Large Frequency Offsets Research Articles

Digital Carrier Recovery Loop Using both Frequency Detector and Phase Detector for MPSK Systems

We propose a digital carrier recovery loop employing both a frequency detector and a phase detector for M-ary phase shift keying (MPSK) systems. A new frequency error correction function is also derived to increase the acquisition range. It is shown through computer simulation that the proposed scheme can reduce the acquisition time at large frequency offsets, unlike the existing ones.

A novel non-data-aided symbol timing recovery technique for OFDM systems

A new highly efficient non-data-aided technique to recover symbol timing of orthogonal frequency-division multiplexing systems is proposed. The algorithm in the proposed work exploits the interference that results due to the loss of orthogonality between subcarriers, where the second-order statistics of the resulting interference is proportional to the offset from the optimum sampling point. The presented technique does not require prior fine carrier synchronization, and it is capable of extracting symbol timing at low E/sub s//N/sub 0/ values with large carrier frequency offsets (CFOs). The system performance was investigated in multipath fading channels with large CFOs and additive white Gaussian noise.

A 2.5-3.125-Gb/s quad transceiver with second-order analog DLL-based CDRs

This paper describes a 2.5-3.125-Gb/s quad transceiver with second-order analog delay-locked loop (DLL)-based clock and data recovery (CDR) circuits. A phase-locked loop (PLL) is shared between receive (RX) and transmit (TX) chains. On each RX channel, an amplifier with user-programmable input equalization precedes the CDR. Retimed data then goes to an 1:8/1:10 deserializer. On the TX side, parallel data is serialized into a high-speed bitstream with an 8:1/10:1 multiplexer. The serial data is introduced off-chip through a high-speed CML buffer having single-tap pre-emphasis. Proposed DLL-based CDR can tolerate large frequency offsets with no jitter tolerance degradation due to its second-order PLL-like nature. Also, this study introduces an improved charge-pump and an improved phase-interpolator. Fabricated in a 0.15-/spl mu/m CMOS process, the 1.9-mm/sup 2/ transceiver front-end operates from a single 1.2-V supply and consumes 65-mW/channel of which 32 mW is due to the CDR. CDR jitter generation and high-frequency jitter tolerance are 5.9 ps-rms and 0.5 UI, respectively, for 3.125 Gb/s, 2/sup 23/-1 PRBS input data with 800-ppm frequency offset.

MINIMIZATION OF NONRESONANT EFFECTS IN A SCALABLE ISING SPIN QUANTUM COMPUTER

The errors caused by the transitions with large frequency offsets (nonresonant transitions) are calculated analytically for a scalable solid-state quantum computer based on a one-dimensional spin chain with Ising interactions between neighboring spins. Selective excitations of the spins are enabled by a uniform gradient of the external magnetic field. We calculate the probabilities of all unwanted nonresonant transitions associated with the flip of each spin with nonresonant frequency and with flips of two spins: one with resonant and one with nonresonant frequencies. It is shown that these errors oscillate with changing gradient of the external magnetic field. Choosing the optimal values of this gradient allows us to decrease these errors by 50%.

Joint compensation of IQ imbalance, frequency offset and phase noise in OFDM receivers

AbstractZero‐IF receivers are getting a lot of attention because of their potential to enable low‐cost OFDM terminals. However, zero‐IF receivers also introduce IQ imbalance which can have a huge impact on the performance. Rather than increasing component cost to decrease the IQ imbalance, an alternative is to tolerate the IQ imbalance and compensate for it digitally. Current solutions either require additional analog hardware or are based on digital algorithms that converge too slowly for bursty communication. Moreover, the impact of a frequency offset and phase noise on the IQ imbalance estimation/compensation problem is not considered. In this paper, we analyze the joint IQ imbalance/frequency offset/phase noise estimation and propose a low‐cost, highly effective, all‐digital mitigation scheme. For large IQ imbalance large frequency offsets and in the presence of phase noise our solution still results in an average implementation loss below 0.5 dB. It, therefore, enables the design of low‐cost, lowcomplexity OFDM receivers. Copyright © 2004 AEI

