Digital codetracking becomes very popular because of the evolution toward all digital modem implementation of DSSS systems.
In this paper, accurate nonlinear analysis for the noncoherent second-order digital codetracking loops is investigated over AWGN channels with the presence of Doppler shift. This modeling of channel finds applications in GPS  and other civilian or military satellite-based DSSS systems, where Doppler shift is due to the relative movement between the satellite and the receiver. In the analysis, based ona regenerative Markov chain modeling of the codetracking process, the lock-in range, transient response, MSE, and MTLL are evaluated more accurately than the traditional analysis. Furthermore, in a digital DLL, the adaptation of codetracking can only be done in discrete steps, i.e., quantized adaptation and that will result in significant changes in the loop performance. In this analysis, the effect of quantized adaptation is evaluated as well.
B. Acquisition Process
As in nearly all literature, the code phase uncertainty 2 of an acquisition system is divided into cells, with a cell size to within the lock-in range of the codetrackingloop used for fine code alignment. The acquisition system then searches through the code phases and determines which cells are the correct cells, according to some type of code phase correlators, search strate- gies, and test methods. As usual, the correct cells will be de- noted as the cells (hypothesis ) and the incorrect cells as the cells (hypothesis ), respectively. In practice, there may be more than one correct cell. For simplicity of presen- tation, however, only the case of one correct cell will be used as an example, although the method applies equally well to the more general case with more than one correct cell. Two types of decision errors may occur when detecting a cell, namely false dismissal of the cell and false alarm of the cells. When a false alarm occurs, the synchronization will be turned to codetrackingfor fine alignment. In this case, it is assumed that the false alarm can always be detected after some fixed or random time, and the synchronization will be turned back to the code
Index Terms— Correlator, delay-locked loop, directsequencespreadspectrum, tracking error variance.
I. S YSTEM D ESCRIPTION AND S IGNAL M ODEL
I N THIS letter, we present acodetracking receiver with less complexity, by employing a differentially coherent tech- nique originally proposed for pseudonoise (PN) acquisition receiver . The proposed differentially coherent delay-locked loop (DCDLL) scheme is shown in Fig. 1. The received signal is first filtered by front-end band-pass filter (BPF) and the bandwidth of BPF is . is set to be chip rate ( , where is the chip duration). Then this proposed DCDLL scheme processes the received signal using a differential decoder with a delay of -chip duration in the delay path. The decoder output is then correlated with the difference of the advanced (early) and retarded (late) versions of the local PN code to produce an error signal. After the error signal is filtered by a low-pass filter (LPF), then it drives the voltage-controlled clock (VCC) through the loop filter and corrects the code phase error of the local PN code generator. In this proposed system, the bandwidth of LPF, denoted as , is set to be the system data rate ( , where is the data bit duration). The processing gain of this direct-sequencespread-spectrum (DS/SS) system is thus given by or . Usually, if the system is applied in ranging, and
II. A SSUMPTIONS AND S IGNAL M ODELS
We consider a scenario where one OFDM desired signal with a single input/single output channel and one cyclo- stationary interference with period MT 0 share a common frequency band, where M is a positive integer known to the receiver and T 0 is the OFDM symbol duration. The receiver is assumed to precisely acquire synchronization information of the desired signal including symbol timing, carrier frequency, carrier phase and channel impulse response, which implies the received waveforms of OFDM sub-carriers are precisely known to the receiver. The channel impulse response of the desired signal is assumed frequency-selectivefading and time- invariant in our observation interval of interest. The maximum delay spread of the channel is assumed smaller than the length of CP such that ISI and ICI can be avoided by removing CP.
Channel modeling simulation tools that enable researchers and designers to accurately predict the performance of wire- less systems become increasingly important as personal com- munications and wireless data services evolve. A basic under- standing of the channel is important not only for designing modulation and coding schemes for robust communication over such channel, but also for investigating the channel fad- ing impact on existing networking algorithms, such as rout- ing and power adjustment which critically depend on channel attenuation. At present, most network protocol simulations and even power control algorithms are using the free space (distance) channel propagation model which is basically only function of transmitter-receiver distance. Typically, for the indoor environment, the channel characteristics are much too complex to be modeled by simple distance functions. Yet, a realistic channel model is essential for network protocol eval- uation, especially in the presence of mobility. Therefore, a more realistic channel fading model which accounts for chan- nel quality variations with movement is needed for network protocol simulation.
Abstract—This brief presents afrequencytrackingloop (FTL) to realize a crystalless wireless sensor node (WSN) for wireless body area network (WBAN). By trackinga remote wireless RF reference for system clock calibration, the proposed FTL allows WSNs to tolerate a large-frequency error from on-chip CMOS oscillators. Moreover, to achieve energy-efficient transmissions in crystalless, a sufficiently accurate convergence clock is required to enable burst overmegabits-per-second system throughput with minimized operation duty cycle. For the dedicated purpose, a comparison-based binary-search tracking scheme, which ensures accurate and robust convergence against noisy wireless channel, is further developed to manage the operation of FTL. The in- termediate frequency back-end part of FTL is implemented in 90-nm CMOS process. Measurement results show that the FTL extends an initial tolerance of system clock error to ±3% and achieves a final quartz-crystal comparable ±50-ppm accuracy.
B ANDWIDTH C OMPARISON R ESULTS B ETWEEN PSPN C ODES AND PN C ODES
Fig. 4. Plot of bandwidth versus toggle rate of the spreading codes.
