Cellular communications systems

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CELLULAR COMMUNICATIONS SYSTEMS η'ϊιι'Η) miwpn nmyn

Eitan, Pearl, Latzer &amp;amp; Cohen-Zedek

P-1465-IL 26676gal.doc 3/3/97

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CELLULAR COMMUNICATIONS SYSTEMS FIELD OF THE INVENTION

The present invention relates to cellular wireless communications systems generally and more particularly to apparatus and methods for cellular communications with base stations.

BACKGROUND OF THE INVENTION

Cellular multiple access communications date back to the early eighties. The nineties witnessed an outburst^ofthis^type of service throughout the world and the introduction of digital technologies. The market is expected to soar and expand into Personal Communication Services (PCS), offering personal service, a host of value added features, and total personal mobility, indoors and outdoors. Broadband services are expected to emerge at the beginning of the next century. These may require a partial renewal of the network infrastructure.

Cellular mobile communication attempts to provide mobility, multi-user capacity (many independent users access the system), coverage (service is offered over a large contiguous area) and grade and quality of service.

Cellular communications are generally limited by local codes to a range of frequencies. A widely used technique of cellular communications employs spatial isolation in order to be able to reuse the same frequencies beyond a given range called a guard zone. The communications of each user is maintained with a base station, whose antenna is elevated above the scenery in order to achieve a well defined and controlled coverage area. Sectorization is achieved by directive antennas that illuminate only one sector, thereby reducing interference, enhancing performance and reducing a pattern of frequency reuse.

Each sector of cellular communications is characterized by a number of calls per unit area, also called area capacity. Area capacity may be increased by reducing the cell size. Small cells that are positioned below roof tops in urban areas are called microcells. These use lower and smaller antennas. The cell hardware is more compact, and in some cases has less circuits. Another technique for microcells involves the antenna and RF circuitry only, remote from the cell equipment and connected via RF, fiber or microwave link, to the cell. Such an arrangement is especially attractive for operators in possession of RF or fiber trunking, like CATV companies. A future trend to increase the capacity of large cells involves smart antennas. These are multibeam array antennas at the base stations, controlled to form narrow beams that are matched to the disposition of the desired user and the sources of interference; These are expected 1

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to enhance the coverage and the capacity. The complexity involved in this technology is expected to be relieved with new cell architectures, including, among others, active antenna modules.

The network infrastructure of a typical modern cellular communications system includes a number of base stations, the actual number being related to the capacity required (measured in Erlangs, which is the number of fully occupied circuits), and to the coverage area. The base stations generally constitute about 80% of the network cost. A typical cost for a full capacity large cell base station is $500,000 - $1,000,000. The infrastructure also includes interconnect trunking, which depends mainly on the total length of interconnect lines, and switching fabric, which depends on the number of cells and calling load (measured in BHCA -Busy Hour Call Attempts). The cost of the basic service of providing airtime depends mainly on the number of base stations and on their cost.

One of the problems of cellular communications systems is transmission losses. The transmit chain of a first generation base station consists of single carrier HPA’s, filtered, combined and relayed by a high power cable to the mast. The losses involved in the chain amount to 8 - 10 dB. The carrier spacing is restricted by the combiner to at least 600 KHz.

In an effort to cut down losses, second generation base stations were developed. A second generation base station includes a MCLPA - Multi Carrier Linear Power Amplifier. This reduces the losses and adds flexibility to the design of the carriers (frequency allocations). A low noise amplifier (LNA) is used in the receive chain in the base station. The LNA reduces cable losses which degrade the system noise figure. An additional receive antenna is typically used for diversity. Recent installations place the LNA on the mast.

However, the MCLPA is an expensive part, running from $10,000 for a minicell to over $100,000 for a full capacity cell. Furthermore, MCLPA’s are currently supplied to the whole market by a limited number of vendors. The MCLPA’s from these vendors are available only in a power range of about 25 to 500 W.

SUMMARY OF THE INVENTION

The present invention seeks to provide a novel base station for cellular wireless communications based on a modular structure.

The present invention includes an active radiator module (ARM) that serves as a basic transmit/receive module in a variety of cellular base station configurations. The active radiator modules follow the trend of cellular architecture development and are designed to meet both current and future needs. It is a novel approach that may reduce the cost of the base station while providing desired flexibility. 2 120364/2

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In the active radiator module system, a combined signal is transmitted in low power through a cable to a mast, where it redistributes to the active radiator modules. The number of active radiator modules needed is a function of both the total effective radiated power (ERP) and gain required. The receive chain includes an LNA in each element, which reduces the noise figure of the system. The same active radiator module can serve in microcells that require small power and low gain antennas. A remote RF unit is the least expensive solution for microcells. Its applicability is limited by the cost of RF trunking. It is the preferred solution for operators that have an access to the CATV or to fiber trunking already laid. This unit includes an amplifier, an LNA, and a transformer to the trunking band. This same module may be a part of a microcell or a picocell, but the RF is included inside the package, while the antenna is typically separate. The modular structure of the base station of the present invention provides readily upgradable base station performance at relatively low cost.

There is thus provided in accordance with a preferred embodiment of the present invention, a modular cellular wireless communication base station including a plurality of active radiator modules located at a desired antenna location, each module including at least one antenna for transmitting and receiving, a transmitter including a power amplifier, and a receiver, a beam forming network controlling the relative amplitudes and phases of each of the modules, and an RF front end transmitting over a low power link with the plurality of active radiator modules via the beam forming network and receiving over a lower power link via a low noise amplifier.

In accordance with a preferred embodiment of the present invention, the RF front end is located remote from the plurality of modules. Preferably each module is self-enclosed.

Additionally in accordance with a preferred embodiment of the present invention at least one of the active radiator modules comprises two separate transmit and receive antenna elements. Preferably the transmit and receive antenna elements are isolated from each other by up to 85 dB.