A low-power all-digital FSK receiver for space applications

A frequency-shift keying (FSK) receiver has been designed for deep space applications which exhibits potential for ultra low power performance. The receiver is based on a novel, almost all-digital architecture. It supports a wide range of data rates and is very robust against large and fast frequency offsets due to Doppler. The architecture utilizes subsampling and 1-bit data processing together with a discrete Fourier transform-based detection scheme to enable power consumption dramatically lower than implementations reported in the literature. Novel and power-efficient algorithms are derived for frequency and timing tracking. Most of the power saving techniques are applicable to a variety of applications, but some are achieved by taking advantage of the asymmetric power constraints for the receiver and the transmitter as well as the absence of adjacent channel interferers. The worst-case bit-error rate (BER) performance of the receiver is just 2.5 dB below that of the optimal uncoded noncoherent FSK receiver at a BER of 10/sup -6/ and better for lower BERs.

Noncoherent adaptive channel identification algorithms for noncoherent sequence estimation

In this letter, two novel noncoherent adaptive algorithms for channel identification are introduced. The proposed noncoherent least-mean-square (LMS) and noncoherent recursive least squares (RLS) algorithms can be combined easily with noncoherent sequence estimation (NSE) for M-ary differential phase-shift keying signals transmitted over intersymbol interference (ISI) channels. It is shown that the resulting adaptive noncoherent receivers are very robust against carrier phase variations. For zero frequency offset, the convergence speed and the steady-state error of the noncoherent adaptive algorithms are similar to those of conventional LMS and RLS algorithms. However, the conventional algorithms diverge even for relatively small frequency offsets, whereas the proposed noncoherent algorithms converge for relatively large frequency offsets. Simulations confirm the good performance of NSE combined with noncoherent adaptive channel estimation in time-variant (fading) ISI channels.

VLSI implementation of a 100-μW multirate FSK receiver

A very low-power frequency-shift keying (FSK) receiver has been designed for dual-purpose operation: deep space applications and general purpose baseband processing. The receiver is based on a novel, almost all-digital architecture. It supports a wide range of data rates and is very robust against large and fast frequency offsets due to Doppler effects. The architecture utilizes subsampling and l-b data processing together with an FFT-based detection scheme to enable power consumption dramatically lower than a conventional implementation., A system/hardware co-design approach allows the use of a number of circuit-level power reduction techniques while still meeting system-level constraints. In particular, we designed a combination of fully parallel and word-serial decimation stages to simultaneously optimize power consumption and silicon area. We also designed a very efficient FFT block that uses approximate arithmetic and pruning to greatly reduce overall complexity. Additional modules, such as direct digital frequency synthesizer (DDFS) and magnitude computation, have also been optimized in view of the targeted system parameters: signal-to-noise ratio and bit-error rate. The entire architecture has been made maximally flexible and power efficient by utilizing local clock gating and simple interstage, handshaking mechanism. The receiver has been implemented in 0.25-/spl mu/m CMOS technology and takes up under 1 mm/sup 2/. The power consumption is below 100 /spl mu/W for data rates below 20 kb/s. Rates up to 2 Mb/s are supported.

Blind self-noise-free frequency detectors for a subclass of MSK-type signals

Frequency offset due to Doppler shift and/or oscillator instabilities degrade the receiver performance. A family of frequency detectors for frequency offset estimation and compensation in digital receivers is introduced. The proposed detectors are best suited for frequency offset compensation of a subclass of binary continuous phase modulation with h=1/2 that includes modulation schemes with nonnegative frequency pulses. For the considered modulation schemes, the modulation-induced self-noise term is absent from the variance of the frequency estimate. The estimator is nondata- and nontiming-aided and its estimation range is either half or a quarter of the bit rate (R). With larger frequency offsets, the estimators that have a /spl plusmn/R/2 estimation range introduce a frequency ambiguity of R that is of no relevance to the performance of a differential detection based receiver.

Data-aided estimation of carrier frequency for burst detection of QAM

To overcome large carrier frequency offsets in burst demodulation of quadrature amplitude modulation, a new Viterbi-type algorithm coupled with a data-aided frequency estimation is developed. This algorithm can quickly acquire carrier offset, then remove it during the process of demodulation. Its performance is confirmed as approaching that of coherent demodulation, with little loss of short pilot data.

