A technique to improve the performance of serial, matched-filter acquisition in direct-sequence spread-spectrum packet radio communications

In this paper, we examine a simple method to improve the performance of serial, matched-filter acquisition in direct-sequence spread-spectrum packet radio communications. Each packet transmission includes an acquisition preamble, and the preamble sequence is changed at the boundaries of predefined time epochs based on a pseudorandom sequence generator. It is shown in previous work that the presence of an intermediate-frequency filter and the characteristics of the automatic gain-control subsystem lead to a probability of not acquiring that is a nonmonotonic function of the signal-to-noise ratio if the acquisition algorithm uses a threshold-crossing detector with a fixed threshold. The acquisition algorithm presented in this paper employs an estimator to adaptively select the acquisition threshold for each test statistic. It is shown that this technique reduces the severity of the nonmonotonicity and substantially improves the acquisition performance.     

Optimum transmission ranges in a direct-sequence spread-spectrummultihop packet radio network

The authors obtain the optimum transmission ranges to maximize throughput for a direct-sequence spread-spectrum multihop packet radio network. In the analysis, they model the network self-interference as a random variable which is equal to the sum of the interference power of all other terminals plus background noise. The model is applicable to other spread-spectrum schemes where the interference of one user appears as a noise source with constant power spectral density to the other users. The network terminals are modeled as a random Poisson field of interference power emitters. The statistics of the interference power at a receiving terminal are obtained and shown to be the stable distributions of a parameter that is dependent on the propagation power loss law. The optimum transmission range in such a network is of the form CK^α where C is a constant, K is a function of the processing gain, the background noise power spectral density, and the degree of error-correction coding used, and α is related to the power loss law. The results obtained can be used in heuristics to determine optimum routing strategies in multihop networks     

Optimizing information efficiency in a direct-sequence mobile packet radio network

We consider a direct-sequence multihop packet radio network using slotted ALOHA in a Rayleigh fading environment. We are interested in optimally choosing the transmission range, code rate, and slotted ALOHA transmission probability to be used by each node. We use a new performance measure, information efficiency, to analyze the network and show that the information efficiency of the low-cost mobile packet radio network can be improved approximately 20 fold by using the optimum parameters. We suggest a practical trellis-coded modulation scheme which comes close to realizing the theoretic limits found.     

A protocol for adaptive transmission in direct-sequence spread-spectrum packet radio networks

Wireless communication channels may change greatly from one transmission to the next, due to variations in propagation loss and interference. The use of fixed transmission parameters for such channels results in wasted energy when channel conditions are good. Adaptation of the power, code rate, and symbol rate reduces energy consumption and interference caused to other systems. Such adaptation requires information about the characteristics of the channel, which is more difficult to obtain in a packet radio network (PRN) or other mobile ad hoc network than in a typical cellular communication system. We develop methods for providing partial information about the channel state from three statistics that are derived by different subsystems in the receiving terminals of a direct-sequence spread-spectrum PRN. We present and evaluate a protocol that uses this information to adapt the transmission parameters in response to changes in interference and propagation conditions in the network. The performance of the new adaptive-transmission protocol is compared with a system with fixed transmission parameters and with an adaptive protocol that is furnished with perfect knowledge of the channel state at the completion of each transmission.     

Packet admission control in a direct-sequence spread-spectrum LEOsatellite communications network

The effect of multiple-access interference on the throughput performance of a direct-sequence spread-spectrum low-Earth-orbiting satellite communications network is discussed. To recognize the effect of interference when their sources are either inside or outside the service area of a satellite, we develop a stochastic model for the location of users. We show that the effect of interference on the performance degradation from users with large propagation distance to their connecting satellites is the dominant factor. Hence, to improve the performance of the system, we propose a method in which the transmissions of packets are controlled according to their distances to connecting satellites as well as the traffic distribution     

A comparison of slow-frequency-hop and direct-sequence spread-spectrum packet communications over doubly selective fading channels

The performance is compared for slow-frequency-hop and direct-sequence spread-spectrum packet communication systems of similar complexity with doubly selective fading channels characterized by several different multipath delay profiles and Doppler spreads. The delay profiles are chosen to reflect experimental measurements of mobile-radio channels, and the Doppler spreads are chosen to represent the range of carrier frequencies and vehicle speeds encountered in common mobile communication systems.     