Further in accordance with a preferred embodiment of the present invention the beam forming network is located adjacent the plurality of active radiator modules, one for transmit and one for receive.

Still further in accordance with a preferred embodiment of the present invention, the modular cellular wireless communication base station includes a CATV up/down converter module. Preferably the CATV up/down converter module comprises a coaxial cable connected to a CATV network, the cable carrying a CATV forward link and reverse link. A CATV diplexer is 3 120364/2

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preferably provided that separates transmit and receive signals. The converter module preferably comprises a mixer, a phased locked oscillator and a band pass filter, thereby to eliminate image and low frequencies.

In accordance with a preferred embodiment of the present invention the RF front end communicates with the beam forming network via a fiber optic link. In one embodiment, at least two separate fibers separately carry transmitter and receiver signals. Alternatively, one fiber carries both transmitter and receiver signals, and a splitter and a filter are provided to split and filter the signals.

Additionally in accordance with a preferred embodiment of the present invention, the transmitter amplifier comprises a first stage comprising a monolithic silicon gain stage and a second stage comprising a hybrid packaged power amplifier.

Further in accordance with a preferred embodiment of the present invention a transmitter filter is provided that reduces transmitter wide band noise in a receiver band. Additionally or alternatively, a transmitter filter reduces spurious signals that interfere with a receiver channel of a cell.

Still further in accordance with a preferred embodiment of the present invention, there are provided a receiver amplifier and a receiver filter, wherein the receiver filter reduces a transmitter signal to a level wherein interfering intermod products are not generated in the receive chain, and the receiver amplifier is not desensitized by saturation. The other purpose of the receiver filter is to reduce interfering signals from other base stations and other systems.

Yet further in accordance with a preferred embodiment of the present invention, a receiver filter is provided that reduces interfering signals from sources external to the wireless communication base station.

In accordance with a preferred embodiment of the present invention, the plurality of active radiator modules are stacked to form an active antenna having desired gain and beam shape determined, by the beam forming network. The modules may be stacked in a vertical array, a planar array or a circular array, for example.

There is also provided in accordance with a preferred embodiment of the present invention, a method for mitigating a fading of signals on a forward link of a CDMA wireless system, the method including splitting a transmission signal to a plurality of transmitter antennas, introducing a delay that is longer than a CDMA chip in a transmit chain of the antennas relative to a first of the antennas, transmitting the signals by all the antennas, receiving the signals with different correlators, and combining the signals, thereby mitigating a fading of the signals.

Preferably each the antenna transmits with approximately equal coverage. 4

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In accordance with a preferred embodiment of the present invention, the step of transmitting comprises transmitting from a plurality of spaced antennas.

Additionally in accordance with a preferred embodiment of the present invention the step of transmitting comprises transmitting from a plurality of antennas that transmit at different polarization.

Further in accordance with a preferred embodiment of the present invention the step of combining comprises combining with natural multipath signals.

There is also provided in accordance with a preferred embodiment of the present invention, a modular dual polarized base station antenna system including a plurality of pairs of orthogonal polarization antennas, wherein one of the pairs is polarized at ±45° and another of the pairs is H-V polarized. Preferably a pair of transmit antennas are polarized at ±45°, and a pair of receive antennas are H-V polarized. Preferably each antenna is fed by a separate amplifier.

In accordance with a preferred embodiment of the present invention at least one isolation structure is provided for increasing isolation between the antenna pairs.

There is also provided in accordance with a preferred embodiment of the present invention, a method for modular dual polarized base station transmission and reception, the method including transmitting with a pair of transmit antennas polarized at ±45°, and receiving with a pair of receive antennas that are H - V polarized.

In accordance with a preferred embodiment of the present invention the transmit signals are split and weights of polarization are applied at a base station. Alternatively, weights of polarization are applied by control of amplifier gain. The weights may be applied at RF, IF or baseband frequencies.

There is also provided in accordance with a preferred embodiment of the present invention, a polarization diversity and matching system for cellular radio, including a dual polarized antenna pair at a base station, each antenna including an appropriate receive channel, and a signal combining and control circuitry that adds polarization diversity to a base station receiver.

In accordance with a preferred embodiment of the present invention, the circuitry is characterized by two time constants, wherein a fast circuit adapts to fading signals on a received reverse link and changes weights of two receive antennas, and a slow circuit follows physical movements of a mobile station and averages fading of a received signal.

Additionally in accordance with a preferred embodiment of the present invention, the slow circuit is driven by information from received signals and matches transmitted signal polarization to that of an incoming signal. 5

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Further in accordance with a preferred embodiment of the present invention, a signal from two receiving antennas are weighed by weights controlled by the signal combining and control circuitry, the weights being fed into a transform circuit that transforms the weights according to polarizations of the transmitting antennas and differences in gain.

There is also provided in accordance with a preferred embodiment of the present invention, a method for increasing transmission gain to a mobile station of a mobile communications system, including substantially simultaneously transmitting from two transmit antennas so as to form a radiation pattern that is characterized by a plurality of radiation lobes, each lobe being characterized by a width inversely proportional to a distance between the antennas, an amplitude of the lobes being bound by the radiation pattern of the antennas, determining a transmission direction to a mobile station, and aiming the pattern so as to produce a maximum in the transmission direction, thereby increasing transmission gain to the mobile station, and reducing scattering into the mobile station from foreign objects.

In accordance with a preferred embodiment of the present invention, the step of determining a transmission direction comprises amplifying and filtering a signal from each antenna, splitting the signals, and changing a phase of the signals relative to one another so as to determine the direction. Alternatively, the step of determining a transmission direction comprises extracting direction information from a receive diversity control for a given antenna channel and correcting for a difference in frequency.

