A RAKE receiver is a radio receiver designed to counter the effects of multipath fading. A signal transmitted from a transmitter encounters physical objects such as buildings, trees, etc. that cause multiple time delayed signals being received at a receiver due to reflection, diffraction, or scattering effects. A RAKE receiver uses multiple sub-receivers called fingers, each assigned to different multipath components. Each finger independently equalizes a single multipath component. The contributions of all fingers are combined in order to make the most use of the different transmission characteristics of each transmission path.
Typically, the signal of interest is coherently summed, while the noise of the different sources is assumed to be independent and therefore will be summed non-coherently. In case of two equal strength signal sources, the Signal-to-Noise Ratio (SNR) will increase by 3 dB (factor 2) after combining (signal amplitude will be doubled, therefore signal energy will be 4-fold while noise energy after addition will b e 2-fold).
In a traditional RAKE receiver, multipath propagation is exploited by collecting signals from individual multipath components and coherently adding them together. This adding or summing of signals leads to the use of more radiated energy from a transmitter. To effectively sum signals, an estimated channel impulse response is needed. Hence, knowledge of amplitude, phase and relative propagation delay from the most relevant multipath components has to be available to the receiver. However, the estimated channel impulse response is not perfect and differs from the true channel impulse response. The discrepancy between true and estimated channel impulse response can be modelled as an Additive White Gaussian Noise (AWGN) source whose variance is constant for all multipath components and the value of the variance of the noise that is present in channel coefficients depends on the SNR of the channel and the channel estimation parameters such as preamble size, estimation algorithm, and estimation resources. The variance can be treated as constant because typically all channel coefficients suffer from the same noise variance. The constant variance of the estimation noise causes a relative larger effect on weak multipath components than on strong multipath components. Due to channel estimation limitations, traditional RAKE receivers suffer from the following problems.
Due to estimation noise on a multipath component, the contribution of a RAKE finger to the SNR may be smaller than expected and in some cases even negative when more noise than signal is added to the combined signal. Further, in Selective RAKE (SRAKE) receivers, fingers are assigned to multipath components using the estimated channel impulse response rather than the true channel impulse response. Estimation noise may cause errors in the selection of the most promising multipath components. Moreover, the selection of multipath components is performed without taking channel statistics into account. A multipath component with a larger delay will likely have a smaller amplitude than a multipath component with a smaller delay.
FIG. 1 shows achieved SNR as a function of the number of used RAKE fingers, depicted in several scenarios. The curves are achieved using one realization of the exponential channel model, simulating a channel with an RMS delay spread of 50 ns. The best performance is achieved when a perfect channel estimate is present, which is (of course) a hypothetical scenario. In the hypothetical scenario, the SRAKE curve 100 shows the fastest growing SNR curve as function of the number of RAKE fingers. The Partial RAKE (PRAKE) curve 102 shows that without selecting/ordering the RAKE fingers, more RAKE fingers are required for achieving a specific SNR. Both the SRAKE curve 100 and PRAKE curve 102 show that after reaching a certain point, SNR does not increase with the addition of more RAKE fingers. In fact, as shown by curves 104, 106, 108, SNR actually decreases when more RAKE fingers are added, due to the estimation noise and addition of more noisy multipath components.
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