(12) United States Patent
`Eidson
`
`USOO6411824B1
`(10) Patent No.:
`US 6,411,824 B1
`(45) Date of Patent:
`Jun. 25, 2002
`
`(54) POLARIZATION-ADAPTIVE ANTENNA
`TRANSMIT DIVERSITY SYSTEM
`
`(75) Inventor: Isld Brian Eidson, San Diego, CA
`
`(73) Assignee: Conexant Systems, Inc., Newport
`Beach, CA (US)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(*) Notice:
`
`(56)
`
`(21) Appl. No.: 09/103,417
`(22) Filed:
`Jun. 24, 1998
`(51) Int. Cl. .................................................. H04B 1/38
`(52) U.S. Cl. ....................... 455/561; 455/562; 455/101;
`370/320; 342/361
`(58) Field of Search ................................. 455/561, 562,
`455/63, 65, 67.6, 5O1, 504,506, 101. 3.43361.
`s was
`343853. 370?320, 376. 375299
`References Cited
`U.S. PATENT DOCUMENTS
`455/562
`4,747,160 A
`5/1988 Bossard
`5.491723. A
`2/1996 Diepstraten. - - - - - - - - - - ... 455/101
`5,548,583 A
`8/1996 Bustamante ................. 370/18
`5,701,591 A * 12/1997 Wong .......................... 455/63
`5,724,666 A * 3/1998 Dent ............
`... 455/562
`5,771,449 A * 6/1998 Blasing et al. .............. 455/562
`5,784,032 A * 7/1998 Johnstone et al.
`... 434/702
`5,884,192 A * 3/1999 Karlsson et al. ............ 455/562
`5,903,238 A * 5/1999 Sokat et al. ......
`... 342/365
`5,933,788 A * 8/1999 Faerber et al. .
`... 455/562
`5,963.874 A * 10/1999 Mabler .........
`... 455/562
`5.999.826 A * 12/1999 Whinnett .................... 455/562
`
`
`
`6,018,317 A * 1/2000 Dogan et al. ............... 342/378
`6,043,790 A * 3/2000 Derneryd et al. ........... 343/853
`6,172,970 B1 * 1/2001 Ling et al. .................. 370/347
`FOREIGN PATENT DOCUMENTS
`EP
`342694
`* 2/1996
`GB
`2310.109
`2/1996 ............ HO4B/7/02
`* cited by examiner
`
`Primary Examiner William Trost
`ASSistant Examiner Tilahun Gesesse
`(74) Attorney, Agent, or Firm- Knobbe, Martens, Olson &
`Bear LLP
`ABSTRACT
`(57)
`A duplex polarization adaptive System is described. The
`System provides polarization diversity for base Station anten
`nas under both receive and transmitting conditions. Since
`the base station provides polarization diversity in both
`transmit and receive modes, no polarization diversity is
`needed in the handheld unit. Even though the handheld unit
`does not provide polarization diversity, a duplex communi
`cation System, that uses polarization diversity for both the
`uplink and the downlink is provided, because the base
`Station provides polarization diversity for the uplink and the
`downlink paths. By installing the two-way diversity at the
`base Station, the overall cost of implementing diversity is
`reduced because one base Station can typically Serve many
`handsets. The base Station antenna determines the polariza
`tion State of Signals received from a remote unit, Such as a
`handheld unit, using a polarization diverse antenna System.
`The base Station then transmits using the same polarization
`State. The System is compatible with time-division duplex
`Systems
`
`35 Claims, 11 Drawing Sheets
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`US 6,411,824 B1
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`1
`POLARIZATION-ADAPTIVE ANTENNA
`TRANSMIT DIVERSITY SYSTEM
`
`BACKGROUND OF THE INVENTION
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`1. Field of the Invention
`The present invention relates to the field of wireless
`communications, and more particularly, to polarization
`diversity Systems for wireleSS communications.
`2. Description of the Related Art
`It can be fairly Said that the age of wireleSS communica
`tions began in 1898 when Guglielmo Marconi broadcast the
`first paid radio program from the Isle of Wight. The system
`used by Marconi was a one-way wireleSS communication
`System comprising a transmitter that Sent messages, carried
`by electromagnetic waves, to one or more receivers. One
`way communications Systems, Such as broadcast radio,
`television, etc., are still widely used today.