There is also, provided in accordance with a preferred embodiment of the present invention, apparatus for increasing transmission gain to a mobile station of a mobile communications system, including two transmit antennas positioned together with a pair of diversity receive antennas, wherein the two transmit antennas transmit substantially simultaneously so as to form a radiation pattern that is characterized by a plurality of radiation lobes, each lobe being characterized by a width inversely proportional to a distance between the transmit antennas, an amplitude of the lobes being bound by the radiation pattern of the transmit antennas, and aiming apparatus that aims the pattern so as to produce a maximum in a transmission direction to a mobile station, thereby increasing transmission gain to the mobile station, and reducing scattering into the mobile station from foreign objects.

In accordance with a preferred embodiment of the present invention, the transmit antennas and the receive antennas comprise a plurality of active radiator module arrays.

Additionally in accordance with a preferred embodiment of the present invention, the transmit antennas are spaced by a distance as required to avoid correlation between fading of signals from remote mobile stations within a coverage area. 6

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Further in accordance with a preferred embodiment of the present invention, the apparatus includes a Multiple Carrier Linear Power Amplifier (MCLPA). Preferably the Multiple Carrier Linear Power Amplifier (MCLPA) is located at one of the antennas and connected thereto without a cable.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated from the following detailed description, taken in conjunction with the drawings in which:

Fig. 1 is a simplified schematic illustration of a modular base station, constructed and operative in accordance with a preferred embodiment of the present invention;

Fig. 2 is a simplified schematic illustration of a base station with an RF section of a second generation base transceiver subsystem (BTS) and an active radiator module constructed and operative in accordance with a preferred embodiment of the present invention;

Fig. 3 is a simplified block diagram illustration of an active radiator module forming part of the apparatus of Fig. 2, constructed and operative in accordance with a preferred embodiment of the present invention;

Fig. 4 is a simplified block diagram illustration of an active radiator module for CATV infrastructure based remote microcells, constructed and operative in accordance with a preferred embodiment of the present invention;

Fig. 5 is a simplified block diagram illustration of an active radiator module remote microcell via fiber, constructed and operative in accordance with a preferred embodiment of the present invention;

Fig. 6 is a simplified block diagram illustration of an active radiator module based high gain antenna array, constructed and operative in accordance with a preferred embodiment of the present invention;

Fig. 7 is a table comparing the transmission path power budget of a second generation prior art BTS and that of an active radiator module array, constructed and operative in accordance with a preferred embodiment of the present invention;

Fig. 8 is a simplified block diagram illustration of a modular design of the active radiator module, constructed and operative in accordance with a preferred embodiment of the present invention;

Fig. 9 is a simplified illustration of mechanical structure of the active radiator module, constructed and operative in accordance with a preferred embodiment of the present invention; 7

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Fig. 10 is a simplified block diagram illustration of one module of an active radiator module constructed and operative in accordance with a preferred embodiment of the present invention;

Fig. 11 is a simplified block diagram illustration of a stack of modules of active radiator modules constructed and operative in accordance with a preferred embodiment of the present invention;

Figs. 12, 13 and 14 are simplified illustrations of three different arrays of stacks of modules, constructed and operative in accordance with three preferred embodiments of the present invention;

Fig. 15 is a simplified illustration of a transmission diversity system in a forward link of a CDMA base station, constructed and operative in accordance with a preferred embodiment of the present invention;

Fig. 16 is a simplified illustration of a modular dual polarized base station antenna system, constructed and operative in accordance with a preferred embodiment of the present invention;

Fig. 17 is a simplified illustration of a polarization diversity and matching system for cellular radio, constructed and operative in accordance with a preferred embodiment of the present invention; and

Figs. 18, 19 and 20 are simplified illustrations of transmission gain with two antennas in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to Fig. 1 which illustrates a modular base station, constructed and operative in accordance with a preferred embodiment of the present invention. A combined signal is transmitted in low power through a cable to a mast, where it redistributes to a plurality of active radiator modules. The number of active radiator modules needed is a function of both the total effective radiated power (ERP) and gain required. The receive chain includes an LNA in each element, which reduces the noise figure of the system. The same active radiator module can serve in microcells that need small power and low gain antennas.

Reference is now made to Fig. 2 which illustrates a base station with an RF section of a second generation base transceiver subsystem (BTS) and an active radiator module constructed and operative in accordance with a preferred embodiment of the present invention. The single channels are combined, after preamplification and channel filtering, and then feed a Multi Carrier Linear Power Amplifier (MCLPA). The combined signal is then band-pass filtered, diplexed and run through a high power, low loss cable, to the antenna array. 8

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A disadvantage of prior art second generation BTS is, inter alia, that the complex comprising the cable and antenna array, serving both transmit and receive signal, is required to be extremely linear and not to generate IMD (Intermod products) higher than about -135 dBc, which puts a high stress on the antenna and the cable connections. The diplexer and band pass filters need about 100 dB of isolation between transmitter and receiver frequencies. The cost of this architecture is a power loss of 3 to 5 dB in the filters, diplexer and cable, that has to be compensated by a high power MCLPA and all its supporting equipment. The cable loss degrades the noise figure on the receive side.

In the present invention, the MCLPA, high power cable, diplexer and broadband superlinear antennas, and LNA are all replaced by an active radiator module. The active radiator module is mounted on the mast and comprises a low power PA an elemental radiator (dipole or a patch) and a corresponding receive element. The active radiator module performs amplification at low level and combines the power in the air, uses two narrow band antennas for transmit and receive, thus reducing the linearization and structural requirements of the antennas, and amplifies the received signal at the antenna terminal with no additional loss. The cables connecting the active radiator module and the BTS are simple and not sensitive to loss, and may be extended as needed.

Reference is now made to Fig. 3 which is a simplified block diagram illustration of an active radiator module forming: part of the apparatus of Fig. 2. The active radiator module includes two separate. transmit and receive antenna elements. This obviates the need for a diplexer, with the associated cost, power loss and occupied volume. Each antenna is preferably narrow banded, typically covering 12.5 MHz (&lt;2%). A separation of preferably approximately 45 MHz provides about 20 dB isolation. Further isolation (up to 85 dB) is provided by the filters on the receiver and the transmitter channels. The transmitter amplifier is low power, 2 W or .2 W, depending on the application. A LNA follows the filter on the receiver channel.

