Diversity for mobile terminals

Polarization diversity is achieved in mobile terminals with three antennas. Each of the three antennas are orthogonal to one another. With three orthogonal antennas, the mobile terminal increases the likelihood that at least one antenna's polarity will match that of the incoming signal's wavefront, regardless of how the user orients the mobile terminal. Once a signal has been received by each of the three antennas, they can be selected or combined using a variety of techniques and/or algorithms. Among these techniques are selection diversity, switching selection diversity, fixed combining diversity, and adaptive combining diversity (e.g., maximal ratio combining and interference rejection combining).

BACKGROUND OF THE INVENTION 
1. Technical Field of the Invention 
The present invention relates to wireless communications and, in 
particular, to implementing polarization diversity in mobile terminals. 
2. Description of Related Art and Objects of the Invention 
Mobile wireless communication is becoming increasingly important for 
safety, convenience, and efficiency. One prominent mobile communication 
option is cellular communication. Cellular phones, for instance, can be 
found in cars, briefcases, purses, and even pockets. Cellular phones, like 
most mobile communication options, rely on the transmission of 
electromagnetic radiation from one point to another. 
In general, a cellular system is composed of many cells, each with a base 
station antenna for receiving transmissions. From the base station, the 
cellular system has interfaces for routing a call through or to the 
land-based, or terrestrial, telephone network, often referred to as the 
public switched telephone network (PSTN). The base stations form one half 
of the cellular system. Cell phones, called mobile stations, mobile 
terminals, or merely terminals, form the second half of the cellular 
system. In short then, electromagnetic radiation transmissions between 
terminals and base stations are an essential component of cellular 
systems, and such transmissions must be optimized by the cellular system 
to maximize cellular phone service, quality, and availability. 
When communicating via the transmission of electromagnetic radiation, 
diversity can be used to counteract signal fading, which occurs when a 
signal's strength decreases. A given radio signal will usually take 
multiple diverse paths from the transmitter to the receiver. These 
multiple paths arise from the signal taking a direct path or any one of 
many reflective paths. As a result, the receiver actually has several 
versions of the same signal from which to choose for processing. Often, 
the different paths will not be fading simultaneously, so if the receiver 
can always be processing the version of the signal with the least fade at 
a given moment, then the overall transmission will be more reliably 
received and processed. This is termed path diversity. Diversity in 
general, however, can be applied to various techniques for creating and/or 
selecting the current optimum version of the signal. 
Referring now to FIG. 1, an example of the benefits of diversity is 
illustrated. A Graph of Time versus Received Signal Level is provided at 
100. Graph 100 represents a mobile terminal with two signal levels from 
which to choose for processing. Signal One is diagramed at 110, and Signal 
Two is diagramed at 120. If Graph 100 represents path diversity, then each 
signal represents the signal arriving via a different path. In Graph 100, 
selection diversity is implemented by selecting the strongest signal at 
any given instant for processing. The strongest signal is diagramed at 130 
as the Diversity Signal. At Point in Time 140, for example, Graph 100 
demonstrates how the mobile terminal switches from processing Signal One 
110 to Signal Two 120. Diversity Signal 130 demonstrates how selecting the 
stronger of Signal One 110 and Signal Two 120 effectively creates a higher 
averaged received signal power. (It is noted that Diversity Signal 130 is 
diagramed above the higher of Signal One 110 or Signal Two 120 instead of 
directly over for diagrammatical clarity.) 
In addition to taking advantage of diverse paths (path diversity as 
described above), the receiver can have two antennas. If the antennas are 
spaced sufficiently far apart (approximately 0.25.lambda. for mobiles and 
10.lambda.-20.lambda. for base stations), then one antenna is often in a 
better reception position than the other antenna at any given instant. 
Using two spaced-apart antennas is termed space, or antenna, diversity. If 
Graph 100 represents space diversity, then each signal of Signal One 110 
and Signal Two 120 represents the signal arriving on a different antenna 
of the two antennas. The Diversity Signal 130 represents the signal 
selected for processing that is currently strongest, and Point in Time 140 
represents a point in which the receiver is switching from one antenna to 
the other. 
