lllllllllllllllllIll"lllllIllllllllllllllIllllIlllllllllllllllllllllllllll
`C
`U8005280472A
`1111 Patent Number:
`5,280,472
`Umted States Patent
`Gilhousen et a1.
`[45] Date of Patent:
`Jan. 18, 1994
`
`
`[19]
`
`[54] CDMA MICROCELLULAR TELEPHONE
`SYSTEM AND DISTRIBUTED ANTENNA
`SYSTEM THEREFOR
`
`[75]
`
`Inventors: Klein S. Gilhousen, San Diego;
`Franklin p, Antonio, Del Mar, both
`of Calif.
`
`[73] Assignee: Qualcomm Incorporated, San Diego,
`Califi
`
`1le Appl- N0-= 849,551
`[22] Filed:
`M813 9, 1992
`
`[63]
`
`Related US. Application Data
`Continuation of Ser. No. 624,118, Dec. 7, 1990, aban-
`doned-
`Int. (31.5 .............................................. H0” 13/00
`[51]
`[52] US. Cl. .3..................................... 370/18; 342/375;
`375/1; 455/2791
`[58] Field of Search ................. 370/13, 93, 19; 375/1;
`380/34; 455/273, 278.1, 279.1, 276.1; 343/353;
`342/375
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`4,383,327
`5/1983 Timor .................................... 370/93
`4,475,215 10/1984 Gutleber .......................... 370/ 18
`
`4,672,658 6/1987 Kavehrad et al.
`.................... 379/63
`8/1988 Hui ...............:.....
`4,761,778
`370/60 1
`
`4,841,527 6/1989 Raychaudhun
`371/32
`4,901,307 2/1990 Gllhousen ................. 370/18
`
`. 455/283 X
`4,920,348 4/1990 Baghdady ......
`
`1/1991 Kaufmann ........... 375/1
`4,984,247
`
`9/1991 Messenger .........
`5,046,066
`370/18
`......... 375/1
`5,056,109 10/1991 Gilhousen at al.
`5,073,900 12/1991 Mallinckrodt ............ 370/18
`
`.
`..... 375/1 X
`3/1992 Gilhousen ct a1.
`5,101,501
`
`5,103,459 4/1992 Gilhousen et a1.
`......... 375/1
`5,109,390 4/1992 Gilhousen ct a].
`..................... 375/1
`Primary Examiner—Douglas W. Olms
`Assistant Examiner—Melvin Marcelo
`Attorney, Agent, or Firm—Russell B. Miller; Katherine
`w. Walker
`ABSTRACT
`[57]
`A code division multiple access (CDMA) communica-
`tion system in which cellular techniques are utilized in a
`wireless Private Branch Exchange (PBX) environment.
`A microcellular arrangement is defined in which a base
`station communicates user information signals using
`CDMA communication signals with subscriber termi-
`nals. A distributed antenna system IS utilized in the
`system to provide multipath signals which facilitate
`signal diversity for enhanced system performance.
`.
`.
`37 Claims, 4 Drawmg Sheets ‘
`
`14
`
` TO / FROM
`
`PSTN
`
`10
`
`WIRED
`' PHONES
`
`VOCODER
`
`CHANNEL
`
` TO / FROM
`
`
` CDMA
`
`V ODER
`0C
`
`CHANNEL
`
`CONTROLLER
`
`18
`
`CLEARWIRE 1006
`
`

`

`US. Patent
`
`Jan. 18, 1994
`
`Sheet 1 of 4
`
`5,280,472
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`US. Patent
`
`Jan. 18,1994
`
`Sheet 2 of 4
`
`5,280,472
`
`40G
`
`TRANSCEIVER
`
` TO / FROM
`
`40A
`
`FIG. 2
`
`V v V v v
`
`
`
`TO / FROM
`TRANSCEIVER
`
`FIG. 3
`
`

`

`US. Patent
`
`Jan. 18, 1994 '
`
`Sheet 3 014
`
`5,280,472
`
`A
`
`100
`
`DIPLEXER
`'
`
`102
`ANALOG ,
`RECEIVER
`
`'DIGITAL' ------- 1
`r—H DATA
`I
`-
`I
`I RECEIVER
`.. ..