The performance of optical fiber direct-sequence spread-spectrummultiple-access communications systems

Because of the random nature of the photodetection process and the multiple-user interference, an exact analysis of avalanche photodiode (APD)-based optical code division multiple-access (CDMA) communications systems is intractable and quite often, Monte Carlo (MC) simulations which yield exact estimates of system performance in terms of bit error rates (BERs) require a prohibitive computational burden. A quick and accurate MC method for simulating APD-based optical CDMA systems is presented. In particular, a performance analysis of optical CDMA systems employing optical orthogonal and prime sequence codes is undertaken     

Performance evaluation of a protocol for packet radio network in mobile computer communications

The need to provide computer network access to mobile terminals and computer communications in the mobile environment has stimulated and motivated the current developments in this area. Packet radio technology has developed over the past decade in response to the need for real-time, interactive communications among mobile users and shared computer resources. In computer communication systems we have a great need for sharing expensive resources among a collection of high peak-to-average (i.e., bursty) users. Packet radio networks provide an effective way to interconnect fixed and mobile resources. The results of an attempt to study the performance of the mobile packet radio network for computer communications over degraded channels are presented. We develop a model under fading conditions and derive a protocol for evaluating the performance of the mobile packet radio network (MPRNET) in terms of the packet error rate, packet delay, throughput and average number of retransmitted packets per cycle. The analytical results are presented and numerical examples are given to illustrate the behavior of these performance criteria as a function of packet transmission rate, packets transmitted per cycle, packet size, and vehicle speed with the help of appropriate plots.     

The near/far effect in local ALOHA radio communications

The ALOHA protocol allows many uncoordinated terminals to transmit data packets through a common channel. The application of ALOHA to short range (indoor or mobile) radio systems containing mobile terminals that send packets to a central base station is studied. In such systems, terminal-to-base distances can be vastly different and there can be pronounced shadowing. This gives rise to a capture effect at the receiver, where the packet arriving with the highest energy has a good chance of being detected accurately, even when other packets are present. It is well known that this capture effect improves the throughput and delay characteristics of the system as a whole. One of the principal findings is that even the user with the weakest signal shares in these improvements. This result is derived for the realistic case in which each terminal has a buffer to store packets that arrive while the terminal is waiting to transmit packets that were destroyed in earlier collisions.     

The application of automatic gain control to microwave radiometers

An automatic gain control system for stabilizing the gain of microwave radiometers is described. System analysis sets forth the requirements for gain and bandwidth of the AGC loop. Two systems are described. One is a continuous AGC system that maintains a constant detector voltage and, as a result, the system gain is a function of the input signal. For input signals which are small compared to the system noise temperatures the nonlinearity due to the AGC system is negligible. In the second system, the output of the detector is sampled when the receiver comparison switch is connected to the reference termination. Therefore, the gain of the system is not affected by the signal and no nonlinearity is caused by the AGC system. The effect of noise in the AGC loop is analyzed and it is shown that the time constant of the AGC system can be made shorter than the final system output filter without increasing the over-all system noise.     

On the radio capacity of TDMA and CDMA for broadband wireless packet communications

Studies of the capacity of cellular systems, stated in terms of the admissible number of remote users, have generally been limited to voice telephony. We address the problem of comparing the interference-limited performance of code-division multiple-access (CDMA) and time-division multiple-access (TDMA) systems in a packet-switched environment. The objective is to determine whether the capacity advantages claimed for circuit-switched CDMA still apply in a packet-switched environment, where the natural time diversity of bursty transmission may be a significant factor. Under a set of specific assumptions about the wireless environment (including path loss, shadow fading, multipath delay spread, cochannel interference, power control, and coding), we evaluate the number of users that can be admitted to the system while maintaining some desired quality-of-service (QoS) level. Four different classes of users with different characteristics and requirements are considered. The system capacity is found to depend significantly on the QoS objectives, which might be stated in terms of availability of some specified signal-to-interference level, packet-loss rate, or mean tolerable delay. The main finding is that strict requirements imposed on the radio access level tend to favor CDMA, whereas if some form of packet recovery is allowed at the higher layers (implying a relaxed set of requirements on the radio interface), then a somewhat higher capacity may be achieved by TDMA.     

