Telecommunications – Carrier wave repeater or relay system – With transmission line
Reexamination Certificate
1999-10-29
2002-12-31
Nguyen, Lee (Department: 2746)
Telecommunications
Carrier wave repeater or relay system
With transmission line
C455S020000
Reexamination Certificate
active
06501942
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates generally to cellular communication systems, and specifically to cellular communication systems for areas where radio-frequency signals have difficulty entering.
II. Description of the Related Art
In cellular communications systems there are typically regions where the coverage is difficult or incomplete, for example, within metal-framed structures and underground. One of the reasons for the difficult coverage is Faraday-cage type shielding, wherein radio frequency (RF) signals have difficulty penetrating an effectively closed conducting structure. Another reason is Rayleigh fading, which is generated by a signal traversing multiple paths between a transmitter and a receiver. Typically the multiple paths are caused by reflections and/or refractions of the signal by objects between the transmitter and the receiver. The multiple paths followed by the signal generate interference effects at the receiver, which effects manifest themselves as differences in measured signal strength at the receiver, the measured signal strength being a function of the different paths followed by the signal. Methods for improving the coverage in regions where Faraday-cage type shielding and Rayleigh fading occur are known in the art.
U.S. Pat. No. 5,404,570, to Charas et al, whose disclosure is incorporated herein by reference, describes a repeater system used between a base transceiver station (BTS) which is able to receive signals and a closed environment such as a tunnel, wherein the environment is closed off to transmissions from the BTS. The system down-converts a high radio-frequency (RF) signal from the BTS to an intermediate-frequency (IF) signal, which is then radiated by a cable and an antenna in the closed environment to a receiver therein. The receiver up-converts the IF signal to the original RF signal. Systems described in the disclosure include a vehicle moving in a tunnel, so that passengers in the vehicle who would otherwise be cut off from the BTS are able to receive signals.
U.S. Pat. No. 5,603,080 to Kallandar et al., whose disclosure is incorporated herein by reference, describes a plurality of repeater systems used between a plurality of BTSs and a closed environment, wherein the environment is closed off to transmissions from the BTSs. Each system down-converts an RF signal from its respective BTS to an IF signal, which is then transferred by a cable in the closed environment to one or more respective receivers therein. Each receiver up-converts the IF signal to the original RF signal. Systems described in the disclosure include a vehicle moving between overlapping regions in a tunnel, each region covered by one of the BTSs via its repeater system. Thus passengers in the vehicle who would otherwise be cut off from one or more of the BTSs are able to receive signals from at least one of the BTSs throughout the tunnel.
U.S. Pat. No. 5,765,099, to Georges et al., whose disclosure is incorporated herein by reference, describes a system and method for transferring an RF signal between two or more regions using a low bandwidth medium such as twisted pair cabling. In a first region the RF signal is mixed with a first local oscillator to produce a down-converted IF signal. The IF signal is transferred to a second region via the low bandwidth medium, wherein the signal is up-converted to the original RF signal using a second local oscillator. The local oscillators are each locked by a phase locked loop (PLL) in each region to generate the same frequency, the locking being performed in each loop by comparing the local oscillator frequency with a single low frequency stable reference signal generated in one region. The reference signal is transferred between the regions via the low bandwidth medium.
One of the methods for overcoming Rayleigh fading, is to use a plurality of spatially-diverse receiving antennas, relying on the fact that statistically the chance of destructive interference occurring at all the antennas for a given signal is small. Using a plurality of antennas (in many cases two antennas are sufficient) enables a corresponding diversity of received signals to be analyzed, and typically the strongest signal is chosen. In the case of two antennas, the received signals are referred to as main and diversity signals.
U.S. Pat. No. 5,513,176, to Dean et al., whose disclosure is incorporated herein by reference, describes a distributed antenna array within a region where reception is difficult. The performance of the antenna array is enhanced by utilizing the signal diversity which is generated by the antennas being spatially distributed. Each antenna has a differential time delay applied to signals received by the antennas, so that the diverse signals are also separated in time. The differentially-delayed signals are preferably down-converted to an intermediate frequency and are combined, and the combined signal is then transferred out of the region via a cable.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide methods and apparatus for improved coverage in regions where cellular communication is inherently difficult.
In preferred embodiments of the present invention, repeater apparatus for use in a cellular communications system comprises a master transceiver unit, which communicates, preferably over the air or alternatively via a cable, with a base transceiver station using master radio-frequency (RF) signals. The master transceiver unit is coupled by one or more cables to one or more slave transceiver units, which are situated in an environment substantially closed off electromagnetically from the environment wherein the base transceiver station is situated. The slave units comprise respective slave antennas, by means of which the slave units communicate with mobile cellular transceivers using slave RF signals. Most preferably, the slave units are located in a region where the master RF signals are not able to penetrate, such as within a building. Signals between the master unit and the slave units are transferred at intermediate frequencies (IF) via the cables.
In both master and slave units, the IF signals are generated by down-converting the respective RF signals, and the RF signals are recovered by up-converting the IF signals. The up-conversion and down-conversion are performed using the same local oscillator (LO) frequency in the master and the slave units. In order to generate the same LO frequency in all units, one local oscillator in the master unit generates the LO frequency, and the generated LO frequency is divided by an integer in the master unit. The divided LO signal is transferred by the cable to the slave units, wherein the divided LO signal is multiplied by the same integer to recover the LO frequency.
Transferring an integer-divided LO frequency throughout the system, for use as a reference, enables recovery of the undivided LO frequency by multiplying by the same integer. Furthermore, the integer-divided LO signal can be distributed with low loss over conventional, inexpensive cable. Thus, one local oscillator supplies the entire system with the same LO frequency, so that problems which might be caused by LO frequency differences within the system are obviated.
In some preferred embodiments of the present invention, the RF signals comprise code division multiple access (CDMA) signals, which include one or more pilot signals associated with cellular channels over which the mobile transceivers are to communicate. The slave units comprise respective transmission delay elements which add a differential time delay to signals transmitted from the master unit to the respective slave units. The differential time delay effectively adds diversity to the signals received by the slave units, so that reception of pilot tone signals transmitted by the master unit is improved. In some preferred embodiments of the present invention, the slave units comprise respective receive delay elements, which add a differential time delay to signals received
Weissman Haim
Yona Eli
Brown Charles D.
Cheatham Kevin T.
Nguyen Lee
Qualcomm Incorporated
Wadsworth Philip
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