / NodeB Antenna Height 3 m 3 m

Power Control OFF ON

Minimum Access Rx Level -110 dBm -

The Guard Band (Δf) represents the operator spectrum allocation on the available bands. The guard band was set to 2.8MHz or 7.6MHz. It is determined by the band separation that avoids overlapping by adjusting the GSM BCCH (Broadcast Control Channel) channel number of 200 KHz bandwidth and the UARFCH (UMTS Absolute Radio Frequency Channel WCDMA Carrier) of 5MHz bandwidth as shown in Figure 3. For example, if the band separation is 200KHz, the Guard Band (Δf) is equal to 2.8MHz which comes from 5MHz (WCDMA carrier) divided by 2 plus

BTS GSM 1900

UL:1895-1900MHz

DL:1975-1980MHzBTS WCDMA 2100

RNC

Spectrum Analyzer Call collect ion

UL:1965-1975MHz

DL:2155-2165MHz

UL WCDMA in DL GSM

DL GSM in UL WCDMA

I nterference

BSC

Figure 2. WCDMA/GSM Inter-band interfering each other

200KHz (band separation) plus 200KHz (GSM carrier) divided by 2.

The UE WCDMA Transmission output power (TXPWR WCDMA) varies from +21dBm to +12 dBm, +5 dBm and –10dBm, and it represents the different needs for a UE to improve its uplink path in a live network. It is related to the level of interference that a WCDMA phone causes on a GSM phone due to the transmission power.

Figure 3. Guard Band (Δf) configuration

III. RESULTS After executing appropriate changes in the lab scenario

configuration altering the variables mentioned in previous chapter, we obtained the following results.

Figures 4 to 7 shows the inter-band interference impact in the MS/UE side which is caused by the interference from WCDMA UE uplink transmission into GSM MS downlink reception. The Figure 8 shows the interference in the BTS/NodeB side which is caused by the interference from GSM BTS DL transmission to the WCDMA NodeB UL reception. The GSM baseline shown in the figures legend means the reference case without interference.

C/I vs RSSI (TXPWR WCDMA: +21dBm, Δf: 2.8MHz)

0

5

10

15

20

25

-120 -110 -100 -90 -80 -70 -60 -50 -40 -30

RXLEV GSM (dBm)

C/I

(dB

)

GSM Baseline 50cm 2m 4m 8m

Figure 4. UE WCDMA Tx power impact on GSM MS C/I for different distances (d) and for different GSM MS RxLev (RSSI)

When either the distance between GSM – WCDMA phones

(d) in Figure 4 or guard band (Δf) in Figure 5 are increased, the Rx signal level (RXLEV GSM) required by the GSM MS to have the same carrier to interference ratio (C/I) is reduced. This indicates that the impact caused by the interband interference from WCDMA phone on the GSM phone is reduced.

C/I vs RSSI (TXPWR WCDMA: +12dBm, d: 1m)

0

5

10

15

20

25

-120 -110 -100 -90 -80 -70 -60 -50 -40 -30

RXLEV GSM (dBm)

C/I

(dB

)

GSM Baseline 2.8MHz 7.6MHz

Figure 5. UE WCDMA Tx power impact on GSM MS C/I for different guard bands (Δf) and for different GSM MS RxLev (RSSI)

On the other hand, in the Figure 6, when the WCDMA phone transmission power (TXPWR WCDMA) is increased, the GSM MS needs higher Rx signal level to have the same performance indicators such as C/I. Hence, the impact of the interband interference from WCDMA phone on GSM MS behavior is increased when TXPWR WCDMA is higher.

C/I vs RSSI (Δf: 2.8MHz, d: 1m)

0

5

10

15

20

25

-120 -110 -100 -90 -80 -70 -60 -50 -40 -30

RXLEV GSM (dBm)

C/I

(dB

)

GSM Baseline +21dBm +12dBm +5dBm -10dBm

Figure 6. UE WCDMA Tx power impact on GSM MS C/I for different GSM MS RxLev (RSSI)

When comparing the GSM baseline (reference case without interference) to most of the test cases with inter-band interference is observed degradation in the GSM phone receiver sensitivity in dBm, as shown in Figure 7. According 3GPP specifications [5] the GSM phone receiver sensitivity is

measured as the minimum signal level at which you obtained a FER performance <= 1% for speech channels. In Figure 7 the FER performance <= 1% was observed for an EFR (Enhanced Full Rate codec of 12.2Kbps) speech channel connection at C/I >=9dB and RxQual <=2.