`l
`I
`I
`I
`l
`l
`I
`I
`‘
`I
`
`104
`
`SEARCHER
`RECEIVER
`
`-
`
`
` ——_——_———__—_—_———._____.-____-_._.
`120
`
`I
`
`W -
`DATA
`DECODER
`RECEIVER
`
`108
`
`20A
`
`_
`
`PILOT / CONTROL
`CHANNEL
`TRANSMIT
`MODULATOR &
`TRANSMIT
`POWER CONTROL
`
`18
`
`TRA
`
`116
`
`“4
`
`SUMMER
`
`22A
`
`VOCODER
`
`FROM OTHER
`TRANSMIT
`MODULATOR / TRANMIT
`POWER CONTROL
`CIRCUITS
`
`118
`
`TO / FROM
`PBX
`
`
`
`FIG. 4
`
`

`

`US. Patent
`
`Jan.18, 1994
`
`Sheet 4 of 4
`
`.
`
`. 5,280,472
`
`214
`
`SEARCHER
`RECEIVER
`
`DIGITAL
`DATA
`RECEIVER
`
`DIPLEXER
`
`COMBINER
`& DECODER
`
`
`
`
`
`
`
`
` DIVERSITY
`
`
`
`
`
`
`
`
`USER
`DIGITAL
`
`BASEBAND
`
`
`CONTROL
`
` ANALOG
`RECEIVER
`
`
`PROCESSOR
`
`
`TRANSMIT
`MODULATOR
`
`
`FIG. 5
`
`

`

`1
`
`5,280,472
`
`CDMA MICROCELLULAR TELEPHONE SYSTEM
`AND DISTRIBUTED ANTENNA SYSTEM
`THEREFOR
`
`This is a continuation of application Ser. No.
`07/624,118, filed Dec. 7, 1990 abandoned.
`
`BACKGROUND OF THE INVENTION
`1. Field of the‘Invention
`The present invention relates to wireless PBX and
`wireless local loop telephone systems. More specifi-
`cally, the present invention relates to a novel and im-
`proved microcellular telephone system and distributed
`antenna system therefor so as to facilitate indoor com-
`munications using spread spectrum communication sig-
`nals.
`
`11. Description of the Related Art
`The use of code division multiple access (CDMA)
`modulation techniques is one of several techniques for
`facilitating communications in which a large number of
`system users are present. Other multiple access commu-
`nication system techniques, such as time division multi-
`ple access (TDMA), frequency division multiple access
`(FDMA) and AM modulation schemes such as ampli-
`tude companded single sideband (ACSSB) are known in
`the art. However the spread spectrum modulation tech-
`nique of CDMA has significant advantages over these
`modulation techniques for multiple access communica-
`tion systems. The use of CDMA techniques in a multi-
`ple access communication system is disclosed in US.
`Pat. No. 4,901,307,
`issued Feb.
`13, 1990, entitled
`“SPREAD SPECTRUM MULTIPLE ACCESS
`COMMUNICATION SYSTEM USING SATEL-
`LITE OR TERRESTRIAL REPEATERS”, assigned
`to the assignee of the present invention, of which the
`disclosure thereof is incorporated by reference.
`In the just mentioned patent, a multiple access tech-
`nique is disclosed where a large number of mobile tele-
`phone system users each having a transceiver communi-
`cate through satellite repeaters or terrestrial base sta-
`tions (also referred to as cell—sites stations, cell-sites or
`for short, cells) using code division multiple access
`(CDMA) spread spectrum communication signals. In
`using CDMA communications, the frequency spectrum
`can be reused multiple times thus permitting an increase
`in system user capacity. The use of CDMA results in a
`much higher spectral efficiency than can be achieved
`using other multiple access techniques.
