Controlling transmitter gain in a wireless telecommunications system

A central terminal (10) in a wireless telecommunications system (1) includes an analog card (206) that combines inputs from a plurality of modem units (204) for a plurality of downlink communication paths. The analog card (206) generates a composite transmit signal (214) that is provided to a radio frequency card (208). The radio frequency card (208) prepares the composite transmit signal (214) for radio frequency transmission from the central terminal (10). A power amplifier (218) in a combining shelf (201) amplifies the composite transmit signal (214) to a desired transmitting level. A detector (240) measures a power output of the power amplifier (218). The power output measurement determined by the detector (240) is collected by a combiner monitor (222) and delivered to a shelf controller (210) of the modem shelf (200). The shelf controller (210) provides the power output measurement to the analog card (206). The analog card (206) compares the power output measurement to power estimates of the inputs from the modem units (204). The analog card (206) generates an adjustment signal (242) to control the power output from the power amplifier (218) by adjusting a gain of the radio frequency card in accordance with the comparison.

TECHNICAL FIELD OF THE INVENTION
 The present invention relates in general to telecommunications technology
 and more particularly to an apparatus and a method of controlling
 transmitter gain in a wireless telecommunications system.
 BACKGROUND OF THE INVENTION
 In order to meet transmitted power specifications, radio frequency gain
 should be accurate in a wireless telecommunications system. However,
 inaccuracies typically occur in the radio frequency gain of wireless
 telecommunications systems. These inaccuracies may occur as a result of
 effects such as shifts in tolerances, temperature variations, and device
 limitations, among others. Therefore, it is desirable to eliminate or
 reduce these effects on the accuracy of the radio frequency gain.
 SUMMARY OF THE INVENTION
 From the foregoing, a need has arisen for a method and an apparatus that
 controls the radio frequency gain in a wireless telecommunications system
 to compensate for inaccuracy causing effects.
 An object of the invention is to provide an apparatus and a method of
 controlling transmitter gain in a wireless telecommunications system that
 substantially eliminate or reduce disadvantages and problems associated
 with conventional wireless telecommunications systems.
 From the foregoing, it may be appreciated that a need has arisen for a
 method and device that controls the radio frequency gain in a wireless
 telecommunications system to compensate for inaccuracy causing effects. In
 accordance with the present invention, there is provided an apparatus and
 method of controlling transmitter gain in a wireless telecommunications
 system that substantially eliminates or reduces disadvantages and problems
 associated with conventional wireless telecommunications system.
 According to an embodiment of the present invention, a method of
 controlling transmitter power in a wireless telecommunications system is
 provided that includes transmitting a composite transmit signal. The
 composite transmit signal carries information from inputs for a plurality
 of downlink communication signals. An output radio frequency power of the
 composite transmit signal is sampled and compared to the inputs for the
 plurality of downlink communication paths. A gain of the output radio
 frequency power is adjusted according to results of the comparison.
 The present invention provides various technical advantages over
 conventional wireless telecommunications systems. For example, one
 technical advantage is to control a gain of an output radio frequency
 power for a composite transmit signal. Another technical advantage is to
 eliminate or reduce effects affecting the accuracy of the gain of the
 output radio frequency power. Yet another technical advantage is to
 maintain a constant gain for the output radio frequency power. Other
 technical advantages are readily apparent to one skilled in the art from
 the following figures, description, and claims.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 is a schematic overview of an example of a wireless
 telecommunications system. The telecommunications system includes one or
 more service areas 12, 14 and 16, each of which is served by a respective
 central terminal (CT) 10 which establishes a radio link with subscriber
 terminals (ST) 20 within the area concerned. The area which is covered by
 a central terminal 10 can vary. For example, in a rural area with a low
 density of subscribers, a service area 12 could cover an area with a
 radius of 15-20 Km. A service area 14 in an urban environment where is
 there is a high density of subscriber terminals 20 might only cover an
 area with a radius of the order of 100 m. In a suburban area with an
 intermediate density of subscriber terminals, a service area 16 might
 cover an area with a radius of the order of 1 Km. It will be appreciated
 that the area covered by a particular central terminal 10 can be chosen to
 suit the local requirements of expected or actual subscriber density,
 local geographic considerations, etc, and is not limited to the examples
 illustrated in FIG. 1. Moreover, the coverage need not be, and typically
 will not be circular in extent due to antenna design considerations,
 geographical factors, buildings and so on, which will affect the
 distribution of transmitted signals.