The block of “frequency divider” generates the clock with data rate. PN code generator generates the PN code. Despreader is used for despreading procedure and the decision circuit detects the signal. The transistor netlist of the blocks in Fig. 6 is implemented. The circuit level simulator Hspice simulates the power consumption of the transistor netlist. All the blocks shown in Fig. 6 are included in this power consumption simulation. The circuit schematics described in Fig. 6 could be operated by different spreading codes with different code lengths. That is to say, this is a soft-coded spreadspectrum system. The simulation results of the power consumption are listed in Table IV. From Table IV, we find the percentages of reduction for power consumption range from 8% to 14% with PSPN codes compared to PN codes. The concept of low toggle rate means low-power consumption has been verified by the simulation results.
Efficient channel estimation is important for multiple-antenna sys- tems especially when the number of antennas increases. To avoid the degradation of estimation accuracy due to interference, an intuitive way is to transmit training sequences for each transmitting antenna in turn . Fora system with M antennas, this scheme requires M times band- width compared with a single antenna transmitter system. However, orthogonal training sequences can be simultaneously applied for each transmitter antenna to estimate the channel efficiently , . Fora single tap coefficient discrete channel model, it is well known that or- thogonal sequences are the optimal training sequences that minimize the estimation errors if the additive noises are identical independent Gaussian random processes. In this case, a Hadamard matrix can be ap- plied. However, in the case of multipath channel, the channel for each pair of transmitting and receiving antennas should be modeled by sev- eral taps. It can be proven that the training sequences should have both good autocorrelation and cross correlation. Existence of such training sequence sets is still an open problem. In this paper, we discuss the existence of such optimal binary training sequence sets and propose a search algorithm.
Fig. 6. Probability distributions of P and P as functions of S with several SNR values.
For the power management module, the functional blocks of noise estimator, sampling rate generator, and threshold gener- ator are included in this module. The operating frequencies of sampling rate generator and threshold generator are exactly the refresh rate of the noise estimator, which is denoted by . is the rate at which the noise estimator update the estimated channel noise level. The refresh frequency depends on the co- herent time of the channel. For example, the refresh frequency of the system with fast-fading channel will be larger than that with slow-fading channel. According to this reason, we assume that the channel is slow-fading channel. There are several prac- tical considerations when designing the refresh rate. The data rate is usually several times of refresh rate. And the sampling rate is also several times of data rate. Because the switch fre- quency of the power management module is much less than that of the PN acquisition structure, the power consumption of this module can be ignored. Therefore, the power consumption of this proposed architecture is dominated by the code acquisition structure, which is mainly depends on the sampling rate.
The difficulties of design BER-minimized STF block codes for the MIMO highly frequency-selective block-fading chan- nels can be discussed in three aspects. Note that we take the IEEE 802.15.3a UWB channel model as an example in this paper. 1) First, the IEEE 802.15.3a channel model has four different sets of parameters, named CM1, CM2, CM3, and CM4. For different channels, we have to design different codes to reflect the channel characteristics. One challenging issue arises: Is there a universal code which is optimal for all the four channel models CM 1–4 for given numbers of subcarriers and transmit antennas? 2) As the numbers of subcarriers and transmit antennas increase, the number of all possible codes becomes astronomical. Thus, the second challenge is how to search the optimal codes efficiently. 3) Because traditional STF coding methods focus on linear codes, it will be challenging to examine if there exist nonlinear optimal STF block codes.
A. Residual FrequencyTracking Error
In the acquisition stage, we assume that the residual fre- quency error after the frequencytracking stage is an integral multiple of the subcarrier spacing. However, there exists a residual frequencytracking error that introduces ICI and de- grades the performance of the acquisition scheme. To explore how the residual frequencytracking error affects the acquisi- tion scheme, a computer simulation is taken. Fig. 15 shows the plots of versus the normalized residual frequencytracking error. The solid-line curve and the dashed-line curve represent the estimated by (22) for SNR dB and SNR dB, respectively. In Fig. 15, the “ ” symbols and the “ ” symbols represent the Monte Carlo simulations results for SNR dB and SNR dB, respectively. To speed up our simulation, the acquisition range is set to ten. We can see that the missed lock probability of the acquisition scheme is still very low even the residual frequencytracking error is as large as 0.47 times of the subcarrier spacing. That is, the proposed acquisition scheme is insensitive to the tracking error. As shown in Fig. 7, the residual tracking error after the frequency detector without averaging process is smaller than 0.15 times of the subcarrier spacing, which is tolerable to the proposed acquisition scheme.
The OFDM (orthogonal frequency division multiplexing) signaling is one kind of "Multi-Carrier" technology, which can slow down delayed transmission especially for operating at frequency non-selectivefadingchannels. Therefore, it gains quite large profit making a study of executing efficiency at different frequency non-selectivefadingchannels. The OFDM system has become the most popular choice of transmission modulation in the new wireless communication field. Hence, in this article utilize a method of OFDM modulation to explore the statistical characteristic of distributions of Rayleigh、Ricean、Weibull while working in fading channel.
Chieh-Ho Lee, Student Member, IEEE, and Chung-Ju Chang, Senior Member, IEEE
Abstract— This letter proposes an approximation method by characteristic function (AM-CF) method to approximate the distribution of interference in DS/CDMA cellular systems. This method considers the effects of frequency-selective multipath fad- ing; it also assumes perfect power control and a rectangular/sinc chip waveform. The AM-CF method can yield results that fit the Monte Carlo simulation results more accurately than the conventional standard Gaussian approximation method.