SYSTEM AND METHOD FOR REDUCING CUMULATIVE NOISE IN A DISTRIBUTED ANTENNA NETWORK

Abstract: A communications system and method are provided for reducing noise in a distributed antenna network. The system includes a plurality of remote antenna units (100n) with each remote antenna unit including a receiver for receiving input signals, a signal strength processor (130n) for determining whether a valid signal is present at its respective remote antenna unit (102n) and an output controller (142n) for switching off the network connection when no valid signal is present. As a result, the cumulative noise generated by uplink stages connected to the remote antenna units (100n) is reduced by switching off the network conncection of remote antennas that do not have to be connected. The communications system and method may further include an output controller for controlling the output for its respective remote antenna unit based on the comparison by the signal strength processor. The output controller may control the network connection of the remote antennas by switching off their connection to the network when the signal strength processor determines that the signal strength level is below a noise threshold level. The output controller may control the output of the remote antenna units by switching off their connection to the network when the signal strength processor determines that the signal strength level is below a noise threshold level and by switching on the connection to the network when the input signal strength level is greater than or equal to the noise threshold level.

Description: SYSTEM AND METHOD FOR REDUCING CUMULATIVE NOISE IN A DISTRIBUTED ANTENNA NETWORK BACKGROUND The present invention is directed to a system and method for reducing the cumulative noise resulting from the uplink stages connected to a plurality of remote antenna units in a distributed antenna network. More particularly, the present invention reduces the cumulative noise by utilizing a signal strength processor. As personal communications services (PCS) evolve as the next generation of cellular telephone technology, systems and techniques for simply and efficiently transmitting and receiving communication signals are being investigated. One known system is a distributed antenna network (which is also referred to as a multicast network) which provides coverage over substantial areas by a plurality of remote antenna units. An example of a distributed antenna network is illustrated in Figure 1 where an individual transceiver unit 10 is connected to a plurality of cells 20 1 ; ...20 n by transmission media 30 which transports radio signals between the transceiver unit 10 and the cells 20,,...20 n . Each of the cells 20],...20 n include remote antenna units 21,,...21 n . The remote active antenna units 21,,...21 n may be connected to the transmission media 30 by frequency converting circuitry 22 ι ,...22 n for certain applications. Various infrastructures are being developed and modifications of existing infrastructures are of great interest as an alternative for PCS because they are fully capable of providing high quality signals at lower costs than traditional cellular infrastructures. For example, CATV infrastructures have been modified for use in PCS. Such modifications include the CATV infrastructures using a hybrid fiber/coax (HFC) cable infrastructure to increase capacity and improve service quality. Although it is theoretically possible for any CATV infrastructure to support PCS with the proper modifications, the HFC cable infrastructure offers an attractive option as an economical alternative to wireless providers seeking to avoid the high cost of network construction. Figure 2 illustrates the basic components of a CATV infrastructure used to support PCS. In Figure 2, base station equipments 50, and 50 2 are connected to a public network, such as a public switched telephone network. Remote antenna signal processors (RASPs) 52, and 52j connect the base station equipments 50, and 50 2 to a fiber equipment 54. The fiber equipment 54 is connected to a fiber node 58 by fiber optic cable 56 and the fiber node 58 is connected to remote antenna driver (RAD) nodes 62 ! and 62 2 by two-way coaxial cable 60. The RAD nodes 62, and 62 2 each include a group of RADs 64, and 64 2 and 66, and 66 2 respectively connected to antennas 68,, 68 2 , 70,, and 70 2 . This CATV infrastructure converts radio frequency signals into CATV frequency signals usable in the existing CATV infrastructure and converts the CATV frequency signals back into radio frequency signals for broadcast. More specifically, the RASPs 52, and 52 2 convert the radio frequency signals from the base station equipments 50, and 50 2 and then send the converted signals in the downlink path toward the appropriate fiber node 58 and onto the coaxial cable 60. The RADs 64,, 64 2 , 66,, and 66 2 are connected to the coaxial cable 60 for converting CATV frequency signals into assigned radio frequency signals. Radio frequency signals may be received by the RADs 64,, 64 2 , 66,, and 66 2 which convert these signals into signals of frequencies suitable for transmission in the uplink path of the CATV infrastructure. Thereafter, the RASPs 52, and 52 2 convert the upstream CATV frequency signals back into radio frequency signals for processing by the base station equipments 50, and 50 2 . This CATV infrastructure may also accommodate equipment for multiple modulation schemes, such as time division multiple access (TDM A), code division multiple access (CDMA), and frequency division multiple access (FDMA). Radio telephony systems