NSMA Conference

Interference Temperature Round Table

May 18, 2004

Les WildingCingular Wireless5565 Glenridge ConnectorAtlanta, GA 30342

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Topics for Discussion

1. Interference Temperature as it Applies to Point-to- Point Microwave Links.

2. Interference Temperature as it Applies to CMRS Networks.

3. Summary

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Interference Temperature and Point-to-Point MW

In the NPRM section the Commission discusses applying the interference temperature concepts to both the FSS and FS services.

Band Segments selected by the Commission for deployment of unlicensed devices are:

•FSS Band Segment 12.75 - 13.25 GHz (excluding 13.15-13.2125 GHz)

•F S Band Segment 6525 MHz to 6700 MHz

Note: In the FS category the Commission selected the lower half of the full (6525-6875 GHz) upper 6 GHz band.

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Interference Temperature and the Digital MW LinkCommission Assumption 1: 126 dB margin

When computing interference, the interfering path (path C to B) must be below the interference objective for the victim microwave link (path A to B).

Discriminationangle for Ant B

Victim Microwave Link

Interfering path

Unlicensed Device

Tx A Rx B

Tx C

Figure 1

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Interference Temperature and Point-to-Point MW

Coordination objectives for interference are based on published T/I curves for the victim receiver and the interfering carrier. The Interference objective for coordinating a typical MW link is:

I (Coordination) = – 75.5 dBm – 34 dB – 5 dB = – 114.5 dBm

Where: 75.5 dBm = the Static BER Threshold of 10-6

34 dBm = the Specific T/I threshold for the victim and interfering bandwidths and frequency offsets.

5 dBm = Multiple Exposure Allowance

The unfaded C/I (Coordination) for a digital link is: {-40 dBm- (-114.5 dBm)} = 74.5 dB

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Interference Temperature and the Digital MW Link

The NPRM describes an unlicensed device that is transmitting on 6600 MHz with an output of -41.25 dBm/MHz and operating 100 meters from a FS receiver with a discrimination angle of 20 degrees.

The formula for calculating the operating margin is;EIRP (A) - Path loss (A to B) - C/I (Coordination) =>

EIRP (C) - Path loss (C to B) + Disc Loss (B to C)

Using the coordination data for the MW path and correcting the EIRP of the unlicensed device for 3.75 MHz bandwidth yields: 64 dBm-140 dB-74 dB => -35.5 dBm -93 dB - 43 dB which = -150 dBm > -171.5 dBm : = 21.5 dB marginThis is the margin for ALL interferers not the 126 dB in the NPRM

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Interference Temperature and the Digital MW LinkCommission Assumption 2: Use of Transmit Power Control TPC

Use of Dynamic Freq. Selection DFS

Interfering pathDiscriminationangle for Ant B

Victim Microwave Link

UnlicensedDevice

Tx/Rx A Rx/Tx B

Tx/Rx C

Tx 6525-6700 MHzRx 6700-6875 MHz

Rx 6525-6700 MHzTx 6700-6875 MHz

Interfering pathfor Ant C

Figure 2

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Interference Temperature and the Digital MW Link

•For this to work, the TPC and DFS detectors in unlicensed device would have to hear the MW signal from site A and make decisions about what is happening at site B in order to set its transmit level.

•The propagation path (A to C) is primarily along the ground and not subject to atmospheric disturbances. Whereas the FS link (A to B) is subject to atmospheric disturbances.

•This could lead to a situation where there is a lot of fading activity on path (A to B) that is not seen by the ground based unlicensed receiver thus causing false decisions and cases of interference into site B.

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•CMRS networks already manage interference in real timeTransmitter Power ControlFrequency hopping & modulation coding techniques

•CMRS operators have greatly reduced self-interference Improved capacity and higher data throughput Increased cell coverage uses total radio channel sensitivity

•Unlicensed Devices are incompatible with CMRS networksCMRS operator can not control external interferenceExternal interference raises noise floor,

Lowers system capacityLowers system/customer data throughputIncreases mobile transmit power/lowers battery lifeLowers quality of service

Interference Temperature and the CMRS Network

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Interference Temperature

Summary:•Interference Temperature is a CONCEPT

It needs to be adequately defined and quantified before it has any value in spectrum management.

•SDR and Cognative Radios are in their infancy and are not ready to take on the tasks envisioned by the Commission.

•Most users of licensed spectrum make use of the total range of receive levels down to and including the thermal noise floor of their communications systems.

•FS and FSS licensed services can not tolerate interference levels greater than the currently authorized Part 15 levels.

