interference in cellular networks

Network Group Interference in Cellular Networks

DRAFT

Network Group

Interference in Cellular Networks

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Contents

1. INTRODUCTION..................................................................................................................1.1 Scope..............................................................................................................................1.2 Objectives.......................................................................................................................1.3 References................................................................................................................... 3

2. PROCEDURE ……………………………………………………………………………………..32.1. Introduction..................................................................................................................32.2. Growth Scenarios........................................................................................................32.3. Frequency Reuse........................................................................................................42.4. Interference..................................................................................................................42.5. Techniques to Minimise Interference...........................................................................92.6. Effects on Coverage and Interference by applying power decrease, antenna height decrease and beam tilting.................................................................................................102.7. Antenna Patterns.......................................................................................................11

3. PROCEDURE DETAILS...................................................................................................134. DEFINITIONS......................................................................................................................................................... 14

Interference in Cellular Networks

1. Introduction

1.1 Objectives

One of the most important limitations in a cellular system is the frequency resource. The challenge is to serve the greatest number of customers with an acceptable system quality using the allocated frequency spectrum. This document provides information to determine the reasons for, and effects of interference on system quality in a cellular network. Several techniques to minimize interference are also discussed.

1. 2 Scope

Inter-modulation, inter-symbol, adjacent and co-channel interference in a cellular mobile radio network will be discussed.

1.3 References

1. Advanced Radio Planning Training Manual, Training Course, Quotient Communications, 1995.

2. CME20 System Survey, Training Document, Ericsson.3. Cell Planning Principles, Training Document, Ericsson.4. M. D. Yacoub, Foundations of Mobile Radio Engineering, CRC Press, 1993.5. R. Steele, J. Whitehead, and W. C. Wong, System Aspects of Cellular Radio, IEEE

Communications Magazine, pp. 80-86, January 1995.

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6. William C. Y. Lee, Mobile Cellular Telecommunications - Analog and Digital Systems, Second Edition, McGraw-Hill International Editions, 1995.

2. Procedure

2.1 Introduction

The number of users a network can support is fundamentally dependent on the Common Air Interface (CAI) over which users communicate. User capacity is dependent on many factors, but the main ones are the amount of spectrum allocated, the size of the radio coverage area from a Base Station (BS), and the amount of interference a particular radio link can tolerate.

2.2 Growth Scenarios

At the conception phase of a cellular network, capacity is not the problem since there are limited users. However, the designer must provide coverage. During this phase, BSs are often sited according to the maximum range that can be obtained. Since capacity is of little importance at this stage, cluster sizes tend to be larger, as this provides negligible co-channel and adjacent channel interference.

As the network matures, capacity becomes increasingly important. Cluster size is decreased while maintaining Signal-to-Interference Ratios (SIRs) that ensure acceptable link quality. To accommodate increased capacity without additional BS sites, omni-directional antennas are replaced by directional ones that partition cells into sectors. Sectorization generally results in an increased SIR, which improves the quality of the radio link.

The minimum acceptable SIR (SIRmin) is system specific. It is believed that a GSM system can be designed with SIRmin of 9 dB, although network designers are advised to use a higher figure. A low SIRmin enables fewer cells per cluster to be used.

2.3 Frequency Reuse

Frequency reuse patterns is a fundamental principle in the design of cellular systems. This is defined as the use of radio channels on the same carrier frequency covering geographically different areas. These areas must be separated from one another by a sufficient distance so that any co-channel or adjacent channel interference that may be encountered does not compromise the radio link quality.

Frequency reuse methods are useful for increasing the efficiency of spectrum usage but can easily result in interference. See Reference [5], Chapter 2 for more information on frequency re-use distances and patterns.

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2.4 Interference

Interference can be defined as an unwanted signal, which is received into the same antenna port as the desired carrier signal and is the major limiting factor in the performance of cellular radio systems.

Sources of interference include another mobile in the same cell, a call in progress in a neighboring cell, another BS operating on the same frequency, or any other non cellular system that leaks energy into the cellular frequency band. Interference on voice channels causes cross talk where the subscriber hears interference in the background due to an undesired transmission. On control channels, interference leads to missed and blocked calls due to errors on the digital signaling. Interference is more severe in urban areas due to the greater RF noise floor and the large number of BSs and mobiles. Interference is a major bottleneck in increasing capacity and is often responsible for dropped calls. Transmitters from competing cellular operators are often a significant source of out of band interference.

The two major interference problems in mobile radio systems are adjacent channel interference and co-channel interference. Other types of interference include intermodulation and intersymbol. These problems will be addressed, but the focus will be on adjacent-channel and co-channel interference.

2.4.1 Inter-modulation interference

This type of interference is generated in any non-linear circuit when the product of two or more signals results into another signal, having a frequency that is equal or almost equal to the required signal. In the transmitter, intermodulation interference usually occurs in the power amplifier, whereas in the receiver, it is produced in the first converter stage.

