Calculate SDR Sensitivity, Link Budget and Maximum Allowable Path Loss

Updated Oct 8, 2021



  • USRP-2954
  • USRP Software Defined Radio Device
  • PXI Vector Signal Transceiver

In RF/Wireless system design, it is required to calculate the System Link Budget between transmitter and receiver which requires to know transmitter power and receiver sensitivity. For Software Defined Radio such as NI-USRP and NI VST (Vector Signal Transceivers), the RF parameters are dynamically defined according to user requirements which are used to define the receiver sensitivity. 

For RF/wireless system to operate stable, the RF propagation path loss must be lower than the Maximum Allowable Path Loss, by the amount of system link margin. Maximum Allowable Path Loss (MAPL) will define the distance between transmitter and receiver, the bigger the MAPL, the longer the distance which is related the RF propagation environment. RF propagation discussion is out of this article scope.

This article discusses how to calculate USRP sensitivity and the system link budget between its transmitter and receiver, and examples of calculating the System Link Budget and Maximum Allowable Path Loss of the USRP system. USRP-2954 is taken as an example for calculations. The method discussed in this article is valid for other RF systems such as NI VST. The intend audiences are NI USRP and VST users who have basic knowledge of RF transmitter/receiver specifications and RF propagation, such as transmitter power, receiver sensitivity, transmitter-receiver system link budget, noise figure, propagation path loss, antenna gain. 

Calculate USRP Sensitivity

  1. The sensitivity is calculated with formula Sev = N_floor  + NF + 10xlog(BW) + SNR, N_floor  = kTB = -174dBm,
N_floor  is the system noise power in 1 Hz,  in room temperature, we call it noise floor, 
k is Boltzmann's constant, k 
= 1.38 × 10-23J K-1
T is the absolute temperature of the receiver also known as noise temperature, consider it equals to 290K at room temperature of 17C
B is the bandwidth of 1 Hz
  1. Find USRP-2954 Noise Figure NF = 6dB, and consider BW = 1MHz, SNR = 5dB for 64QAM modulation with 5% BER, 
  2. Calculate the sensitivity using the formula given in step 1. Sev = -174 + 6 + 10xlog(1,000,000) + 5= -103dBm

Calculate USRP System Link Budget

  1. Find USRP-2954 transmitter power from data sheet: 17dBm 
  2. USRP-2954 receiver sensitivity is calculated above: -103dBm
  3. Calculated system link budget with formula S_LB = Tx_pwr - Sev                          
          S_LB = 17 - (-103) = 120dB
S_LB is the System Link Budget
Tx_pwr is the transmitter power
Sev is the receiver sensitivity


Calculate Maximum Allowable Path Loss - MAPL 

  1. Define the link margin according to system design requirements, consider 18 dB 
  2. Get the system link budget calculated above: 120dB
  3. Get Transmitter antenna gain, consider 12dB 
  4. Get the receiver antenna gain, consider 8dB
  5. Calculated maximum allowable path loss (MAPL) with formula MAPL = S_LB + G_tx + G_rx - LKM
         MAPL  = 120 + 12 + 8 - 18 = 122dB

G_tx: transmitter antenna gain,
G_rx: receiver antenna gain, 
S_LB: system link budget, consider 120dB as shown above
LKM: link margin

Here we summarize the calculated results in below table, high lighted row is the summary of the results discussed in above steps. 

Calculate RF system sensitivity and link budget.png