In another article on correlation, we said that correlation is the Master Algorithm for a digital communication system, not only for data detection but for parameter estimation of various kinds as well. We applied it to derive the matched filter for optimal detection. Here, we apply the principle of maximum correlation for solving our phase synchronization problem. The discussion in this context usually considers QPSK modulation but the extension to higher-order modulation schemes is a straightforward task. A similar framework is used for finding and compensating for other distortions as well such as carrier frequency offset and symbol timing offset, the examples of which we will see later.
In order to implement correlation, one signal is obviously the Rx signal . It is correlated with a clean version of what is expected at the Rx, the expected signal template. That is the most logical solution to detect the signal and estimate the desired parameters. For example, in case of a carrier phase offset, the Rx signal will be correlated with a perfect Tx signal but rotated in phase by an unknown offset. This procedure then generates an algorithm to estimate that phase offset.
where (in a DFT size of ) corresponds to carrier frequency and and are the inphase and quadrature waveforms (each time we define the inphase and quadrature waveforms, there is a reason behind choosing index or index that becomes clear as we progress through that particular derivation).
We face three problems in implementing the above relation.
- [1. Observation interval] The above sum is computed from to , which implies that we could theoretically continue computing this correlation with every received sample . However, the useful component of the received signal is given by a span of symbols while every sample outside this interval is noise only. The time duration of such symbols is
Thus, the finite observation interval is given by
where is samples/symbol, or . Therefore, the observation window consists of samples. It is understood in this calculation that the group delay arising from pulse shaping and matched filtering on either side of the transmission sequence has been taken care of by the Rx.
- [2. Start of frame] The second problem is to determine the starting point of the useful component in a sampled signal, as many of its initial samples are noise only. It is the job of a frame synchronization unit to determine the symbol level boundaries of the Rx signal, while the symbol timing synchronization block establishes the optimal sampling instant within a symbol. This will be explained in detail in another article but Figure below illustrates this alignment problem for a visual clarification.
- [3. Distortion at edges] Due to the pulse shape spreading beyond a single symbol and actually extending to symbols in each direction, there is a distortion introduced at the edges of the observation interval in the correlation result. However, it can be ignored if is sufficiently long.
We also assume downconversion with perfect frequency recovery, i.e., Tx and Rx oscillators have the same frequency , to emphasize on phase recovery problem. After perfect downconversion, our expected signal template is the complex sequence , with which the correlation of needs to be implemented. The overall setup is drawn in Figure below.
Continuing from the article on effect of a phase offset and ignoring the double frequency terms (the higher frequency terms as a product of downconversion process are either filtered by an analog filter in the RF frontend or a digital filter in the digital frontend and furthermore, the matched filter also suppresses such terms, as it is also a lowpass filter due to the pulse shape originally designed to restrict the spectral contents of the wireless signal), the downconverted complex signal is expressed as
where the conjugate arises due to the signals being complex. Having determined through frame synchronization block, we can write Eq (5) as
Recall that two complex signals are most similar when the inphase part of their correlation at time zero is maximum. Consequently, we ignore the component above and plug the definition of from Eq (4) above.
Ignoring the distortion at the edges of the summation, the above relation can be simplified recalling the fact that are the samples of a square-root Nyquist pulse with support samples.
Notice that after the received signal is downconverted to baseband, it is correlated with a similar pulse shape to the Tx, . As we saw in the details of matched filter, another way of implementing this correlation is through filtering the signal with its flipped version, (convolution operation in a filter flips again back to and the operation becomes correlation). This is known as matched filtering. Clearly, after summing over the sample index , we are left with a signal with symbol rate spaced samples at times .
The above expression is important. As we proceed, this Eq (9) will form the basis of many types of phase estimators, both in feedforward and feedback schemes.
A question at this stage is: starting from correlation, what have we achieved so far? While not clear as of now, we are on our way to solve the fundamental problem of the synchronization process. In a few other articles, we will discuss data-aided, decision-directed and non-data-aided techniques for phase synchronization.