may utilize this CATV infrastructure by operating on available portions of the radio frequency spectrum over fiber optic and coaxial cables, which are widely available in urban areas, so that such systems may be installed to take advantage of this existing infrastructure. The large installed base of fiber optic and coaxial cables used by CATV operators may thereby be effectively exploited at a minimal cost by this infrastructure which distributes the signals to the appropriate antenna locations. The antenna network used in CATV infrastructures is commonly termed a distributed antenna network or multicast network because coaxial or heliaxial cable is used to feed a plurality of antennas distributed within a coverage area. The use of multiple antennas effectively increases and controls the size of the coverage area. When used in a two-way communication system, a distributed antenna network suffers from some problems. Low power urban cellular base stations and low power PCS handsets of limited range may introduce noise to the network which limits the coverage area by the network. Also, each remote antenna contributes noise that is relatively more important in the uplink. These noise problems significantly increase as the number of remote antennas in the distributed antenna network increases. The cumulative noise is the sum of the individual noise contributions of the remote antennas. Thus, having numerous remote antennas can severely degrade the performance of a communication system in which the signal-to-noise ratio is a critical parameter. As existing CATV networks continue to be modified for use in telephony communication, it is desirable to reduce noise generated in distributed antenna networks so that a system using many remote antennas is useful for telephony communication over large coverage areas. SUMMARY It is, therefore, one object of the present invention is to provide a system and method for reducing the cumulative noise generated by uplink stages connected to a plurality of remote antenna units in a distributed antenna network. Another object of the present invention is to provide a system and method for determining whether or not a valid signal is present at each remote antenna in a distributed antenna network and switching off the network connection to each remote antenna when no valid signal is present. Accordingly, the cumulative noise of the network is reduced without adversely affecting the communication performance of the system. A further object of the present invention is to provide a system and method for detecting the signal strength level at each remote antenna in a distributed antenna network and comparing this detected level to a noise threshold level. When this comparison determines that the detected signal level is below the noise threshold level at any of the remote antennas, the network connection for those remote antennas are switched off so that unnecessary noise is not introduced to the network. According to one embodiment of the present invention, the foregoing and other objects are attained in a communications system and method for reducing noise in a distributed antenna network. The system comprises a plurality of remote antenna units with each remote antenna unit including a receiver for receiving input signals, a signal strength processor for determining whether a valid signal is present at its respective remote antenna unit and an output controller for switching off the network connection when no valid signal is present. As a result, the cumulative noise generated by uplink stages connected to the remote antenna units is reduced by switching off the network connection of remote antennas that do not have to be connected. According to another embodiment of the present invention, the foregoing and other objects are attained in a communications system and method for reducing noise in a distributed antenna network. The system comprises a plurality of remote antenna units with each remote antenna unit including a receiver for receiving input signals, and a signal strength processor for detecting and comparing the signal levels with a noise threshold level at its respective remote antenna unit. The system further comprises an output controller for controlling the network connection for its respective remote antenna unit based on the comparison by the signal strength processor. The output controller controls the network connection of the remote antennas by switching off their connection to the network when the signal strength processor determines that the signal level is below the noise threshold level. The output controller additionally controls the network connection of the remote antenna units by switching on the connection to the network for remote antenna units when the signal strength processor determines that the signal level is greater than or equal to the noise threshold level. In the above-described system, the cumulative noise added by uplink stages in a distributed antenna network is greatly reduced without adversely affecting the communication capabilities of the system. Because each remote antenna unit in the distributed antenna network contributes to the cumulative noise when its output is connected to the network, whether or not the remote antenna unit has a valid signal, the performance of the system may be unnecessarily degraded when the output of remote antenna units that do not have valid signals present are connected to the network. The system and method of the present invention reduce the cumulative noise by switching off the connection to the network of the remote antenna units that do not have a valid signal and do not have to be connected to the network for communication purposes. Thereby, connections to the network are switched on only for remote antenna units that have valid signals present and are necessary to be connected to the network for communication purposes. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully understood by reading this description in conjunction with the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the present invention, wherein: Figure 1 illustrates a conventional distributed antenna network; Figure 2 is a block diagram of a known CATV infrastructure which supports PCS; Figures 3(a) and 3(b) are block diagrams showing embodiments of the system for reducing noise in a distributed antenna network according to the present invention; and Figure 4 is a block diagram showing a system infrastructure in which a reduced noise system according to an embodiment of the present invention is implemented. DETAILED DESCRIPTION This invention is directed to a system and method which reduces the cumulative noise generated by uplink stages connected to a plurality of remote antenna units in a distributed antenna network. In these systems, it is essential to reduce the noise contribution of each remote antenna unit to avoid increasing the cumulative noise to such a point that performance is severely degraded as more remote antenna units are added to the network for increasing the coverage area. Because noise is contributed to the network by each of the remote antenna units being simply connected to the network, even when no signals are present, the cumulative noise may be reduced by connecting only those remote antenna units that are necessary for communication purposes, i.e., remote antenna units having valid signals present. A signal strength processor detects the signal strength level and determines whether a valid signal is present at each of the remote antenna units in the network and then switches off the network output at its respective remote antenna unit when no valid signal is present. Accordingly, the cumulative noise generated in the uplink stages of the network is reduced because only the remote antenna units receiving or transmitting valid signals are connected to the network. The cumulative noise reduction system and method according to this invention are based upon this principle. Figure 3(a) illustrates one example of a system that is used in each remote antenna unit for reducing noise in a distributed antenna network according to the present invention. In this system, a plurality of remote antenna units 100 π are connected to the network 150 but only a representative remote antenna unit 100 n is shown in Figure 3(a). An antenna 102 n provides an RF input signal to transceiver circuitry 110 n in this system. The transceiver circuitry 110 π is connected to a signal strength processor 130 n which controls the connection between the antenna 102 π and the network 150. The signal strength processor 130 n determines whether a valid signal is present at the remote antenna 102 n . If a valid signal is present, the antenna 102 n is connected to the network 150 by an output controller 142 n . If a valid signal is not present, the output controller 142 n does not connect the antenna 102 n to the network 150. The signal strength processor 130 n determines whether a valid signal is present at the antenna 102 π by analyzing the signal strength level. The signal strength processor 130 n measures the signal strength level at the antenna 102 n and compares this measured level with a noise threshold level. If the measured signal strength level is greater than or equal to the noise threshold level, the processor 130 n determines that a valid signal is present at the antenna 102 n ; if the measured signal strength level is below the noise threshold level, a valid signal is considered not to be present at the antenna 102 n . Figure 3(b) illustrates the components of Figure 3(a) in more detail. The transceiver circuitry 110 n includes a low noise amplifier 112 n connected to the antenna 102 n for amplifying RF signals gathered by the antenna 102 n . The amplified signal is further processed by a preselecting filter 114 n , a mixer 116 n connected to a first local oscillator 118 n , and an intermediate frequency filter 120 n . A standard coupler 122 π sends this processed signal from the intermediate frequency filter 120 n to the signal strength processor 130 n and also to a mixer 124 π . The mixer 124 n is connected to a second local oscillator 126 n and outputs signal 128 n to the output controller 142 n . The signal strength processor 130 n includes a signal strength detector 132 n for receiving an output signal 127 n from the coupler 122 n . The signal strength detector 132 n preferably includes a plurality of log amplifiers 134 n which are connected to a rectifier 136 n . Although the log amplifiers 134 n are not necessary for enabling the signal strength to be detected, they are used to provide calibration in dB instead of linear calibration. In detecting the signal strength level, the signal strength detector 132 π amplifies the signal level to compensate for the loss, typically on the order of 10 dB, due to the coupler 122„. A conventional integrated circuit chip, such as Phillips NE/SA625 and SA626 ICs, may perform these signal strength detection functions. The signal strength detected by the signal strength detector 132 n is input to a comparator 140 n which compares this detected signal strength level with a noise threshold level. The noise threshold level is generated by a threshold reference