Two other examples of diversity are frequency and time. Frequency diversity 
requires that the transmitter transmit the same signal over two different 
frequencies because when the frequencies are sufficiently far apart, their 
fading should vary sufficiently to allow one signal to frequently be 
strong when the other is fading. Time diversity requires that the 
transmitter transmit the same signal at two different times because when 
the duration between the transmissions is sufficiently long, their fading 
should differ sufficiently whereby the earlier or later signal will be 
strong while the other is fading. With respect to FIG. 1, Signal One 110 
and Signal Two 120 would represent signals transmitted either on different 
frequencies or at different times. While both frequency and time diversity 
are useful techniques, they require duplicative transmissions, which waste 
transmitter resources, e.g., power transmitted. 
Another example of diversity is polarization diversity. Polarization 
diversity improves the average power of the processed signal because 
signals transmitted on orthogonal polarizations exhibit uncorrelated 
fading. For example, with respect to FIG. 1, the vertically polarized 
electromagnetic signal may be represented by Signal One 110, and the 
horizontally polarized electromagnetic signal may be represented by Signal 
Two 120. Diversity Signal 130 then represents the selection of the better 
of the two orthogonally polarized signals. 
Polarization diversity reception has occasionally been used at base 
stations. Referring now to FIG. 2, polarization diversity reception at a 
base station is illustrated at 200. Base station tower 210 includes a 
vertically polarized antenna 220 and a horizontally polarized antenna 230. 
For mobile terminals (which in this application encompasses all portable 
communication devices, including, but not limited to, cellular phones, 
citizen band radios, walkie-talkies, etc.), however, there is normally 
only one transceiving antenna. Those mobile terminals that do have a 
second antenna (for example, for space diversity) typically employ 
antennas with low gain, vertical polarization, and omni-directionality. 
These antennas are usually of monopole or dipole derivatives. 
Mobile terminals, whether they are cellular radios, mobile radios, or other 
types of mobile terminals, can be positioned at any orientation. If they 
are portable-sized (e.g., approximately 10.times.8.times.3 inches), then 
they might be either laid flat or set upright. If they are 
hand-held-sized, then they can potentially be held at any orientation 
during use. Unfortunately, because prior art mobile terminal antennas have 
a fixed polarization (usually vertically polarized when held in hand and 
sitting straight up), non-optimum reception results when mobile terminals 
are held at an orientation other than the one for which they were 
designed. This becomes an even more acute, system-wide concern for 
critical communications to terminals such as those used by public safety 
and similar agencies because these terminals must function adequately even 
in a network's fringe areas. 
As discussed immediately above, hand-held mobile terminals may be held at 
any orientation during a call. This causes non-optimum reception because 
most base stations transmit vertically polarized signals. Furthermore, the 
environment through which electromagnetic signals travel can cause 
scattering and fading. Scattering can alter the polarization of a signal's 
wave front before it reaches the mobile terminal's antenna. Fading 
exacerbates the reception difficulties because it can cause the signal 
with the best polarization to be the weakest. In other words, the 
strongest signal may not be in the orientation that the mobile terminal's 
antenna was designed for. A device or technique for increasing the 
likelihood that the strongest signal can be received by a correctly 
polarized antenna is needed. 
In summary, mobile terminals have heretofore only incorporated at most two 
antennas, both of which were of the same polarization. Also, polarization 
diversity reception has heretofore only been used at base stations and, 
even then, with only two diversity branches. 
A non-exhaustive list of objects of the invention follows: 
An object of the invention is to provide a mobile terminal that uses 
polarization diversity. 
An object of the invention is to provide a mobile terminal that uses 
polarization diversity with three branches. 
Another object of the invention is to provide a mobile terminal whose 
polarization diversity branches are combined using a variety of techniques 
and algorithms. 
Another object of the invention is to implement a three-branch polarization 
diversity receiver capable of being well-matched to any incoming 
transmission, regardless of polarity. 
Yet another object of the invention is to improve a mobile terminal's 
reception with respect to polarization diversity. 
Yet another object of the invention is to improve an entire communication 
system by implementing polarization diversity in a mobile terminal. 
