Method and Apparatus for Achieving Antenna Receive Diversity With Wireless Repeaters

The present invention relates to wireless telecommunications in general, and, more particularly, to a method and apparatus for achieving antenna receive diversity with a wireless repeater.

FIG. 1 depicts a schematic diagram of base station 101 that comprises a single receive antenna, antenna 103, through which base station 101 receives signals transmitted from wireless terminal 107. As is well known to those skilled in the art, the use of a single receive antenna at base station 101 leaves the received signal quality vulnerable to a natural phenomenon known as Rayleigh fading.

Rayleigh fading occurs when the multipath components of a radiated signal destructively interfere at the receive antenna with the result that the signal-to-noise ratio of the composite received signal is below an acceptable threshold. Typically, a small change in the position of the wireless terminal changes the amplitude and relative phase of the respective multipath components at the receive antenna with the result that the signal-to-noise ratio of the received signal may be substantially improved. Unfortunately, another small change in position may degrade the signal-to-noise ratio. Empirically, as a wireless terminal moves and the path between the wireless terminal and base station changes with time, the received signal quality vacillates. This vacillation is known as Rayleigh fading.

To mitigate the effects of Rayleigh fading, base stations are often constructed with two, spatially diverse receive antennas. FIG. 2 depicts two antennas, receive antenna 203 and receive antenna 205, which are typically separated by some multiple (or submultiple) of the wavelength of the received signal's carrier signal. Because the signal path from wireless terminal 207 to each of receive antenna 203 and receive antenna 205 is always different, the multipath components at each receive antenna have different amplitudes and phases and, therefore, it is rare that the signal-to-noise ratio of each composite signal is simultaneously low at both receive antennas. Empirically, when the signal-to-noise ratio at one receive antenna is low, the signal quality at the other is typically satisfactory. This fact enables the base station to receive the transmitted signal at both antennas, to compare the relative signal quality at each antenna and to select, at each moment in time, the better signal. This technique is known as antenna receive diversity and it typically improves the overall received signal quality by 3 to 12 dB, or more.

Occasionally, the geographic area serviced by a base station is so large that the signals transmitted by a wireless terminal at the fringe of the area cannot be satisfactorily received, without assistance, by the base station. Alternatively, sometimes the geographic area serviced by a base station contains hills and valleys such that the signals transmitted by a wireless terminal to a base station are shadowed by the terrain. To alleviate this problem, wireless repeaters are employed.

FIG. 3 depicts a schematic diagram of base station 301, with receive antenna 303 and receive antenna 305, wireless terminal 307 and repeater 309, in the standard repeater configuration. The purpose of repeater 309 is to receive signals from wireless terminal 307, amplify them and re-transmit them to base station 301, when the distance or terrain makes it unlikely that wireless terminal 307 can transmit directly to base station 301.

Repeater 309 has one receive antenna for receiving signals from wireless terminal 307 and one transmit antenna for re-broadcasting signals those signals to base station 301. Because repeater 309 has only one receive antenna, the quality of the signals it receives are subject to Rayleigh fading. Furthermore, the fact that base station 301 incorporates receive antenna diversity becomes largely superfluous because the position of repeater 309 with respect to base station 301 does not change with time. Therefore, although repeater 309 is beneficial in that it boosts the overall signal strength of the signals from wireless terminal 307, the improvements in signal quality afforded by antenna receive diversity at base station 301 are lost.

Embodiments of the present invention are capable of re-broadcasting multipath signals while avoiding many of the costs and restrictions associated with prior methods. In particular, embodiments of the present invention can be fabricated inexpensively that preserve the spatial diversity of multipath signals emanating from a wireless terminal.

These results are obtained in an illustrative embodiment comprising a first antenna for receiving a first signal from a wireless terminal and for transmitting said first signal along a first path; a second antenna for receiving a second signal from said wireless terminal and for transmitting said second signal along a second path; a summing circuit for receiving said first signal from said first path and said second signal from said second path, for summing said first signal and said second signal to form an aggregate signal, and for outputting said aggregate signal along a third path; a delay in said second signal that makes said second signal path longer than said first signal path; and a third antenna for receiving said third signal from said third path and for transmitting said third signal.