Reference is now made to Fig. 4 which illustrates an active radiator module for CATV infrastructure based remote microcells, constructed and operative in accordance with a preferred embodiment of the present invention. The basic active radiator module is preferably combined with a CATV up/down converter module to establish the CATV infrastructure based remote microcells. This special application active radiator module will make use of the existing CATV network as an RF trunk for remote RF Microcells. Such an existing CATV network is in use in U.S. markets with a great cost and capacity advantage. A similar product is being offered by Lucent Technologies. 9

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The CATV up/down converter module input is preferably a coaxial cable connected to the CATV network and carrying the CATV standard forward link (typically 450-650 MHz) and reverse link (typically 5-52 MHz). A bandwidth of 10 MHz for each forward and reverse links is preferably dedicated to cellular active radiator module use.

The CATV diplexer within the converter separates the transmitter and receiver signals. These are then converted to the appropriate cellular frequencies. Each of these converters includes a mixer, phased locked oscillator and a band pass filter to eliminate image and low frequencies. The up/down converter module is attached directly to the active radiator module in this application.

Reference is now made to Fig. 5 which illustrates an active radiator module remote microcell via fiber, constructed and operative in accordance with a preferred embodiment of the present invention. A fiber/RF converter module is attached to the basic active radiator module for fiber-optics trunking for remote active radiator module microcells. RF trunking via fiber is an efficient method for microcells layout, proposed for both in-buildings and outdoors microcell distribution. The fiber/RF transducer module preferably includes both transmitter fiber/RF converter and receiver RF/fiber converter within the same module. The input to this module is preferably either one fiber carrying both transmitter and receiver signals, split and filtered within the module, or two separate fibers, depending on fiber infrastructure. The fiber/RF converter module is attached directly to the active radiator module.

Reference is now made to Fig. 6 which illustrates an active radiator module based high gain antenna array. Large cellular cells require both high ERP (Effective Radiated Power) and antenna gain. Arrays composed of active radiator module elements provide both effectively, at lower BTS cost, and higher flexibility and reliability. The ERP generated by a linear array composed of N active modules, each transmitting p Watts, is N2p. There is no additional loss, otherwise included in the link budget due to the BFN (Beam Forming Network), cable and diplexer.

Reference is now made to Fig. 7 which is a table comparing the transmission path power budget of a second generation prior art BTS and that of an active radiator module array, constructed and operative in accordance with a preferred embodiment of the present invention. It may be appreciated that a 10 element array, a 100 W MCLPA, with the associated high power cable and diplexer, may be replaced by 10 active radiator modules, each transmitting 2 W. A similar advantage is obtained on the receive path.

Reference is now made to Fig. 8 which illustrates a modular design of the active radiator module in accordance with a preferred embodiment of the present invention. Each 10

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module can be attached to other modules to establish a new product matched to specific customer requirements.

The active radiator module preferably comprises five basic building blocks and the integrating enclosure. The basic building blocks are: 1. transmitter amplifier 2. receiver amplifier 3. transmitter/receiver band pass filters 4. transmitter/receiver antenna element 5. power supply

Before describing the amplifier of the active radiator module of the present invention, a brief discussion of the prior art will now be presented. Generally in the prior art, stringent intermod products (IMP) specifications are imposed on the BTS transmission, in order to avoid interference to its own receivers and to adjacent cells and systems. These impose linearity requirements on the transmission chain beyond the channel filters. The MCLPA for a large cell is thus specified not to exceed -70 dBc IMP. These constraints do not apply for a single channel amplifier, and are relaxed for microcells, where the dynamic range of the cell is reduced by over 30 dB. The IMP requirements for CDMA systems are less stringent. A class A amplifier with proper backoff (3 to 5 dB) may serve the requirements for CDMA microcells and for other systems’ low capacity microcells and cells (“minicells”). Higher linearity requires linearization techniques. Pre-distortion results in 7 to 10 dB higher 3rd ICP (third order intercept point) and enables 10 to 20 dB lower IMP. A cost factor of 10 is considered today practical compared to the class A amplifier, owing to the hybrid design of the latter, as compared to discrete components and manual tuning of the pre-distorted amplifier. The cost may be reduced by resorting to a similar technology, justified for large quantities. Further linearization requires feed-forward techniques as used in the high power MCLPA. It is expected that these expensive techniques will not be needed in any of the active radiator module applications.

The amplifier of the active radiator module of the present invention will now be described. A class A amplifier, operating at 3 to 5 dB backoff, may be used for the first generation of active radiator module. Such an amplifier offers low cost and a short development time. The amplifier accommodates the CDMA microcells and minicells and low capacity microcells and minicells for other systems, which constitute a major portion of the market.

The transmitter amplifier preferably comprises two stages. The first stage is preferably a monolithic silicon class A gain stage. The second stage is preferably a hybrid packaged power amplifier. The amplifier with all of its matching and biasing networks are ll

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preferably assembled using SMT technology on a RF printed board within the transmitter amplifier enclosure.

Typical transmitter amplifier specifications are presented here for purposes of description of best mode, but the present invention is not limited to these values.

Frequency band.....................870-895 MHz (AMPS, TDMA, CDMA) 935-960 MHz (GSM)

Output power (average)...........2 w, saturated for AMPS

1 w, for TDMA GSM 0.6 w, for CDMA 1 dB compression........................32-33 dBm 3rd order intercept point.............+43 dBm min.