`In contrast to one-way Systems that can only Send mes
`Sages from one person to another, duplex (two-way) wireless
`communications Systems, Such as cellular telephones, cord
`leSS telephones, etc., allow two-way communication
`between two or more parties. In its simplest form, a duplex
`communication System is the combination of two one-way
`Systems. In a duplex communication System, each party is
`equipped with a transceiver (a transmitter combined with a
`receiver) So that each party can both send and receive
`messages. Communication is two-way because each trans
`ceiver uses its transmitter to Send messages to the other
`transceivers, and each transceiver uses its receiver to receive
`messages from the other transceivers.
`AS with normal conversation between people, duplex
`communication Systems typically use Some technique to
`minimize the interference that occurs when two parties try to
`transmit (i.e., talk) at the same time. As with normal
`conversation, many duplex Systems use Some form of a Time
`Division Duplexing (TDD) algorithm, wherein only one
`party at a time is allowed to transmit. Each party transmits
`only during its allotted time interval, and during that time
`interval, all other parties are expected to receive the trans
`mission (i.e., listen). Other division techniques, Such as, for
`example, frequency division, code division, etc., are also
`used to Separate transmissions between parties.
`TDD systems include the Digital European Cordless
`Telephone (DECT), the Personal Handy phone System
`(PHS), the Personal ACcess System (PACS), and the Per
`sonal Wireless Telecommunications (PWT) system. DECT
`is a 2nd generation cordleSS telephone Standard, designed to
`be capable of Supporting very high traffic densities at
`1895–1906 MHz (private) and 1906–1918 MHz (public),
`with a proposed extension to a 300 MHz frequency band.
`DECT uses a TDMA/TDD access technique and a GMSK
`modulation technique, making it Suitable for low mobility
`high capacity concentrated usage environments Such as city
`center offices and transport hubs. PHS, developed in Japan,
`operates at 1880–1900 MHz, uses a TDMA/TDD access
`technique and a JL/4 QPSK modulation technique. PACS,
`developed by Bellcore, uses both TDMA/FDD (Frequency
`Division Duplex) and TDMA/TDD. PWT is the new name
`60
`for the licensed DT 1900 as well as the unlicensed WCPE
`cordless technologies found in the United States.
`In both one-way and duplex communication Systems, the
`transmitter provides Radio Frequency (RF) Signals to a
`transmitting antenna that converts the RF signals into Elec
`troMagnetic (EM) waves. The EM waves propagate to a
`receiving antenna where the EM waves are converted back
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`into RF signals that are provided to the receiver. Ideally, the
`EM waves travel in a single path directly from the trans
`mitting antenna to the receiving antenna, without any exter
`nal influences or perturbations, and without taking multiple
`paths. Unfortunately, ideal conditions are rarely found in the
`real-world and thus the EM waves that propagate from the
`transmitting antenna to the receiving antenna are often
`disturbed by external influences. These disturbances often
`reduce the strength of the EM waves that reach the receiving
`antenna, and thus impair the performance of the communi
`cations System. Fluctuation in the Strength of the received
`Signal is known as Signal fading. The impairment caused by
`Signal fading can include reduced range, higher noise, higher
`error rates, etc. Fading is usually caused by destructive
`interference of multipath waves. In theory, the reduction in
`Signal Strength at the receiving antenna can be offset by
`increasing the strength of the EM wave produced by the
`transmitting antenna. However, the strength of the EM wave
`produced by the transmitting antenna is usually limited by
`various factors, including, government regulations, the size/
`cost/weight of the transmitter, the size/cost/weight of the
`transmitting antenna, and the power available to operate the
`transmitter. The power available to the transmitter is par
`ticularly important in battery operated devices, Such as
`handheld cellular telephones, where battery life is an impor
`tant aspect of overall System performance.
`Two common types of Signal fading are multipath fading
`and polarization mismatch fading. Multipath fading occurs
`when the EM waves take two or more paths to travel from
`the transmitting antenna to the receiving antenna. The waves
`arriving at the receiving antenna along different paths will
`often interfere with each other, Such that a wave arriving
`from a first path will tend to cancel a wave arriving from a
`Second path. Receive-antenna position-diversity is a method
`often used to mitigate the effects of multipath fading. In
`Systems with receive-antenna position-diversity, Several
`receiving antennas are positioned Such that the phase centers
`(i.e., positions) of the antennas are physically separated by
`a few wavelengths. The receiving antennas are used to
`receive the EM waves, and the output from each receiving
`antenna is provided to the receiver for Special processing.
`Receive-antenna position-diversity works because the
`destructive interference is typically a localized phenomenon.