generator 138 n and may be variably set. A comparison signal 141 n from the comparator 140 n is then used to control the connection of the processed signal from the antenna 102 n to the network 150. More specifically, a signal 128 n output from the transceiver circuitry 110 n for the antenna 102 n is only connected to the network 150 when the comparator 140 n determines that the detected signal strength level is greater than or equal to the noise threshold level. Thereby, the comparison signal 141 n causes the output controller 142 π to connect the processed signal 128 n to the network 150. The output controller 142 n may be an electrical switch, such as a transistor, controlled by the comparison signal 141 n . To determine whether a valid signal is present at the remote antenna units, the noise threshold level used for the comparison with the detected signal strength level should preferably be set slightly above the noise floor of the uplink prior to the connection of the transmission network. The noise threshold level may be set with a digital potentiometer or the like so that remote adjustments may be performed. Referring to Figure 3(b), the threshold reference generator 138 n may be variably adjusted to achieve the optimum noise threshold level setting at slightly above the noise floor. This noise level setting may be factory set in the remote antenna units by measuring the output signal 128 n without an RF signal being input (the noise present without a transmitting carrier). The noise floor is measured so that the noise threshold level may be monitored. If a remote antenna unit is being switched on or off improperly, a central unit for controlling the entire system may be used by a system operator to adjust the noise threshold level or the adjustment may be performed manually at the remote antenna unit. The signal strength detector 132 n and the output controller 142 n should be fast enough to track fades and the ramp up time of individual bursts (in the case of TDM A) so as not to distort the data. The bandwidth for measuring the signal strength should preferably be narrower than the channel bandwidth or otherwise compensated to maximize the sensitivity of the measure branch. In other words, an optimum point exists such that the bandwidth of the measuring device is narrower than the channel bandwidth but allows a response time fast enough to track the fades and the burst ramps. In order to minimize false switching, hysteresis may be used at the optimum point. More specifically, a voltage range is utilized about the optimum point such that the remote antenna unit is always on when the detected level is above the voltage range and is always off when the detected level is below the voltage range. The hysteretic effect is provided when the detected level is within the voltage range. If the voltage range is reached when the remote antenna unit is on, the remote antenna unit will not be switched off until the minimum level of the voltage range is reached. Similarly, if the voltage range is reached when the remote antenna unit is off , the remote antenna unit will not be switched on until the maximum level of the voltage range is reached. Figure 4 illustrates an example of a CATV system infrastructure that may advantageously utilize Applicants' system and method for reducing noise. The infrastructure includes fiber nodes 200,,...200 n which are connected to a CATV headend 300 by respective optical fibers 250,,...250 n . The CATV headend 300 may support a number of fiber nodes 200,,...200 n , each of which may generally support approximately 500 to 1500 homes or subscribers through a plurality of remote antenna units 202,,...202 n . The remote antenna units 202,,...202 n are respectively connected by the optical fibers 250,,...250 n to the CATV headend 300. Also, a plurality of the remote antenna units 202,,...202 n are connected to an amplifying and optical/electrical converting circuit 206 n by coaxial cabling 204 n . The CATV headend 300 includes an amplifying and electrical/optical converting circuit 302 which is used for interfacing the CATV headend 300 to the optical fibers 250,,...250 n . The amplifying and electrical/optical converting circuit 302 is connected to combining and splitting circuitry 304 which is connected to a video source 306 and cable access processing (CAP) units 308,, ...308 n which at least correspond in number to the fiber nodes 200,,...200 n of the system. The CAP units 308,, ...308 n are connected to radio base stations (RBS) 314,,...314 n and a hub 310. The hub 310 is further connected to a remote antenna management system (RAMS) 312. The CAP units 308,,...308 n are also connected to an operation support system/ switching center (OSS/SC) 350. The OSS/SC 350 includes a mobile switching center 352, a base station controller 354, an operation support system 356 and its own RAMS 358. The RAMS 358 of the OSS/SC 350 may communicate with the RAMS 312 of the CATV headend 300 so that a plurality of CATV headends may be controlled at a central location. The base station controller 354 is also connected to a macro cell 360 which includes a RBS 362. The CAP units 308,, ...308 n provide the frequency conversion and power level adjustments for placing telephony carrier signals on the CATV infrastructure as well as controlling and monitoring the status of the remote antenna units 202,,...202 n . The remote antenna units 202,,...202 n are located in desired areas and convert the CATV base communication signals back to their proper air frequencies and power levels. Each of the remote antenna units 202,,...202 n is associated with