Linewidth measurements of the JET energetic ion and alpha particle collective Thomson scattering diagnostic gyrotron

Spectral purity of the transmitter source of a collective Thomson scattering (CTS) system is vitally important to insure that measured signals only originate from the plasma and not from stray source light. A number of high power (up to 500 kW), 140 GHz gyrotron tubes used with the Joint European Torus (JET) CTS system have been found to have one or more spurious modes and many harmonics in the output spectrum. The CTS diagnostic receiver system was used to make measurements of the gyrotron spectrum. It was comprised of a homodyne part from MIT for frequency sidebands <500 MHz, and a heterodyne part constructed at JET for frequency sidebands from 0.1 to 6 GHz. One tube at high power produced a strong 25 MHz mode and its harmonics to large frequency offsets, unsuitable for CTS measurements. Only at reduced power of approximately 100 kW was this tube’s spectrum sufficiently clean for CTS. Another tube at JET operated at 500 kW output power with only low level parasitic modes, indicating that higher power gyrotrons may be available for future alpha particle measurements. The main receiver was tested with a low power test setup which simulated the gyrotron stray source light, the thermal ion feature and plasma electron cyclotron emission.

Blind least-squares estimation of carrier phase, Doppler shift, and Doppler rate for m-PSK burst transmission

A novel blind algorithm is proposed for joint carrier phase/Doppler shift least-squares (LS) estimation for burst transmission of m-PSK modulated data. The algorithm can be extended to include Doppler rate estimation. The performance of the algorithm is contrasted against that of known algorithms, and it is shown to be advantageous particularly for short bursts and large frequency offsets. The signal-to-noise ratio penalty with respect to the ideal estimation case is shown to be below 1 dB for a frequency error as large as 0.1 of the symbol rate and bursts as short as 20 symbols.

Correlation branch selection for extended tracking range delay‐locked loops

AbstractIn spread spectrum systems the delay‐locked loop is commonly used to track the spreading code. A problem is to design the delay‐locked loop to be both accurate and robust against disturbances which normally requires a tradeoff. The use of extended tracking range delay‐locked loops increases the ability of the system to maintain lock even for large Doppler frequency offsets. The extended tracking range is achieved by using more than two correlators to form the error detector characteristic. However, the noise level in the synchronization loop is increased by every additional correlator leading to larger tracking jitter. In this paper an algorithm is proposed which reduces the noise level in the loop considerably. This is done by selecting only those correlation branches with an useful control signal. The investigated performance criteria are the tracking jitter and the mean time to loose lock. Both analytical and simulation results are presented. The extended tracking range delay‐locked loop with correlation branch selection shows very promising performance results for both criteria at medium to high signal‐to‐noise ratios. It approaches the performance of the classical delay‐locked loop for the tracking jitter and has a much larger mean time to loose lock than the classical delay‐locked loop.

A FREQUENCY OFFSET ESTIMATION AND COMPENSATION ALGORITHM FOR K/Ka-BAND COMMUNICATIONS

This paper presents a new open-loop technique for estimating and correcting Doppler frequency shift in K/Ka-band communication systems with special reference to the advanced communications technology satellite (ACTS) mobile terminal (AMT) modem, which utilizes square-wave pulse-shaped, binary differential phase shift-keyed (DPSK) modulation. The novelty of this estimation scheme is that it exploits the Doppler-induced phase shift over a fraction of a symbol interval to provide an estimate of the Doppler offset, without requiring symbol synchronization. Furthermore, by utilizing time-differential detection (delay-and-multiply), the proposed technique can tolerate much larger frequency offsets than existing open- or closed-loop techniques. Analytical results are provided for the variance of the above estimator and the error probability performance of the AMT is evaluated in the presence of the Doppler correction. Practical design considerations are also discussed, including a method for modifying the front end, digital bandlimiting filter in such a way that Doppler bias effects in the new estimator are eliminated. Simulation results reveal that, in general, performance improves with increasing data rates, i.e., the new frequency offset estimation/compensation algorithm induces a degradation from ideal of approximately 1 dB at a 6 dB energy per data symbol (bit) and a 2.4 kbps data rate. However, there is no appreciable degradation when the data rate is increased to 9.6 or 19.2 kbps.