`Even if one of the receiving antennas is experiencing
`multipath fading, it is likely that another receiving antenna
`located Several wavelengths away will not experience fad
`ing. The Separation between the antennas is desirable
`because the probability of having all of the received signals
`for all of the receiving antennas faded at one time becomes
`increasingly Small as the number of antennas are increased.
`Receive-antenna position-diversity is commonly used in
`wireleSS base Stations where antenna Size, weight, and cost
`are leSS important than in handheld units. Antenna position
`diversity is rarely used in handheld units because of the Size,
`weight, and cost associated with multiple receiving antennas
`Spaced Several wavelengths apart. For example, conven
`tional analog cellular telephones operate using EM waves
`having a frequency of approximately 1 GigaHertz (GHz). A
`1 GHz EM wave in air has a wavelength of approximately
`1 foot. Thus, an effective position-diversity antenna System
`would be Several feet acroSS. This is clearly impractical for
`a handheld telephone, but very practical for a base Station
`antenna mounted on a large tower.
`Various techniques are used to process the antenna
`outputs, including, for example, Antenna Switching
`Diversity, and Maximal Ratio Combining. Antenna Switch
`ing Diversity Systems Simply pick the receiving antenna that
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`is currently receiving the Strongest EM wave and use that
`antenna as the receiving antenna.
`Maximal Ratio Combining Systems combine the outputs
`of one or more receiving antennas into a Single output Signal.
`The outputs of the antennas are coherently phased and
`weighted to provide maximum power in the output Signal.
`Maximal Ratio Combining typically offers better perfor
`mance than Antenna Switching Diversity because it com
`bines the antenna outputs, thus bringing in more signal while
`tending to average out the noise. This results in a higher
`Signal-to-Noise Ratio (SNR).
`The combination of antenna-position diversity and maxi
`mal ratio combining is closely related to the technique of
`antenna-pattern diversity. In antenna pattern diversity, the
`antenna typically comprises Several antenna elements. The
`transmitter provides RF signal to each antenna element Such
`that the EM radiation from the antenna elements is focused
`in a particular direction, much like the focused beam from
`a flashlight. In Some locations, Such as Japan, regulatory
`constraints favor the less effective technique of antenna
`Switching rather than maximal ratio combining. In the
`Japanese PHS System for example, So-called "Smart anten
`nas' which provide antenna-pattern diversity, are only
`allowed if they also reduce the maximum power output
`provided by each antenna element by an amount propor
`tional to the number of antenna elements. For example, if
`four antenna elements are available, the maximum output at
`each antenna element is limited to one-fourth of the legally
`mandated maximum output power from a single antenna
`element. A possible rationale for this regulation is that the
`Japanese PHS system allows competitive service providers
`to share the same frequency bands. If one competitor is
`allowed to focus EM waves in one direction, then a nearby
`base Station operated by another competitor, and Servicing
`mobile users along the same radiation path, would experi
`ence interference. By reducing the maximum power avail
`able to each antenna element in an array of antenna
`elements, the total power output of the array is limited. This,
`unfortunately, greatly reduces the effectiveness of transmit
`diversity using antenna combining by up to 3 dB for a
`two-antenna System, and up to 6 dB for a four-antenna
`System. With these constraint losses, antenna-Switching
`tends to outperform maximal ratio combining (at least from
`a diversity reception Standpoint; maximal ratio combining
`does reduce the interference Seen by other users not in the
`paths of its beams).
`Polarization mismatch fading occurs when the polariza
`tion of the EM wave that arrives at the receiving antenna
`does not match the polarization of the receiving antenna. For
`example, polarization mismatch fading is common when
`using a mobile handset because different users will orient the
`handset at different angles. Base Station antennas are typi
`cally designed for a vertically oriented linear polarization.
`Most typical handheld units have a Small whip antenna
`(more precisely, a monopole antenna) that is also linearly
`polarized, with a polarization vector that is parallel to the
`antenna. Thus, in theory, most handheld units provide the
`least polarization mismatch fading when the antenna is held
`vertically. Unfortunately, the wireless handset is rarely held
`So that the antenna is vertical. The handset is usually held
`diagonally So that the mouthpiece (microphone) is close to
`the user's mouth, and the earpiece (loudspeaker) is over the
`user's ear. If the user is Standing or Sitting, the vertical axis
`of the mobile handset is therefore often 45 degrees or more
`off of true Vertical. If the user is reclining, the handset may
`be almost completely horizontal.