a specific transceiver radio unit (TRU) in the RBS 314,,...314 n so that the CATV network is totally transparent to the radio operation. The operating frequency of the CAP units 308,, ...308 n is set to exactly match the frequency of the remote antenna transmitter. Up to six TRUs in a specific one of the RBS 314 n feed one of the CAP units 308,,...308„ which in turn serves several fiber nodes 200,,...200 n in the CATV network. A combiner within the CAP units 308,,...308 n provide six transmit input ports from the RBS 314,,...314 n in order to support a maximum of six TRUs. The CAP units 308,,...308 n convert the telephony carrier signals to CATV frequency signals and feed them to the CATV headend 300 over a bidirectional coaxial cable. In the CATV headend 300, the telephony carrier signals are combined with the video signals and both signals are transmitted over the fiber optic cables 250,,...250 n to fiber nodes 200,,...200 π . At the fiber nodes 200,,...200 n , the signals are converted back to electrical signals and distributed over a tree and branch coaxial network. Each individual one of the remote antenna 202, , ...202 n taps off the coaxial cabling 204, filters the carrier signals, converts the frequency and transmits the carrier signals over the air interface. The video signals continue unaltered to each CATV subscriber. For the uplink, the remote antenna units 202,,...202 n receive two diversity signals from the air, down convert each to a different frequency, and transmit the diversity carrier signals over the CATV infrastructure. In the CATV infrastructure at the fiber nodes 200,,...200 n , the electrical signals are converted to optical signals and transmitted back to the CATV headend 300. These signals are then converted back to electrical signals and routed to the CAP units 308,,...308 π over the bidirectional cable. The CAP units 308,,...308 π convert the uplink carrier signals back up to frequencies for the input to the TRUs. The CAP units 308,,...308 n receive digital control information from the RAMS 312 and forward remote antenna control information to the desired one of the remote antenna units 202,,...202 n . Because each one of the remote antenna units covers a small area, multicasting provides a more efficient use of radio resources to cover larger areas. The number of remote antenna units in a multicast group is limited by the addition of receiver thermal noise resulting from each remote antenna unit in the uplink. By the signal strength detection system and method of this invention, the number of remote antenna units that are used in a multicast group may be increased because the cumulative noise resulting from the uplink stages is reduced. For example, if 50 remote antenna units are included in a multicast group, but only two remote antenna units are receiving valid signals, the 48 remote antenna units that are not receiving a valid signal may be shut off from the network connection and the cumulative noise introduced to the network by these remote antenna units is eliminated. Therefore, the only noise contribution in this case is from the two remote antenna units that have a valid signal and the cumulative noise is significantly reduced. The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention, which is determined by the following claims. All such modification that would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims: WHAT IS CLAIMED IS: 1. A communication system for reducing noise in a distributed antenna network, comprising: a plurality of remote antenna units, each of said remote antenna units including, a receiver for receiving input signals, a signal strength processor for detecting levels of said input signals for said remote antenna unit and comparing the levels of said input signals with a noise threshold level, and an output controller for switching off a network connection for said remote antenna unit when said signal strength processor determines that the levels of said input signals are less than said noise threshold level and switching on the network connection for said remote antenna unit when said signal strength processor determines that the levels of said input signals are greater than or equal to said noise threshold level. 2. A communication system according to claim 1, wherein said noise threshold level is set slightly greater than the noise floor of the uplink for the remote antenna units. 3. A communication system according to claim 1 , wherein said signal strength processor comprises: a signal strength detector for detecting the level of said input signals; a threshold reference generator for variably setting said noise threshold level; and a comparator for comparing the levels of said input signals with said noise threshold level. 4. A communication system for reducing cumulative noise in the uplink stages of a distributed antenna network, comprising: transceivers for sending and receiving information; a plurality of remote antenna units operatively communicative with said transceiver, each of said remote antenna units including, a receiver for receiving input signals, a signal strength processor for detecting levels of said input signals for said remote antenna unit and comparing the levels of said input signals with a noise threshold level, and an output controller for switching the output of said remote antenna unit on and off in response to the comparison by said signal strength processor. 