Microwave Radio System Gain

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  • Topic: Noise, Decibel, Signal-to-noise ratio
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  • Published : February 18, 2013
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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

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)
(Link Margin)
* 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

Flat 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.

Interference Fade Margin
* 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.

Dispersive Fade Margin
* Dependent on the type of equipment and modulation used. * These are gains in the equipment which are factored in because of technical improvements on the system and how they improve the information signal itself * It is defined to be the fade depth exceeded for the same number of seconds at a threshold error rate (the threshold error rate is defined to residue at the value of interest for which the dispersion signatures were created).

Dispersive Fade Margin
* DFM is calculated based on the W-curves using computation * DFM = 17.6 – log10 (Sw/158.4)

Composite Fade Margin
* This is the total of all fade margins

RECEIVER THRESHOLD (Receiver Sensitivity)
What is Receiver Threshold?
* Receiver threshold means the lowest signal your receiver will pick up and still operate. When nearing threshold, radio will sound noisy with static, TV will show...
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