`Polarization mismatch fading often occurs when the user
`orients the handheld unit So that the antenna is not vertical.
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`This polarization mismatch fading Sometimes goes unno
`ticed because most communication Systems are designed
`with a power budget that provides a large exceSS power
`margin. By holding the antenna at less than optimal
`orientation, the user is merely unconsciously using up Some
`of the power budget designed into the System. However, at
`the far fringe of a reception area, most of the power budget
`is used up just getting the EM waves from the transmitter to
`the receiver. Thus, at the fringe of a reception area, the user
`will notice the effects due to polarization mismatch.
`ASSuming line of Sight propagation, a 45 degree polar
`ization mismatch between a single base Station antenna and
`mobile unit antenna results in only half of the power (3 dB)
`being delivered to the receiver; a 90 degree mismatch results
`in (theoretically) no power being delivered to the receiver.
`Many Studies have been done on Signal Strength versus
`antenna orientation in the mobile unit. For example K. Li
`and S. Mikuteit, “Characterization of Signal Polarization
`Near 900 MHz in and on Vehicles and Within Buildings”,
`Proceedings of ICUPC 1997, pp. 838–842, indicates that,
`indeed, a mobile unit antenna oriented toward the vertical
`tends to offer higher performance than those oriented toward
`the horizontal. However, this study also found that in
`complex, non-line-of-Sight (e.g., multipath) environments,
`the difference between the horizontal and vertical polariza
`tion signal Strengths can be Small. Moreover, in Strong
`multi-path conditions, the above Study reports that a circu
`larly polarized antenna (which mixes horizontal and vertical
`polarizations) performs best. Size, cost, and complexity
`considerations typically prohibit the incorporation of a cir
`cularly polarized antenna into the handset. Likewise, cost,
`antenna Switching losses, and antenna Separation consider
`ations tend to disfavor the incorporation of multiple anten
`nas into the handset.
`Recently, receive-only base Station antenna polarization
`diversity has been investigated in the hope of improving
`performance of the path from a handset to a base Station Such
`as a cellular tower. This path is often called the uplink.
`Unfortunately, in the receive-only context, perceived gains
`have been Seen, but they are not Sufficient to justify receive
`only diversity in many applications. M. Nakano, T. Satoh,
`and H. Arai, “Up Link Polarization Diversity and Antenna
`Gain Measurement of a Hand-Held Terminal', IEEE Anten
`nas and Propagation Society International Symposium, Jun.
`18–23, 1995, vol. 4 pp. 1940–1943, describes the results of
`field experiments on the received polarization of 900 MHz
`Signals. This paper notes that the average Signal level of the
`horizontal (H) polarization component received from a
`handheld phone is, in general, greater than the vertical (V)
`component. Moreover, the paper indicates that the correla
`tion coefficient between horizontal and Vertical Signals under
`fading conditions is less than 0.3, which is important Since
`the diversity antennas should be as uncorrelated as possible
`in order to reap maximum gains.
`A. Turkmani, A Arowojolu, P. Jefford, and C. Kellet, “An
`Experimental Evaluation of the Performance of Two-Branch
`Space and Polarization Diversity Schemes at 1800 MHz,
`IEEE Transactions on Vehicular Technology, vol. 44, no. 2,
`May 1995, pp. 318-326, describes results similar to Nakano
`et al., but using 1800 MHz signals. Turkmani et al. con
`cluded that receive-only polarization-diversity outperforms
`receive-only position diversity. In particular, Turkmani et al.
`found that a 45-degree oriented handset induced mismatch
`losses averaging 6 dB, while using two vertical antennas for
`receive-only antenna-position diversity. By contrast, Turk
`mani et al. found that a polarization-diverse receiver Setup
`Suffered less fading, and showed that the total advantage of
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`using receive-only polarization diversity appears to be
`approximately 6 dB when the handset is tilted at 45 degrees.
`K. Cho, T. Hori, H. Tozawa, and S. Kiya, “Bidirectional
`Base Station Antennas with 4-Branch Polarization and
`Height Diversity', Proceedings of ISAP'96, Chiba Japan, pp.
`357-360, reports results which tend to corroborate the
`results discussed above. Cho et al. describe measured data
`for a number of handset antenna inclinations. The results
`indicate that the combined Statistic of overall signal power
`and diversity gain favors polarization diverse antennas for
`mobile handset tilts greater than (approximately) 27 degrees
`from the vertical.