5. A communication system according to claim 4, wherein said output controller switches on the connection between the network and said remote antenna unit when said signal strength processor determines that the levels of said input signal are greater than or equal to said noise threshold level and said output controller switches off the connection between the network and said remote antenna unit off when said signal strength processor determines that the levels of said input signal are less than said noise threshold level. 6. A communication system according to claim 4, wherein said receiver in each of said remote antenna units comprises: an antenna for receiving an RF input signal; a low noise amplifier and a preselecting filter for amplifying and filtering said RF input signal; a first local oscillator, a first mixer and an intermediate filter for processing the filtered RF input signal; and a second local oscillator and a second mixer for further processing the signal output from said intermediate filter. 7. A communication system according to claim 4, wherein said signal strength processor comprises: a signal strength detector for detecting the levels of said input signals; a threshold reference voltage generator for variably setting said noise threshold level; and a comparator for comparing the levels of said input signals with said noise threshold level. 8. A communication system according to claim 7, wherein said signal strength detector comprises a predetermined number of logarithmic amplifiers and a rectifier. 9. A communication system according to claim 4, wherein said output controller comprises a switch for controlling the connection between the output of said remote antenna unit and the network. 10. A communication system according to claim 4, wherein said transceivers are connected to said remote antenna units by an analog transmission media which transports radio signals therebetween. 11. A communication system according to claim 4, wherein said noise threshold level is set to be slightly above the noise floor of the uplink for each of said remote antenna units and the network. 12. A communication system for reducing noise in a distributed antenna network, comprising: a plurality of remote antenna units, each of said remote antenna units including, a receiver for receiving input signals, a signal strength processor for determining whether a valid signal is present at the remote antenna unit, and an output controller for disconnecting the remote antenna unit from the network when said signal strength processor determines that a valid signal is not present. 13. A method for reducing noise in a distributed antenna network of a communication system comprising the steps of: (a) receiving input signals by a plurality of remote antenna units; (b) detecting levels of said input signals and comparing the levels of said input signals with a noise threshold level; and (c) switching off the connection between the network and said remote antenna units when said step (b) determines that said input signals are less than said noise threshold level and switching on the connection between the network and said remote antenna units when said step (b) determines that said input signals are greater than or equal to said noise threshold level. 14. A method according to claim 13, wherein said noise threshold level is set slightly greater than the noise floor of the uplink for the remote antenna units. 15. A method according to claim 13, wherein said step (b) further comprises the steps of: (b)(1) detecting the level of said input signals; (b)(2) variably setting said noise threshold level; and (b)(3) comparing the level of said input signal with said noise threshold level. 16. A method for reducing cumulative noise in the uplink stages of a distributed antenna network of a communication system, comprising the steps of: (a) sending and receiving information; (b) receiving input signals sent at said step (a) by a plurality of remote antenna units; (c) detecting the level of said input signals and comparing the level of said input signals with a noise threshold level; and (d) switching the connection between the network and said remote antenna units on and off in response to the comparison by said step (c). 17. A method according to claim 16, wherein said step (d) connects the network and said remote antenna unit when said step (c) determines that the level of said input signal is greater than or equal to said noise threshold level and said step (d) disconnects the network and said remote antenna unit when said step (c) determines that the level of said input signal is less than said noise threshold level. 18. A method according to claim 16, wherein said step (c) further comprises the steps of: (c)(1) detecting the level of said input signals; (c)(2) variably setting said noise threshold level; and (c)(3) comparing the level of said input signal with said noise threshold level. 19. A method according to claim 16, wherein said noise threshold level is set to be slightly above the noise floor of the uplink stage at the output for each of said remote antenna units and the network. 20. A method for reducing noise in a distributed antenna network of a communication system, comprising the steps of: (a) receiving input signal by a plurality of remote antenna units; (b) determining whether or not a valid signal is present at the remote antenna units; and (c) disconnecting the remote antenna units from the network when said signal strength processor determines that a valid signal is not present at the remote antenna units.

Inventors: ECKSTEIN LARRY, HUFFMAN JACQUE, NAIDU ARUN, ECKSTEIN, LARRY, HUFFMAN, JACQUE, NAIDU, ARUN
Applicants: ERICSSON GE MOBILE INC, ERICSSON INC.
Application year: 1997
Main classification: H04B7
IPC codes: H01Q3, H04B7