 The central terminals 10 for respective service areas 12, 14, 16 can be
 connected to each other by means of links 13, 15 and 17 which interface,
 for example, with a public switched telephone network (PSTN) 18. The links
 can include conventional telecommunications technology using copper wires,
 optical fibres, satellites, microwaves, etc.
 The wireless telecommunications system of FIG. 1 is based on providing
 fixed microwave links between subscriber terminals 20 at fixed locations
 within a service area (e.g., 12, 14, 16) and the central terminal 10 for
 that service area. In a preferred embodiment each subscriber terminal 20
 is provided with a permanent fixed access link to its central terminal 10.
 However, in alternative embodiments demand-based access could be provided,
 so that the number of subscribers which can be serviced exceeds the number
 of telecommunications links which can currently be active.
 FIG. 2 illustrates an example of a configuration for a subscriber terminal
 20 for the telecommunications system of FIG. 1. FIG. 2 includes a
 schematic representation of customer premises 22. A customer radio unit
 (CRU) 24 is mounted on the customer's premises. The customer radio unit 24
 includes a flat panel antenna or the like 23. The customer radio unit is
 mounted at a location on the customer's premises, or on a mast, etc., and
 in an orientation such that the flat panel antenna 23 within the customer
 radio unit 24 faces in the direction 26 of the central terminal 10 for the
 service area in which the customer radio unit 24 is located.
 The customer radio unit 24 is connected via a drop line 28 to a power
 supply unit (PSU) 30 within the customer's premises. The power supply unit
 30 is connected to the local power supply for providing power to the
 customer radio unit 24 and a network terminal unit (NTU) 32. The customer
 radio unit 24 is also connected to via the power supply unit 30 to the
 network terminal unit 32, which in turn is connected to telecommunications
 equipment in the customer's premises, for example to one or more
 telephones 34, facsimile machines 36 and computers 38. The
 telecommunications equipment is represented as being within a single
 customer's premises. However, this need not be the case, as the subscriber
 terminal 20 preferably supports either a single or a dual line, so that
 two subscriber lines could be supported by a single subscriber terminal
 20. The subscriber terminal 20 can also be arranged to support analogue
 and digital telecommunications, for example analogue communications at 16,
 32 or 64 kbits/sec or digital communications in accordance with the ISDN
 BRA standard.
 FIG. 3 is a schematic illustration of an example of a central terminal of
 the telecommunications system of FIG. 1. The common equipment rack 40
 comprises a number of equipment shelves 42, 44, 46, including a RF
 Combiner and power amp shelf (RFC) 42, a Power Supply shelf (PS) 44 and a
 number of (in this example four) Modem Shelves (MS) 46. The RF combiner
 shelf 42 allows the four modem shelves 46 to operate in parallel. It
 combines and amplifies the power of four transmit signals, each from a
 respective one of the four modem shelves, and amplifies and splits
 received signals four way so that separate signals may be passed to the
 respective modem shelves. The power supply shelf 44 provides a connection
 to the local power supply and fusing for the various components in the
 common equipment rack 40. A bidirectional connection extends between the
 RF combiner shelf 42 and the main central terminal antenna 52, typically
 an omnidirectional antenna, mounted on a central terminal mast 50.
 This example of a central terminal 10 is connected via a point-to-point
 microwave link to a location where an interface to the public switched
 telephone network 18, shown schematically in FIG. 1, is made. As mentioned
 above, other types of connections (e.g., copper wires or optical fibres)
 can be used to link the central terminal 10 to the public switched
 telephone network 18. In this example the modem shelves are connected via
 lines 47 to a microwave terminal (MT) 48. A microwave link 49 extends from
 the microwave terminal 48 to a point-to-point microwave antenna 54 mounted
 on the mast 50 for a host connection to the public switched telephone
 network 18.