`These Studies, and others, use polarization-diversity that
`is implemented at the receiving antenna because that is, in
`effect, where the problem arises. In general, the transmitting
`antenna has no "knowledge” of the location, polarization, or
`even existence of a receiving antenna. The transmitting
`antenna merely creates an EM wave which radiates in many
`directions. A Single EM wave radiated by the transmitting
`antenna may be received by Several receiving antennas, each
`receiving antenna having a different polarization. Even if the
`transmitting antenna transmits an EM wave that is properly
`polarized for a particular receiving antenna, multipath
`effects, diffraction from objects Such as buildings, and other
`25
`propagation effects can rotate the polarization of the EM
`wave such that the polarization of the EM wave that arrives
`at the receiving antenna no longer matches that antenna.
`Although, performance of a communication System can
`be improved by using a receive-only polarization-diversity,
`the gains are modest and may not justify the additional cost
`and complexity of implementation. Moreover, implement
`ing receive-only diversity in the base Station only improves
`the communication path from the handset to the base Station
`(the uplink path). Polarization-diversity in the base station
`receiving antenna does nothing to improve the communica
`tion path from the base Station to the handset unit (the
`downlink path). Thus, the benefits of base station diversity
`are one-sided. In many communications Systems, there is
`little benefit to increasing the uplink performance if down
`link performance is not similarly increased, and Vice versa.
`Two-way polarization diversity can be implemented by
`building a handset unit with a polarization-diverse receiving
`antenna. Unfortunately, as discussed above, implementing
`antenna diversity in the handset unit is typically not practical
`due to problems related to cost, weight, Size, and complexity.
`SUMMARY
`The present invention solves these and other problems by
`disclosing polarization diversity for base Station antennas
`under both receive and transmitting conditions. Since the
`base Station provides polarization diversity in both transmit
`and receive modes, no polarization diversity is needed in the
`handheld unit. Even though the handheld unit does not
`provide polarization diversity, a duplex communication
`System, that uses polarization diversity for both the uplink
`and the downlink is provided, because the base Station
`provides polarization diversity for the uplink and the down
`link paths. By installing the two-way diversity at the base
`Station, the overall cost of implementing diversity is reduced
`because one base Station can typically Serve many handsets.
`The base Station antenna determines the polarization State
`of Signals received from a remote unit, Such as a handheld
`unit, using a polarization diverse antenna System. The base
`Station then transmits using the same polarization. In a
`preferred embodiment, this System is used with a time
`division duplex System.
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`In one embodiment the base Station has a polarization
`diverse antenna comprising Several antenna elements con
`figured to receive EM waves having different polarization
`States. In one embodiment the antenna elements are config
`ured to receive EM waves that are cross-polarized. In
`another embodiment, a first antenna element is configured to
`receive horizontally polarized waves and a Second antenna
`element is configured to receive vertically polarized waves.
`During receive mode, the power and phase of the output
`Signal from each antenna element is measured. A diversity
`receiver combines the output Signals to achieve diversity
`gain. Upon going into transmit mode, the base Station
`transmitter weight the antenna output powers in a ratio
`corresponding to their received power measurements, and
`with relative phases which are reversed from the received
`phases. By So doing, the base Station effectively tracks the
`polarization of the Signal transmitted by the mobile unit Such
`that the same polarization State is used for both transmit and
`receive functions. The base Station adopts a transmit polar
`ization that is better Suited to the polarization of the antenna
`on the handset unit, regardless of the orientation of the
`handset.
`In another embodiment, predictive algorithms are used to
`predict a polarization State for the next re-transmission.
`The present invention may be used in many wireleSS
`systems including, for example, DECT, PHS, PACS-UA,
`PACS-UB, PWT, PWT(E), and in third-generation wireless
`systems, such as the proposed CDMA/TDD System.
`BRIEF DESCRIPTION OF THE DRAWINGS
`The advantages and features of the disclosed invention
`will readily be appreciated by perSons skilled in the art from
`the following detailed description when read in conjunction
`with the drawings listed below.
`FIG. 1 is a block diagram of a wireleSS communications
`System showing an uplink path, a downlink path, and noise.
`FIG. 2 is a block diagram of a wireleSS communications
`System showing antenna orientations between two handsets
`and a base Station.
`FIG. 3 is a block diagram of a wireleSS communications
`System showing a multipath Signal environment.
`FIG. 4 is a timing diagram showing the operation of a time
`division duplex (TDD) system.