Statistical description of non-coherent automatic gain control

Automatic gain control (AGC) is an important function in many different systems. It is the objective of this paper to introduce an analysis method for the statistical tracking performance of non-coherent AGC. The method presented can easily be adapted to a wide variety of different applications and systems, e.g. the power control in communications networks such as CDMA systems, or the design of gain control circuits along receiver down-conversion chains possibly including analogue-to-digital conversions. We describe the fluctuation of the controlled gain by stochastic differential equations. For some typical operational cases the physical probability density function of the AGC gain variation is compared with different approximation techniques, i.e. the Fokker-Planck (F–P) approach and the assumption of Gaussian distributions. The limitations of the approximation methods are emphasized because in telecommunications literature the F–P method is often applied without sufficient verification of the actual conditions for the method's applicability.     

基于光电倍增管的自动增益控制技术研究

为了压缩水下无线光通信系统中接收光信号的动态范围、提高有效通信距离,从自动增益控制技术的基础理论入手,利用光电倍增管增益可随工作电压改变的特性,设计出基于光电倍增管实现的自动增益控制电路,其数字控制部分以单片机为核心,并在空气中模拟水下实验。结果表明,该电路可以对接收光信号实现40dB的压缩,提高了水下无线光通信的有效范围。     

The impact of mobile radio communications

Future industry trends are discussed, using an analogy between the present and the industrial revolution. The scale of the industry is examined. Small conformal antennas are seen as key devices, since many antennas will come into use, and conventional antennas would spoil the beauty of the urban environment     

一种GPS接收机中自动增益控制的设计

在GPS接收机中,由于不同的传输距离以及多路径衰减的效应,使得接收机收到的信号强度是不固定的,所以采用一个自动增益控制电路来将信号强度处理成一个固定的强度大小。本文的目的在于,分析与设计一个应用与GPS卫星导航接收机的自动增益控制电路。介绍了自动增益控制的原理,对相关的电路设计进行了分析,并给出了仿真结果。     

Spectral efficiency of direct-sequence code-division multiple-access in cellular mobile radio

In this paper, the spectral efficiency achieved by a direct-sequence code-division multiple-access (DS-CDMA) system operating in a cellular mobile radio environment is discussed. In contrast to previous works, here, spectral efficiency is examined in terms of channel capacity per user, estimated in an average sense, and the system's inherent diversity potential is considered. The analysis covers only the forward link, assuming a static model of operation for the considered DS-CDMA system. In addition, it is shown that the application of a power control scheme leads to spectral efficiency enhancement due to cochannel interference reduction. Hence, the derived expressions provide a means for quantitative evaluation of cellular DS-CDMA spectral efficiency with respect to multiple-access and cochannel interference. © 1997 John Wiley & Sons, Ltd.     

基于AD8367的自动增益控制电路分析

简要介绍了ADI公司的对数放大器AD8367的特性以及利用AD8367实现自动增益控制（AGC）,通过建立简化的等效原理图分析了该AGC电路的数学特性,得到该电路的输入输出关系,以及实现自动增益控制时的输入信号幅度的范围。实验验证了该分析的正确性。     

Development of blind algorithm with automatic gain control

Wireless Networks - In this paper, the concept of blind algorithm with automatic gain control (AGC) is introduced in adaptive antenna system for signal optimization with an aim to estimate the...     

The strongly connecting problem on multihop packet radio networks

The problem of strongly connecting a multihop packet radio network by using a minimal total amount of transmission power is investigated. This problem is shown to be NP-complete. An approximation algorithm with the same computational complexity as that of finding a minimum spanning tree is given. It is also shown that the approximation algorithm can find a solution no greater than twice that of the optimal solution. Experimental results show that the approximation solution may be close to the optimal solution     

A call admission control scheme for packet data in CDMA cellular communications

In wireless cellular communication systems, call admission control (CAC) is to ensure satisfactory services for mobile users and maximize the utilization of the limited radio spectrum. In this paper, we propose a new CAC scheme for a code division multiple access (CDMA) wireless cellular network supporting heterogeneous self-similar data traffic. In addition to ensuring transmission accuracy at the bit level, the CAC scheme guarantees service requirements at both the call level and the packet level. The grade of service (GoS) at the call level and the quality of service (QoS) at the packet level are evaluated using the handoff call dropping probability and the packet transmission delay, respectively. The effective bandwidth approach for data traffic is applied to guarantee QoS requirements. Handoff probability and cell overload probability are derived via the traffic aggregation method. The two probabilities are used to determine the handoff call dropping probability, and the GoS requirement can be guaranteed on a per call basis. Numerical analysis and computer simulation results demonstrate that the proposed CAC scheme can meet both QoS and GoS requirements and achieve efficient resource utilization.