ADJACENT BAND COMPATIBILITY BETWEEN GSM AND CDMA

Presented By

Sheikh Danish Mazhar Assistant Director (Monitoring) Frequency Allocation Board

Main Points

Introduction Methodologies to Determine Interference Case Study Interference Scenarios Mitigation Factors Example of Pakistan Conclusions

Introduction

GSM:

GSM (Global System for Mobile Communications) is the most popular 2G standard for mobile phones. GSM-900 uses 890–915 MHz to send information from the mobile station to the base station (uplink) and 935–960 MHz for the other direction (downlink).

GSM-1800 uses 1710–1785 MHz to send information from the mobile station to the base transceiver station (uplink) and 1805–1880 MHz for the other direction (downlink).

Introduction (Contd.)

CDMA:

CDMA (Code Division Multiple Access) is a mobile digital radio technology. CDMAOne (IS95A, JSTD008, IS95Band) and CDMA2000 are the wireless standards proposed by 3GPP2. CDMA2000 is considered a 2.5G technology.

CDMA2000 is the technology used in WLL (Wireless Local Loop) systems deployed in Pakistan. Frequency bands (450 MHz, 479 MHz, 900 MHz,1900 MHz,3.5 GHz) are allocated for WLL services in different regions throughout pakistan.

What is Adjacent Band Compatibility

Adjacent band compatibility means the operation of two technologies belonging to different standards in adjacent bands, such that they don’t interfere with each other.

Identification of Potential Interference Scenarios:

a) CDMA2000 downlink adjacent to the GSM uplink in 880 – 915

MHz.

b) CDMA2000 uplink in 1850 - 1910 MHz and the GSM downlink in 1805 - 1880 MHz

Methodologies to Determine Interference

For evaluating interference between GSM and CDMA in adjacent bands, two methodologies are used as mentioned below:

1. Minimum Coupling Loss 2. Monte Carlo Theory

1. The Minimum Coupling Loss (MCL) method calculates the isolation required between interferer and victim to ensure that there is no interference.

Methodologies to Determine Interference

The method is simple to use and does not require a computer for implementation. The primary drawback is that it is a worst case analysis and produces a spectrally inefficient result.

2. Monte Carlo theory is a statistical technique based upon the consideration of many independent instants in time and locations in space. For each simulation trial, a scenario is built up using a number of different random variables i.e. where the interferers are with respect to the victim, how strong the victim's wanted signal strength is, which channels the victim and interferer are using etc.

Case Study (Deployment of CDMA in 900 MHz Band)

The case study examines the interference that can impact GSM Base station (BS) receiver below 915 MHz when a CDMA system is deployed with its downlink band 917 – 921 MHz adjacent to the GSM uplink band 890 – 915 MHz.

A minimum frequency separation of 2.15 MHz is assumed between the two bands in order to reduce the amount of interference from a CDMA BS leaking into the GSM.

Case Study (Deployment of CDMA in 900 MHz Band)

GSM Uplink CDMA Downlink

921MHz914.9 + 2. 15 MHz (Transition Band)

880 MHz 914.9

Figure 1: GSM and CDMA systems with frequency separation around the duplex transition frequency at 915 MHz

Case Study (Deployment of CDMA in 900 MHz Band)

A link budget methodology approach is used for the scenario where CDMA is the interferer and GSM is the victim.

The study focuses on the deployment of up to three CDMA carriers with centre frequencies 917.675 , 918.925, and 920.175 MHz each of 1.25 MHz bandwidth.

Case Study (Deployment of CDMA in 900 MHz Band)

The possibility of deployment of a fourth carrier is not considered here because the noise from the CDMA transmitter increases to a much higher level at a frequency separation below 800 kHz between CDMA and GSM bands.

For the GSM system, RF requirements from the 3GPP specifications 3GPP TS 45.005 are applied, in order to make the results less dependent on specific GSM BS implementation choices and/or operator specific deployment conditions.