Figure 7. UE WCDMA Tx power impact on GSM MS Sensitivity for diferent sets of distances (d) and guard bands (Δf)

The GSM MS receiver sensitivity shown in figure 7 is a variable that affects the downlink RF link budget calculation to obtain the maximum path loss allowed. As a consequence, its degradation decreases the coverage radius of the site planned. Thus, additional sites will be necessary to have the original coverage area, when no inter-band interference is present.

In the Figure 7 when the GSM carrier is close to WCDMA carrier at minimum separation (Guard Band (Δf) of 2.8MHz) and at maximum UE WCDMA power transmission (21dBm), the losses in MS sensitivity vary from 35dB to 50dB depending on distance separation between phones (for our test cases: 0.5m, 1m, 2m, 4m. see Section II ).

-40

-38

-36

-34

-32

-30

-28

-26

-24

-22

-20

MAX MAX - 6 MAX - 10 MAX - 16 MAX - 20 MAX - 25 MAX - 30

DL TXPWR BTS GSM1900 (From MAX=+39dBm)

UL

WC

DM

A T

XPW

R (d

Bm

)

Baseline 2.8MHz 27.6MHz

Figure 8. Impact on UE WCDMA Tx Power due to Inter-band interference from DL GSM BTS transmission for different Guard Band (Δf)

The interference caused by DL Transmission from the GSM BTS to the WCDMA NodeB reception in the UL path was observed in Figure 8. The baseline is the scenario without interference from GSM BTS and was measured the WCDMA UE UL transmission power. The GSM BTS full power

transmission is +49dBm and is indicated as MAX in the Figure 8. Some tests were done for 200KHz and 25MHz band separations, which corresponds to Guard Band (Δf) of 2.8MHz and 27.6MHz respectively, in order to quantify impacts on this scenario. At the worst condition, when the GSM BTS full power transmission is used, the WCDMA UE needs to increase its power to 5dB (for Δf equal to 27.6MHz) and 15dB (for Δf equal to 2.8MHz), when compared to the baseline (case without interference).

This impact on UE WCDMA Tx power showed in Figure 8 affects battery life and UL cell capacity. UE has to deliver more power in order to compensate Uplink interference, therefore battery life is reduced and the overall uplink noise increase which degrades capacity.

IV. CONCLUSIONS Implementations of GSM and WCDMA networks on the

same region with proximity on spectrum allocation can impact each other. This proximity causes the loss of phone sensitivity and the increment on power transmission, which in turn leads to degradation in the network performance and capacity. It is strongly recommended that RF planning take into consideration these impacts since preliminary assumptions on the number of sites can be modified after an inter-band interference scenario is deployed. Further investigation should be done to evaluate impact on relevant KPIs (Key Performance Indicators) on both systems.

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Generation Mobile Communications, 3rd ed., Wiley, 2004. [2] A. Richardson, WCDMA Design Handbook, 3rd ed., Cambridge

University Press, 2008 [3] J. Laiho, A. Wacker, T. Novosad, Radio Network Planning and

Optimization for UMTS, 2nd ed., Wiley, 2006. [4] 3GPP TS 25.101 v.7.5.0, UE Radio Transmission and Reception (Ch 6),

3rd Generation Partnership Project (3GPP) Std. [5] 3GPP TS 45.005 v.7.8.0, GSM/EDGE Radio Transmission and

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Malkov, Z. Li, “Interference Analysis of Coexistence Scenarios in TV White Spaces”. in International Telecommunications Symposium - ITS, Brazil, Sep 2010

[7] Recommendation ITU-R SM.329-10 - Unwanted emissions in the spurious domain, ITU Radiocommunication Bureau (BR), Feb 2003.

[8] F.-L. Lin, S.-F. Chen, L.-F. Chen, and H.-R. Chuang, “Computer simulation and measurement of error vector magnitude (EVM) and adjacentchannel power ratio (ACPR) for digital wireless communication RF power amplifiers,” in IEEE Vehicular Technology Conference (VTC99-Fall), vol. 4, 1999, pp. 2024–2028.

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