SUMMARY OF THE INVENTION 
According to a preferred embodiment of the invention, polarization 
diversity is achieved in a mobile terminal by using three antennas. Each 
one of the three antennas is orthogonal to the other two. With three 
orthogonal antennas, the mobile terminal increases the likelihood that at 
least one antenna's polarity will match that of the incoming signal's wave 
front, regardless of how the user physically orients the mobile terminal. 
Once a signal has been received by each of the three antennas, they can be 
selected or combined using a variety of techniques and algorithms. 
Selection diversity operates by selecting the signal with the strongest 
current signal. Switching selection diversity, on other hand, operates by 
remaining (i.e., continuing) with the currently selected antenna until a 
predetermined power threshold is no longer met; then, another antenna is 
tested. Fixed combining diversity co-phases each signal and then sums them 
while adaptive combining diversity can properly weight each signal before 
summing them (maximal ratio combining algorithm) or can steer a null 
toward interference (interference rejection combining algorithm). 
By receiving a transmitted signal on three orthogonal antennas, a mobile 
terminal increases the probability that one signal is received by an 
antenna with a polarity matching that of the strongest incoming signal. 
After appropriately selecting from or combining these three different 
signals, a better signal is attained as compared to not utilizing 
polarization diversity. The attained signal is then forwarded for further 
receive processing.

DETAILED DESCRIPTION OF THE DRAWINGS 
A preferred embodiment of the present invention and its advantages are best 
understood by referring to FIGS. 3A-8B of the drawings, like numerals 
being used for like and corresponding parts of the various drawings. 
Mobile terminals operate in a three dimensional environment. For instance, 
the user can rotate the mobile terminal in any direction. According to a 
preferred embodiment of the invention, three antennas are provided for a 
mobile terminal. Referring now to FIG. 3A, mobile terminal 310 is 
illustrated. Three antennas 320 are preferably connected to the mobile 
terminal 310 at diverse angles/orientations. With the three orientations 
of the three antennas 320, the likelihood that one antenna's orientation 
closely matches the wave polarization of the strongest incoming signal is 
increased. 
Though the three antennas 320 are pictured external to mobile terminal 310, 
one, two, or all three antennas 320 can be located within the housing of 
mobile terminal 310. Each of the three antennas 320 can be of monopole, 
dipole, loop, strip, microstrip, or patch antenna derivatives, or any 
other form of antennas suited for the application of the mobile terminal. 
It should be noted that all three antennas 320 need not be of the same 
type; they may be any desirable combination. 
Referring now to FIG. 3B, a presently preferred embodiment of a set of 
three antennas is illustrated. Each of three antennas 350 are orthogonal 
(.alpha.=90.degree.) to one another. Preferably, each of three antennas 
350 are approximately orthogonal to each other to maximize the probability 
that at least one antenna will be properly polarized with the wave 
polarization of the incoming signal, regardless of the orientation of the 
mobile terminal. It should be reiterated that any type of antenna is 
within the scope of the invention, i.e., three loop antennas, for example, 
could replace the three antennas 350 as pictured. It is noted here that 
the invention's proper scope also covers polarization diversity (i) of two 
branches in mobile terminals and (ii) of three branches in general. 
Furthermore, at least some improvement can be attained when using more 
than three polarization branches. The diversity selection and combination 
techniques and algorithms described hereinbelow also apply to terminals 
with two polarization diversity branches from two antennas. 
As explained hereinabove, diversity is beneficial to wireless 
communication. According to one embodiment of the invention, the sources 
for the diverse signals arise from three approximately orthogonal 
antennas. To ultimately achieve any benefits from diversity, the diverse 
signals must be combined (or selected or otherwise processed) in some 
manner. In short, the best signal or the best combination of signals 
should be extracted. 
Referring to FIG. 3C, another aspect for achieving diversity is pictured. 
Three Polarizationally Diverse Antennas (TPDA) 360 are linked to 
Selection/Combination Unit 370. Selection/Combination Unit (SCU) 370 
applies a technique and/or an algorithm (as described hereinbelow) to 
optimally select or produce a signal from TPDA 360. The produced signal, 
Output Signal 380, is then sent to Signal Processing Unit 390 for 
decrypting, decoding, etc. by the associated mobile terminal. 
