Multiple stable lock points in the S-curve of a decision-directed loop

Resolving Phase Ambiguity through Unique Word and Differential Encoding and Decoding

In the context of carrier synchronization, we have discussed the Costas loop and other techniques before. Today, we discuss the significance of differential encoding and decoding for phase ambiguity resolution. Keep in mind that this topic is different than differential detection. In the former case, the data bits are encoded before modulation and decoded after demodulation in a differential manner. Nevertheless, the demodulation is still coherent (i.e., it requires carrier synchronization). In the latter case, the data symbols are detected during demodulation through differential operations, thus canceling the effect of channel phase and eliminating the need for carrier synchronization. Let

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Output from the Costas Loop block after phase convergence

Costas Loop for Carrier Phase Synchronization

Costas loop is a carrier phase synchronization solution devised by John Costas at General Electric Company in 1956 [1]. It had an enormous impact on modem signal processing in general and carrier synchronization in particular. At that time, it was customary to send a pilot tone for carrier synchronization along with the data signal which consumed a significant amount of power. Costas was one of the earliest scientists to demonstrate that the carrier phase could be reliably recovered from the Rx signal without the need of a pilot tone. In words of Costas, "It is unfortunate that many engineers tend

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Experimental setup for low SNR receiver

Design of a Low-SNR Receiver

Wireless communication is energy inefficient due to the nature of the medium that spreads out energy in an unguided manner, as opposed to guided media like optical fiber and coaxial cable. To avoid wastage of power, one solution is to lower the transmit (Tx) power but then the receiver is left with the herculean task of efficiently demodulating the receive symbols at a low SNR. This article describes the design and implementation of one such receiver. Background The physical layer of a receiver system consists of three major parts, namely the frontend, the inner receiver, and the outer receiver. The

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A block diagram for the implementation of the feedforward phase estimator

How to Estimate the Carrier Phase

In this article, I will describe how to estimate the carrier phase from an incoming waveform in a feedforward manner. This algorithm utilizes a sequence of known pilot symbols embedded within the signal along with the unknown data symbols. Such a signal is sent over a link in the form of separate packets in burst mode wireless communications. In most such applications with short packets, the phase offset $\theta_\Delta$ remains constant throughout the duration of the packet and a single shot estimator is enough for its compensation. Here, the primary task of the designer is to develop this closed-form expression

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Classification of carrier frequency synchronization algorithms

Classification of Carrier Frequency Synchronization Techniques

We have discussed before that carrier phase synchronization is done at the end of the Rx signal processing chain due to the very nature of the DSP implementation. And that almost all DSP based phase synchronization algorithms are timing-aided. Timing acquisition implies knowing the symbol boundaries in the Rx sampled waveform which is equivalent to identifying the optimal sampling instants where the eye opening is maximum and Inter-Symbol Interference (ISI) from the neighbouring symbols is zero. In the case of Carrier Frequency Synchronization (CFO), this is not true. From a previous post on the effect of CFO, we know that

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