`The terrestrial channel experiences signal fading that
`is characterized by Rayleigh fading. The Rayleigh fad-
`ing characteristic in the terrestrial channel signal
`is
`caused by the signal being reflected from many different
`features of the physical environment. As a result, a
`signal arrives at a mobile unit receiver from many direc-
`tions with different transmission delays. At the UHF
`frequency bands usually employed for mobile radio
`communications,
`including those of cellular mobile
`telephone systems, significant phase differences in sig-
`nals traveling on different paths may occur. The possi-
`bility for destructive summation of the signals may
`result, with on occasion deep fades occurring.
`Terrestrial channel fading is a very strong function of
`the physical position of the mobile unit. A small change
`in position of the mobile unit changes the physical de-
`lays of all the signal propagation paths, which further
`results in a different phase for each path. Thus, the
`motion of the mobile unit through the environment can
`
`,
`
`2
`result in a quite rapid fading process. For example, in
`the 850 MHz cellular radio frequency band, this fading
`can typically be as fast as one fade per second per mile
`per hour of vehicle speed. Fading this severe can be
`extremely disruptive to signals in the terrestrial channel
`resulting in poor communication quality. Additional
`transmitter power can be used to overcome the problem
`of fading. However, such power increases effect both
`the user, in excessive power consumption, and the sys-
`tem by increased interference.
`The CDMA modulation techniques disclosed in US.
`Pat. No. 4,901,307 offer many advantages over narrow
`band modulation techniques used in communication
`systems employing satellite or terrestrial repeaters. The
`terrestrial channel poses special problems to any com-
`munication system particularly with respect to multi-
`path signals. The use of CDMA techniques permit the
`special problems of the terrestrial channel to be over-
`come by mitigating the adverse effect of multipath, e.g.
`fading, while also exploiting the advantages thereof.
`In a CDMA cellular telephone system, the same wide
`band frequency channel can be used for communication
`in all cells. The CDMA waveform properties that pro-
`vide processing gain are also used to discriminate be-
`tween signals that occupy the same frequency band.
`Furthermore the high speed pseudonoise (PN) modula-
`tion allows many different propagation paths to be sepa-
`rated, provided the difference in path delays exceed the
`PN chip duration, i.e. l/bandwidth. If a PN chip rate of
`approximately 1 MHz is employed in a CDMA system,
`the full spread spectrum processing gain, equal to the
`ratio of the spread bandwidth to system data rate, can
`be employed to discriminate against paths that differ by
`more than one microsecond in path delay from each
`other. A one microsecond path delay differential corre-
`sponds to differential path distance of approximately
`1,000 feet. The urban environment typically provides
`differential path delays in excess of one microsecond,
`and up to 10—20 microseconds are reported in some
`areas.
`
`In narrow band modulation systems such as the ana-
`log FM modulation employed by conventional tele-
`phone systems, the existence of multiple paths results in
`severe multipath fading. With wide band CDMA mod-
`ulation, however, the different paths may be discrimi-
`nated against in the demodulation process. This discrim-
`ination greatly reduces the severity of multipath fading.
`Multipath fading is not
`totally eliminated in using
`CDMA discrimination techniques because there will
`occasionally exist paths with delayed differentials of
`less than the PN chip duration for the particular system.
`Signals having path delays on this order cannot be dis-
`criminated against in the demodulator, resulting in some
`degree of fading.
`It is therefore desirable in the CDMA cellular tele-
`phone system that some form of diversity be provided
`which would permit a system to reduce fading. Diver-
`sity is one approach for mitigating the deleterious ef-
`fects of fading. Three major types of diversity exist:
`time diversity, frequency diversity and space diversity.
`Time diversity can best be obtained by the use of
`repetition, time interleaving, and error detection and
`correction coding which is a form of repetition. The
`present invention employes each of these techniques as
`a form of time diversity.
`CDMA by its inherent nature of being a wideband
`signal offers a form of frequency diversity by spreading
`the signal energy over a wide bandwidth. Therefore,
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`3
`frequency selective fading affects only a small part of
`the CDMA signal bandwidth.
