Multi-path equalizer configuration technique

A technique for arranging a receiver device (20) includes equally spacing adjacent rake receivers (26-30) within an observation window (OW). A disclosed technique for updating an observation window includes changing the position of one of the rake fingers (24, 30) while maintaining a position of other ones of the rake fingers. The disclosed technique for arranging rake fingers in a receiver device (20) does not depend on searching and tracking channel conditions and reduces computational complexity without sacrificing receiver performance.

FIELD OF THE INVENTION

This invention generally relates to communications. More particularly, this invention relates to wireless communication.

DESCRIPTION OF THE RELATED ART

Wireless communications are used in a variety of situations. For communications in cellular networks, there is a need for a variety of receiving devices that receive wirelessly transmitted signals. One type of receiver is known as a rake receiver, which includes a plurality of rake fingers arranged to provide a desired performance. Rake receivers operate in a well known manner. Arranging the rake fingers in such a receiver is typically based upon the properties of the wireless propagation channel.

Two properties that have to be addressed for most situations are the propagation delay, which is caused by the distance between the transmitter and the receiver, and the delay spread, which is caused by the multi-path channel.

A well-known device for compensating for delay spread is a so-called equalizer. In advanced CDMA receivers, for example, an equalizer is typically applied before the de-spreading stage. Computing equalizer filter coefficients requires knowledge of the channel co-efficients of the wireless communication channel. In a CDMA system, for example, the channel coefficients can be obtained by a channel estimation algorithm, which usually operates on de-spreaded pilot symbols in the control channel.

Measuring the timing properties of the wireless channel is usually accomplished using a correlator unit acting as a searcher on the control channel containing the pilot symbols. The searcher obtains the channel energy response of the multi-path channel.FIG. 1Aschematically shows an example channel energy response10.FIG. 1Bshows a corresponding rake finger assignment12where a rake finger is assigned to each delay of the detected multi-path. ComparingFIG. 1AandFIG. 1B, for example, shows the correspondence between the position of the rake finger arrangement12and the position of the delays of the detected multi-path. While such an approach has proven useful for many situations, there are drawbacks.

For example, an equalizer must have knowledge of the channel coefficients for making appropriate determinations. De-spreading and channel estimation introduce some latency in the signal processing chain. Accordingly, the output of the channel estimation algorithm is typically not time-aligned with the input signal from the antenna. If an equalizer is placed before a de-spreader, the time delay of the channel estimates must be compensated by including a buffer such as a first in, first out (FIFO) buffer in front of the equalizer or the receiver performance will suffer from the timing-misalignment.

Moreover, it is computationally intense to have the searcher or correlator unit monitoring the control channel. Implementing such a searcher requires significant processing power when realized through software and a significant amount of logic when realized through hardware. In either event, the realization is relatively costly and causes high power consumption.

There is a need for a more efficient arrangement for processing received signals in wireless communication systems. This invention addresses that need.

SUMMARY OF THE INVENTION

An exemplary method of configuring a receiver includes equally spacing each of a plurality of rake fingers from an adjacent one of the rake fingers.

In the disclosed example, the equal spacing comprises a selected amount of time. In one example, the amount of time is less than an inverse of a received signal bandwidth.

A disclosed example includes determining a relationship between power at the plurality of rake fingers and time. An observation window containing the equally spaced rake fingers can be adjusted responsive to the determined relationship according to selected criteria.

An exemplary receiver device includes a plurality of rake fingers having an equal spacing between each rake finger and an adjacent one of the rake fingers.

The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.

DETAILED DESCRIPTION

The following examples demonstrate how equally spacing rake fingers using a constant or uniform timing separation between rake fingers for processing a received signal reduces computational complexity and facilitates more efficient signal processing.

FIG. 2schematically shows an example receiver device20including an antenna22for receiving at least one signal. The example receiver device20includes four rake fingers24,26,28and30. The rake finger24, for example, includes a portion32for down sampling a received signal. In this example, each portion32performs down sampling by two. A de-spreading and de-channelization portion34includes a filter36and a down sampling portion38. In this example, the filter36corresponds to scrambling and channelization codes. The down sampling portion38in this example down samples by the spreading factor using a known technique.

