In a wireless communication system, especially in a mobile communication system, fading occurs from times to times. Buildings, mountains, and foliage on the transmission path between a transmitter and a receiver can cause reflection, diffraction, and scattering on a propagating electromagnetic wave. The electromagnetic waves reflected from various large objects, travel along different paths of varying lengths. If there is an obstacle with sharp irregularities on the transmission path, the secondary waves resulting from the obstructing surface are present around the obstacle. Also if there are small objects, rough surfaces, and other irregularities on the transmission path, scattered waves are created. All these waves will interact with each other and cause multipath fading at specific locations.
The multipath fading can seriously deteriorate the quality of a communication system. In a multi-channel communication system, the multipath fading could be more serious. Each component of a multipath fading signal on a particular channel only not interferes with the other components of the signal, but also interferes with each component of the signals on other channels.
Direct sequence spread spectrum system is multipath resistant due to the fact that the delayed versions of the transmitted pseudo-noise (PN) signal have poor correlation with the original PN signal. Together with a RAKE receiver, a direct sequence spread spectrum system can combine the information obtained from several resolvable multipath components and therefore improve the system performance.
In a RAKE receiver, usually there are a searcher, several fingers, and a signal combiner. A searcher is a device to detect each major component of a multipath signal and obtain the information about the component, such as signal strength, phase, and time relation. A finger is a device to track a particular component of a multipath signal. A signal combiner is a device to combine the various components of a multipath signal together.
One can build a searcher based on either correlator or matched filter. A searcher based on correlator shifts its local reference signal by an amount of time and then compares the input signal with the shifted local reference signal for a period of time. It finds the multipath information by repeating the process of shifting and comparing. Usually, the smaller the amount of time shifted, the higher the time resolution on a multipath signal; the longer the period of time compared, the more reliable the detection result. A searcher based on matched filter compares a fixed section of reference signal with a section of input signal while the input signal keeps coming. The lengths of both sections are equal. It can detect a component of a multipath signal immediately when the component is coming. Generally speaking, to have about the same confidence on detection result, a searcher based on correlator needs less hardware but more time, while a searcher based on matched filter needs less time but more hardware.
In a packet-switched communication system, the packet received could come from total different source than the previous one and generally there is no any relation between two adjacent packets. In order to obtain multipath information quickly, one may desire to use matched filter especially when transmission rate is very high. Also even there are some similarities between a searcher and a finger, in many communication systems, searchers and fingers are built separately, which results in more hardware. In order to save hardware, one may desire to use the same set of matched filters to serve both searcher and fingers.
A portion of a searcher based on regular matched filter is shown in FIG. 1.
The signal register 105 consists of a plurality of registers with the first register connected to input signal Sin and each of the rest registers connected to its previous one. Usually these registers are driven by a same clock such as the sampling clock of input signal Sin. There is a tapped output after every several registers and together there are M tapped outputs. The signals at the M outputs are replicas of the input signal Sin with different delays. The delay between any two adjacent output signals is the same.
In a digital implementation of a matched filter, it is necessary to align up the input signal and the reference signal in time domain in order to make the matched filter to work. That is, there must be a same number of samples from the input signal and from the reference signal for comparison at any moment and the sampling periods for both the input signal and the reference signal must be the same.
The fixed reference signal 110 also has M output signals invariant with the time. They are a sampled version of a local reference signal in a predetermined time interval with delay between any two adjacent samples being equal to the delay between two adjacent output signals of signal register 105. When the local reference signal is a PN sequence, the fixed reference signal is a section of the PN sequence. Usually the section is a whole sequence of a short PN sequence are section of a long PIN sequence. In the latter case, since the M outout signals of the fixed reference signal 110 are invariant with the time to provide a fixed reference in a regular matched filter, the section is a fixed section and does not change with the time. Having a fixed reference signal, a regular matched filter does not change its reference section and therefore it usually is used for detecting if a signal is matched to a whole sequence of a short sequence or if a signal is matched to a particular fixed section of a long sequence.
A section of input signal Sin is represented by M output signals from the signal register 105 and a section of the local reference signal is represented by M output signals from the fixed signal 110. Since the M outout signals of the fixed reference signal 110 are invariant with the time, the section of the local reference signal used for detection is a fixed section of the local reference signal or a whole sequence of the local reference signal. In other words, the reference section does not change with the time.
The matched filter 115 consists of a plurality of multipliers and an adder. Each of the multipliers 1201 to 120M will multiply an output signal from fixed reference signal 110 with a corresponding output signal from signal register 105. The adder 125 will add the products from all of the multipliers together. Matched filter 115 takes the summation as its output, which is the correlation between the section of input signal Sin and the section of the reference signal.
Since FIG. 1 is part of a searcher, it is desirable that either the M signals from local fixed reference signal 110 correspond to a unique word specially for finding various components of a multipath signal or M is very large so that there is no confusion to identify a multipath component. To save hardware, one may want to use the same hardware to serve a finger. As FIG. 1 has to be part of a finger, the M output signals from local fixed reference signal 110 can not be the same unique word required by searcher, otherwise the word is not unique. Also, the M output signals should correspond to one symbol period otherwise it is difficult to separate the contribution of the first data symbol from the contribution of the second data symbol. However, when the M output signals from local fixed reference signal 110 correspond to one symbol period and do not correspond to a unique word, the searcher based FIG. 1 can not detect a multipath signal spanned over several symbol periods. Because there is no way to tell that if a detected component is a component of first symbol or a component of second symbol.
To detect and track a multipath signal spanned over more than 1 symbol period correctly based on a same set of matched filters, it is necessary for the reference signal to be different in several adjacent symbol periods. In regular matched filter, the reference signal is fixed as shown in FIG. 1. Therefore when the reference signal varies from symbol period to symbol period, the structure based FIG. 1 does not apply. To distinguish from regular matched filter, one can refer a matched filter with a variable reference signal or with a modified input signal as a dynamic matched filter.
It would, therefore, be desirable to build dynamic matched filters in a receiver for detecting, tracking, and combining the various components of a multipath signal spanned over several symbol periods.