`Space or path diversity is obtained by providing mul-
`tiple signal paths through simultaneous links from a
`mobile user through two or more cell-sites. Further-
`more, path diversity may be obtained by exploiting the
`multipath environment through spread spectrum pro-
`cessing by allowing a signal arriving with different
`propagation delays to be received and processed sepa-
`rately. Examples of path diversity are illustrated in
`copending US. patent application entitled “SOFT
`HANDOFF IN A CDMA CELLULAR TELE-
`PHONE SYSTEM”, Ser. No. 07/433,030, filed Nov. 7,
`1989, now US. Pat. No. 5,101,501 issued Mar. 31, 1992
`and copending US. patent application entitled “DI-
`VERSITY RECEIVER IN A CDMA CELLULAR
`TELEPHONE SYSTEM”, Ser. No. 07/432,552, also
`filed Nov. 7, 1989, now US. Pat. No. 5,109,390 issued
`Apr. 28, 1992, both assigned to the assignee of the pres-
`ent invention.
`
`15
`
`20
`
`The deleterious effects of fading can be further con-
`trolled to a certain extent in a CDMA system by con-
`trolling transmitter power. A system for cell-site and
`mobile unit power control is disclosed in copending
`US. patent application entitled “METHOD AND AP-
`PARATUS FOR CONTROLLING TRANSMIS-
`SION POWER IN A CDMA CELLULAR MOBILE
`TELEPHONE SYSTEM”, Ser. No. 07/433,031, filed
`Nov. 7, 1989, now US. Pat. No. 5,056,109, issued Oct.
`8, 1991, also assigned to the assignee of the present
`invention.
`
`The CDMA techniques as disclosed in US. Pat. No.
`4,901,307 contemplated the use of coherent modulation
`and demodulation for both directions of the link in
`mobile-satellite
`communications. Accordingly,
`dis-
`closed therein is the use of a pilot carrier signal as a
`coherent phase reference for the satellite-to-mobile link
`and the cell-to-mobile link. In the terrestrial cellular
`environment, however, the severity of multipath fad-
`ing, with the resulting phase disruption of the channel,
`precludes usage of coherent demodulation technique
`for the mobile-to-cell link. The present invention pro-
`vides a means for overcoming the adverse effects of
`multipath in the mobile-in-cell link by using noncoher-
`ent modulation and demodulation techniques.
`The CDMA techniques as disclosed in US. Pat. No.
`4,901,307 further contemplated the use of relatively
`long PN sequences with each user channel being as-
`signed a different PN sequence. The cross-correlation
`between different PN sequences and the autocorrelation
`of a PN sequence for all time shifts other than zero both
`have a zero average value which allows the different
`user signals to be discriminated upon reception.
`However, such PN signals are not orthogonal. Al-
`though the cross-correlations average to zero, for a
`short time interval such as an information bit time the
`cross-correlation follows a binomial distribution. As
`such, the signals interfere with each other much the
`same as if they were wide bandwidth Gaussian noise at
`the same power spectral density. Thus the other user
`signals, or mutual interference noise, ultimately limits
`the achievable capacity.
`The existence of multipath can provide path diversity
`to a wideband PN CDMA system. If two or more paths
`are available with greater than one microsecond differ-
`ential path delay, two or more PN receivers can be
`employed to separately receive these signals. Since
`these signals will typically exhibit independence in mu]-
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`4
`tipath fading, i.e., they usually do not fade together, the
`outputs of the two receivers can be diversity combined.
`Therefore a loss in performance only occurs when both
`receivers experience fades at the same time. Hence, one
`aspect of the present invention is the provision of two or
`more PN receivers in combination with a diversity _
`combiner. In order to exploit the existence of multipath
`signals, to overcome fading, it is necessary to utilize a
`waveform that permits path diversity combining opera-
`tions to be performed.