A first technique for selecting from among the TPDA 360 in SCU 370 is 
Selection Diversity. Selection Diversity outputs the diversity branch with 
the highest signal to noise ratio (SNR). Referring now to FIG. 4A, TPDA 
360 each receive a signal. Variable Gain Units (VGU) 420 are adjusted to 
provide the same average SNR for each antenna/polarization branch. 
Selector 430 then connects the branch with the highest instantaneous SNR 
to the Output 440. It is noted that it is often easier to select the 
branch with the largest (S+N)/N [where S is signal and N is noise] because 
measuring SNR is difficult in practice. It is also noted that the 
instantaneous SNR value is acquired in practice by using selection 
circuitry whose time constants are shorter than the reciprocal of the 
signal fading rate. 
Referring to FIG. 4B, a flowchart explaining the process of selection 
diversity is diagramed at 450. Step 460 provides for the equalization of 
the average SNR of each branch by adjusting the gains with VGUs 420. Step 
470 provides for the selection of the polarization branch with the highest 
instantaneous SNR for output to the processing circuitry. Referring again 
to FIG. 1, it is noted that Diversity Signal 130 is the output produced by 
a selection diversity technique applied to Signal One 110 and Signal Two 
120. Thus, the output 440 of the selector 430 represents the combined 
diversity signal 130 of the antennas 360. 
A second technique for selecting from among the TPDA 360 in SCU 370 (FIG. 
3C) is Switching Selection Diversity, also called feedback or scanning 
diversity. This switching selection technique advantageously requires only 
one receiving chain, thus simplifying the mobile terminal. Referring now 
to FIG. 5A, Default Primary Antenna (DPA) 510 is the preferred antenna 
under all conditions. First Secondary Antenna 512 and Second Secondary 
Antenna 514 complete the TPDA 360. Each antenna 510, 512, and 514 can be 
selected by Switch Selector 516 in response to instructions received via 
Control Line 518 from Terminal Transceiver System (TTS) 520. 
TTS 520 operates according to the following set of rules illustrated in 
FIGS. 5B and 5C. FIG. 5B illustrates Switch Selecting Criteria (SSC) graph 
530, and FIG. 5C illustrates Switching Selection Diversity flowchart 550. 
Initially, DPA 510 is selected by Switch Selector 516 under the 
instruction of TTS 520 via Control Line 518 as indicated by Step 552. The 
signal level on DPA 510 is monitored. If the signal level maintains a 
certain threshold, for example it stays above "L", then the TTS 520 makes 
no changes as indicated by Decision Step 554. If the signal level drops 
below a certain level, for example "L" (by dropping from point A to point 
B as shown in SSC graph 530), then TTS 520 instructs Switch Selector 516 
to switch to another antenna, e.g., First Secondary Antenna 512, as 
indicated by Decision Step 554 and Decision Step 556. 
The signal level on the First Secondary Antenna 512 is tested at Decision 
Step 556 to determine whether it exceeds a certain threshold, for example 
"H". If the signal level on the First Secondary Antenna 512 does 
exceed"H", then TTS 520 instructs the Switch Selector 516 to select the 
First Secondary Antenna 512 at Step 558. If the signal level on the First 
Secondary Antenna 512 does not exceed "H", then at Decision Step 560 the 
Second Secondary Antenna 514 is tested to determine whether it exceeds a 
certain threshold, for example "H". If the signal level on the Second 
Secondary Antenna 514 does exceed "H", then TTS 520 instructs Switch 
Selector 516 to select the Second Secondary Antenna 514 at Step 562. If 
the signal level on the Second Secondary Antenna 514 does not exceed "H", 
then TTS 520 instructs Switch Selector 516 to switch to and to select DPA 
510 as indicated by Decision Steps 560 and 552. 
