Hughes Systique Corporation wiki / Digital_Receiver

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Introduction

All Digital communication system requires some degree of synchronization to incoming signals by the receiver. Synchronization ensures that operations occur in the logically correct order, and is a critical component in ensuring the correct and reliable system operation. The Synchronization process helps in reliable estimation of the transmitted signal.
Two important types of synchronization needed are

The synchronization is implemented at the demodulator present in the receiver.
Figure below shows a typical structure of the demodulator

In the figure above the first process block does the frequency translation. The incoming IF sequence is multiplied by the quadrature phase, un-modulated sine wave (which may or may not be phase locked to the incoming signal) and which has the same frequency as the IF carrier. The output of this process is the in-phase and quadrature component. The I and Q components are composed of wanted modulated signal at baseband and an unwanted high frequency signal component.
The second block represents the process of matched filtering. This process serves two purpose

It eliminates the high frequency component generated during frequency translation processs.
It shapes the signal in time and frequency domain to ensure optimum data detection.

The third process is known as Symbol Timing Recovery (STR). This can be done using feedback scheme where the signal is sampled` after matched filter. Alternatively it may be sampled prior to, or directly after frequency translation process. Also a feedforward STR method could be used.
After the STR has been established the Carrier Recovery(CR) is done. If the feed forward carrier recovery is done then symbol recovery must be established prior to Carrier Recovery. Consequently, the STR must operate on sample which has unknown carrier phase and frequency offset. In this scheme the STR scheme chosen must be phase independent. Alternatively, a feedback CR may be chosen as shown by dotted line from CR function to frequency translation oscillator. If the phase lock is achieved prior to symbol recovery a phase dependent STR must be employed.
The final process is that of data detection where the received matched filtered and synchronized samples are processed to determine the values of transmitted data sample

Classifications in Synchronization
Figure below shows analog,hybrid and digital models for synchronization
Analog Recovery

Hybrid Recovery

Digital Recovery

Traditionally, analog or hybrid analog/digital solutions have been used for carrier and timing recovery.

Adjusting directly the demodulator local oscillator or sampling clock
Phase-locked loops (PLLs) and related circuits have been used as key elements.

Digital solutions are preferred now:

Better in terms of size or power consumption or economy
Better solutions can be found in terms of jitter performance and/or tracking speed
Convergence speed is very important in high speed transmissions. Long feedback loops can be avoided (e.g., in hybrid solutions, loops from the digital part back to the analog part are needed)

All-Digital Synchronisation Concept

Free-running local oscillators for demodulation and frequency conversion
Free-running sampling clock
Errors are compensated in digital part

Thus all the synchronization functions can be implemented using digital techniques

Synchronization principles
Synchronization principle can be based on two categories

Data-aided (DA)

Data aided synchronization is based on the fact that reference symbol sequences are known to the receiver apriori(training signals, preambles/midambles, pilot frequencies, etc.). This makes the scheme relatively simpler to implement . However it suffers from severe drawbacks as transmitter must allocate some of the available power to the transmission of clock signal. Even greater disadvantage being a fraction of avaiable channel bandwidth must be allocated for the transmission of clock signal. This method is mostly used in telephone systems that employs large bandwidths to transmit the signals of many users. In such case transmission of clock signal is shared in the demodulation of signal among many users.

Non-data-aided (NDA)

The goal of non-data aided algorithms is to recover the transmitted signal in noisy channel without using explicit signaling information. This technique does not rely on known or detected symbol values. The demodulator extracts the clock signal from received data which is used for detection process. This helps in preserving power and bandwidth. Blind detection increases data capacity by eliminating the need for signaling. Other advantages include source identification and noise identification, which can be used to adjust the coefficients of adaptive filters to detect source symbols correctly, in presence of changing channel parameters. This is particularly useful for frequency selective fading channels.

Synchronizers can be classified in yet another way

Feedback mechanism
Feedforward mechanism

Feedback mechanism uses Phase Locked Loop principle to ensure that the phase and frequency of sampler are locked to the incoming signal. Feedback algorithms are usually based on simple stochastic approximation approach: for instance, timing error detector gives a signal which is a monotonic function of the error. Based on this signal, the timing is adjusted in such a way that the error is reduced. Figure below explains
the principle involved in feedback synchronization.

Feedforward methods derive an estimate of the timing phase by applying non linear process to the received signal. The estimate then can be used to adjust the sample timing to the optimum location which use unsynchronized. Figure below explains the principle involved in Feedforward synchronization.

Comparing feedback and feed-forward configurations
In feedforward configuration, accuracy of the estimator determines directly the accuracy of the detection.
Feedforward algorthm suffers from cycle slips although it is faster. Feedback mechanism is comparively slow and takes longer to lock into the correct symbol

Influencing factors for Synchronizers

Convergence speed

In mobile and high speed satellite communication systems, the symbol timing recovery functions have to be able to follow very rapid changes. In TDMA systems timing recovery are carried out for each burst independently of the others.

Timing error

In case of timing recovery, the estimation errors appear as timing jitter, random fluctuation of the timing strobes due to noise and non-idealities of the algorithm. The errors are partly due to the noise in the received signal and partly due to non-idealties of the algorithm.

Hang-up, cycle slip

Different types of misbehavior of the synchronizer, e.g. locking to a wrong phase or frequency or loosing a symbol or a complete cycle of the carrier.