Link Budget

ECE-T512 – Wireless Systems Link Budget/Receiver Sensitivity

Receiver Sensitivity/Link Budget • Receiver Sensitivity refers to the ability of the receiver to detect radio signals. • Receivers must detect radio signals in the presence of noise – External: Atmospheric (lightning), cosmic, man-made (e.g. automobile ignition) – Internal: Thermal noise

• Ratio of desired signal power to thermal noise power before detection is carrier to noise ratio,


Receiver Sensitivity/Link Budget •
is a function of link parameters such as: – Transmitter power – Path loss – Antenna gains – Effective input-noise temperature of receiver

• Link Budget - Formula relating above parameters to


Link Budget (in Watts – linear scale) Pt Gt Gr Pr = Lrx L path N = kT0 Bw F Pr Carrier to
= Noise Ratio N Ec Bw Modulated Symbol Signal =
N0 Rc to Noise Ratio  Ec S rx = Lrx kT0 FRc   N0

   min

Pt/r = transmit/receive power Gt/r = transmit/receive antenna gains Lp = Path loss Lrx = receiver losses N = input noise k = Boltzman’s constant T0 = noise temperature Bw = noise bandwidth F = noise figure – degradation of SNR from input to output (Typical 5-6 dB)
= carrier to noise ratio Ec = energy per modulated signal N0 = PSD of white noise Rc = modulated symbol rate Srx = receiver sensitivity

Maximum Allowable Path Loss Ec Pt G t G r = N 0 kT0 R c FL rx L path


Ec   Srx = L rx kT0 FR c   N 0  min

Using logarithmic scale, in dB:

 Ec     N0 

=P + +




t ( dB) G t ( dB) G r ( dB) kT0 ( dBm / Hz ) R c ( dBHz ) F( dB) L rx ( dB) L path ( dB)

dB

Srx

( dB )

=L

 Ec    N0 

+ + + + rx ( dB) kT0 ( dBm / Hz ) R c ( dBHz ) F( dB)  

dB

Typical design process: 1. Find minimum Ec/N0 – From knowledge of modulation and coding 2. Solve for Srx and then solve for Lpath to find maximum allowable path loss:

L path,max

Can be translated to distance =P +G +G
S t ( dB) t ( dB) r ( dB) rx ( dBm ) to find maximum cell radius ( dB )

Additional Link Budget Parameters • Margin for system loading -- Interference Loading/Margin • Shadow margin • Handoff gain First two factors reduce maximum allowable path loss, LMAX, but the third increases it

Reducing LMAX corresponds to decreasing design specification of cell range Current discussion is around relative link budget adjustments (for comparing different system concepts) Absolute maximum allowable path loss requires other factors specific to the system (coding techniques, use of arrays, etc.)

Interference Loading / Margin • Cell breathing - Cell size fluctuation caused by varying subscriber load causing different levels of co-channel interference and adjacent channel interference • Interference Margin introduced into link budget to account for system degradation due to high traffic --- otherwise coverage will be increasingly poor near planned cell boundaries • Near/far effect - power control • If CCI and ACI are assumed to be white noise (accurate for these purposes) the net effect of cell breathing effects will be to increase total noise power by multiplicative factor of LI (Interference Margin) •To account for Interference Loading, we reduce LMAX by LI (both in dB scale) •Required LI is a function of the ability of the system to deal with interference • In general LI is greater for CDMA (more interference limited) than TDMA (where overlapping BW not used) • Recall: Reducing LMAX corresponds to decreasing design specification of cell range

Shadow Margin • To maintain an acceptable outage probability in the presence of shadowing effects, we introduce shadow margin • Quality of radio link is acceptable only when received signal power is greater than a threshold • Edge outage probability - Probability that received power is less than threshold at cell edge

• Area outage probability - Probability that received power is less than threshold power averaged over entire cell area

• MSHAD is the value reducing maximum allowable path loss (decreasing cell design range) to maintain acceptable outage probability when shadowing effects are taken into account

Handoff Gain • Macrodiversity concept - Link to serving basestation may be shadowed such that the power from the serving basestation is below a critical threshold --however, the link to an adjacent basestation may be above threshold

• Handoff reduces required shadow margin (of a single cell) by Handoff Gain - GHO • Hard handoff - Different radio channels assigned by a neighboring basestation to takeover handling of mobile user call (TDMA systems) • Soft handoff - Mobile switching center selects between instantaneous received signals from a variety of basestations (CDMA systems)