# Microwave Radio System Gain

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• Topic: Noise, Decibel, Signal-to-noise ratio
• Pages : 7 (1714 words )
• Published : February 18, 2013

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MICROWAVE RADIO SYSTEMS GAIN

System Gain
* Gs is the difference between the nominal output power of a transmitter (Pt) and the minimum input power to a receiver (Cmin) necessary to achieve satisfactory performance; * Must be greater than or equal to the sum of all gains and losses incurred by a signal as it propagates from a transmitter to a receiver * In essence, system gain represents the net loss of a radio system, which is used to predict the reliability of a system for a given set of system parameters.

* Ironically, system gain is actually a loss.
* Losses are much higher than the gains.
* Therefore, the net system gain always equates to a negative dB value (i.e., a loss) * Because system gain is defined as a net loss
* individual losses are represented with positive dB
* individual gains are represented with negative dB
* Mathematically, system gain in its simplest form is
* Gs = Pt - Cmin

where
Gs = system gain (dB)
Pt = transmitter output power (dBm or dBW)
Cmin = minimum receiver input power necessary to achieve a given reliability and quality objective

* Gs = Pt - Cmin > = losses – gains
* Pt - Cmin >= FM(dB) + Lp(dB) + Lf(dB) + Lb(dB)- At(dB)- Ar(dB) Gains:
* At= transmit antenna gain relative to an isotropic radiator (dB) * Ar = receive antenna gain relative to an isotropic radiator (dB) Losses
* FM = fade margin for a given reliability objective (dB) * Lp = free-space path loss in (dB)
* Lf= transmission line loss in (dB)
* Lf= total coupling or branching loss in (dB)
*
* The reduction in receive signal level;
* Reduction in signal strength at the input to a receiver; * It applies to propagation variables in the physical radio path that affect changes in the path loss between transmit and receive antennas

What Is Fade Margin?
* Considers the non-ideal and less predictable characteristics of radiowave propagation, such as multipath propagation and terrain sensitivity; these characteristics cause temporary, abnormal atmospheric conditions * Under interference-free conditions, the fade margin is defined as the difference between the received signal level under ”normal” wave propagation conditions (fade-free time) and the receiver’s threshold level at a given bit-error level * The fade margin in the absence of frequency selective fading within the bandwidth of the receiver

Types of Fade Margin
* Flat Fade Margin
* Interference Fade Margin
* Dispersive Fade Margin
* Composite Fade Margin

* In flat fading, the coherence bandwidth of the channel is larger than the bandwidth of the signal. Therefore, all frequency components of the signal will experience the same magnitude of fading.

* Based on congestion of systems within the path using the same band of frequencies. Taken from graphs from a specific location and varies over time.