Background of the Invention A typical problem encountered by remote units within wireless communication systems is that of multi-path fading. During multipath fading, a signal that is transmitted to a remote unit is canceled (via destructive interference) by the same signal that has been reflected off of an object. This is illustrated in FIG. 1. As shown in FIG. 1, signal 103 that is transmitted to remote unit 101 is reflected off surface 102. In many cases, the reflected signal 104 will destructively interfere with incoming signal 103, causing a multi-path fade. At the point where the two signals destructively interfere, remote unit 101 may be unable to receive incoming signal 103, possibly resulting in a dropped call. Therefore a need exists for a method and apparatus for transmitting a signal within a communication system that reduces the chance that a signal will undergo multi- path fading.

Brief Description of the Drawings FIG. 1 is an illustration of multi-path scattering.

FIG. 2 is a block diagram of a signal transmitter in accordance with the preferred embodiment of the present invention.

FIG. 3 is an illustration of multi-path scattering in accordance with the preferred embodiment of the present invention.

FIG. 4 is a flow chart showing operation of the signal transmitter of FIG. 2 in accordance with the preferred embodiment of the present invention.

Detailed Description of the Drawings To address the need for a method and apparatus for reducing multi-path fading a method and apparatus for transmitting a signal within a communication system is provided. A properly modulated carrier signal exits transmission circuitry and enters a first and a second mixer. The modulated carrier is mixed with a first and a second function by the first and the second mixers. The functions are generated by a first and a second signal generator. The mixed signals exit the mixers, and are amplified, to be radiated by an antenna. The antenna comprises two orthogonal antenna elements that are in close proximity with one another (although not in contact). One of the mixed signals is radiated on a first element and the other mixed signal is radiated on the second element. The resulting signal transmitted from the antenna is the original carrier signal having the plane of polarization constantly changing. Thus a reflected signal emitted an instance earlier cannot interfere with a wave currently emitted.

The present invention encompasses a method for transmitting a signal in order to reduce multipath fading. The method comprises the steps of receiving on a first signal path, a carrier signal having a first phase and carrier frequency, and receiving the carrier signal on a second signal path. The carrier signal is mixed with a first function on the first signal path, wherein the first function has a frequency differing from the carrier frequency, and the carrier signal is mixed with a second function on the second signal path, wherein the second function has a frequency differing from the carrier frequency.

The present invention additionally encompasses a method for transmitting a signal. The method comprises the steps of receiving on a first and second signal path, a carrier signal operating at a carrier frequency (coc), and have an associated amplitude (A) and phase ( (D). The carrier signal is mixed with a first function on the first signal path, and a second function on the second signal path. In the preferred embodiment of the present invention the first function has a frequency

(c35) differing from the carrier frequency, and the second function has a frequency equal to Ms.

The present invention additionally encompasses an apparatus comprising a first mixing circuit having a carrier signal with a carrier frequency as an input, and also having a first function having a second frequency as an input, and outputting the carrier signal mixed with the first function. The apparatus additionally comprises a second mixing circuit having the carrier signal as an input, and also having a second function having the second frequency as an input, and outputting the carrier signal mixed with the second function.

FIG. 2 is a block diagram of signal transmitter 200 in accordance with the preferred embodiment of the present invention. As shown, transmitter 200 comprises standard transmission circuitry 201, signal generators 207-209, mixers 203-205, amplifiers 211-213, phase delay circuitry 219, and antenna 221. In the preferred embodiment of the present invention transmission circuitry is a standard base station infrastructure transmitter such as a CDMA BTS SCTM-4812T. More particularly, transmission circuitry 201 utilizes a Code Division Multiple Access (CDMA) system protocol as described in Cellular System Remote unit-Base Station Compatibility Standard of the Electronic Industry Association/Telecommunications Industry Association Interim Standard 95 (TIA/EIA/IS-95A), which is incorporated by reference herein. (EIA/TIA can be contacted at 2001 Pennsylvania Ave. NW Washington DC 20006). In alternate embodiments transmission circuitry 201 may utilize other analog or digital cellular transmission protocols such as, but not limited to, the Narrowband Advanced Mobile Phone Service (NAMPS) protocol, the Advanced Mobile Phone Service (AMPS) protocol, the Global System for Mobile Communications (GSM) protocol, the Personal Digital Cellular (PDC) protocol, or the United States Digital Cellular (USDC) protocol.

