Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1997-11-07
2001-09-25
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C370S342000
Reexamination Certificate
active
06295311
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to radio communication systems, and more particularly to a method and apparatus for performing phase estimation of a noncoherently detected signal transmitted in a mobile communication system such as a code-division multiple access communication system.
BACKGROUND OF THE INVENTION
Spread spectrum is a digital radio frequency signaling technique in which a synchronized code is used by a transmitter and receiver pair to respectively spread and de-spread a transmitted data sequence over a predefined bandwidth. As the name suggests, spread spectrum systems utilize more bandwidth than other conventional signaling techniques such as time-division multiple access (TDMA). However, the improved security and noise rejection attained by spread spectrum systems compensates for the increased bandwidth requirements.
Spread spectrum modulation was originally developed for military use, where secure communications were required. However, due to its unique multipath and interference rejection characteristics, spread spectrum has civilian applications in mobile radio environments. Spread spectrum techniques are especially well suited for applications where a number of independent users need to share a common band of frequencies without the benefit of an external synchronizing mechanism, such as in code-division multiple access (CDMA) cellular radio systems.
CDMA techniques are well known. Some well known CDMA systems employ coherent detection for both directions of the communication path (i.e., base station to mobile and vice versa). Coherent detection (with phase) offers significant advantages over noncoherent detection (without phase). However, for the conventional phase locked loop type of phase synchronization, a stable signal with a high signal-to-noise ratio is required to track the unknown phase. For the reverse link (mobile unit to base station) of CDMA systems, several reasons make such phase tracking impractical. First, the spreaded signal has very low signal-to-noise ratio which cannot be used for tracking the phase of the signal. Second, fast fading makes the signal unstable and shifts the unknown phase. Thus, a very fast tracking loop is required to maintain a good estimation of the phase. In addition, the bursty nature of voice activity disrupts the signal. Further, excessive power consumption precludes transmitting a pilot signal from the mobile. Moreover, a rake receiver is typically used to combine multipath fading. If a plurality of multipath signals are used for the demodulation, the signal-to-noise ratio per path to achieve the desired performance will be significantly reduced. However, each path experiences independent phase shift. Thus, the phase tracking loop has to work with a very low signal-to-noise ratio for each path. Finally, any phase tracking circuitry built to overcome the above difficulties, will be complicated and expensive. Rake receivers comprise multiple data receivers, thus, the complexity of the circuitry required for phase tracking is dependant upon the number of demodulated paths which is typically relatively high.
Because of the difficulties associated with coherent detection, noncoherent detection is typically used in the reverse link of CDMA systems. In order to achieve decoding in many CDMA systems, both an orthogonal code and a convolutional code have to be soft-decision decoded. If orthogonal coding were the outer layer (first step in transmit and last step in receive), its soft decision decoding would be straightforward. However, in order to soft-decision decode the convolutional code, the decoder of the orthogonal code must develop a likelihood for each convolutional code symbol.
Prior art approaches to this issue have certain drawbacks. For instance, prior art approaches do not deliver the signal-to-noise ratios achievable if coherent detection could be used. What is needed therefore, is a method for estimating the unknown phase and strength of a noncoherently detected signal in order to increase the received signal-to-noise ratio of symbols transmitted in code-division multiple access communication systems and, thus, the capacity of such systems.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the invention, a method for estimating the phase of a noncoherently detected orthogonal signal is provided. The method comprises the steps of: sampling first and second received signals, wherein each of the first and second received signals is representative of a transmitted signal; and respectively determining first, second, third and fourth values from the first, second, third, and fourth received signals wherein the first and second values and the third and fourth values are respectively indicative of a likelihood that first and second estimated signals correspond to the transmitted signal. The method also comprises the steps of developing an index from the first, second, third and fourth values; selecting a first selected value from the first and second values based on the index; developing a first weight from the first selected value; selecting a second selected value from the third and fourth values based on the index; developing a second weight from the second selected value; scaling the first and second values with the first weight; and, scaling the third and fourth values with the second weight.
In some preferred embodiments, the first received signal is an in-phase signal from a first finger of a rake receiver and the second received signal is a quadrature-phase signal. In other preferred embodiments, the first received signal is an in-phase signal from a first finger of a rake receiver and the second received signal is from a second finger of a rake receiver.
In any of the foregoing embodiments, the step of determining a first value may optionally be performed via a Hadamard Transform; the step of developing an index may comprise summing the first and third values and summing the second and fourth values and selecting the index of the larger of the two sums; and/or the step of scaling the first and second values comprises multiplying the first and second values by the first weight.
In some embodiments, the step of developing the weight comprises the step of accumulating a plurality of the selected values over a predefined time period and calculating an average of the plurality. In such an instance, the average may optionally be calculated based on a subset of the plurality.
In any of the foregoing embodiments, the method may also include the step of combining the scaled first value with the scaled third value and combining the scaled second value with the scaled fourth value.
In accordance with a further significant aspect of the invention, an apparatus for compensating for phase differences between first and second received signals corresponding to a transmitted signal is provided. The apparatus comprises a first selector for selecting a first value from first and second likelihoods that first and second estimates correspond to the transmitted signal. The first and second likelihoods are based on the first received signal. The apparatus also includes a first estimator for developing a first weight from the first value; and a second selector for selecting a second value from third and fourth likelihoods that third and fourth estimates correspond to the transmitted signal. The third and fourth likelihoods are based on the second received signal. In addition, the apparatus is provided with a second estimator for developing a second weight from the second value.
In some preferred embodiments, the apparatus is further provided with a first buffer for buffering the first value. In such embodiments, the apparatus may optionally be provided with a second buffer for buffering the second value; a first queue for queuing the first and second likelihoods; and/or a second queue for queuing the third and fourth likelihoods.
In some embodiments, the first received signal is an in-phase signal from a first finger of a rake receiver. In such an embodiment, the second receiv
Bocure Tesfaldet
Hughes Electronics Corporation
Sales Michael W.
Whelan John T.
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