While the Switch Selector 516 has selected either the First Secondary 
Antenna 512 or the Second Secondary Antenna 514, the signal level is 
continually tested to determine whether it falls below a given threshold, 
for example "L". While the signal level is above the given threshold "L", 
the selector continues with the current secondary antenna as indicated by 
Step 564. If the signal level falls below the threshold level "L", for 
example from point C to point D as shown in SSC graph 530, then the TTS 
520 instructs Switch Selector 516 via Control Line 518 to return to DPA 
510 as indicated by Step 564 and Step 552. 
The instruction to toggle between any two antennas 510, 512, or 514 from 
TTS 520 to Switch Selector 516 can be designed to occur only upon a 
certain event. For example, one criterion could be requiring that the 
switching occur just prior to the beginning of a time slot. Other 
threshold schemes could be implemented without departing from the spirit 
and scope of the invention. For instance, the toggling between antennas 
can be triggered during the fall from "H" to "L" somewhere along the curve 
delineated by point A and point B (or point C and point D). The process 
reflected by the flowchart can be altered so that when either secondary 
antenna's signal is dropping, the opposite secondary antenna is tested 
before trying DPA 510. 
A third technique for selecting from among the TPDA 360 using SCU 370 (FIG. 
3C) is Fixed Combining Diversity, sometimes called Equal Gain Combining 
Diversity. The Fixed Combining Diversity technique can advantageously 
produce an acceptable signal from a number of unacceptable input signals. 
Referring now to FIG. 6A, each antenna from a First TPDA 605 is connected 
to one co-phaser of a First Three Co-phasers 610. Usually, equal gain 
combining diversity requires variable co-phasing. Here, however, when the 
First TPDA 605 are all orthogonal to one another, the ideal phase 
difference is 90 degrees. Hence, the appropriate phasing is diagramed at 
the First Three Co-phasers 610. Each of the three inputs are summed at a 
First Summer 615. The three inputs are combined in this manner and can 
constitute the output to TTS 620. 
In FIG. 6A, a second set of TPDA is included. Each antenna in the Second 
TPDA 635 is connected to one co-phaser of the Second Three Co-phasers 640 
and then combined by summing at a Second Summer 645. The Switch Selector 
630 can toggle from the First TPDA 605 (at the output of the First Summer 
615) to the Second TPDA 635 (at the output of the Second Summer 645) based 
on instructions from the TTS 620 over the Control Line 625. The Second 
TPDA 635 can be, for example, spatially separated from the First TPDA 605. 
The Switch Selector 630 can then choose the better of the two TPDA 605 and 
635 for reception and subsequent processing. In other words, a second 
diversity (e.g., space or antenna diversity) may be implemented in 
addition to polarization diversity in FIG. 6A. 
Although two sets of TPDA 605 and 635 are depicted in FIG. 6A and included 
in the process diagramed in a Fixed Combining flowchart 650, fixed 
combining can be implemented with one set of antennas (one level of 
diversity). Now referring to FIG. 6B, the Fixed Combining flowchart 650 is 
illustrated. In Step 655, signals are received on both the First TPDA 605 
and the Second TPDA 635. In Step 660, the received signals are co-phased 
by the First Three Co-phasers 610 and the Second Three Co-phasers 640. The 
co-phased signals for each of the First TPDA 605 and the Second TPDA 635 
are then summed in Step 665. Finally, in Step 670, the better of the two 
outputs from the First Summer 615 and the Second Summer 645 is selected 
and sent to the TTS 620 for further processing as instructed by the TTS 
620 over the Control Line 625. The better of the two outputs can be 
determined according to many techniques, e.g., space diversity. 
A fourth technique for selecting from among the TPDA 360 in SCU 370 (FIG. 
3C) is Adaptive Combining Diversity. With the rapid advancement of 
processing abilities in mobile terminals, Adaptive Combining Diversity can 
be a reality in many mobile communication situations, e.g., cellular, 
Personal Communication System (PCS), Personal Communication Network (PCN), 
Universal Mobile Telecommunication System (UMTS), and International Mobile 
Telecommunication 2000 (IMT2000). For instance, integrated circuit 
technology makes it possible to use multiple receiving chains, even in 
handheld terminals. Among the many Adaptive Combining Diversity algorithms 
that benefit from improvements in digital signal processing (DSP) 
abilities are maximal ratio combining (MRC) and interference rejection 
combining (IRC). Using an adaptive combining algorithm advantageously 
permits a reduction in base station transmitter power, increases 
reliability, and reduces overall system cost. Each of the adaptive 
combining algorithms are preferably realized in a DSP unit. 
