OC Optimum Combining
OC (Optimum Combining), also known as maximal ratio combining (MRC), is a signal processing technique commonly used in wireless communication systems to improve the quality and reliability of received signals. OC aims to mitigate the effects of fading, interference, and noise by combining multiple received signals in an optimal manner.
Wireless communication channels are subject to various impairments that can degrade the quality of the received signal. One such impairment is fading, which is the fluctuation of the received signal strength caused by multipath propagation. In multipath propagation, the transmitted signal reaches the receiver through multiple paths due to reflections, refractions, and diffractions. As a result, these multiple versions of the transmitted signal arrive at the receiver with different amplitudes and phases, leading to signal fading.
In a wireless communication system utilizing OC, multiple copies of the transmitted signal are received at the receiver through different antennas or spatially diverse elements. Each received signal undergoes independent fading due to the multipath propagation. The basic idea behind OC is to combine these received signals in a way that maximizes the received signal power and minimizes the effects of fading, interference, and noise.
The process of OC involves two main steps: signal combining and weighting. In the signal combining step, the received signals from multiple antennas or spatially diverse elements are combined. In the weighting step, appropriate weights are assigned to each received signal before combining them. These weights determine the contribution of each received signal to the combined signal.
To understand the principle behind OC, let's consider a system with N receive antennas and N independent received signals, denoted as r₁, r₂, ..., rₙ. Each received signal rₙ can be represented as the sum of the desired signal s and the interference and noise components. Mathematically, we can express this as:
rₙ = s + nₙ
where nₙ represents the interference and noise for the nth received signal. The goal of OC is to estimate the desired signal s as accurately as possible by combining the received signals.
In the signal combining step, the received signals are combined using appropriate weights. The combined signal y is given by:
y = w₁r₁ + w₂r₂ + ... + wₙrₙ
where w₁, w₂, ..., wₙ are the weights assigned to each received signal. These weights can be complex numbers, allowing for phase adjustments as well.
The optimal weights that maximize the combined signal power can be obtained using the principle of maximum ratio combining. The maximum ratio combining takes into account the fading characteristics of the individual received signals. The optimal weight for the nth received signal is proportional to the complex conjugate of its fading coefficient, denoted as hₙ. Mathematically, the weight can be expressed as:
wₙ = (hₙ*) / √(Σ|hₙ|²)
where hₙ* is the complex conjugate of hₙ, and Σ|hₙ|² represents the sum of the squared magnitudes of the fading coefficients.
The use of optimal weights in OC provides a gain in the combined signal power compared to other combining techniques, such as selection combining or equal gain combining. This gain results from the exploitation of the constructive combination of signals with diverse fading characteristics.
The weighted combining process in OC results in a combined signal with improved signal-to-noise ratio (SNR) and reduced interference. Since the desired signal components add constructively while the interference and noise components are averaged out, the combined signal is less affected by fading, interference, and noise, leading to improved communication performance.
It's worth noting that OC assumes knowledge of the fading coefficients of the received signals. In practice, these coefficients are estimated using channel estimation techniques. The accuracy of the estimated coefficients affects the performance of OC. If the channel estimation is inaccurate, the weights assigned to the received signals may deviate from the optimum, leading to performance degradation.
In summary, OC (Optimum Combining) is a signal processing technique used in wireless communication systems to improve the quality and reliability of received signals. By combining multiple received signals in an optimal manner, OC mitigates the effects of fading, interference, and noise. The technique relies on assigning weights to the received signals based on their fading characteristics, allowing for constructive combination and improved communication performance.