Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
1999-08-19
2003-05-27
Pham, Chi (Department: 2631)
Pulse or digital communications
Equalizers
Automatic
C375S147000
Reexamination Certificate
active
06570918
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a receiver for recovering data from spread spectrum radio signals. The invention furthermore relates to a method for recovering data from spread spectrum radio signals.
Data is communicated using spread spectrum radio signals by combining the data with a spreading code which has an effect of increasing the bandwidth of the radio signals on to which the data and spreading code combination are modulated. Properties of the spreading code facilitate detection of the radio signals and recovery of the data at a receiver, despite the presence of contemporaneously detected like modulated spread spectrum radio signals generated by other transmitters. As a result, spread spectrum radio systems are used in mobile radio telephone systems to provide contemporaneous communication of data from a plurality of mobile stations. This is known as code division multiple access, and has been selected for both second and third generation mobile radio communication systems.
A characteristic of radio communications within a part of the radio frequency spectrum allocated to mobile radio communication systems, is that the radio signals propagate from transmitters to receivers via a plurality of paths. As a result, the radio signals detected by the receiver are superimposed. Where a temporal difference in the propagation time between at least two paths differs by more than a symbol period, inter-symbol interference results which must be mitigated in the receiver in order for data communicated by the radio signals to be recovered.
A known receiver which operates to recover data from received spread spectrum radio signals is known to those skilled in the art as a “rake” receiver. The rake receiver is known to be provided with a plurality of rake fingers. Each of the rake fingers is assigned on a pre-determined basis to one of a plurality of delays corresponding to the relative propagation delay experienced by the radio signals travelling via propagation paths along which the received radio signals may have traveled between the transmitter and the receiver. Thus the rake fingers are positioned in a temporal relationship with respect to each other and within each rake a cross-correlator is provided to cross-correlate the received radio signals with a locally generated version of the user specific spreading code. The cross-correlation after a symbol period, generated by each of the rake fingers, is thereafter combined in order to generate an estimate of the data symbols communicated by the radio signals.
A disadvantage of known rake receivers is that substantial parts of the energy of the radio signals may reach the receiver via paths which have a propagation delay corresponding to a relative temporal displacement falling between the temporal position of the fingers of the rake receiver. As such self-interference occurs within the rake receiver in that the energy corresponding to paths not in correspondence with the temporal position of the rake fingers causes interference with the correlation of the received radio signals within the rake fingers. Additionally each path received by a finger even produces self-interference to all other fingers. The self-interference is governed approximately by the auto-correlation function of segments of the spreading code.
Another known disadvantage of the rake receiver is caused by what is known as the “near/far” problem. The “near/far” problem is known to those skilled in the art as an effect whereby radio signals transmitted by a transmitter close to the receiver, having a relatively strong received signal strength, have an effect of suppressing radio signals transmitted by a transmitter further away, having a relatively weak signal strength. This has an effect of further exacerbating self-interference, especially where several paths of approximately the same amplitude which are not in synchronization with the corresponding temporal position of the rake fingers are present in the received signals.
A spread spectrum radio signal is generated by combining the data symbols to be communicated with a spreading code and modulating the resulting combination onto a radio frequency carrier signal. The spreading code typically comprises a plurality of symbols known as chips which are combined with the data by modulating the spreading code with the data in some way. Furthermore, in order to provide appropriate spectral shaping, the chips of the spreading code are combined with a modulation filter such as, for example, a root raised cosine filter. A root raised cosine modulation filter is well known to those skilled in the art. It is known that by passing the received signal through a receiver filter having a corresponding root raised cosine filter, a raised cosine pulse shape results, with the effect that if the signal is sampled at the symbol rate or in this case the chip rate, then no inter-symbol interference is present in the received signal samples. This applies, of course, only under the condition that there is no inter-symbol interference caused by the channel.
Multi-path propagation is a characteristic of radio communications in a frequency band used by mobile radio communication systems. As a consequence, inter-symbol interference resulting from the transmit and receive filters will be present at each of the correlators of the rake fingers as a result of paths causing self-interference. In other words, the self-interference problem has the further effect that the received chips are superimposed, causing residual inter-symbol interference in the received signal. In order to prevent non-linear distortions by aliasing when decimating the signal to the chip rate a very high initial sample rate is required so as to allow a fine time resolution of the decimation process. In order to effect this time resolution, the received signal must be oversampled at the chip rate. However, since the chip rate is already many times greater than the symbol rate, oversampling at the chip rate is undesirable.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a receiver for recovering data from spread spectrum radio signals and a method for recovering data from spread spectrum radio signals which overcome the above-mentioned disadvantages of the heretofore-known receivers and methods of this general type and which recover data from spread spectrum radio signals in which inter-symbol interference is substantially reduced, without requiring a sampling of the spread spectrum signal at a high rate.
With the foregoing and other objects in view there is provided, in accordance with the invention, a receiver for recovering data from spread spectrum signals, comprising:
an equalizer for filtering spread spectrum signals in accordance with an impulse response adapted to the spread spectrum signals and generating filtered signals, the equalizer mitigating at least a portion of an inter-symbol interference present in the spread spectrum signals; and
a data symbol estimator connected to the equalizer for estimating data by de-spreading the filtered signals with a spreading code.
In accordance with another feature of the invention, a receiver controller includes a data processor for adapting a plurality of impulse response coefficients for an impulse response estimate of a communications channel through which the spread spectrum signals have passed, the data symbol estimator detects data symbols from the spreading code in combination with the impulse response estimate.
In accordance with yet another feature of the invention, the data symbol estimator is a rake detector having at least one rake finger for correlating the filtered signals with respect to the spreading code at a delay determined from the impulse response estimate at relative temporal displacements corresponding to at least one propagation path and scaled by at least one of the impulse response coefficients.
In accordance with a further feature of the invention, the spread spectrum signals include a given signal formed with a pilot spreading
Greenberg Laurence A.
Mayback Gregory L.
Nguyen Dung X.
Pham Chi
Siemens Aktiengesellschaft
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