Two tones IMP...........................-40 dBc for 0.2 w per tone -46 dBc for 0.1 w per tone

Gain..........................................30 dB, @ small signal

Gain flatness.............................+/- 0.5 dB over any 1.25 MHz

Absolute phase variation............+/- 5° over any 1.25 MHz AM/PM conversion...................0.25°/dB Max up to 3 dB below 1 dB cp

Noise figure..............................7.5 dB Max

Input VSWR.............................1.5 : 1 @50 ohm system output VSWR............................1.3 : 1 @50 ohm system

DC supply voltage......................8 to 18 volts DC DC supply current.....................4.3 to 1.9 Amp

Operating temperature...............-20° to +60 °C

The receiver amplifier within the active radiator module is intended to ensure that the base station sensitivity will not be degraded because of long coaxial cables losses or other media losses and noise add in between the antenna element and base station front end.

The receiver amplifier preferably has enough gain, low enough noise figure, high enough compression and intercept points to eliminate sensitivity, inter-channel interference and non-linear multi-channel distortion degradation.

Typical receiver amplifier specifications are presented here for purposes of description of best mode, but the present invention is not limited to these values.

Frequency range.................................................800-950 MHz

Noise figure.......................................................3.5 dB 12

Gain.......................................

Input 1 dB compression point. Input 3 rd order Intercept Point

Class of operation...................

Voltage supply.......................

Current requirement...............

Operating Temperature...........

Technology............................. .30 dB -10 dBm

.0 dBm A

.+8v regulated .150 mA ,-20°C to +60°C SMT of MMIC

Transmitter and receiver filters of the active radiator module establish, together with transmitter/receiver antennas separation, a diplexer which isolates transmitter and receiver signals from each other. Specifications for transmitter and receiver filters are directly driven from performance requirements of active radiator module per system and application.

Typical specifications are presented here for purposes of description of best mode, but the present invention is not limited to these values.

Noise power input to ARM @ receiver band................................-155 dBm/Hz

Transmitter channel noise figure............................................................10 dB

Transmitter channel gain........................................................................35 dB

Channel Band Width (for AMPS, TDMA)................................30 KHz

Transmitter output noise @ transmitter antenna over channel B.W...................-74 dBm (-155 +10+35+35=-74)

For the receive channel: MDS( Minimum Detectable Signal) for AMPS, TDMA)..........-125 dBm

Input 3rd Intercept Point..........................................................0 dBm

Channel B.W (for AMPS, TDMA)...........................................30 KHz

Receiver Noise Figure......................................................................4 dB

Receiver noise power at ARM input................................................-135 dBm (KTBF=-174+3 5+4=-13 5)

The transmitter filter has two roles within the active radiator module. The first is to reduce transmitter wide band noise in a receiver band. The second is to reduce spurious signals which might interfere with a receiver channel of the same cell or other cells or other systems. The more demanding requirement is the first one and it dictates the transmitter filter performance and thus transmitter filter structure.

In order for the transmitter noise and leakage into the receiver channel input to be lower than the receiver noise floor, an isolation of 60 dB (-74+135) is required. 20 dB of the 13 required isolation is attributed by transmitter/receiver antenna isolation and the other 40 dB plus 10 dB of safety margin , are given by receiver band rejection of the transmitter filter. The same reasoning holds for CDMA systems where the values differ but the ultimate results hold.

The receiver filter has two roles within the active radiator module. The first is to reduce the transmitter signal to a level where interfering intermod products are not generated in the receive chain, and the receiver amplifier is not desensitized by saturation. The other purpose of the receiver filter is to reduce interfering signals from other base stations and other systems. The more demanding requirement is the first one and it will dictate the receiver filter performance and thus the filter’s structure.

In order for the transmitter leakage not to interfere with received signal, it should be kept at a much lower level than receiver channel compression for systems with no AGC or when AGC is at minimum. For example, if the 1 dB compression point at the receiver antenna terminal is -60 dBm, the transmitter leakage is preferably below -70 dBm. For transmitter average output power of +33 dBm and transmitter/receiver antenna isolation of 20 dB, the receiver filter rejection of transmitter band is preferably 85 dB. The same reasoning holds for CDMA systems where the values differ but the ultimate results hold.

Typical specifications for the active radiator module filters are presented here for purposes of description of best mode, but the present invention is not limited to these values. Transmitter Filter:

Pass band ..................................10 MHz over 870-895 MHz (AMPS) or 935-960 MHz (GSM)

Rejection ..................................-50 dB @35 MHz below pass band -40 dB @ 35 MHz above pass band

Insertion loss................................-1 dB max. @ pass band

Ripple within band........................0.4 dB max. over any 1.25 MHz band

Group delay variation 0.5 nsec max. over any 1.25 MHz

Receiver Filter

Pass band ...................................10 MHz over 825-850 MHz (AMPS) or 890-915 MHz (GSM)

Rejection ..................................-85 dB @35 MHz above pass band -40 dB @ 35 MHz below pass band

Insertion loss................................-1 dB max. @ pass band

Ripple within band........................0.4 dB max. over any 1.25 MHz band

Group delay variation 0.5 nsec max. over any 1.25 MHz 14

Reference is now made to Fig. 9 which illustrates mechanical structure of an active radiator module 100 in accordance with a preferred embodiment of the present invention. Active radiator module 100 preferably includes a housing 102 typically constructed of aluminum. A plurality of tuning elements 104 and I/O connectors 106 are preferably mounted on outside surfaces of housing 102. Input from transmitter and receiver antenna elements 108 and 110, respectively, is fed to the I/O connectors 106 and output connections are preferably directly made with the amplifier’s circuit boards (not shown). Disposed inside housing 102 are transmitter and receiver filters 112 and 114, respectively. Each of the transmitter and receiver filters 112 and 114 preferably includes a 6-coaxial-resonators elliptic filter in combine structure.