A FREQUENCY OFFSET ESTIMATION AND COMPENSATION ALGORITHM FOR K/Ka‐BAND COMMUNICATIONS

This paper presents a new open-loop technique for estimating and correcting Doppler frequency shift in K/Ka-band communication systems with special reference to the advanced communications technology satellite (ACTS) mobile terminal (AMT) modem, which utilizes square-wave pulse-shaped, binary differential phase shift-keyed (DPSK) modulation. The novelty of this estimation scheme is that it exploits the Doppler-induced phase shift over a fraction of a symbol interval to provide an estimate of the Doppler offset, without requiring symbol synchronization. Furthermore, by utilizing time-differential detection (delay-and-multiply), the proposed technique can tolerate much larger frequency offsets than existing open- or closed-loop techniques. Analytical results are provided for the variance of the above estimator and the error probability performance of the AMT is evaluated in the presence of the Doppler correction. Practical design considerations are also discussed, including a method for modifying the front end, digital bandlimiting filter in such a way that Doppler bias effects in the new estimator are eliminated. Simulation results reveal that, in general, performance improves with increasing data rates, i.e., the new frequency offset estimation/compensation algorithm induces a degradation from ideal of approximately 1 dB at a 6 dB energy per data symbol (bit) and a 2⋅4 kbps data rate. However, there is no appreciable degradation when the data rate is increased to 9⋅6 or 19⋅2 kbps.

A new pattern jitter free frequency error detector

One method to enable fast acquisition of a phase-locked loop (PLL) in spite of large frequency offsets and small PLL bandwidths is to use an additional AFC (automatic frequency control) loop. A suitable frequency error detector (FED) for large frequency offsets is the well-known balanced quadricorrelator. This FED is shown to produce great pattern jitter if it is to work with digital modulated MQAM and MPSK signals (M>2) and random data. The authors describe how this pattern jitter for MQAM and MPSK signals can be overcome completely. Another understanding of the balance quadricorrelator is also given.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

New Phase-Lock Loop Circuit Providing Very Fast Acquisition Time

We present a new circuit configuration for second-order phase-lock loops that provides, for large initial frequency offsets, acquisition times several orders of magnitude shorter than those achieved using conventional phase-lock loops. This new circuit also provides frequency locking almost instantaneously when the time delay around the loop is small. Furthermore, it can, without losing lock, sustain frequency changes several hundred times faster than those which can be sustained by a conventional circuit.

Frequency Detectors for PLL Acquisition in Timing and Carrier Recovery

A significant problem in phase-locked loop (PLL) timing and carrier extraction is the initial acquisition. Very narrow loop bandwidths are generally required to control phase jitter, and acquisition may depend on an extremely accurate initial VCO frequency (VCXO) or sweeping. We describe two simply implemented frequency detectors which, when added to the traditional phase detector, can effect acquisition even with very small loop bandwidths and large initial frequency offsets. The first is the quadricorrelator, previously applied to timing recovery by Bellisio, while the second is new, and called a rotational frequency detector. The latter, while limited to lower frequencies and higher signal-to-noise ratios, is suitable for many applications and can be implemented with simpler circuitry.

Unlock Characteristics of the Optimum Type II Phase-Locked Loop

This paper describes a study of the statistical unlock characteristics of the commonly used optimum type II phase-locked loop taking into account the nonlinearity of the phase detector. The unlock behavior is investigated by simulating the loop with random noise inputs on a digital computer. A criterion of loop unlock is evolved using the phase portrait of the loop and consists of crossing of separatrices by loop error and a measure of loop error rate. Approximately 100 statistically independent trials were made for each of a number of combinations of initial phase error, initial frequency error, and input noise amplitude. From these computer runs, cumulative frequency distributions of unlock time and curves of mean time to unlock were obtained and are presented. The phase portrait of the loop containing time information also is included in the paper, along with a plot of capture characteristics for large initial frequency offsets.