 A personal computer, workstation or the like can be provided as a site
 controller (SC) 56 for supporting the central terminal 10. The site
 controller 56 can be connected to each modem shelf of the central terminal
 10 via, for example, RS232 connections 55. The site controller 56 can then
 provide support functions such as the localisation of faults, alarms and
 status and the configuring of the central terminal 10. A site controller
 56 will typically support a single central terminal 10, although a
 plurality of site controllers 56 could be networked for supporting a
 plurality of central terminals 10.
 As an alternative to the RS232 connections 55, which extend to a site
 controller 56, data connections such as an X.25 links 57 (shown with
 dashed lines in FIG. 3) could instead be provided from a pad 228 to a
 switching node 60 of an element manager (EM) 58. An element manager 58 can
 support a number of distributed central terminals 10 connected by
 respective connections to the switching node 60. The element manager 58
 enables a potentially large number (e.g., up to, or more than 1000) of
 central terminals 10 to be integrated into a management network. The
 element manager 58 is based around a powerful workstation 62 and can
 include a number of computer terminals 64 for network engineers and
 control personnel.
 FIG. 3A illustrates various parts of a modem shelf 46. A transmit/receive
 RF unit (RFU--for example implemented on a card in the modem shelf) 66
 generates the modulated transmit RF signals at medium power levels and
 recovers and amplifies the baseband RF signals for the subscriber
 terminals. The RF unit 66 is connected to an analogue card (AN) 68 which
 performs A-D/D-A conversions, baseband filtering and the vector summation
 of 15 transmitted signals from the modem cards (MCs) 70. The analogue unit
 68 is connected to a number of (typically 1-8) modem cards 70. The modem
 cards perform the baseband signal processing of the transmit and receive
 signals to/from the subscriber terminals 20. This includes 1/2 rate
 convolution coding and.times.16 spreading with CDMA codes on the transmit
 signals, and synchronisation recovery, despreading and error correction on
 the receive signals. Each modem card 70 in the present example has two
 modems, each modem supporting one subscriber link (or two lines) to a
 subscriber terminal 20. Thus, with two modems per card and 8 modems per
 modem shelf, each modem shelf could support 16 possible subscriber links.
 However, in order to incorporate redundancy so that a modem may be
 substituted in a subscriber link when a fault occurs, only up to 15
 subscriber links are preferably supported by a single modem shelf 46. The
 16th modem is then used as a spare which can be switched in if a failure
 of one of the other 15 modems occurs. The modem cards 70 are connected to
 the tributary unit (TU) 74 which terminates the connection to the host
 public switched telephone network 18 (e.g., via one of the lines 47) and
 handles the signalling of telephony information to, for example, up to 15
 subscriber terminals (each via a respective one of 15 of the 16 modems).
 The wireless telecommunications between a central terminal 10 and the
 subscriber terminals 20 could operate on various frequencies. FIG. 4
 illustrates one possible example of the frequencies which could be used.
 In the present example, the wireless telecommunication system is intended
 to operate in the 1.5-2.5 GHz Band. In particular the present example is
 intended to operate in the Band defined by ITU-R (CCIR) Recommendation
 F.701 (2025-2110 MHz, 2200-2290 MHz). FIG. 4 illustrates the frequencies
 used for the uplink from the subscriber terminals 20 to the central
 terminal 10 and for the downlink from the central terminal 10 to the
 subscriber terminals 20. It will be noted that 12 uplink and 12 downlink
 radio channels of 3.5 MHz each are provided centred about 2155 MHz. The
 spacing between the receive and transmit channels exceeds the required
 minimum spacing of 70 MHz.
 In the present example, as mentioned above, each modem shelf will support 1
 frequency channel (i.e. one uplink frequency plus the corresponding
 downlink frequency). Up to 15 subscriber links may be supported on one
 frequency channel, as will be explained later. Thus, in the present
 embodiment, each central terminal 10 can support 60 links, or 120 lines.
 Typically, the radio traffic from a particular central terminal 10 will
 extend into the area covered by a neighbouring central terminal 10. To
 avoid, or at least to reduce interference problems caused by adjoining
 areas, only a limited number of the available frequencies will be used by
 any given central terminal 10.