`FIG. 5 is a block diagram of a wireless communication
`System with a base Station antenna that provides three-axis
`polarization diversity.
`FIG. 6A is a System block diagram of the communications
`system shown in FIG. 5, which uses the same antenna
`elements for transmit and receive functions.
`FIG. 6B is a system block diagram of the communications
`system similar to the system shown in FIG. 5, but with
`Separate antenna elements for transmit and receive func
`tions.
`FIG. 7 is a diagram of one embodiment of a base station
`antenna that Supports two-axis polarization diversity.
`FIG. 8 is a System block diagram of a communications
`System that provides two-axis polarization diversity.
`FIG. 9 is a system block diagram of a diversity system that
`achieves polarization diversity through the use of antenna
`Switching.
`FIG. 10 is a system block diagram of a diversity system
`that achieves polarization diversity through the use of maxi
`mal ratio combining.
`In the drawings, the first digit of any three-digit number
`generally indicates the number of the figure in which the
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`ERICSSON v. UNILOC
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`element first appears. Where four-digit reference numbers
`are used, the first two digits indicate the figure number.
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`US 6,411,824 B1
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`8
`base Station 110. The extracted message is typically pro
`vided to a loudspeaker so that the user 104 can hear the
`meSSage.
`In an analog communications System, the user 104 will
`typically hear the message accompanied by the noise (e.g.
`Static). If the message is loud enough in relation to the noise
`then the user 104 will be able to ignore the noise and listen
`to the message. However, if the noise is loud in relation to
`the message, the user 104 will have difficulty extracting the
`message from the noise. At Some point, the noise can
`become so loud in relation to the message that the user 104
`is unable to discern the message. The ratio of the Strength of
`the desired signal (the message Signal) to the noise is called
`the Signal to Noise Ratio (SNR). The SNR is an important
`measure of the quality and reliability of an analog commu
`nication System. A SNR greater than one is desirable, and
`indicates that the message Signal is Stronger than the noise
`Signal. SNR leSS than one is undesirable, and indicates that
`the message Signal is weaker than the noise Signal.
`Information theory teaches that the desired message can
`no longer be extracted from the noise when the SNR drops
`below -2 dB. However, a SNR of 0 dB (unity) is often
`considered to be a practical lower desired limit for real
`World Systems. Analog communications Systems tend to fail
`gradually as the SNR drops close to unity. In an analog
`System, as the SNR drops from Some large value to unity, the
`user 104 will hear more and more static but the system will
`typically still work and the user 104 will be able to discern
`at least part of the message. Unlike analog Systems, digital
`communication Systems typically do not fail as gradually.
`Many digital communication System use masking So that as
`the SNR drops, the user 104 will typically not hear any
`increase in noise, but at Some point, the SNR will drop to a
`point where the System will Stop operating, and the user will
`hear periods of Silence.
`Since SNR is the ratio of Signal Strength to noise Strength,
`the SNR of a communication system can be improved by
`either increasing the Signal Strength, reducing the noise
`strength, or both. The strength of the noise 116 and 118 is
`typically determined by environmental factors that are
`beyond the control of the communication System designer.
`Thus, in many circumstances, the best method for improving
`the SNR is to increase the Signal Strength. For example, the
`Signal Strength at the handset 102 can be increased by
`increasing the strength of the EM wave radiated by the base
`Station antenna 106. Unfortunately, government regulations
`typically limit the strength of the EM wave radiated by the
`base station 110.
`Increasing the strength of the EM wave radiated by the
`base station 110 only increases the SNR for the downlink
`114. To increase the SNR for the uplink 112, the signal
`Strength at the base Station 110 can be increased by increas
`ing the strength of the EM wave radiated by the handset
`antenna 103. Here again, government regulations often limit
`the maximum radiated power. Moreover, other power
`considerations, such as battery drain, often limit the EM
`Signal Strength that can be produced by the handset 102.
`Thus, other methods for increasing the Strength of the
`received signal, both at the base station 110 and the handset
`102, are desirable.
`One method for improving the strength of the received
`signal, and thus the SNR, is to improve the EM coupling
`between the base station antenna 106 and the handset
`antenna 103. In particular, the polarization of the EM signal
`radiated by the base station antenna 106 should match the
`polarization of the handset antenna 103 (and vice versa).
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`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`FIG. 1 is a block diagram of a typical duplex wireleSS
`communications System 100 showing two-way communica
`tion between a handset 102 and a base station 110. Th