Both transmitter and receiver antenna elements 108 and 110 are preferably printed patch elements. Transmitter and receiver elements 108 and 110 are preferably printed on the same base material (typically polyurethane material) and covered by a sheet of epoxy-fiberglass or other protective cover that withstands the environment, including UV radiation. Both elements 108 and 110 are preferably designed to ensure the required isolation between the elements. The design is preferably compatible with array-stacking of elements for a high-gain antenna, as will be described further below.

Typical specifications are presented here for the active radiator module antenna for purposes of description of best mode, but the present invention is not limited to these values. Frequency band..................Transmitter: 870-895 MHz for AMPS, TDMA, CDMA

935-960 MHz for GSM

.................Receiver: 825-850 MHz for AMPS, TDMA, CDMA

890-915 MHz for GSM

Transmitter/receiver elements isolation.............20 dB min. VSWR........................................1.4 : 1 Max

Size............................................150x150 mm

An active radiator module power supply preferably supplies all DC power requirements of the transmitter and receiver amplifiers and includes all protection means needed for a tower top mounted device. Since the active radiator module power supply is preferably mounted on top of the antenna tower and cable connecting the base station and the active radiator module does not have a prefixed length, a DC-DC converter is needed within the power supply. DC supply is preferably through transmitter or receiver coaxial cables, which means a BIAS-T should be implemented within the active radiator module power supply. The DC supply source is preferably within the base station. This way of DC supply is convenient for the modular approach 15 where each module (CATV converter or Fiber/RF converter) has an independent power supply, all consuming DC power from the same source through connecting coaxial cables.

Typical specifications are presented here for the power supply for purposes of description of best mode, but the present invention is not limited to these values.

Input voltage..............................18-32 V DC

Output voltages............................15 V DC @ 2 Amp (or other voltage with same power) .............................8 V DC @0.2 Amp

Operating temperature ..................-20°C to +60°C

Input connection...........................Through an internal BIAS-T

The integrating enclosure of active radiator module is based on a transmitter/receiver filters block. This block occupies the main volume of the active radiator module. The mechanical structure is preferably divided into two main mechanical parts: the main block of the filters with printed antenna and radome mount as one part and filters cover with transmitter amplifier, receiver amplifier and power supply compartments as second part. Both mechanical parts are preferably made of die cast aluminum and screwed to one another with sealing conductive O-ring in between the parts. Overall size of active radiator module structure is preferably around 150 x 150 x 150 mm.

The active radiator module aluminum structure is preferably designed to dissipate heat from the transmitter and receiver amplifiers and power supply. Overall heat dissipated within the active radiator module is about 30 W and the temperature rise above ambient temperature is approximately 10 °C or less.

The antenna radome is preferably at the front of the active radiator module, while the transmitter and receiver connectors are preferably on the rear side. Several active radiator module units may be interconnected to form an array for the higher gain and higher power antennas.

As mentioned above, the active radiator modules are compatible with arraystacking of elements for a high-gain antenna. Reference is now made to Fig. 10 which illustrates a block diagram of one module of an active radiator module constructed and operative in accordance with a preferred embodiment of the present invention. The active radiator module preferably includes a power amplifier, band pass filter, diplexer and LNA in conjunction with a radiating element.

Reference is now made to Fig. 11 which illustrates a block diagram of a stack of modules of active radiator modules constructed and operative in accordance with a preferred 16 «Ο embodiment of the present invention. A low power cable is used with the transmit beam forming network.

Reference is now made to Figs. 12, 13 and 14 which illustrate three different possible arrays of stacks of modules, constructed and operative in accordance with a preferred embodiment of the present invention. Fig. 12 illustrates a vertical array, Fig. 13 illustrates a planar array and Fig. 14 illustrates a circular array. It is appreciated that other configurations are possible in the scope of the invention.

The following list summarizes some of the advantages of the active radiator module of the present invention: • A common module for many cellular applications • A major cost saver for large cells (reduces the power requirement by 3 to 6 dB as compared to MCLPA at the base station (BS) compartment, reduces the need for high power low loss cable, and enables longer distance between the antenna and the BS). • Takes a substantial “bite” out of the BS market, while avoiding the need for expensive technology licenses associated with the BS production. • Reduces the BS noise figure by eliminating the cable losses. • Reduces antenna Intermod Products (IMP) by separating the transmitter and receiver antennas and by eliminating cable.connections.. • A failure of a module in an array does not cause a catastrophic damage to the BS, but only a graceful degradation. • A building block for multiple-beam and smart antennas.

In accordance with another embodiment of the present invention, a method and apparatus are provided whereby a fading of a mobile channel can be mitigated on a forward link of a CDMA wireless system, by transmission from two or more spaced antennas, or from two antennas that transmit at different polarization. Part of this concept has been applied to distributed antennas, as described in United States Patent 5,280,472, issued Jan 18, 1994, the disclosure of which is incorporated herein by reference. The present invention is different, however, in that it applies to the transmit antennas only, particularly to large base stations, and for cellular service, while the referred patent applies to PBX indoors, and a serial chain of elements, each serving both for transmit and receive.

By way of background, it is noted that the forward and the reverse links in most cellular systems are transmitted at different frequencies (what is called FDD - frequency division duplex). The typical spacing for the 800 or 900 MHz systems is 45 MHz, which is larger than the coherence bandwidth of the typical terrestrial cellular communications channel. There is not 17

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•υ enough correlation between the fading in both channels, and the fading information from the reverse link cannot be used to control a diversity transmission in the forward link. A unique feature of the CDMA system is the “rake receiver”, which consists of a number of digital correlators that enable the reception of different transmissions, and in particular, the same transmission with a different delay. The receivers in the IS 95 system, for example, are equipped with 4 such correlators (called “fingers”) that, in the intended mode of operation, search for delayed multipath transmissions. This feature can be used for intentionally delayed transmission from two or more antennas whose fadings are not correlated, e.g., antennas that are sufficiently spaced apart, or that transmit with different polarization.

