Effects of Narrowband Interference on Direct Sequence Systems

This paper discusses the possible sources of narrowband interference and their effects on the operations of direct sequence systems. We analyze the effects of narrowband interference on the performance of the systems under perfect synchronization condition and compare the analytical results with those of experimental observations. Techniques for suppressing such interference are also discussed briefly. Narrowband interference may originate from internal and/or external sources, which can cause false locks on code and tracking circuits and degrade the performance of the correlator at the receiver. The interference are spread by the reference PN code in the correlation processes and their power spectra appear in form of [sin x/x]^ function at the correlator input. The reduction of the signal-to-noise ratio due to a narrowband interference depends on how close the interference frequency is with the carrier frequency, the power level of the interference, and the IF band­ width of filtering. The effects of narrowband inter­ ference obtained from the analyses have been verified by experimental observations. Techniques for suppress­ ing narrowband interference are mentioned briefly and a technique has been proposed based on the results of analyses. 1Introduction Direct sequence systems are the major type of spread spectrum systems. One of the significant features of such systems is the ability to reject interference, but the systems may still be vulnerable to certain types of interference under certain conditions. At first we might tend to think that a wideband rather than a narrowband interference would be more harmful to the systems. This is, however, not true except that the wideband interference has a high cross-correlation with the desired signals. In fact, a narrowband interference such as a continuous wave (CW) can be detrimental to the operation of direct sequence systems. Such interference may be produced as a result of incomplete code clock and carrier suppressions at the modulators of the system itself or may come from outside sources such as intentional and unintentional systems or jamming. Narrowband interference can affect the performance of the systems either by causing false locks in code and carrier tracking loops of the systems and/or by reducing the signal-to-noise ratio of the correlator output when the synchronization between the trans­ mitter and the receiver has already been established. This paper discusses possible sources of narrow­ band interference from both internal and external sources and their effect on code and carrier tracking loops. However, the major attention is to analyze the effect of CW interference on the performance of direct sequence systems under perfect synchronization conditon, to show how and to what extent CW inter­ ference affects the system under certain conditions, and to discuss techniques of preventing or minimizing the effects of the interference on the systems. 2. Sources of Narrowband Interference For direct sequence systems, pseudonoise random (PN) codes are used to modulate the carrier by phaseshift keying at the code rate. Balanced modulation is always used to produce suppressed carrier signals to achieve signal hiding and to prevent unauthorized interception. In addition, it has the advantage that no power will be wasted in transmitting the carrier. Any incomplete suppression can cause harmful and even detrimental interference to the receiver. As a result, a great care must be exercised to achieve complete code clock and carrier suppressions in the modulators of both the transmitter and the receiver of the systems. 2.1 Internal Sources Narrowband interference can be originated from the system itself. Fig. 1 (a) and (b) show the simplified diagrams of the transmitter and the receiver of a direct sequence system, respectively. ( a ) T R A N S M I T T E R