`A method and system for constructing PN sequences
`that provide orthogonality between the users so that
`mutual interference will be reduced is disclosed in co-
`pending U.S. patent application entitled “SYSTEM
`AND METHOD FOR GENERATING SIGNAL
`WAVEFORMS IN A CDMA CELLULAR MO-
`BILE TELEPHONE SYSTEM”, Ser. No. 07/543,496,
`filed Jun. 25, 1990, now US. Pat. No. 5,103,459 issued
`Apr. 7, 1992 also assigned to the assignee of the present
`invention. Using these techniques in reducing mutual
`interference allowing higher system user capacity and
`better link performance. With orthogonal PN codes, the
`cross-correlation is zero over a predetermined time
`interval, resulting in no interference between the or-
`thogonal codes, provided only that
`the code time
`frames are time aligned with each other.
`In an such a CDMA cellular mobile system as dis-
`closed in Ser. No. 07/543,496, signals are communi-
`cated between a cell-site and mobile units using direct
`sequence spread spectrum communication signals. In
`the cell-to-mobile link, pilot, sync, paging and voice
`channels are defined. Information communicated on the
`cell-to-mobile link channels are, in general, encoded,
`interleaved, bi-phase shift key (BPSK) modulated with
`orthogonal covering of each BPSK symbol along with
`quadrature phase shift key (QPSK) spreading of the
`covered symbols. In the mobile-to-cell link, access and
`voice channels are defined. Information communicated
`on the mobile-to-cell link channels are, in general, en-
`coded,
`interleaved, orthogonal signalling along with
`QPSK spreading. Using orthogonal PN sequences does
`in fact reduce mutual interference, thereby permitting
`greater user capacity,
`in addition to supporting path
`diversity so as to overcome fading.
`The above mentioned patent and patent applications
`disclose a novel multiple access technique wherein a
`large number of mobile telephone system users commu-
`nicate through satellite repeaters or terrestrial base sta-
`tions using code division multiple access spread spec-
`trum modulation that alloWs the spectrum to be reused
`multiple times. The resulting system design has a much
`higher spectral efficiency than can be achieved using
`previous multiple access techniques.
`In cellular telephone systems, a large geographic area
`is provided with mobile telephone service by installing
`a number of cell-sites situated so as to provide coverage
`of the entire geographic area. If service demand exceeds
`the capacity that can be provided by a set of cell-sites
`that just provides coverage, the cells are subdivided
`into smaller cells. This process has been carried out to
`the extent that some major metropolitan areas have
`nearly 200 cell-sites.
`The technique described in US. Pat. No. 4.90l,307
`uses CDMA to achieve a very high capacity by provid-
`ing marginal isolation gain through the exploitation of
`system characteristics and functions such as multiple
`steerable antennas, speech activity and reuse of the
`entire frequency band in every cell of the system. The
`
`

`

`5,280,472
`
`5
`result is a significantly higher system capacity than
`provided by other multiple access techniques such as
`FDMA and TDMA.
`
`In a further development of the cellular telephone
`idea, it is desired to provide a number of very small
`cells, called microcells, which would provide coverage
`of a very limited geographic area. Usually, it is consid-
`ered that such areas are limited to a single floor of an
`office building and the mobile telephone service can be
`viewed as a cordless telephone system that may or may
`not be compatible with the mobile cellular telephone
`system. The rationale for providing such a service is
`similar to the reasoning for use of Private Branch Ex-
`change (PBX) systems in business offices. Such systems
`provide for low cost phone service for a large number
`of calls between phones within the business while pro-
`viding simplified dialing for internal phone numbers. A
`few lines are also provided to connect the PBX system
`to the public telephone system, allowing calls to be
`made and received between phones in the PBX system
`and telephones located elsewhere. It is desirable for the
`microcell system to provide a similar level of service
`but with the added feature of cordless operation any-
`where within the service area of the PBX.
`In applications such as the wireless PBX or Wireless
`local
`loop telephone systems path delays are much
`shorter in duration than in cellular mobile systems. In
`buildings and other indoor environments where PBX
`systems are used it is necessary to provide a form of
`diversity which will enable discrimination between
`CDMA signals.