Reference is now made to Fig. 15 which illustrates a transmission diversity system in a forward link of a CDMA base station, constructed and operative in accordance with a preferred embodiment of the present invention. The embodiment is for a base station built according to the Interim Standard IS 95, and related standards, but is not limited to these standards only. The transmission signal is preferably split to the number of antennas to be used for transmission, typically two antennas 131 and 133. A delay that is longer than the CDMA chip (a term known to those familiar with CDMA technology) is inserted in the transmit chain of each of the other antennas relative to the first antenna. This delay may be inserted in the RF chain, by a delay line 301 in Fig. 15, (a term known to those familiar with FRF technology) or in the digital base band. With the IS 95, the required delay is higher than 1.5 microseconds. The signal is then transmitted by all antennas; each having about the same coverage. These signals are then received by the mobile station with different “fingers” and optimally combined, as with natural multipath signals (a process known to those familiar with CDMA technology). The diversity gain thus obtained may be significant.

The typical delay profile depends on the environment. In urban and suburban environments, the first delay cluster may extend to about 2 microseconds. An optimal delay is preferably selected for the transmission in order to minimize interference from natural multipaths arriving at the same time.

In accordance with another embodiment of the present invention, dual polarized active antennas are provided for BS of mobile communication systems. This dual polarized pair for transmit, and one for receive, allow the BS to use polarization diversity on receive, and polarization matching on transmit.

Diversity is a powerful way to mitigate the multipath in mobile communications systems. Most of the cellular systems in use today employ antenna diversity in the base station for receiving the reverse link - transmission from the mobile station (MS). However, the mobile 18

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stations do not include multiple antennas and diversity receivers for reasons of cost and complexity. Diversity in use by most is space diversity, whereby the two (or more ) receive antennas are spaced apart in order to reduce the correlation between their fading.

The correlation between orthogonally polarized signals is also low, particularly for urban and indoors propagation, and polarization diversity is also an efficient mitigation means against channel fading. One very important advantage of polarization diversity is the compactness of the antenna arrangement involved.

Dual polarized antennas may also be used at the BS for polarization matching of the forward link (transmission from the BS to the MS), as described further hereinbelow. The use of modular active antennas in the BS is also described further hereinbelow.

Reference is now made to Fig. 16 which illustrates a modular dual polarized base station antenna system, constructed and operative in accordance with a preferred embodiment of the present invention. The system preferably includes two pairs of orthogonal polarization antennas. The embodiment of Fig. 16 exhibits a compact arrangement, whereby one pair 202 is polarized at ±45° while another pair 204 is H-V (horizontal - vertical) polarized. This particular arrangement has a relatively high isolation between the transmit and receive pairs, and each pair may be tuned to its desired frequency range. Other arrangements are possible, e.g. patches.

The transmit antennas are typically polarized at ±45°. In one embodiment, each antenna is fed; by a separate amplifier. The transmit signal splitting and weights of each polarization are performed at the base station at a low RF level, or IF or at the baseband, thus avoiding RF losses. One other alternative is to apply the weights by control of the amplifiers gain. The use of two amplifiers allows for the summation of their transmit power in the air, for any polarization determined by the polarization matching circuit.

The receive antenna is typically polarized at H-V. In one embodiment, each antenna preferably feeds an LNA (Low Noise Amplifier). The weights to each signal are applied after amplification, at the BS, in RF, IF or baseband frequencies.

Each active radiator module unit consists of two dual polarization antenna pairs, as exemplified in Fig. 16. Isolation structures, as shown by lines 206, are used in one embodiment for increasing the isolation between the antenna pairs.

In accordance with another embodiment of the present invention, a method and apparatus are provided whereby a mobile cellular communications system can mitigate the fading environment and compensate for the loss due to mismatch of the polarization of the BS and the mobile terminals. 19

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The electromagnetic radiation is polarized, and allows for two orthogonal polarization states. Any antenna cannot be matched simultaneously to both polarizations. The propagation of the signals through a nonhomogeneous medium may transfer part of the signals to an orthogonal polarization. This is the case for terrestrial communications, for example, and in particular in urban areas, where the signals encounter multipaths from objects on the way. The transfer of polarization has been found to be typically -6 to -10 dB in rural areas, and much more (-3 dB or higher) in urban areas and indoors. The orthogonal polarization components have been found to have an independent fading pattern, with correlation of less than 0:6.

Cellular systems operate traditionally in vertical polarization. Most of the mobile terminals, mounted on vehicles, have vertically polarized antennas. The polarization of hand-held terminals is variable, however, and depends on the particular terminal, its orientation relative to a head and the base station. There is an a priori mismatch of the polarization between the hand-held terminal and the BS antenna.

The forward (from the BS) and the reverse (from the MS) transmissions do not have correlated fading, because of the frequency difference between them. Diversity schemes, used for mitigating the fading, are therefore applicable on receive only, due to lack of information to control transmission so as to reach the other receiver without fade. In most of the present day systems, only the base stations are equipped with more than one antenna for receiving the signal from each mobile station. Mobile stations that are equipped with only one antenna are unable to use antenna diversity for reception.

Reference is now made to Fig. 17 which illustrates a polarization diversity and matching system for cellular radio, constructed and operative in accordance with a preferred embodiment of the present invention. A dual polarized antenna pair at a base station, with an appropriate receive channel for each, and a signal combining and control circuitry, adds polarization diversity to a base station receiver, by itself or in addition to other diversities. In the present invention the adaptive combining control circuit has two time constants. First, a fast circuit adapts to fading signals on a received reverse link and changes weights of two receive antennas. Second, a slow circuit follows only the physical movements of the MS, but averages fading of the received signal. This control is applied to a dual polarization transmission antenna pair, with their appropriate transmission channels. This slow control, driven by the information from the received signals, matches the transmitted signal polarization to that of the incoming signal. This enhances the forward link significantly in situations of large polarization mismatch, that may be frequent in normal operation. 20

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The signal from the two receiving antennas are weighed by weights 150, controlled by the measurement and control circuit. This can be applied in RF, IF or at the baseband. These weights are also fed into a transform circuit, that transforms the weights according to the different polarization of the transmitting antennas (e.g. tilted linear, or circular) and differences in gain. The low-pass filter averages the fast control variations and responds only to the slow variations resulting from the physical attitude changes of the MS. Weights 152 may be applied to the transmitted signal at the base band, at the IF, prior to power amplification, as shown, or after amplification, or applied to the amplifiers gain control. The same antennas may also be used for both reception and transmission. In such a case the signals are separated by a diplexer.