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Interference Temperature

In short, I commend the Commission for thinking outside the box

BUT

the Commission has put the licensees feet into the water before the ARK has been built to protect the licensed users from drowning in a sea of interference.

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Interference Temperature and Point-to-Point MWThe Digital Microwave Link

Digital microwave paths are designed to meet a desired performance measured in:

•Error free seconds. •Unavailability in seconds per year.

The components that make up this performance are:•Its composite fade margin (CFM). •Its threshold to interference (T/I) margin.

The link’s CFM is primarily the sum of the flat (thermal) and dispersive fade margins.

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Interference Temperature and Point-to-Point MWThe Digital Microwave Link

Figure 1

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Interference Temperature and the Digital MW LinkCommission Assumption 1. 126 dB of margin and

S/I Requirement of 30 to 50 dB

Equipment used in a typical MW linkThe NEC 2600 series MW is typical of the radios used in this band

Transmitter output power + 28.5 dBm maxModulation = 128 QAMModulation bandwidth = 3.75 MHzReceiver threshold BER of 10-6 = -75.5 dBmT/I Co-channel = 34 dBT/I adjacent channel = -5 dBReceiver outage threshold BER of 10-3 = -78.5 dBm (assumed to be 3 dB worse than the static threshold)

The Andrew PAR6-65 antenna is a typical Category A antenna

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Interference Temperature and the Digital MW LinkCommission Assumption 1. 126 dB of margin and

S/I Requirement of 30 to 50 dB

Coordination of a typical MW linkMicrowave transmitter output power: + 28.5 dBmTypical losses on the transmit side: - 3.0 dBTypical gain for a 6 foot MW dish: + 38.8 dBi

Typical EIRP: = + 64.3 dBm

The unfaded receive signal level for a typical 40 km path is:+ 64.3 dBm -140 dB (path loss) +38.8 dBi - 3 dB (feeder) = -39.9 dBm

In the digital world, EIRP’s range from 60 dBm to 75 dBm based on a typical 0.5 watt to 2 watt transmitter and a 6 to 8 foot diameter standard performance dish.

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Interference Temperature and the Digital MW Link

Maximum Interferer Power vs MW Ant Height above Ground

0.00

1000.00

2000.00

3000.00

4000.00

5000.00

6000.00

7000.00

8000.00

0.1 0.5 0.9 3 5 7

Distance from Tower (km)

Max

imu

m I

nte

rfer

er P

ow

er

(Mic

row

atts

) MW ant @ 30 MetersMW ant @ 50 MetersMW ant @ 70 MetersAveraged levels

Figure 3

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The FCC’s Definition of Interference Temperature

Originally proposed by the Spectrum Policy Task Force (SPTF) as an “Interference Management Concept” to:

•Establish a maximum level of interference that can be tolerated by a receiver but ONLY after a systematic and thorough study of the existing RF environment.

•Establish a clear definition of the spectrum users Rights and Responsibilities

•Establish a clear quantifiable definition of Harmful Interference

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The FCC’s Definition of Interference Temperature

There is no basis for the term or a definition of “Interference Temperature” in any of the Standards documents. The Commission used the cookbook approach by taking parts of the definitions of Antenna Temperature and System Noise Temperature to cook up their Interference Temperature dish.

The FCC’s Interference Temperature Metric as defined in ET Document 03-237 is:

A measure of the RF power generated by undesired emitters plus noise sources that are present in a receiver system (I+N) per unit of bandwidth. More specifically, it is the temperature equivalent of this power measured in units of Kelvin.

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The FCC’s Concept of Interference ControlThe Commission would have you believe that a licensee would be agreeable to accepting some guaranteed maximum level of interference (Interference Temperature Limit)

To accomplish this the FCC is proposing to employ one or more of the following techniques:

•Use of Software Defined Radios to select frequencies and/or modulation modes that would avoid interference. •Use of Cognitive Radio technologies (smart radios).•Deployment of remote monitoring receivers to sense the RF environment.•Incorporation of RF interference monitoring technology into the licensed receiver to detect the presence of interfering signal levels.

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The FCC’s Concept of Interference Control

•Deployment of a grid of monitoring stations that would sense the interference temperature and broadcast this data to all unlicensed devices.

Problems with the monitoring scenarios •These approaches, with the exception of equipping the licensed receiver with a monitor, all fail to accurately identify the interference conditions as seen by the victim licensed receiver.

•They all require spectrum for communications with/between the monitoring device(s) and the unlicensed devices.