`The primary problem solved by the disclosed inven-
`tion is the provision of a simple antenna system that
`provides high capacity, simple installation, good cover-
`age and excellent performance. Another problem is to
`achieve the above limited coverage while maintaining
`compatibility with the mobile cellular system and while
`taking a negligible amount of capacity away from the
`mobile system. This is achieved in the disclosed inven-
`tion by combining the capacity properties of CDMA
`with a new distributed antenna design that confines the
`radiation to a very limited and carefully controlled area.
`The implementation of spread spectrum communica-
`tion techniques, particularly CDMA techniques, in a
`PBX environment therefore provides features which
`vastly enhance system reliability and capacity over
`other communication system techniques. CDMA tech-
`niques as previously mentioned further enable problems
`such as fading and interference to be readily overcome.
`Accordingly, CDMA techniques
`further promote
`greater frequency reuse,
`thus enabling a substantial
`increase in the number of system users.
`
`SUMMARY OF THE INVENTION
`
`A key aspect in the wireless PBX and the wireless
`local loop of the present invention is the CDMA distrib-
`uted antenna. In this concept, a set of simple antennas
`are fed by a common signal with only time delay pro-
`cessing to distinguish signals. The transmit output of the
`cell transmitter is fed down a coaxial cable to a string of
`radiators. The radiators are connected to the cable
`using power splitters. The resulting signals, amplified as
`necessary, are fed to the antennas. The salient features
`of this antenna concept are: (l) extremely simple and
`inexpensive; (2) neighboring antennas have time delays
`inserted in feed structure so signals received from two
`antennas are distinguishable by PN temporal process-
`ing; (3) exploits direct sequence CDMA’s ability to
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`discriminate against multipath; and (4) creates deliber-
`ate multipath that satisfies discrimination criteria.
`In the distributed antenna processing, each antenna
`taps into the distribution cable somewhat like a cable
`TV system. Broadband gain is provided as needed at the
`antennas or at the cable taps. Note that the cable system
`will usually consist of two cables, one for transmit sig- ’
`nals and one for receive signals. In many cases, the
`necessary delay will be provided naturally by the distri-
`bution cable and no additional delay elements will be
`necessary. When additional delay is necessary, it will
`usually prove simplest to coil up a length of coaxial
`cable.
`
`A very important feature of this architecture is that
`no signal specific processing is necessary. In particular,
`no filtering, mixing translation or other complex signal
`processing operations need be performed. Only amplifi-
`cation is needed and that is provided “in bulk” to all of
`the signals in the cable with a single amplifier.
`Another advantage is that little site specific engineer-
`ing is required for installation. Normally, antenna place-
`ment will be determined only by physical constraints,
`together with the requirement that every location desir-
`ing service must be covered by at least one antenna.
`There is no concern for overlap. In fact, overlapping
`coverage is desirable in that it provides diversity opera-
`tion to all terminals in the overlap area. Overlap is,
`however, not required.
`The advantages of the distributed antenna concept
`become clear when considering the inherent simplicity
`of the cell equipment required to support a wireless
`PBX, wireless local loop or wireless home extension
`phone.
`For the initial installation of a wireless PBX in a hotel
`or office building it is probable that a system capable of
`handling up to 40 simultaneous calls will be adequate.
`For a system of this capacity, only a single wideband
`(1.25 MHz bandwidth) receiver/transmitter needs to be
`provided. A single receiver/transmitter is then coupled
`into the antenna system drive cable. As described, this
`can be a single serial string of antenna elements.
`Another possible antenna approach allows two or“
`more cables to be driven in parallel by the receiver/-
`transmitter with the necessary delay elements located
`with the receiver/transmitter equipment. As the capac-
`ity demand for a single system grows beyond 40 simul-
`taneous calls, the system can be expanded in two differ-
`ent dimensions.
