Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
2000-02-25
2004-09-28
Fan, Chieh M. (Department: 2634)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S316000, C375S347000
Reexamination Certificate
active
06798850
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method and arrangements in a radio receiver system with several standards. More specifically, the invention relates to a method and arrangements of receiving signals in a radio receiver system at the same time with several signal format standards, such as different CDMA, TDMA and FDMA standards. Furthermore, the invention relates to a rake receiver for use in such systems.
DESCRIPTION OF RELATED ART
Different channel access methods exist for the sending and receiving of digital signals. In TDMA, Time Division Multiple Access, a channel consists of a time slot in a periodic train of time intervals over the same frequency. In FDMA, Frequency Division Multiple Access, a communication channel is a single radio frequency band. Interference with adjacent channels is limited by the use of band pass filters which only pass signal energy within the specified frequency band. In contrast, Code Division Multiple Access, CDMA, allows signals to overlap in both time and frequency. Thus, several CDMA signals can share the same frequency band, but the CDMA receiver can also operate at several frequency bands.
In CDMA-technique, simultaneous connections can thus make use of a common frequency band. The selection, i.e. discrimination, between the desired signal and other signals is carried out by suitable signal processing, which is based on that the desired signal is coded. All simultaneous connections have different codes.
In the CDMA system, a radio frequency signal is received by an antenna unit and is downconverted to an intermediate frequency in one or several stages in the analog part of the radio. The intermediate signal is sampled and digitized by an AID converter and filtered in a channelizer to filter out each channel sufficiently well so that signals of other frequencies do not interfere. The result is a bandlimited signal that can be fed to base band processing to reproduce the sent digital data stream by a demodulating unit, which uses for example band spreading technique.
In band spreading, radio signals are transmitted by expanding the bandwidth of the information signal by means of an independent code signal. The most common type of band spreading is direct sequence, in other words direct modulation of the carrier wave with a code or signature sequence, also called pseudo-noise sequence or PN-sequence. The signature sequence usually comprises N bits which are called chips. The entire N-chip sequence is referred to as a transmitted symbol.
In traditional CDMA with direct spreading, the band spreading is achieved so that each fed information bit is replaced by a code sequence that consists of N chips and a spread spectrum is formed.
A plurality of coded information signals modulate a radio freqency carrier and are jointly received as a composite signal at a receiver. If the receiver is authorized and has a synchronous code signal, then the composite signal is correlated with one of the unique codes, and the corresponding information signal can be isolated and demodulated by the actual decoding technique.
There are specifications for the connection between two units with respect to the mechanical connection, the electrical and logic properties of the signals and the rules for the signal sequences. The logic or physical border between two functions is called interface.
In a mobile radio system, the existence of such signal reflecting surfaces as walls, building structures, hills, mountains, etc. are liable to cause the receiver of a radio signal perceiving the signals to be dispersed in time, wherewith signals that carry the same information will arrive at the receiver at different time delays.
To optimally detect the transmitted symbols the rays received must be combined, whereby the signal becomes diversity amplified. Diversity gain can be achieved in advanced signal processing, for example with a rake receiver, wherein several signal components can be used.
A rake-receiver is a radio receiver which utilizes this feature of time dispersed signals. The rake-receiver includes a plurality of independent receiver units, so called rake taps or fingers, each of which receives and tracks, or locates, a respective ray. The rake-receiver also includes means for combining the received signals, and means for delaying these signals so that they will be brought into phase prior to combining the signals. The rake-receiver can usually combine components with separation more than one or equal to one chip-period. The rake fingers or rake taps of a rake receiver in a CDMA-system is adjusted in accordance with the arrival time of the signal.
An ideal sampling is carried out at the strongest place of the signal. If a sample is taken only once during one chip duration, and it is not taken wherein the signal is strongest, then the sampling is not taken optimally in every chip or symbol with non-optimal detection as a result and a systematic error is achieved.
The probability for correct sampling is increased by oversampling, in which several samples are taken for each chip period and the best sample, a decision sample, for each chip can be chosen, which has a start time that indicates the phase of the signal.
The oversampling ratio is defined as the ratio of the signal's sample rate to the symbol or chip rate. In bandlimiting it means that, the more bandlimited the signal is, the more samples there exist for each chip and thus the oversampling ratio is increased with decreasing bandwidth.
The systematic error connected to the sampling of each chip gets smaller with increased oversampling ratio, but, on the other hand, the oversampling ratio out from the channelizer should be as low as possible so that the receiver system would not need complicated hardware. The lower the oversampling ratio, the farther from the optimum placement one gets, with decreasing sensitivity as a consequence. This sensititvity reduction is called “detector loss”. Thus, the systematic error also depends on the data rate and the equipment performance of the system.
The sampler (i.e., the A/D converters), has a predetermined sample period, which is adjusted in relation to the symbol or chip rate of the radio receiver in a simple way. The adjusted rate is usually a small integer multiple of the symbol rate, f-symbol, or in case of a direct sequence CDMA system, a small integer multiple of the chip rate, f-chip.
With a resampler it is possible to resample from a sample rate that is not an integer times the chip rate to a proper sample rate.
Resampling is usually performed by interpolation between points in the original data sequence, using different techniques. Some of the techniques use extensive computation, but the more bandlimited the signal is (consisting of only relatively low frequencies) the simpler are the satisfactory techniques.
The simplest interpolation techniques are different “hold” circuits, where first order hold uses the first derivative of the signal to interpolate between points and zero order hold just approximates the desired sample with the closest sample in the original data stream. Zero order hold is of course simpler, but requires a more strongly bandlimited signal in order not to give too high approximation errors. Other resampling techniques often involve higher order filtering or polynomial interpolation.
Resamplers are previously described in for instance U.S. Pat. No. 5,598,439 and U.S. Pat. No. 5,513,209 for demodulation of digitally modulated signals in communication systems. CDMA and rake receivers are decribed in for instance U.S. Pat. No. 5,640,416 and JP-08256084. A previous solution for a multi-rate CDMA communication is presented in EP-814581.
A base station that supports different radio transmission standards is known from EP-815698. The system of this patent can support CDMA and TDMA signals at the same time by using at least two digital channel devices. The channelizer of this patent can only work with one bandwidth and an own channelizer is needed for each standard. A system with several standards having this solution needs quite ext
Hellberg Richard
Wedin Anders
LandOfFree
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