The rake fingers24-30are separated from each other in time and each of the rake fingers is equally spaced from an adjacent one of the rake fingers. The example ofFIG. 2includes a functional block40that corresponds to a spacing Ts, which corresponds to a selected amount of time between the rake fingers24and26. Another functional block42introduces a spacing 2Tsbetween the rake finger24and the rake finger28. Accordingly, a spacing Tsexists between the rake fingers26and28. Another functional block44introduces a spacing 2 Tsin addition to that provided by the functional block40such that the rake finger30is separated from the rake finger28by Ts.

FIG. 3schematically shows a rake finger configuration50consistent with the example ofFIG. 2. In this example, each of the rake fingers24,26,28and30is spaced from an adjacent one of the rake fingers by a spacing Ts. The equal spacing Tsbetween the rake fingers does not depend on tracking delays on a detected multi-path and then assigning the positions of the rake fingers. Instead, equally spaced rake fingers are arranged for processing a received signal. The spacing Tsis selected independent of the type of searching and tracking that has otherwise been required.

Selecting the Tsspacing between the rake fingers may vary to meet the needs of a particular situation. In one example, a three quarter chip spacing is used between the rake fingers. The spacing Tsneed not be an integer fraction of the chip duration. One example technique for selecting the spacing Tsincludes ensuring that the spacing between the rake fingers is smaller than the inverse of the signal bandwidth of the received signal.

In the example ofFIG. 3, the rake fingers24-30are spaced within an observation window shown at OW. The length of the observation window in this example is 3Ts. For receiver devices having K rake fingers, the observation window length OW will be (K−1)Ts.

Assigning the positions of the rake fingers in this manner reduces the computational complexity of a receiver device, in part, because it is no longer necessary to track the channel for determining how to position rake fingers. The size of the observation window in general will depend on the delay spread, which depends on the cell size and the environment.

An example like that shown inFIGS. 2 and 3is especially useful for relatively small cells or so-called microcells because the delay spread is small and there is less variation in the propagation delay under such circumstances. Given this description, those skilled in the art will realize how to implement an embodiment of this invention to meet the needs of their particular situation including selecting an appropriate observation window size and appropriate spacing between the rake fingers.

Referring again toFIG. 2, this example includes an equalizer portion60that uses a known technique for processing the received signal back to its original form after the de-spreading and de-channelization, for example. An equalizer in this position of such a receiver device is unique.

The example ofFIG. 2also includes timing control at61for known reasons and functional blocks62that provide power measurement information at64that is useful for updating the observation window. If a linear type equalizer is used, the power measurement for single rake fingers can be alternatively approximated by taking the magnitude squares of the equalizer weights, for example.

The equalizer in one example is a linear type such as an LMMSE. In another example, the equalizer60is a non-linear type such as a decision-feedback-equalizer or a maximum likelihood-equalizer. The power measurements schematically shown at64inFIG. 2are useful, for example, with non-linear type equalizers.

Adjusting the observation window can be useful for maintaining a desired relationship between a center of energy of a received signal and a center of the observation window. For discussion purposes, consider a notation k for each of the rake fingers, a finger timing tkand a power measurement for each of the rake fingers Pk. Assuming that the timing for the earliest or first in time of the rake fingers τeis provided by another instance of the receiver (e.g., from the preamble detector), at the start of reception the timing of all rake fingers is initialized using the following relationship:
[t1, t2, . . . , tK]=[τe, τe+Ts, τe+2Ts, . . . τe+(K−1)Ts]

Based on the power measurements Pkprovided by the rake fingers24-30, the center of energy over time τpcan be determined from the following equation:

The center of rake finger timing τrcan be determined from the following equation:

In this example, the previous equation provides a mean result because τedoes not necessarily start at zero. This example compensates for such a situation.

Depending on a determined relationship between power at the plurality of rake fingers and time, the observation window may be adjusted. In one example, if the center of energy over time and the center of the rake finger timing differ by more than a selected threshold, the observation window will be adjusted. In one example, the selected threshold is one-half of the timing separation Ts(i.e., Ts/2).

FIG. 4schematically shows an example where the difference between the center of energy and the center of the rake finger timing exceeds the selected threshold and the observation window is adjusted by changing the position of the rake finger30. In this example, the rake finger30is the last in time of the rake fingers of the original observation window. The position of the rake finger30is changed to become the first in time of the rake fingers in the adjusted observation window. In other words, the rake finger30is essentially moved as schematically shown at70into a position at the beginning of the adjusted observation window.