 FIG. 5A illustrates one cellular type arrangement of the frequencies to
 mitigate interference problems between adjacent central terminals 10. In
 the arrangement illustrated in FIG. 5A, the hatch lines for the cells 76
 illustrate a frequency set (FS) for the cells. By selecting three
 frequency sets (e.g., where: FS1=F1, F4, F7, F10; FS2=F2, F5, F8, F11;
 FS3=F3, F6, F9, F12), and arranging that immediately adjacent cells do not
 use the same frequency set (see, for example, the arrangement shown in
 FIG. 5A), it is possible to provide an array of fixed assignment
 omnidirectional cells where interference between nearby cells can be
 avoided. The transmitter power of each central terminal 10 is set such
 that transmissions do not extend as far as the nearest cell which is using
 the same frequency set. Thus each central terminal 10 can use the four
 frequency pairs (for the uplink and downlink, respectively) within its
 cell, each modem shelf in the central terminal 10 being associated with a
 respective RF channel (channel frequency pair).
 With each modem shelf supporting one channel frequency (with 15 subscriber
 links per channel frequency) and four modem shelves, each central terminal
 10 will support 60 subscriber links (i.e., 120 lines). The 10 cell
 arrangement in FIG. 5A can therefore support up to 600 ISDN links or 1200
 analogue lines, for example. FIG. 5B illustrates a cellular type
 arrangement employing sectored cells to mitigate problems between adjacent
 central terminals 10. As with FIG. 5A, the different type of hatch lines
 in Figure SB illustrate different frequency sets. As in FIG. 5A, FIG. 5B
 represents three frequency sets (e.g., where: FS1=F1, F4, F7, F1; FS2=F2,
 F5, F8, F11; FS3=F3, F6, F9, F12). However, in FIG. 5B the cells are
 sectored by using a sectored central terminal (SCT) 13 which includes
 three central terminals 10, one for each sector S1, S2 and S3, with the
 transmissions for each of the three central terminals 10 being directed to
 the appropriate sector among S1, S2 and S3. This enables the number of
 subscribers per cell to be increased three fold, while still providing
 permanent fixed access for each subscriber terminal 20.
 A seven cell repeat pattern is used such that for a cell operating on a
 given frequency, all six adjacent cells operating on the same frequency
 are allowed unique PN codes. This prevents adjacent cells from
 inadvertently decoding data.
 As mentioned above, each channel frequency can support 15 subscriber links.
 In this example, this is achieved using by multiplexing signals using a
 Code Division Multiplexed Access (CDMA) technique. FIG. 6 gives a
 schematic overview of CDMA encoding and decoding.
 In order to encode a CDMA signal, base band signals, for example the user
 signals for each respective subscriber link, are encoded at 80-80N into a
 160 ksymbols/sec baseband signal where each symbol represents 2 data bits
 (see, for example the signal represented at 81). This signal is then
 spread by a factor of 16 using a respective Walsh pseudo random noise (PN)
 code spreading function 82-82N to generate signals at an effective chip
 rate of 2.56 Msymbols/sec in 3.5 MHz. The signals for respective
 subscriber links are then combined and converted to radio frequency (RF)
 to give multiple user channel signals (e.g., 85) for transmission from the
 transmitting antenna 86.
 During transmission, a transmitted signal will be subjected to interference
 sources 88, including external interference 89 and interference from other
 channels 90. Accordingly, by the time the CDMA signal is received at the
 receiving antenna 91, the multiple user channel signals may be distorted
 as is represented at 93.
 In order to decode the signals for a given subscriber link from the
 received multiple user channel, a Walsh correlator 94-94N uses the same
 pseudo random noise (PN) code that was used for the encoding for each
 subscriber link to extract a signal (e.g, as represented at 95) for the
 respective received baseband signal 96-96N. It will be noted that the
 received signal will include some residual noise. However, unwanted noise
 can be removed using a low pass filter and signal processing.
 The key to CDMA is the application of orthogonal codes that allow the
 multiple user signals to be transmitted and received on the same frequency
 at the same time. Once the bit stream is orthogonally isolated using the
 Walsh codes, the signals for respective subscriber links do not interfere
 with each other.
 Walsh codes are a mathematical set of sequences that have the function of
 "orthonormality". In other words, if any Walsh code is multiplied by any
 other Walsh code, the results are zero.