`
`First, and simplest is the use of additional wideband
`frequency channels. In the cellular telephone applica-
`tion, the total 12.5 MHz bandwidth available for each
`direction for each carrier is subdivided into up to ten
`different 1.25 MHz wideband channels. For example, to
`double capacity to 80 simultaneous calls without chang»
`ing the antenna system, a second'receiver/transmitter
`unit would be added along with the necessary digital
`channel unit/vocoder equipment. If the entire spectrum
`of ten channels is not required for CDMA, then the
`remainder can be used by analog PM (or even digital
`TDMA) using the standard 30 kHz channelization.
`If it is desired to increase capacity without using
`additional frequency spectrum then the antenna subsys-
`tem can be subdivided into “pseudo-sectors.” In this
`architecture, the antenna drive cable in divided so as to
`provide two or more ports. Normally, you would try to
`have the antennas in each of the pseudosectors to be
`relatively disjoint from each other, although this is not
`critical. Each of the pseudo-sector is then provided
`
`

`

`5,280,472
`
`7
`with its own receiver/transmitter unit. The digitized
`sample bus output of the receiver/transmitters is fed to
`all of the channel units.
`
`The channel units, as they are designed for cellular
`service, provide for up to three sector bus connections.
`In cellular service, this would allow three adjoining
`sectors of a cell to be connected to a channel unit. The
`channel unit provides diversity combining of signals
`from all three sectors at the symbol level, thus provid-
`ing a very high level of diversity combining. In the
`wireless PBX application, three antenna strings serving
`adjoining service areas could be connected to these
`three busses. This would allow a “soft handoff” without
`switch intervention to be accomplished between any of
`the antennas in the three antenna strings. This has the
`advantage of “hiding" the handoff ’process from the
`switch and allows the switch to be a generic PBX.
`Clearly, the above architecture could be allowed to
`grow to great size. With ten wideband channels in use in
`three “pseudo-sectors”, approximately 1200 simulta-
`neous calls could be processed. This could be serving on
`the order of 15,000 lines corresponding to a large size
`central office capacity. Growth beyond this capacity is
`also possible but the switching architecture would, of
`necessity, begin to take on some of the characteristics
`and requirements of a cellular system.
`The CDMA system described above for the Wireless
`PBX application can also be applied essentially un-
`changed to the wireless local loop problem. In wireless
`local loop applications it is desired to provide improved
`telephone service to a (generally) built up area with
`small cost and ease of installation of the necessary infra-
`structure. The wireless local loop equipment would be
`co-sited with the central office switch serving the area.
`The vocoders, channel units and receiver/transmit-
`ters would all be located together at the same facility as
`the switch. The receiver/transmitter(s) would be cou-
`pled to the distributed antenna system as described
`above. In this system, the RF signals for both iii-bound
`and out-bound signals pass through a pair of cables. The
`cables are tapped periodically to drive radiating ele-
`ments. The cable taps may or may not require the use of
`amplification to maintain signal levels.
`The home telephone unit for interface with the wire-
`less local loop would consist of a low cost CDMA
`mobile phone modified for use with mains power and a
`simple fixed antenna. The telephone handset would
`plug into this RF unit. The simplicity of the user equip-
`ment would be completely consistent with user installa-
`tion. The customer would simply take it home, open the
`box, plug it in and begin to make calls.
`The architecture of the system allows for a simple
`evolution as the market is penetrated. The service could
`begin with a single omnidirectional antenna located at
`the equipment site. This antenna would be mounted on
`a high tower so as to provide coverage of the area. Note
`that the first objective with initial service is universal
`coverage of the service area so that all customers desir-
`ing service may subscribe.
`Then, as demand causes the need for additional ca-
`pacity, the antennas could be sectorized. As demand
`grows still greater, the most dense sectors can be re-
`placed with the distributed antenna. The distributed
`antenna will allow higher capacity because interference
`from adjacent cells is reduced and because subscriber
`units may operate a lower power and generate less
`interference to neighboring cells.
`
`5
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`10
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`15
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`20
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`25
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`30
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`35
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`45
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`50
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`55
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`65
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`8
`Mobile service can also be provided by this system if
`provision is made for appropriate connections between
`adjacent central offices for handoff if the user moves
`from one central office’s service area to another. This
`handoff can be made soft in the manner provided by the
`CDMA cellular system with the use of the appropriate .
