Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
1999-01-11
2001-01-30
To, Doris H. (Department: 2745)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S434000, C342S368000
Reexamination Certificate
active
06181955
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention is that of digital cellular mobile radio systems.
To be more precise, the invention concerns a method of transmitting a control signal by a base station of a digital cellular mobile radio system and a corresponding base station.
The invention applies in particular, but not exclusively, to transmitting BCCH signals in mobile radio systems of the following types: GSM 900 (Global System for Mobile communications operating in the 900 MHz band), DCS 1800 (Digital Cellular System operating at 1800 MHz) or PCS 1900 (Personal Communication System).
A digital cellular mobile radio system is usually implemented within a network of geographical cells through which mobile stations move. A base station is associated with each cell and a mobile station communicates via the base station associated with the cell in which it is located.
Each base station uses a number of signals (also called carriers) at predetermined frequencies on the downlink (from the base station to the mobile stations). One of these signals, called the BCCH (Broadcast Control Channel) signal, is a control signal carrying in particular (generally in the first time slot of each frame) a broadcast control channel BCCH specific to the cell and supplying all mobile stations with general information about the network and information about the broadcasting cell and adjacent cells.
The problem addressed here is transmission of the BCCH signal or more generally of any control signal by the base station. Because all of the cell must be covered, transmission of the signal must comply with a number of constraints. First of all, it must be isotropic (or quasi-isotropic). Also, it must be effected at a sufficiently high power, continuously and at a fixed frequency.
For the above constraints to be complied with, the BCCH signal is conventionally transmitted by an omnidirectional antenna associated with a power amplifier.
However, proposals have been made to replace omnidirectional antennas with antenna arrays in order to increase the size of cells and/or reduce base station transmit power levels. Antenna arrays of this kind form beams dynamically and therefore optimize use of resources.
Initially, only traffic signals have benefited from being transmitted on beams. In other words, each base station has had one or more antenna arrays for transmitting traffic signals and has retained an omnidirectional antenna for transmitting the BCCH signal. The constraints that transmission of the BCCH signal has to comply with are such that for a long time it has been thought that only an omnidirectional antenna could be used.
However, more recently, proposals have been made to transmit the BCCH signal using one or more antenna arrays. The number of antenna arrays is generally directly related to the number of sectors that the cells comprises, one antenna array covering each sector.
Patent document WO/9617486 proposes a base station comprising means for forming a plurality of fixed beams, each covering a part of the cell, and means for assigning the BCCH signal to each of the fixed beams in succession. In this way, transmission of the BCCH signal corresponds to coverage of the cell by cyclic scanning. The above document recommends that the BCCH signal is transmitted in each beam for the duration of one time slot of the frame structure. In this way each beam receives the BCCH signal every N time slots where N is the total number of beams.
The solution proposed in the above patent document is not satisfactory.
First of all, although the BCCH signal is transmitted isotropically, this cannot be considered to be true broadcasting because of the long interval (N time slots) between two transmissions on the same beam.
What is more, because of this long interval, it is difficult for a mobile station to maintain accurate knowledge of the better adjoining base stations. Also, the mobile station may have moved between two successive pointings in its direction, which increases the probability of loss of the BCCH signal and the complexity of handover between sectors and/or between cells.
SUMMARY OF THE INVENTION
An object of the present invention is to overcome the various drawbacks of the prior art.
To be more precise, one object of the present invention is to provide a method of transmitting a control signal by a base station of a digital cellular mobile radio system which optimizes the necessary transmission resources.
Another object of the invention is to provide a method of the above kind which reduces manufacture and maintenance costs.
Another object of the invention is to provide a method of the above kind which broadcasts the control signal substantially isotropically.
Another object of the invention is to provide a method of the above kind retaining all the advantages associated with beam forming (reduced interference, reduced power and/or cell size, etc.).
The above objects, and others that become apparent below, are achieved in a preferred first implementation of the invention by a method of transmitting a control signal by a base station of a digital cellular mobile radio system, said base station comprising at least one array of a plurality of antennas for dynamically creating a beam transmitting said control signal, said transmit beam being adapted to assume different angular positions within a predetermined angular range, a plurality of transmit subsystems being associated with said plurality of antennas, each transmit subsystem including phase modulator means and phase-shifter means, each antenna transmitting a signal supplied by an associated one of said transmit subsystems on the basis of said control signal and contributing to the creation of said beam, which method is characterized in that it includes the following step:
allocating to said control signal, in at least one of said transmit subsystems, a predetermined specific time shift so that said transmit beam, called a vibrating beam, assumes in succession at least some of said different angular positions and enables isotropic broadcasting of said control signal in said angular range.
Thus this first embodiment of the invention forms a beam that vibrates (i.e. takes different angular positions) quickly, at the phase modulation rate of the signal.
Note that this vibration is smoothed by the demodulators and channel decoders of the terminals.
A conventional beam does not vibrate and points in a fixed angular direction. The time shift introduces into one or more transmit subsystems an additional phase-shift of the signals supplied thereby. This additional phase-shift induces the required variation in the direction of the beam. The additional phase-shift must be small enough to prevent any cancellation through addition of signals from the transmit subsystems transmitted by the antennas. Avoiding cancellation of the transmitted signal enables power summing of the signals generated using different time shifts.
Time shift means either a time-delay or a time-advance.
The time shift can be applied before, during or after phase modulation, but before any conversion into the analog domain.
The isotropic effect of the vibration stems from the random nature of the phase modulation bits. Isotropic broadcasting is accentuated in the event of encoding (encryption and/or encipherment). The vibration rate is directly related to the phase modulation rate.
Note that the control signal is broadcast better with the vibrating beam than conventionally with an omnidirectional antenna. The vibration reduces fading phenomena because of overlap between the various angular positions adopted by the vibrating beam.
Introducing a time shift represents a virtually zero cost. Operating in the digital domain, it is sufficient to select certain samples from a plurality of samples.
The invention benefits from all the advantages associated with beam forming by an antenna array.
The difference between two time shifts assigned to two transmit subsystems associated with two adjacent antennas is advantageously equal to a fixed predetermined basic time shift dT.
This shift is, f
Alcatel
Sughrue Mion Zinn Macpeak & Seas, PLLC
To Doris H.
LandOfFree
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