 FIG. 7 is a schematic diagram illustrating signal transmission processing
 stages as configured in a subscriber terminal 20 in the telecommunications
 system of FIG. 1. The central terminal is also configured to perform
 equivalent signal transmission processing. In FIG. 7, an analogue signal
 from one of a pair of telephones is passed via a two-wire interface 102 to
 a hybrid audio processing circuit 104 and then via a codec 106 to produce
 a digital signal into which an overhead channel including control
 information is inserted at 108. The resulting signal is processed by a
 convolutional encoder 110 before being passed to a spreader 116 to which
 the Rademacher-Walsh and PN codes are applied by a RW code generator 112
 and PN Code generator 114, respectively. The resulting signals are passed
 via a digital to analogue converter 118. The digital to analogue converter
 118 shapes the digital samples into an analogue waveform and provides a
 stage of baseband power control. The signals are then passed to a low pass
 filter 120 to be modulated in a modulator 122. The modulated signal from
 the modulator 122 is mixed with a signal generated by a voltage controlled
 oscillator 126 which is responsive to a synthesizer 160. The output of the
 mixer 128 is then amplified in a low noise amplifier 130 before being
 passed via a band pass filter 132. The output of the band pass filter 132
 is further amplified in a further low noise amplifier 134, before being
 passed to power control circuitry 136. The output of the power control
 circuitry is further amplified in a further low noise amplifier 138 before
 being passed via a further band pass filter 140 and transmitted from the
 transmission antenna 142.
 FIG. 8 is a schematic diagram illustrating the equivalent signal reception
 processing stages as configured in a subscriber terminal 20 in the
 telecommunications system of FIG. 1. The central terminal is also
 configured to perform equivalent signal reception processing. In FIG. 8,
 signals received at a receiving antenna 150 are passed via a band pass
 filter 152 before being amplified in a low noise amplifier 154. The output
 of the amplifier 154 is then passed via a further band pass filter 156
 before being further amplified by a further low noise amplifier 158. The
 output of the amplifier 158 is then passed to a mixer 164 where it is
 mixed with a signal generated by a voltage controlled oscillator 162 which
 is responsive to a synthesizer 160. The output of the mixer 164 is then
 passed via the de-modulator 166 and a low pass filter 168 before being
 passed to an analogue to digital converter 170. The digital output of the
 A/D converter 170 is then passed to a correlator 178, to which the same
 Rademacher-Walsh and PN codes used during transmission are applied by a RW
 code generator 172 (corresponding to the RW code generator 112) and a PN
 code generator 174 (corresponding to PN code generator 114), respectively.
 The output of the correlator is applied to a Viterbi decoder 180. The
 output of the Viterbi decoder 180 is then passed to an overhead extractor
 182 for extracting the overhead channel information. The output of the
 overhead extractor 182 is then passed via a codec 184 and a hybrid circuit
 188 to a two wire interface 190 where the resulting analogue signals are
 passed to a selected telephone 192.
 At the subscriber terminal 20, a stage of automatic gain control is
 incorporated at the IF stage. The control signal is derived from the
 digital portion of the CDMA receiver using the output of a signal quality
 estimator.
 FIG. 9 is a block diagram of central terminal 10 in wireless
 telecommunications system 1. Central terminal 10 includes a modem shelf
 200 and a combining shelf 201. Modem shelf 200 includes a tributary unit
 202, a plurality of modem units 204, an analog card 206, a radio frequency
 card 208, a shelf controller 210, and a shelf alarm card 212. Tributary
 unit 202 terminates connections to a host telephone network and handles
 the signalling of telephony information to preferably fifteen subscriber
 terminals 20. Modem units 204 perform the baseband signal processing of
 the transmit and receive signals to and from subscriber terminals 20.
 Analog card 206 performs analog to digital and digital to analog
 conversions, baseband filtering, and vector summation of the fifteen
 transmit signals from modem units 204. Radio frequency card 208 receives a
 composite transmit signal 214 from analog card 206 and generates a
 modulated transmit RF signal 215 therefrom. Radio frequency card 208 also
 recovers and amplifies baseband RF signals from subscriber terminals 20
 for application to modem units 204 through analog card 206. Shelf
 controller 210 manages the operation of modem shelf 200. Shelf alarm card
 212 indicates the operational status of modem shelf 200.
 Combining shelf 201 includes a low noise amplifier 216, a power amplifier
 218, a power supply 220, a shelf monitor 222, and a branching unit 224.