A more complete understanding of the invention may be obtained from consideration of the following description in conjunction with the drawings, in which:

FIG. 1 depicts a schematic diagram of a wireless terminal communicating with a wireless base station;

FIG. 2 depicts a schematic diagram of a wireless terminal communicating with a wireless base station that employs receive antenna diversity;

FIG. 3 depicts a schematic diagram of a wireless terminal communicating with a wireless base station that employs receive antenna diversity, via a wireless repeater;

FIG. 4 depicts a schematic diagram of a wireless terminal communicating with a wireless base station that employs receive antenna diversity, via an illustrative embodiment of the present invention;

FIG. 5 depicts a schematic diagram of one embodiment of the present invention; and

FIG. 6 depicts a schematic diagram of a second embodiment of the present invention.

FIG. 4 depicts a schematic diagram of an illustrative embodiment of the present invention, which comprises repeater 402. As shown in FIG. 5, one embodiment of repeater 402 comprises two receive antennas, receive antenna 411 and receive antenna 413, which are advantageously made and used in the standard configuration for receive antenna diversity. The output of receive antenna 411 is fed into low-noise amplifier 407 and then into summing circuit 417 through a first signal path, and the output of receive antenna 413 is fed into low-noise amplifier 409 and then into summing circuit 417, but through a second signal path. Summing circuit 417 advantageously performs an unweighted vector addition on both signals in well-known fashion. The composite signal from summing circuit 417 is fed into amplifier 421 and transmit antenna 415 along a third signal path. Because repeater 409 does not move with respect to base station 401, transmit antenna 415 is advantageously a directional antenna (e.g., a parabolic or high-gain antenna) pointed directly at base station 401.

Delay element 419 is advantageously in the second signal path between receive antenna 413 and summing circuit 417. Delay element 419 is advantageously a length of conducting material (e.g., coaxial cable, a section of strip line, a microstrip, etc., or a lumped delay line) that causes the second signal path to be longer than the signal path taken by the first signal. When the signals being re-broadcast by repeater 409 are direct sequence spread spectrum signals, as used in IS-95 CDMA wireless telecommunications systems, and thus comprise thousands of temporal "chips," the delay through delay element 419 is advantageously at least two chips, which enables rake receivers in base station 401 to resolve and correlate the two signals. It will be clear to those skilled in the art how to make and use the rake receivers in base station 101. U.S. Pat. No. 5,237,586, entitled "Rake Receiver with Selective Ray Combining," issued Aug. 17, 1993; U.S. Pat. No. 5,566,206; entitled "Method and Apparatus for Determining Data Rate of Transmitted Variable Rate Data in a Communications Receiver," issued Oct. 15, 1996; and U.S. Pat. No. 5,305,349, entitled "Quantized Coherent Rake Receiver," issued Apr. 19, 1994 teach how to make and use rake receivers and are hereby incorporated by reference. The rake receivers in base station 401 enable repeater 402 to convert the spatially diverse signals coming from wireless terminal 407 into temporally diverse signals, which appear as multipath signals at base station 401.

It should be noted however, that the vector addition of the two signals by summing circuit 417 creates a 3 dB signal-to-noise loss, and, therefore, embodiments of the present invention are more advantageous when the signal-to-noise gain from employing the antenna diversity at repeater 409 is greater than the 3 dB signal-to-noise loss from summing circuit 417.

FIG. 6 depicts a schematic diagram of an alternative embodiment of the present invention, in which the spatially diverse antennas of FIG. 5 are replace with polarization diversity antenna 611. According to this embodiment, one lead from polarization diverse antenna 611 is fed into low-noise amplifier 607 and then into summing circuit 617 and the other lead is fed into low-noise amplifier 609 and then into delay element 619. In this embodiment, the polarization diversity of the signals from wireless terminal 407 (FIG. 4) is converted into temporally diversity, which is processed by repeater 402 and base station 401 in the same manner as that described with respect to the embodiment in FIG. 5.

Again, it should be noted that this embodiment of the present invention is more advantageous when the signal-to-noise gain from employing the polarization diversity antennas is greater than the 3 dB signal-to-noise loss from summing circuit 417.

It is to be understood that the above-described embodiments are merely illustrative principles of the invention, and that many variations may be devised by those skilled in the art without departing from the scope of this invention. It is, therefore, intended that such variations be included within the scope of the claims.