Noise Figure, Noise Noise Figure, Noise Factor and SensitivityFactor and Sensitivity

Wireless Systems Instructional Wireless Systems Instructional Design Design

NoiseNoise ““Any unwanted input”Any unwanted input” Limits systems ability to process weak signalsLimits systems ability to process weak signals Sources:Sources:

1.1. Random noise in resistors and transistorsRandom noise in resistors and transistors

2.2. Mixer noiseMixer noise

3.3. Undesired cross-coupling noiseUndesired cross-coupling noise

4.4. Power supply noisePower supply noise

Dynamic range – capability of detecting weak Dynamic range – capability of detecting weak signals in presence of large-amplitude signalssignals in presence of large-amplitude signals

Noise (contd.)Noise (contd.)

powernoiseaverage

powersignalaverage

fN

fSSNR

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““noisiness” of the signal measure = signal-to-noisiness” of the signal measure = signal-to-noise ratio (frequency dependant)noise ratio (frequency dependant)

Random noise Random noise ExternalExternal

• AtmosphericAtmospheric• InterstellarInterstellar

Receiver internalReceiver internal• ThermalThermal• Flicker noise (low frequency)Flicker noise (low frequency)• Shot noiseShot noise

““Sky” NoiseSky” Noise

Noise factorNoise factor

IEEE Standards: IEEE Standards: “The noise factor, at a “The noise factor, at a specified input frequency, is defined as the specified input frequency, is defined as the ratio of (1) the total noise power per unit ratio of (1) the total noise power per unit bandwidth available at the output port when bandwidth available at the output port when noise temperature of the input termination is noise temperature of the input termination is standard (290 K) to (2) that portion of (1) standard (290 K) to (2) that portion of (1) engendered at the input frequency by the input engendered at the input frequency by the input termination.”termination.”

sourcetoduenoiseoutputavailable

powernoiseoutputavailableF

Noise factor (cont.)Noise factor (cont.)

It is a measure of the degradation of SNR It is a measure of the degradation of SNR due to the noise added -due to the noise added -

Implies that SNR gets worse as we Implies that SNR gets worse as we process the signalprocess the signal

Spot noise factorSpot noise factor The answer is the bandwidthThe answer is the bandwidth

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Noise factor (cont.)Noise factor (cont.)

Quantitative measure of receiver Quantitative measure of receiver performance wrt noise for a given performance wrt noise for a given bandwidthbandwidth

Noise figureNoise figure Typically 8-10 db for modern receiversTypically 8-10 db for modern receivers

Multistage (cascaded) systemMultistage (cascaded) system

)log(10 FNF

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SensitivitySensitivity

Minimum detectable input signal level for a Minimum detectable input signal level for a given output SNR (also called given output SNR (also called noise floornoise floor) )

Not necessarily related to required output Not necessarily related to required output SNRSNR

ExampleExample

Link BudgetLink Budget

““Quick and dirty” way of estimating RF link Quick and dirty” way of estimating RF link performanceperformance

Prx,Ptx – received and transmitted power (dB)Prx,Ptx – received and transmitted power (dB) Grx,Gtx – antenna gain (dBi)Grx,Gtx – antenna gain (dBi) L – path lossL – path loss Amisc – miscellaneous attenuation Amisc – miscellaneous attenuation

miscrxtxtxrx A - L- G G P P

Link Budget (cont.)Link Budget (cont.)

Path loss (Friis formula):Path loss (Friis formula):

L = 40 dB + 20log(d) @ 2.4 GHzL = 40 dB + 20log(d) @ 2.4 GHz L = 48 dB + 20log(d) @ 5.7 GHzL = 48 dB + 20log(d) @ 5.7 GHz

Transmit power:Transmit power: 15 – 20 dBm (30 – 100 mW)15 – 20 dBm (30 – 100 mW)

Antenna gain: given in decibels over an isotropic Antenna gain: given in decibels over an isotropic antenna (dBi)antenna (dBi)

0dBi (isotropic), 8 dBi (biquad), 15 dBi (helix), 24 dBi (parabolic)0dBi (isotropic), 8 dBi (biquad), 15 dBi (helix), 24 dBi (parabolic)

20log(d)20log(F) 92.45 L(dB)

Link Budget (cont.)Link Budget (cont.)

Received power (senitivity)Received power (senitivity)

OrinoccoOrinocco Aironet Aironet 350350 SNRSNR

11Mbps11Mbps -82 dBm-82 dBm -85 dBm-85 dBm 16 dB 16 dB

5.5 Mbps5.5 Mbps -87 dBm-87 dBm -89 dBm-89 dBm 11 dB 11 dB

2 Mbps2 Mbps -91 dBm-91 dBm -91 dBm-91 dBm 7 dB 7 dB

1 Mbps1 Mbps -94 dBm-94 dBm -94 dBm-94 dBm 4 dB 4 dB

Link Budget (cont.)Link Budget (cont.)

Amisc:Amisc: Cables (@ 2.4 GHz)Cables (@ 2.4 GHz)

• RG 174: 2 [dB/m] RG 174: 2 [dB/m] • RG 58: 1 [dB/m]RG 58: 1 [dB/m]• RG 213: 0.6 [dB/m]RG 213: 0.6 [dB/m]• IEEE 802.3: 0.3 [dB/m] IEEE 802.3: 0.3 [dB/m] • LMR-400: 0.22 [dB/m] LMR-400: 0.22 [dB/m]

Connectors (BNC, N, SMA)Connectors (BNC, N, SMA)• 0.1 – 1 dB loss0.1 – 1 dB loss

ProjectProject

Link characterization with Network Link characterization with Network Analyzer Analyzer