Spread Spectrum Interference

1WRAP 0975C

Spread Spectrum Interference in WRAP

• DS: Direct Sequence systems. Resulting S/I after de-spreading:

S/Idespread = S/Ispread + Processing Gain

• FH: Frequency hopping systems. Interference is expressed as the percentage of time (“hops”)

Pint[%] = Probability that S/I < Required S/I

• Hybrid DS/FH systems. Interference is expressed as for FH, with calculations accounting for the DS processing gain.

2WRAP 0975C

Parameters• Direct Sequence: Processing gain [dB] given as

a receiver parameter• Frequency hopping: Type of frequency

utilisation– Table-defined: Table of frequencies– Band-defined: Table of band limits, frequency separation– Duty cycle

• Common for all systems:– Transmitter spectrum mask, receiver selectivity– Intermodulation, harmonics, IF, image suppression– Receiver sensitivity, required S/I ratio, transmitter EIRP– Antenna gain and horizontal/vertical radiation pattern– Deployment geometry

3WRAP 0975C

Equipment: Receivers

4WRAP 0975C

Equipment: Transmitters

5WRAP 0975C

FH Interference• Table-defined/Table-defined frequency hop

set– Determine minimum frequency separation Δfint for non-

interference (by iteration)– Determine number of frequencies in the hop set with

less than Δfint

– Perform for each receiver frequency. Add all numbers of interfered frequencies, divide with total number of frequencies, multiply with transmitter duty cycle

– Account for all transmitters that may interfere.

6WRAP 0975C

Table-defined, details

7WRAP 0975C

TXm

TXmmRXfk n

ndP intint 100

RXn

kRXfk

RXTXm P

nP

1intint

1

FF/BH Interference

• For Fixed frequency/Band-hopping cases– For TX1, sub-band 1, determine Δfint

– Determine number of frequencies in sub-band 1 with less than Δfint

– Continue with sub-band 2, 3 etc– Add numbers of interfered frequencies from each sub-

band, divide with total number of frequencies, multiply with transmitter duty cycle

– Form combined interference from all transmitters:

8WRAP 0975C

PintTot = 1 – (1 – PintTX1)(1 – PintTX2)…(1 – PintTXn)

TH/BH Interference

• For Table-hopping/Band-hopping cases– As for the Fixed-frequency/Band-hopping case,

calculated for all frequencies in the table– Determine interference from TX no. M (n is the number

of frequencies in the table) as

9WRAP 0975C

n

kRXfk

mTXm P

n

dP

1intint

1

100

PintTot = 1 – (1 – PintTX1)(1 – PintTX2)…(1 – PintTXn)

BH/BH Interference

• For Band-hopping/Band-hopping cases– As for the Table-hopping/Band-hopping case, by re-

defining the hop bands to table-defined

10WRAP 0975C

Considered cases

• In the Interference tool:– Only co-channel/adjacent channels interference

• In the Collocation Interference tool:

11WRAP 0975C

TX RX Harmonics IF Blocking Intermod Image Co/Adj

FH FF Yes Yes Yes No Yes Yes

FH FH No Yes Yes No No Yes

FF FH No Yes Yes No No Yes

FF FF Yes Yes Yes Yes Yes Yes

Result presentation [%]

• In the Interference tool:– Total interference PintTot

– Individual contribution Pint from each transmitter

• In the Collocation Interference tool:– Total interference PintTot

– Individual contribution Pint from each transmitter

– Individual contribution Pharmonics etc from each transmitter

12WRAP 0975C

Result interpretation

• PintTot gives the percentage of time that the interference is higher than the receiver can handle without sensitivity reduction

• The approximate average number of interference pulses per second can be calculated as

Where

IRate: Interference pulses per second

fhr: Hop rate, hops/s (same rate for all transmitters/receivers)

d: Duty cycle of transmitter [%]

13WRAP 0975C

d

PfI Tot

hrRateint