 Low noise amplifier 216 is designed to overcome losses in the antenna
 feeder, circulator, RF filter, and receive splitter. Power amplifier 218
 amplifies the RF modulated composite transmit signal 215 to a desired
 transmit level. Power supply 220 supplies power to active components in
 combining shelf 201. Shelf monitor 222 reads operation and maintenance
 information and passes the information to shelf controller 210. Branching
 unit 224 provides combiner and RF filtering operations on the transmit
 side and circulating, RF filtering, and splitting functions in the receive
 side.
 Central terminal 10 also includes feeder cables 230, antennas 232, an
 equipment power interface shelf 234, and an element manager 236. Antennas
 232 provide the mechanism to receive and transmit radio frequency signals
 in conjunction with feeder cables 230. Equipment power interface shelf 234
 provides connection to a local DC power supply and the fusing of various
 supply distribution channels within central terminal 10. An alarm system
 is also provided to detect faulty components within central terminal 10.
 Element manager 236 provides external control capability of central
 terminal 10 functions. Element manager 236 is designed to handle small or
 large networks of subscriber terminals 20 within wireless
 telecommunications system. For transmitting operation, tributary unit 202
 receives telephony information from the telephone network. Tributary unit
 202 provides telephony information to modem units 204 over a transmit
 timeslot bus. Telephony information from each modem unit 204 is received
 at analog card 206. Analog card 206 combines the telephony information
 from each modem unit 204 into a composite transmit signal 214. Composite
 transmit signal 215 is modulated into a radio frequency signal by radio
 frequency card 208. Modulated composite transmit signal 214 is amplified
 by power amplifier 218 for wireless transmission over antennas 232.
 Power amplifier 218 includes a detector 240. Detector 240 measures an
 output radio frequency power of modulated composite transmit signal 215
 from power amplifier 218. Detector 240 may also be used to measure output
 voltage from power amplifier 218 which is proportional to the output radio
 frequency power. The measured output radio frequency power is sent to
 shelf controller 210 through combiner monitor 222. Shelf controller 210
 provides the measured output radio frequency power to analog card 206.
 Analog card 206 determines a power estimate from the inputs of each modem
 unit 204 representing the downlink communication paths from central
 terminal 10 to subscriber terminals 20. Analog card 206 compares the power
 estimate to the measured output radio frequency power. Alternatively,
 analog card 206 may determine a voltage estimate from the inputs of each
 modem unit 204 for comparison to the output voltage measured by detector
 240. In response to this comparison, analog card 206 generates an
 adjustment signal 242. Adjustment signal 242 adjusts a gain of radio
 frequency card 208 to control the output radio frequency power from power
 amplifier 218.
 Adjusting the radio frequency gain allows wireless telecommunications
 system 1 to meet transmitted power specifications. Element manager 236 may
 be used to set the gain of radio frequency card 208 to an initial nominal
 value. For stability purposes, central terminal 10 establishes a desired
 transmit power level of 20 db+/-0.5 db per each subscriber terminal within
 wireless telecommunications system 1.
 In summary, a central terminal controls a radio frequency gain by measuring
 an output radio frequency power of a modulated composite transmit signal.
 The modulated composite transmit signal carries the inputs from a
 plurality modem units representing the downlink communication paths from
 the central terminal to corresponding subscriber terminals. The output
 radio frequency power is compared to a power estimate of the inputs from
 the modem units. A radio frequency gain is adjusted in response to the
 comparison. Adjustment of the radio frequency gain maintains a constant
 output radio frequency power of the composite transmit signal, providing
 improved accuracy for the wireless telecommunications system.
 Thus, it is apparent that there has been provided, in accordance with the
 present invention, an apparatus and method of controlling transmitter gain
 in a wireless telecommunications system that satisfies the advantages set
 forth above. Although the preferred embodiment has been described in
 detail, it should be understood that various changes, substitutions, and
 alterations can be made herein. For example, though an output power of the
 composite signal is measured and compared, other measurements and
 comparisons can be made to provide adjustments to the radio frequency
 gain. Other examples are readily ascertainable to one skilled in the art
 and could be made without departing from the spirit and scope of the
 present invention as defined by the following claims.