Impact of Power Levels on Bit Error Rate using Frequency Shift Keying
Frequency Shift Keying (FSK) is a digital modulation scheme that uses discrete frequency changes to represent digital data. It is widely used in various communication systems, including FM transmitters and C4FM or System Fusion. However, one of the challenges faced in FSK modulation is the Bit Error Rate (BER), which can increase at higher power levels. This phenomenon may seem counterintuitive since higher power typically suggests a stronger signal and better performance. So, why does BER increase with power in FSK?
The BER is a critical parameter in digital communication systems, representing the fraction of bits that are received in error over a communication channel. In an ideal scenario, increasing the power of a transmitted signal should improve the signal-to-noise ratio (SNR) and reduce the BER. However, in practical scenarios, especially with FSK using an FM transmitter, several factors can lead to an increase in BER at higher power levels.
Non-linearities in the transmitter and receiver
At higher power levels, the transmitter and receiver components may exhibit non-linear behavior, which can distort the signal and increase the BER. As the power increases, the components may enter a non-linear operating region, causing distortion in the transmitted signal. This distortion can lead to a spread in the frequency spectrum of the signal, which can interfere with adjacent channels and result in increased BER[i].
Consider a 4-FSK signal with non-linearities in the transmitter and receiver. The non-linearities can cause the frequency levels to shift and overlap, making it difficult for the receiver to accurately decode the symbols[ii]. This can result in a higher BER as the receiver may incorrectly interpret the received signal.
To better understand the impact of non-linearities on BER, try to visualize the frequency spectrum of the ideal and distorted 4-FSK signals. The ideal signal would have sharp peaks at the four frequency levels, while the distorted signal would have broader and shifted peaks due to non-linearities.
Intermodulation distortion (IMD)
Intermodulation distortion occurs when two or more signals pass through a non-linear system. This creates additional signals at frequencies that are sums and differences of the original frequencies and their harmonics[iii]. Higher power levels can cause IMD, which can create additional frequencies that were not present in the original signal. In other words, different frequency components of the signal mix and create unwanted frequencies. This can interfere with the desired signal and increase the BER.
Bandwidth Limitations
Higher power levels while using C4FM can lead to a wider spread of the signal spectrum. If the bandwidth of the communication channel is limited, this spread can cause parts of the signal to fall outside the channel’s passband, effectively getting cut off and leading to errors in reception.
In digital narrow mode, bandwidth limitations can lead to reduced voice quality and increased BER. The signal may also become more susceptible to noise and interference, affecting overall communication performance. High power levels can cause spectral regrowth, making the signal more susceptible to noise and interference, affecting overall communication performance.
Multipath fading
In a mobile communication environment, higher power levels can exacerbate multipath fading. Multipath fading occurs when a signal takes multiple paths to reach the receiver, causing phase shifts and constructive or destructive interference. This can result in a fluctuating received signal strength, making it difficult for the receiver to accurately demodulate the FSK signal and leading to a higher BER.
Conclusion
While higher power levels can theoretically improve the performance of an FSK system, practical limitations and non-linearities in the system can lead to an increase in BER. Understanding these factors is crucial for optimizing the performance of using C4FM on local repeaters around the area. Amateur radio operators should keep in mind that higher power is not necessarily better. There is a line between too high and too low of power when using C4FM.
If a station is experiencing broken transmission while using C4FM or sees increased BER on a repeater dashboard, drop your transmit power level. Decreased power should correct issue. It has been witnessed on the Somerset repeaters. Dropping the power from 50 watts to 5 watts has enhanced the quality of voice transmission using C4FM.
[iii] https://rfindustries.com/pdfs/white-papers/Intermodulation-Distortion-in-RF-Connectors.pdf