One example includes using the following relationship
τP−τR,<−Ts/2 thent1:=t1−KTswhere 1=argmax{tk}
for making an adjustment as schematically shown inFIG. 4. The following relationship
τP−τR>Ts/2 thente:=te+KTswheree=argmin{tk}
is used in an example to adjust the observation window in a manner schematically shown inFIG. 5. In this example, as schematically shown at72, the rake finger24, which was the first in time of the rake fingers in the observation window, is moved behind (in time) the rake finger30, which was the last in time of the rake fingers in the observation window. The adjusted observation window includes the second in time of the rake fingers (i.e., rake finger26) as the first in time of the rake fingers in the adjusted observation window. The rake finger24becomes the last in time of the rake fingers within the adjusted observation window.

One example includes determining whether to adjust the observation window every frame (e.g., every 10 m sec). Given this description those skilled in the art will be able to select appropriate timing.

An advantage to the techniques shown inFIGS. 4 and 5is that in either situation three of the four rake fingers remain in the same position. Leaving rake fingers in the same position reduces computation and processing required for resetting fingers. By only changing the position of one of the rake fingers, the observation window can be adjusted to achieve a desired performance while keeping the computational complexity at a minimum. Additionally, the channel estimation filters and measurement averaging buffers need not be reset for fingers remaining at the same position. This avoids otherwise potential performance loss during the initialization phase of rake fingers. This is a significant advantage compared to the standard procedure where rake finger positions have to be continually estimated based upon continually tracking the delays on a channel.

FIG. 6schematically shows an advanced example receiver device20′ employing an equalizer before the despreading stage from a known signal processing technique. In this example, functional blocks80,82,84,86,88and90represent the spacing between rake fingers such that the rake fingers are equally spaced. One difference between the example ofFIG. 6and that ofFIG. 2is that the example ofFIG. 6utilizes a linear combiner92followed by a resampling module94. An example linear combiner is a linear equalizer. One example includes an LMMSE equalizer. The re-sampling module94performs down sampling from the sampling rate to the actual symbol rate. Another example is shown inFIG. 7where a receiver device20″ includes a plurality of re-sampling modules94A-94H in front of the linear combiner92′. In the examples ofFIGS. 6 and 7, the output samples are at the symbol rate.

The disclosed examples provide several advantages. The output of the channel estimation is not time-aligned to the input signal from the antenna. Therefore, when placing the equalizer after the de-spreader, only the latency of the channel estimation has to compensated for. It becomes possible to employ a FIFO buffer for compensating for such latency, which operates on the symbol rate. Because the symbol rate of a de-spreaded signal is much lower than the double chip rate, the amount of memory required for such a FIFO buffer is much lower than was previously possible. Accordingly, the disclosed examples reduce complexity while still allowing for optimum performance.

Another advantage of the disclosed examples is that assigning the rake fingers with a constant or equal timing separation between them creates an observation window. Maintaining the timing separation below the inverse of the signal bandwidth provides a useful timing separation between rate fingers. With such an observation window, it is no longer necessary to search and track the position of every single multi-path. The observation window of the disclosed examples renders searcher and per-finger trackers used in conventional rake receivers redundant. This provides significant savings in terms of computations in the receiver. Again, computational complexity is significantly reduced but optimum performance is still achievable.

For situations where the observation window is larger than the maximum delay spread, which will occur in most small cells, the observation window can be updated as described above. The disclosed example is very simple to implement in terms of computational complexity and is very efficient. Additionally, it allows for maintaining at least some of the rake fingers in a constant position, which reduces complexity and prevents performance losses for otherwise resetting them.

Changing only one rake finger at a time to update the observation window depending on a desired direction of the timing update is also advantageous. The disclosed example technique is particularly useful for situations that rely on medium to long-term measurements of the rake fingers. In such arrangements, the averaging buffers and measurements that otherwise need to be restarted for a rake finger when it is reassigned can be reduced. The disclosed example adjustment technique for updating an observation window reduces the number of re-assignments to a minimum and further reduces computational complexity of a receiver device.