Operation of transmitter 200 in accordance with the preferred embodiment of the present invention occurs as follows: A properly modulated carrier signal exits transmission circuitry 201 and enters mixers 205 and 203. As one of ordinary skill in the art will recognize, carrier signals typically operate at a carrier frequency ( (oc), and have an associated amplitude (A) and phase (0). Both the amplitude and the phase may be functions of time, depending upon the particular type of modulation scheme being utilized. Such a modulated carrier signal can be

represented by the equation A (t) sin (coct+q) (t)). The modulated carrier is mixed with a first and a second function by mixers 203 and 205. In particular, in a first embodiment of the present invention first signal path 225 of the modulated carrier is mixed with sin (cost), and second signal path 227 of the modulated carrier is mixed with cos (colt). The functions sin (co, t) and cos (must) are generated by signal generators 207 and 209, respectively. The mixed signals exit mixers 203-205 and are amplified (via amplifiers 211-213), to be radiated by antenna 221. As shown, antenna 221 comprises two orthogonal antenna elements 215 and 217, that are in close proximity with one another (although not in contact). One of the mixed signals is radiated on element 215, and the other mixed signal is radiated on element 217.

In the first embodiment of the present invention a carrier signal of frequency c3c is mixed with sin (colt) and radiated on a first antenna element. The carrier signal is additionally mixed with cos (colt) and radiated on an orthogonal antenna element in close proximity to the first antenna element. Because of the principle of superposition of signals, the resulting signal transmitted from antenna 211 is the original carrier signal having the plane of polarization constantly rotating in a circular pattern at frequency Ms. With a constantly rotating plane of polarization, a reflected signal emitted an instance earlier cannot interfere with a wave currently emitted. Thus, at any remote unit, a reflected wave will not interfere with a transmitted signal because the waves have different polarization states. This is illustrated in FIG. 3 with incoming signal 303 having a polarization 305 differing from the polarization 306 of reflected signal 304.

In a second embodiment of the present invention phase delay circuitry 219 is utilized to delay the phase of one signal path by a constant amount with respect to the other signal path. In a preferred embodiment, the phase of the second signal path is delayed by 7r/2 from the phase of the first signal path. The resulting signal transmitted from antenna 221 is the original carrier signal having the polarization state evolving between horizontal linear, circular, and vertical linear polarization at frequency wus. In other words, unlike the first embodiment, where the polarization continuously rotates in a circular pattern, in the second embodiment, the state of elliptic polarization of the carrier changes.

In a third embodiment of the present invention signal generators 207-209 generate sine and cosine functions of a random number (R) that changes with

respect to time (t) and is held constant for a period of time (T). In a preferred embodiment, R is a random number varying between 0 and 1, that changes with respect to time, and once changed, is held constant for a period of T microseconds (e. g.,. 1 to 1 microseconds). In the third embodiment, signal generator 207 generates sin (27cR (T, t)) that is mixed with the carrier on the first signal path, and signal generator 209 generates cos (27iR (T, t)) that is mixed with the carrier on the second signal path. The resulting signal transmitted from antenna 221 is the original carrier signal having the plane of linear polarization changing randomly.

In other words, unlike the first and second embodiments, where the polarization continuously rotated in a circular or evolved through elliptical polarization states, in the third embodiment, the polarization of the carrier does not rotate at all, but randomly flips, and remains constant for a period of time (T).

In a fourth embodiment of the present invention signal generators 207-209 do not modify the carrier signal (i. e., signal generators 207-209 generate a constant value of 1), however phase delay circuitry 219 changes the relative phase of a signal path by a time-varying amount (f (t)). The resulting signal transmitted from antenna 221 evolves through following polarization states: linear at 71/4, elliptic, circular, linear at-7E/4.

FIG. 4 is a flow chart illustrating operation of the signal transmitter of FIG.

2 in accordance with the preferred embodiment of the present invention. The logic flow begins at step 401 where a carrier signal enters a first and a second signal path. At step 403 the carrier signal on the first signal path is mixed with a first function and at step 405 the carrier signal on the second signal path is mixed with a second function. At step 407 the phase of the mixed signal on the first signal path is delayed by a first amount. Table 1 shows the first function, second function, and phase delay amount for each embodiment described above. Embodiment First function Second function Phase delav amount 1sin (mst) cos (o) st) 0 2 sin (cst) cos (w$t) Constant 3 sin (27cR (T, t)) cos (27rR (T, t)) 0 4 1 1 f (t) Table 1: First function, second function, and phase delay amount for four embodiments of the invention.

Continuing, at step 409 the carrier mixed with the first function, and phase delayed a first amount is radiated on a first antenna element, and at step 411 the carrier signal mixed with the second function is radiated on a second antenna element. As described above both the first and the second antenna elements are orthogonal to one another, and are in close proximity.

The descriptions of the invention, the specific details, and the drawings mentioned above, are not meant to limit the scope of the present invention. For example, although the preferred embodiments of the invention were described above utilizing five differing mixing functions (1, sin (colt), cos (must), sin (27rR (p, T)), and cos (27cR (p, T))), and three differing phase delay amounts (0, Constant, and f (t)), any multitude of mixing and phase delay functions may be utilized without varying from the scope of the invention. It is intended that all such modifications come within the scope of the following claims and their equivalents.