2.4.2 Inter-symbol Interference

Time dispersion causes inter-symbol interference (ISI) where consecutive symbols interfere with each other making it difficult for the receiver to determine which symbol is the correct one. The interfering symbol is a result of the signal being reflected off an object that is far away eg. mountains, hilly cities, high metal covered buildings etc. See Figure 1 below. ISI can be considered as co-channel interference, however in this case the interferer is a time delayed reflection of the wanted signal. If the time delay is smaller than 15us (ie. 4 bits or approximately 4.4km), the equaliser can solve the problem. The location of the BTS is crucial in combatting ISI and antennas should be pointed away from reflecting objects.

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Figure 1: Time Dispersion

2.4.3 Co-channel Interference

Co-channel interference is a complication that arises in mobile systems using cellular architecture, because channels are used simultaneously in as many cells as possible, with the minimum acceptable separation, in order to increase the reuse efficiency. A base station receiving the required signal from a mobile station within its cell may also receive unwanted signals (interferers) from mobiles within other cell clusters using the same channel.

The determination of the frequency reuse distance, and hence the cell repeat pattern, has a direct influence on the co-channel interference levels ie. a bigger reuse distance implies smaller co-channel interference. However, this also implies a larger quantity of cells per cluster, resulting in smaller reuse efficiency. Consequently, a trade-off between interference and efficiency must be made to provide the required bandwidth at the best possible radio quality.

The performance parameter of special interest is the carrier to co-channel interference (C/I) ratio. The C/I ratio is defined as the ratio of the level of the received required signal to the level of the received unwanted signal. See Figure 2. The C/I ratio is a random variable, affected by the random phenomena such as : Location of mobile Rayleigh fading Log normal shadowing Antenna characteristics Cell site location

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Figure 2: Co-channel interference

The ultimate objective of estimating the C/I ratio is to determine the reuse distance and, the repeat cell pattern. See table below for Ericcson and GSM recs. for C/I for non-hopping and hopping networks.

Non-hopping HoppingGSM recs. >9dB >6dBEricsson >12dB >9dB

Table 1

2.4.4 Adjacent channel Interference

Adjacent channel interference occurrence is due to equipment limitations such as frequency instability, receiver bandwidth, filtering, etc. which results in nearby frequencies leaking into the passband. Adjacent channel interference can be minimised by careful channel assignment and utilising high Q cavity filters.

Figure 3: Adjacent channel interference

The carrier to adjacent C/A ratio is defined as the relation in dB in signal strength between the serving and an adjacent frequency, for example 200 kHz apart. See table below for Ericcson and GSM recs. for C/A for non-hopping networks.

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Non-hoppingGSM recs. > -9dBEricsson > -3dB

Table 2

2.4.4.1 Intra-cell Adjacent Channel Interference

If adjacent channels are used in the same cell site, a situation may arise where a mobile station, transmitting from a short distance to the base station, will strongly interfere with the signal from another mobile, transmitting from a long distance to the same base station, on an adjacent channel. This is known as intracell adjacent channel interference. One possible solution to this problem is to avoid the use of adjacent channels with the same cell.

2.4.4.2 Inter-cell Adjacent Channel Interference

Even in the situation where adjacent channels are not used in the same cell but in adjacent cells, interference may still occur. For instance, consider two mobiles near the cell border, each one transmitting to its own base station through adjacent channels. Each base station receives the required signal and a certain interference level of the unwanted signal.

Adjacent channel interference may occur because of two main factors: Both signals experience attenuation and fading. The fadings of both signals are uncorrelated (the signals travel through different

trajectories) so that the interfering signal may become larger than the unwanted signal.

One possible solution to this problem is to avoid the use of adjacent channels in adjacent cells.

2.4.4.3 Adjacent Channel Interference at the Mobile

The conditions causing inter-cell adjacent channel interference at the mobile are depicted in Figure 4. The mobile using channel may suffer interference from channel +1 and/or channel -1 if they are active in adjacent cells.

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Figure 4: Adjacent Channel Interference at the Mobile

2.4.4.3 Adjacent Channel Interference at the Base Station

The conditions causing inter-cell adjacent channel interference at the base station are depicted in Figure 5. BS B1 receives the wanted signal from mobile unit M1 and interfering signal(s) from M2 and/or M3 that are transmitting on adjacent channels to B2 and B3

respectively. Note that besides the strategic location of M2 and M3, near the cell borders, M1

must be located in a weak signal area of cell1.

Figure 5: Adjacent Channel Interference at the Base Station

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2.5. Techniques to Minimise Interference

The techniques listed below can be used to minimize interference in cellular networks. The application and success of the method used is greatly dependent on the unique situation.

Antenna Diversity Frequency Hopping Power Control Discontinuous Transmission Intracell Handover

2.5.1. Antenna Diversity

This method requires 2 receive antennas at the BS independently receiving the same signal. Antenna diversity is only possible in the uplink path, hence no reduction in frequency re-use distance is possible. However, diversity will improve the mean C/I ratio by exploiting spacial independence of Rayleigh fading. At 900Mhz, it is possible to increase C/I by 3dB with a distance of 56m between receiving antennas.