Interference Scenarios

The following interference scenarios are possible due to adjacent band deployment:

1. Blocking will occur where the incoming power from the CDMA transmitter is above the specified GSM blocking level -110 dBm, this will desensitize the GSM BS receiver.

2. The Unwanted Spurious Emission and Wide Band Noise from the CDMA transmitter that is above the receiver sensitivity of -104 dBm will desensitize the GSM BS receiver such that low level signals may not be received.

Interference Scenarios (Contd.)

3. Desensitization of the GSM BS receiver because of 3rd order Intermodulation Distortion (IMD3) will occur if two or more RF signals exceed the specified levels and if the mixed frequencies contain a frequency component at the GSM BS receive frequency.

a. IMD3 due to mixing of two received CDMA carriers operating in the adjacent frequency range.

b. IMD3 due to a single CDMA carrier in immediate adjacency to the GSM uplink band.

Interference Scenarios (Contd.)

IMD3 product

913.300 MHz

Downlink band 925 – 960 MHz

Transition Frequency 915 MHz

1 2 3

CDMA Carriers

917.675 MHZ

920.175 MHz

Uplink band 870 - 915 MHz

Figure 2: Depiction of IMD3 case (a)

MCL Calculations

Tx. Power (Watts)

Losses (dB)

Tx. Ant. Gain (dB)

Distance (m)

Free Space Propagation

(dB)

25 3 15 20 57.67

GSM Rx. Antenna Gain

(dB)

Feeders etc. (dBm)

Interference Power (dBm)

Required Attenuation

(dB)

15 3 -16 26.31

Mitigation Factors

Following are the different techniques that would enable CDMA to operate without producing harmful interference into the GSM BS receivers:

1. Physical Separation Distance2. Frequency Separation3. Filters4. Frequency planning and Coordination

Mitigation Factors

1. Physical Separation Distance:

The use of physical separation is a typical way of achieving the majority of the necessary attenuation. It is also one of the most cost effective means of establishing the required coupling loss between the CDMA BS transmitter and the GSM BS receiver. Physical separation may be feasible in rural and suburban areas. But in urban areas its not easy to find sites/locations fulfilling this criteria.

Mitigation Factors (Contd.)

2. Frequency Separation:

Use of frequency separation as a single solution to achieve the necessary attenuation of both the power (blocking) and wide band noise from CDMA requires a frequency separation extending outside the allocated band. This is because the wide band noise roll-off of CDMA is fairly slow.

The difficulties in network planning and especially re-planning for optimisation of the network, make frequency separation a very unattractive solution.

Mitigation Factors (Contd.)

3. Filters:

The performance of both the CDMA transmitter and the GSM receiver can be improved using filters. But to allow the filters to operate, a guard band is considered necessary.

It is important to use a filter with a low insertion loss in the GSM BS receiver range to limit any desensitization due to the filter.

Mitigation Factors (Contd.)

Where it is impossible to establish sufficient physical separation to eliminate blocking and desensitization by wide band noise of the GSM BS receiver, additional filters could be used, subject to evaluation of their impact on the receiver’s performance.

4. Frequency Planning and Coordination:

It is necessary that frequency band used for CDMA is coordinated between the GSM and CDMA operators when they are operating in adjacent bands, also taking into account any necessary transition band.

FIGURES IN MHz

880 915

895.1 895.7 905.3889.9 900.5885.1880

SCO

(WLL)

PAKTEL SCO(GSM

)

UFONE WARID TELENOR

MOBILINK

910.1 914.9

925 960

940.1 940.7 950.3934.9 945.5930.1

PAKTEL UFONE WARID TELENOR

MOBILINK

955.1 959.9

SCO(GSM

)835 840

SCO

(WLL)

GSM 900

Example of Pakistan

Frequency Allocation Plan For AJK & Northern Areas

Example of Pakistan

Conclusions

To avoid undue risk of interference from CDMA system deployed in adjacent band with GSM, mitigation factors mentioned should be implemented.

Coordination between both the GSM and CDMA operators in this regard is very important.

Thank you