In accordance with another embodiment of the present invention, a method and apparatus are provided whereby a mobile communications system can enhance the transmitted signal to each mobile station(MS) while reducing the multipath fading, and the interference to other MS.

Transmission in a terrestrial environment encounters multipaths due to scattering from various objects. The multipath components, arriving at the receiver, interfere with each other to form typical fading of the signal, when either transmitter, receiver or scattering obstacles are in motion. This is one of the most detrimental effects on the communications in this service. Diversities are employed to mitigate the fading, whereby propagation routes which have an independent fading (non-correlated fading) are received separately and then combined in a way that reduces the variations in the signal. Base stations in the cellular service typically employ two receive antennas, spaced apart so as to have no correlated reception from the mobile stations in their coverage area. Diversity on the forward link (transmission from the BS) is not implemented in most systems because of the excess complexity needed in the MS by introducing two antennas and the accompanying circuitry. In transmission diversity from the BS, two or more transmit antennas are positioned far apart to avoid correlation, and their transmitted signals are controlled in amplitude and phase in correspondence to the output of the adaptive control circuitry of the receiving antennas. Such an arrangement does not apply to the cellular systems where the forward and the reverse links transmission have different frequencies, far enough to decorrelate the fading between the two links.

Narrow beam transmission from the BS has been proposed, in order to increase the signal strength received by the MS, and reduce the interference to other MS. Narrow beam transmission also reduces the multipath interference to the BS by limiting the illuminated area around the BS. Such a narrow beam has to track the desired MS direction. Algorithms proposed for this task make use of the direction information obtained from the diversity antennas. A BS that 21

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communicates with many mobile stations needs to form simultaneously many narrow beams so as to optimize the transmission to each station. Such an arrangement requires a multibeam antenna array which is a relatively complex system.

Reference is now made to Figs. 18, 19 and 20 which illustrate transmission gain with two antennas in accordance with a preferred embodiment of the present invention.

Simultaneous transmission from two antennas forms a radiation pattern that is characterized by many radiation lobes, the width of each is inversely proportional to the distance (in wavelengths) between the antennas. The amplitude of these lobes is bound by the radiation pattern of the individual antennas. Such a pattern, when aimed in a way to produce a maximum in the direction of a mobile station, has a gain of 2 (3 dB) in that direction. The narrow lobe around this maximum also reduces the scattering into the mobile station from the objects in the area that is now illuminated less. The typical signals arriving at the mobile station from the scattering objects tend to diminish roughly in proportion to the second power of the distance of these scatterers from the mobile stations. Scatterers that are illuminated by the adjacent lobes contribute much less to the fading at the mobile stations. The contribution of the multilobe antenna to the mitigation of fading, when its radiation is properly aimed, is significant and comes close to that of a multibeam array whose size matches the distance between the antennas. This is illustrated in Fig. 18.

The interference, to other mobile stations is also reduced. Though some mobile stations, positioned at the peaks of other radiation lobes, receive the same signal strength as the desired mobile stations, other mobile stations within over 50% of the coverage area receive signals that are at least 3 dB lower, and over 10 dB lower within 20% of the area.

The application of such an arrangement is especially advantageous when the transmit antenna pair is positioned together with the diversity receive pair, on the same installation. A preferred embodiment is a pair of active radiator module arrays, each provided with receive and transmit antennas and amplifiers.

Reference is now particularly made to Fig. 19. The signal from each channel is amplified, filtered and then split to two active radiator modules. A phase control changes the relative phase of one active radiator module in respect to the other, to form the desired lobe direction. The information for directing the lobe may be achieved in different way. One preferred embodiment is by extracting direction information from the receive diversity control for that channel and correcting for the difference in frequency. Phase ambiguity is not important in this case, as it is with true direction finding, because the purpose is to point one of the lobes’ peaks 22

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toward the MS and it is not important to determine which lobe. There are many estimation algorithms that can apply.

Referring to Figs. 19 and 20, a pair of transmitting antennas 401 and 408 are preferably co-located with base station receiving antennas. Antennas 401 and 408 are preferably spaced by a distance as required to avoid correlation between fading of signals from remote mobile stations within the coverage area (411 in Fig. 20), typically 10 wavelengths. A cable 402 is preferably provided to carry high power transmit signals (RF). A Multiple Carrier Linear Power Amplifier (MCLPA) 403 is preferably provided, as well as a cable 404 that carries low power transmit signals. In one preferred embodiment, MCLPA 403 is located at one of the antennas and connected thereto without a cable 402, thus avoiding additional losses. A passive combiner 405 preferably combines signals from the individual carriers, as is known in the art. A channel filter 406 and a single channel low power amplifier 407 (pre-amplifier), both well known in the art, are preferably provided for the transmitted signal. A signal control element 409 may be implemented in the RF, as shown in the figure, by a phase shifter, as is known in the art. In an alternative embodiment, signal control element 409 may be implemented in the IF or at the base band. The lobe of the coverage is referenced by numeral 410, and is created by the interference between the transmission signals of antenna 401 and 408.

It is appreciated that various features of the invention which are, for clarity, described in the contexts of separate: embodiments may also be provided in combination in a single: embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment may also be provided separately or in any suitable subcombination.

It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims which follow: 23