Radio receiver sensitivity determines the input level of the weak radio signal that can be processed by a radio receiver
to meet performance requirements. For analog receivers performance measure is given in terms of required SNR, usually specified at the input to the detection circuit. For digital systems performance measure is given in terms of BER or other parameters that can be derived from BER requirement, depending on the type of radio service. Detection algorithm includes coding, interleaving, equalization, diversity and other signal processing functions used to improve radio reception.
Receiver sensitivity is usually expressed by minimum detectable signal level for a required SNR, which includes the knowledge of the system bandwidth and is defined as
RS[dBm] = — 174dBm + 10log(NF [dB]) + 10log(BW [Hz]) + SNR[dB]
where NF is the overall system noise figure referred to the input of the receiver and BW is the system noise bandwidth. For digital systems system requirement is commonly given in terms of energy per bit to noise density ratio Eb/No. Relation between Eb/No and SNR is given by
Eb/No = SNR BW T
where T is the bit duration (23). Depending on the particular radio system and targeted operating environment, performance measure ofthe system (SNR or BER) can be evaluated under fading conditions or under static conditions.
In the first case fading margin is derived from the statistics of the large-scale signal variation such as log-normal fading. Small-scale signal variations are accounted for implicitly during BER evaluation. When BER of the system is specified under static conditions, fading margin is derived from combined statistics of both large-scale and small-scale variation. Fading margin effectively defines the increase in SNR required to preserve the same BER as in the static case.
Radio sensitivity specifies the lowest detectable signal level at the radio input; however, both desired and unwanted signals may be present at the radio receiver input at high levels, and it is important to specify the performance for undesired response rejection. The unwanted strong signal may reduce a nearby weak (desired) signal resulting in radio desensitization. Desensitization level is specified for a 1 dB weak signal reduction and is a consequence of radio front-end saturation.
Another factor that may severely limit the performance of a radio receiver is intermodulation distortion, where unwanted strong signals produce a number of intermodulation products due to receiver front-end nonlinearities.
IMD products, particularly third-order products, may fall directly in the bandwidth of the desired signal. To limit the level of the IMD products receiver is required to have high intercept point of the particular order.
Dynamic range (DR) of the receiver defines the range of input signals that a radio receiver can process. Different considerations may be taken into account depending on the radio service; however, the most common spurious-free dynamic range (SFDR) is defined as
SFDR[dB] = — (IIP 3[dBm] — RS[dBm])
where IIP3 is the third-order input intercept point.