229765436：Code acquisition with diversity combining for FFH

229765436：Code acquisition with persity combining for FFH-MFSK spread spectrum systems

Code acquisition with persity combining for FFH/MFSK spread spectrum systemsW.H. Sheen C.-C. Tseng

Indexing terms: Code ucquisition, FFH/MFSK sprrud spectrum, Muliipuihfading, Multitone jamming, Diversity combining

Abstract: Pseudonoise code acquisition based on hard-decision and clipped persity combining is investigated for fast frequency-hopped MFSK systems under the effects of white gaussian noise, band multitone jamming and multipath fading. Multipath fading may affect the signal or jamming channel or both. An analytical method is developed to evaluate the mean acquisition time and the operating characteristics of the acquisition system for various jamming and channel conditions. Numerical results show that the time persity offered by a fast hopper can provide a significant performance improvement and may be indispensable in some cases to obtain a satisfactory performance; the clipped persity combining outperforms the hard-decision persity combining, and under some jamming and channel conditions the performance difference of these two methods can be significant; and multipath fading on the signal path degrades the acquisition performance. 1 4 dB degradation is observed, whereas fading on the jamming path reduces the effectiveness of a jamming signal. Computer simulations are also conducted to substantiate the analytical results.

1

Introduction

Frequency hopping (FH) spread spectrum signalling with forward error correction coding (FEC) is the technique of choice for antijam protection against a threat with a large jamming power[1-31. By hopping more than once over a symbol duration, the fast F H (FFH) spread spectrum signalling provides an easy-to-implement form of time persity that can further improve the system performance[14]. A vast volume of research has been devoted to devise various persity combining receivers for FFH spread spectrum systems under various jamming, interference and channel conditions, all with the assumption of perfect PN (pseudonoise) code synchronisation[4-111. However, for a spread spectrum system, accurate PN code synchronisation is essential before any attempt of data demodulation. PN code synchronisation is achieved in twoQ IEE, 1998 IEE Procerdingr online no. 19982446 Paper first received 4th Apnl and in revised form 12th December 1997 The authors are with the Department of Electncal Engneenng, National Chung a e n g University, Chid-Yi, 621 Taiwan, Republic of ChinaIEE Proc -Commun, Vol. 145, No. 6, December 1998

stages: code acquisition followed by code tracking. Code acquisition is a coarse code alignment that aligns the received and local sequences of hopping frequencies to within a hopping duration or less. Code tracking, a fine code alignment, is to maintain a fine synchronism of the hopping boundaries of the two hopping sequences[I-31. In this paper the issue of fast PN code acquisition with perity combining for FFH/MFSK systems is investigated. PN code acquisition has been one

of the most challenging tasks in the design of a spread spectrum receiver. Many persity combining methods have been proposed for data demodulation of an F F H spread spectrum signal[ 4 1 1] . Some examples are the clipped persity combining[5, 71, the noise- and self normalisation persity combining[5, 61, the ratio-statistic persity combining[SI, moment-methods persity combining, list metric persity combining, and many others[9-111. Unfortunately, the persity combining methods designed for data demodulation may not be applicable directly to the problem of PN code acquisition. For example, the ratio-statistic persity combining and self-normalisation persity combining are useless for PN code acquisition owing to a very large probability of false alarm. Very recently, a serial search acquisition system with noise-normalisation persity combining was analysed for FFH/BFSK system under the effects of multipath fading, partial band noise jamming and additive white gaussian noise[13]. In this study, we investigate serial search acquisition systems based on clipped and hard-decision persity combining for FFH/MFSK systems under the effects of band multitone jamming, multipath fading and additive white gaussian noise. The clipped and hard-decision persity combining are considered to be more practical than the noise-normalisation persity combining because it is often difficult to estimate accurately the noise power that is required in the noise normalisation persity combining[5, 131.2

System descriptions

2.1 System and channel models By fast frequency-hopping, we mean that there are more than one hop per MFSK symbol. Let T, and T h denote the symbol and hopping period, respectively. Then T,= LTh, L is a positive integer. To maintain the orthogonality among M-ary signal tones, the adjacent modulation tones are set to be l/Thapart, and a contiguous band of M frequency slots each with a bandwidth of l/Th is used to transmit MFSK signals. The MFSK bands of frequency slots are assumed to be nonoverlap431

229765436：Code acquisition with persity combining for FFH-MFSK spread spectrum systems

ping. As a result, if W,,,Hz is the total available bandwidth, there are Q= W,;ThIM MFSK bands to hop on. The channel considered is a slow multipath Rician fading channel with a coherence bandwidth less than the minimum hopping (frequency) distance. Consequently, MFSK signals with different hopping frequencies are affected by independent fading. By slow fading we mean that the channel is remained unchanged during one hopping period. In addition to inherent additive white gaussian noise, the transmitted signal is also corrupted by the worstcase (in the Houston sense) band multitone jamming, i.e. at most one jamming tone is possible on an MFSK band which may or may not experience channel fading. The fading that affects, if any, the …… 此处隐藏：30256字，全部文档内容请下载后查看。喜欢就下载吧 ……