`software and hardware between the central office
`switches.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The features, objects, and advantages of the present
`invention will become more apparent from the detailed
`description set forth below when taken in conjunction
`with the drawings:
`FIG. 1 is a schematic overview of an exemplary
`CDMA wireless PBX telephone system;
`FIG. 2 is an illustration of an exemplary antenna
`pattern for an distributed antenna system of FIG. 1;
`FIG. 3 is a schematic diagram of an alternative dis-
`tributed antenna system for use with the system of FIG.
`1;
`
`FIG. 4 is a block diagram of the exemplary microcell
`equipment as implemented in a CDMA wireless PBX
`telephone system; and
`FIG. 5 is a block diagram of the mobile unit tele-
`phone configured for CDMA communications in the
`CDMA wireless PBX telephone system.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`In a CDMA wireless telephone system, a microcell
`has a controller, a plurality of spread spectrum modula-
`tor-demodulator units, which are also referred to as
`channel unit or modems, a transceiver and a distributed
`antenna system. Each channel unit consists of a digital
`spread spectrum transmit modulator, a digital spread
`spectrum data receiver and a searcher receiver. Each
`modem at the microcell is assigned to a mobile unit as
`needed to facilitate communications with the assigned
`mobile unit. The term “mobile unit” or subscriber ter-
`minal as used with reference to the microcellular system
`is generally a CDMA telephone set that is configured as
`a hand-held personal communications device, a porta-
`ble CDMA telephone or even a CDMA telephone that
`is fixed at a specific location.
`In the CDMA wireless PBX or local loop telephone
`system, the microcell transmits a “pilot carrier” signal.
`The pilot signal is used by the mobile units to obtain
`initial system synchronization and to provide robust
`time, frequency and phase tracking of the microcell
`transmitted signals. Each microcell also transmits
`spread spectrum modulated information, such as mi-
`crocell
`identification, system timing, mobile paging
`information and various other control signals.
`Upon acquisition of the pilot signal, i.e. initial syn-
`chronization of the mobile unit with the pilot signal, the
`mobile unit searches for another carrier intended to be
`received by all system users in the cell. This carrier,
`called the synchronization channel, transmits a broad-
`cast message containing system information for use by
`the mobiles in the system. The system information iden-
`tifies the microcell and the system in addition to con-
`veying information which allows the long PN codes,
`interleaver frames, vocoders and other system timing
`information used by the mobile mobile unit to be syn-
`chronized without additional searching. Another chan-
`nel, called the paging channel may also be provided to
`transmit messages to mobile units indicating that a call
`
`

`

`5,280,472
`
`9
`has arrived for them, and to respond with channel as-
`signments when a mobile initiates a call.
`When a call is initiated, a pseudonoise (PN) code
`address is determined for use during the course of this
`call. The code address may be either assigned by the
`microcell or be determined by prearrangement based
`upon the identity of the mobile unit.
`In FIG. 1, wireless base station 10 is illustrated which
`includes PBX switch 12 and microcell l4. PBX switch
`12 is used in interfacing base station 10 to the public
`switched telephone network (PSTN) and/or PBX sys-
`tem wired telephones. PBX switch 12 serves in routing
`telephone calls to/from microcell 14 which communi-
`cates the calls via CDMA communication signals with
`the appropriate mobile unit. Microcell 14 includes
`CDMA controller 18, a plurality of channel units
`20A-20N and corresponding vocoders 22A—22N, trans-
`ceiver 24 and distributed antenna system 26.
`PBX switch 12 couples calls to and from a particular
`available vocoder—channel unit pair. PBX switch 12 is
`preferably a device which is capable of providing con-
`trol in transfer of signals therefrom to various vocoders.
`PBX switch 12 may be a digital device which provides
`analog or digital voice signal, in addition to digital data
`signals on a common bus, via well known techniques
`such as a time multiplexed format, to and from the vari-
`ous vocoders. Voice calls received from PBX switch 12
`are digitally en