Referring now to FIG. 7A, an example of the MRC algorithm is illustrated. 
Signals are received on the TPDA 360 and then are input to the Adapting 
Variable Gain Units (AVGU) 710. In this feedback circuit, a Detector 730 
produces Received Signal Strength Indications (RSSIs) and feeds the RSSIs 
back into the AVGU 710 via an Adaptive Control and RSSIs Path 740. The 
Detector 730 can be either m detectors or one detector undergoing m 
iterations (with m=3 here because of the TPDA 360). The signals from all 
three branches are weighted according to their individual signal voltage 
to noise power ratios (in the AVGU 710) and then co-phased (in a 
Co-phasing and Summing Unit (CSU) 720). 
After co-phasing, the signals are summed in the CSU 720. The output of the 
CSU 720 is input to the Detector 730, which serves to provide the control 
feedback signal (via the Adaptive Control and RSSIs Path 740) as well as 
the output signal to be forwarded for receive processing. The MRC 
algorithm produces an output SNR from the Detector 730 equal to the sum of 
the individual SNRs and can, in the process, produce an output with an 
acceptable SNR even when none of the individual input signals are 
themselves acceptable. 
Referring now to FIG. 7B, the MRC flowchart 750 is illustrated. The RSSI 
for each input antenna (the TPDA 360) is detected at Step 755 by the 
Detector 730. Each input signal is weighted according to the RSSI at Step 
760 by the AVGU 710. At Step 765, each weighted signal is co-phased and 
summed by the CSU 720. Meanwhile, the Detector 730 is providing the ACS 
740 to the AVGU 710 at Step 775. Finally, the co-phased and summed signal 
that is output from the Detector 730 can be processed at Step 770. 
Referring now to FIG. 8A, an example of the IRC algorithm is illustrated. 
In contrast to MRC, which excels at processing decorrelated multipass 
signals, IRC excels at processing signals that are highly correlated. IRC 
achieves optimum performance with k antennas and no more than k-1 
interfering signals. Incoming signals are received on the TPDA 360 and are 
input to an Interference Detector (IFD) 810. IFD 810 relies on the high 
correlation of the incoming signals along with a control feedback signal 
(the Adaptive Control Signal (ACS) 840) from a Detector 830 to detect one 
or more interferers. 
Preferably, with three receive antennas, no more than two interferers will 
need to be rejected. An Interference Rejector (IFR) 820 electronically 
moves the antenna reception beam to steer at least one null to the 
interferer(s). The Detector 830 can subsequently detect an 
interference-reduced incoming signal, provide the ACS 840 to the IFD 810, 
and output a signal for further receive processing. 
Referring now to FIG. 8B, the IRC flowchart 850 is illustrated. 
Interference of signals incoming on the TPDA 360 is detected by the IFD 
810 at Step 855. The IFR 820 alters the receiving pattern to steer a null 
to interference (e.g., co-channel interference) at Step 860. At Step 865, 
the Detector 830 detects the interference-reduced signal and forwards the 
output signal to enable further receive processing at Step 870. Meanwhile, 
the Detector 830 is also providing the ACS 840 to the IFD 810 at Step 875 
for feedback control. It is noted that the IRC algorithm performs well in 
interference-limited environments such as cellular communications systems. 
Although a preferred embodiment of the method and apparatus of the present 
invention has been illustrated in the accompanying Drawings and described 
in the foregoing Detailed Description, it will be understood that the 
invention is not limited to the embodiment disclosed, but is capable of 
numerous rearrangements, modifications and substitutions without departing 
from the spirit of the invention as set forth and defined by the following 
claims. For example, selection/combination techniques and/or algorithms 
other than those explicitly described above can be used without departing 
from the spirit and scope of the polarization aspect of the invention. 
Additionally, any algorithm (e.g., Viterbi or recursive least squares, as 
well as MRC and IRC) can be utilized regardless of the combining 
technique.