2.5.2. Frequency Hopping

As with diversity, frequency hopping will also improve the mean C/I performance by exploiting frequency independence of Rayleigh fading. See Figure 6. The thin solid and thin dashed lines indicate receive levels for two different frequencies and the thick solid line shows the smoothing effect when hopping between these two frequencies. Frequency hopping makes fading dips appear shallower. It is effective for both uplink and downlink, but shows more improvement for slow-moving mobiles.

As can be seen from Figure 7, antenna diversity and frequency hopping combined result in a remarkable improvement in C/I performance.

Figure 6: Schematic picture of multipath fading at two different frequencies and at frequency hopping between the two frequencies for a slow moving mobile

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Figure 7: C/I Performance with Antenna Diversity and Frequency Hopping

2.5.3. Intra-cell Handover

Intracell handover is a feature that aims at obtaining a good, even speech quality although temporary interference may be present. When bad quality is measured on either the up or downlink at the same time that the signal strength is high, there is reason to believe that there is severe interference, and that the interference only exists on the serving channel. In this situation, the probability of finding a better channel within the same cell is rather high. With intracell handover, poor quality of signal (as opposed to weak signal) will be detected by the system and may result in a change in carrier frequency or time slot.

2.5.4. Power Control

Power control enables the output power of the mobile or BS to be adjusted according the distance the mobile is away from the BS ie. the nearer the mobile is to the BS, the lower the mobile and BS transmitted power is. This results in the total amount of radiated power in the network being reduced uplink/downlink and co/adj-channel interference in the network is reduced. There is one limitation with power control in that it cannot be used on the BCCH carrier.

2.5.5. Discontinuous Transmission (DTX)

DTX allows the radio transmitter to be switched off during speech pauses. In a normal conversation, this leads to a decrease in transmission time of about 50%. DTX can be used on the uplink and the downlink to reduce interference in the system. DTX also cannot be used on the BCCH carrier.

2.6. Effects on Coverage and Interference by applying power decrease, antenna height decrease and beam tilting

Situations are sometimes encounted where coverage must be reduced to compensate for interference. There are several ways of doing this viz. Reorienting the directional antenna patterns Changing the antenna beam width

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Synthesizing the antenna pattern Decreasing the power Decreasing the antenna height

The latter two are quite effective and will be discussed further.

Power Decrease. As long as the set-up of the antenna configuration at the cell site remains the same, and is the cell site transmitted power is decreased by 3dB, then the reception at the mobile unit is also decreased by 3dB. This is a one-on-one (ie. linear) correspondence and thus easy to control.

Antenna Height Decrease. When antenna height is decreased, the reception power is also decreased. However, the formula

Antenna height gain (or loss) =

is based on the difference between the old and new effective antenna heights and not on the actual antenna heights. Therefore, the effective antenna height is the same as the actual antenna height only when the mobile unit is traveling on flat ground. It is easy to decrease antenna height to control coverage in a flat-terrain area. Decreasing antenna height in a hilly area results in signal-strength contours different from the situation of power decrease. A decrease in antenna height would effect the coverage area. Instead of a almost constant decrease in signal-strength along the border of the coverage (like with power reduction), the coverage area changes to take on completely different signal-strength contours.

(a) (b)

Figure 7: The signal-strength effect as measured by different parameters, (a) different antenna heights, (b)signal-strength changes with power changes.

2.7. Antenna Patterns

The design of cells using different antenna patterns for minimum interference is discussed in detail in Reference [5], Chapter 5. The choice of antenna patterns is important and should be based on the terrain contour, the population and building density, and other

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conditions within a given area. Implementation of antenna tilting or use of an umbrella pattern might be necessary in certain areas to reduce interference. Sidelobe control (i.e., control of secondary lobe information in an antenna radiation pattern) is also very critical in the implementation of a directional antenna.

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3. Procedure Details

Originator Name: Hilton Goodhead

Location: Head Office

Title of Originator: National Manager : Planning and Optimisation

Author Name: Bernard van der Merwe

Location: Head office

Title of Author: Planning and Optimization Engineer

Originator’s Manager: Karel Pienaar

Location: Head office

Title: Group Executive

Quality manager:

Creation Date:

Revision Number: 1.0

Last Saved On: 1 November 1999

Last Saved By: Thaigan Govender

Last Printed On:

Distribution List: Network Group

Authorization:

Designation: Nat. Manager Radio Network Planning & Optimization

Originator’s Manager

Document Originator/Author

DCS Administrator

Name:

Signature:

Date:

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4. Definitions

BS : Base StationCAI : Common air interfaceC/A : Carrier to adjacent channel interference ratio in dBC/I : Carrier to co-channel interference ratio in dBCell Cluster : group of cells within which no frequency reuse is possibleDTX : Discontinuous Transmission - transmission stops while user is not speakingD/R : Frequency Reuse Distance - separation between the mid-points of two co-channel cells, normalized to the cell radiusISI : Inter-symbol interferenceSIR : Signal to interference ratio

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interference in cellular networks

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