The relation between modulation, capacity, and S/N ratio in microwave links can be explained as follows:
• Modulation is the process of changing the characteristics of a carrier wave (such as amplitude, frequency, or phase) according to the information signal (such as voice, data, or video) that needs to be transmitted. Modulation allows the efficient use of the limited frequency spectrum and enables multiple signals to share the same channel.
• Capacity is the maximum amount of information that can be transmitted over a channel in a given time. Capacity depends on various factors such as bandwidth, modulation, coding, interference, noise, and signal quality.
• S/N ratio is the ratio of signal power to noise power, measured in decibels (dB). S/N ratio indicates the quality of the received signal and the level of interference and distortion in the channel. A higher S/N ratio means a clearer and stronger signal, while a lower S/N ratio means a weaker and noisier signal.
The relation between modulation, capacity, and S/N ratio in microwave links is based on the following principles:
• As the modulation order increases, the number of symbols (distinct states or values) that can be represented by each carrier wave also increases. For example, in binary phase-shift keying (BPSK), each carrier wave can represent one bit (0 or 1) by changing its phase by 180 degrees; while in quadrature amplitude modulation (QAM), each carrier wave can represent multiple bits by changing both its amplitude and phase.
• As the number of symbols increases, the number of bits transmitted per symbol also increases. For example, in BPSK, one symbol can carry one bit; while in 16-QAM, one symbol can carry four bits. This means that higher modulation schemes can transmit more information in the same bandwidth and time, thus increasing the capacity of the channel.
• However, as the number of symbols increases, the probability of interference and errors also increases. This is because higher modulation schemes require more precise and accurate detection of the carrier wave characteristics at the receiver end. Any slight deviation or distortion in the amplitude or phase of the carrier wave can cause a symbol error, which means that one or more bits are incorrectly received. Therefore, higher modulation schemes require higher S/N ratios to ensure reliable communication.
• To solve this issue, adaptive modulation can be used. Adaptive modulation is a technique that dynamically adjusts the modulation scheme according to the channel conditions and requirements. For example, when the channel quality is good and the S/N ratio is high, a higher modulation scheme can be used to increase the capacity; while when the channel quality is poor and the S/N ratio is low, a lower modulation scheme can be used to reduce the errors.
• S/N ratio is an important parameter that represents the quality of the received signal at the receiver end of the link. It should satisfy the planned value to ensure good performance. The planned value depends on various factors such as link distance, antenna size, transmitter power, receiver sensitivity, frequency band, atmospheric conditions, and interference sources.
• As the modulation order of the link increases, the required S/N ratio also increases. This is because higher modulation schemes need more signal power to overcome noise power and maintain a clear distinction between different symbols. For example, BPSK requires a minimum S/N ratio of 9.6 dB; while 1024-QAM requires a minimum S/N ratio of 30 dB.
• In addition to S/N ratio, another parameter that affects the performance of microwave links is Rx threshold. Rx threshold is the minimum signal strength or power level that is required at the receiver end to decode the information signal correctly. Rx threshold depends on various factors such as modulation scheme, coding scheme, error rate, bandwidth, and receiver sensitivity.
• As the modulation order of the link increases, the Rx threshold also increases. This is because higher modulation schemes need more signal strength or power level to achieve a certain error rate and bandwidth. For example, at 4-QAM (also known as QPSK), the Rx threshold required is -85 dBm; while at 1024-QAM, the Rx threshold required is -60 dBm.
• Therefore, if you need to increase the link data rate or capacity, you can increase
the modulation order but you have to think also about the Rx threshold. You have to ensure that there is enough signal strength or power level at
the receiver end to support higher modulation schemes. You may have to increase the transmitter power, use larger or more directional antennas, reduce the link distance, or avoid sources of interference and attenuation.
To summarize, the relation between modulation, capacity, and S/N ratio in microwave links is as follows:
• As modulation increases, capacity of link increases, but bit errors also increase, Rx threshold required increases, and S/N ratio required increases.
• As S/N ratio increases, signal quality increases, but noise decreases, interference decreases, and distortion decreases.
• As Rx threshold increases, signal strength required increases, but transmitter power required increases, antenna size required increases, or link distance required decreases.
