Telecommunications – Transmitter and receiver at same station – With frequency stabilization
Reexamination Certificate
2002-02-04
2004-12-07
Chin, Vivian (Department: 2682)
Telecommunications
Transmitter and receiver at same station
With frequency stabilization
C455S075000, C455S113000, C455S119000, C455S255000, C455S257000, C455S258000, C455S266000, C375S316000, C375S326000, C375S327000, C375S344000, C375S219000, C375S376000
Reexamination Certificate
active
06829469
ABSTRACT:
SCOPE OF INVENTION
The present invention relates to radio transmitters and receivers. The object of the invention is to present a new method for producing a low frequency output signal from a frequency hopping input signal and a second method for producing a frequency hopping output signal from a low frequency input signal.
In particular, the invention relates to a method for producing, in a receiver, a first output signal, which has a specific first output frequency, from a first input signal the frequency of which changes quickly in steps of the size of the channel spacing or a multiple of the channel spacing of a mobile communication system within the frequency range of the mobile communication system. Furthermore, the invention relates particularly to a method for producing, in a transmitter, an output signal the frequency of which belongs to the frequency band of a mobile communication system, from a second input signal, which has a specific second input frequency and, further, for changing the second output frequency in steps of the size of the channel spacing or a multiple of the channel spacing of the mobile communication system, on the mobile communication system frequency band.
Further, the invention relates particularly to a receiver, a transmitter and a frequency synthesiser, wherein the method according to the invention is applied to a frequency synthesis, which implement the methods according to the invention. Methods according to the invention, the receivers and transmitters implementing the methods, as well as the frequency synthesiser based on the methods, can be used, in addition to the GSM system, in any radio system where the transmitting and receiving frequency is being changed.
BACKGROUND OF INVENTION
In transmitters and receivers used in mobile communication systems, it must be possible to quickly change the transmitting and receiving frequency. This is particularly important in cellular networks that use frequency hopping. Frequency hopping is an efficient countermeasure for preventing harmful phenomena due to the radio channel's properties and interference.
Rayleigh fade is a major problem especially in data transmission from a base transceiver station to a terminal. This can be seen particularly as a large variation in the received signal level. The variation in the level of the received signal is a problem that occurs particularly in slowly moving terminals. This means that a terminal may stay for a long time in a place where the received signal level is poor.
Rayleigh fade is frequency-dependent by its nature. In frequency hopping, successive bursts are sent at different frequencies. If the difference between these frequencies is sufficient, the fading properties of the frequencies in a radio channel can be considered to be independent of each other. In this case, the probability that two erroneous bursts are received decreases.
A second benefit to be gained by frequency hopping is interference diversity. In a busy traffic area, for example, in big cities, a network's capacity is limited due to interference resulting from the re-use of frequencies. When the same frequency is being used in adjacent cells simultaneously, the frequencies interfere with each other. When using slow frequency hopping, the probability that the received signal and the interfering signal are simultaneously at the same frequency is reduced.
Adjustable local oscillators used in radio transmitters and receivers are often frequency synthesisers. A frequency synthesiser is normally based on a phase-locked loop the operation of which is based on an oscillator locked in a reference frequency.
A frequency synthesiser has a certain frequency resolution and settling time. The frequency of a signal produced by a frequency synthesiser can be changed in steps of the size of the frequency resolution or a multiplier of the frequency resolution. The changing of the frequency and locking in the desired frequency take place during the settling time. The settling time can be shortened by broadening the bandwidth of the filter of a phase-locked loop or by increasing the phase-comparison frequency. A disadvantage is that the increasing of the bandwidth of the phase-locked loop inevitably follows an increase in phase error. The phase-comparison frequency of conventional frequency synthesisers is equal to the frequency resolution. A fractional-n synthesiser has improved the settling time of conventional frequency synthesisers. The synthesiser in question is based on a conventional solution the phase-comparison frequency of which has been increased n-fold. Currently, mainly 5- and 8-fold fractional-n synthesisers are being used. For example, a basic solution, the frequency resolution and phase-comparison frequency of which are 200 kHz, can be speeded up by using, for example, a fractional-n synthesiser the frequency resolution of which is 200 kHz and the phase-comparison frequency 1 MHz (modulo-5).
There are several different means of implementation for the change of receiving and transmitting frequencies. In the basic solution of a Superheterodyne receiver, a local oscillator is used in a first intermediate frequency stage, the frequency of a local oscillator signal produced by it being adjustable. A frequency synthesiser, the frequency resolution of which is equal to the frequency resolution of the received signal, is used as the local oscillator. An input signal is down-converted into an intermediate frequency signal by mixing the intermediate frequency signal and the local oscillator signal. The intermediate frequency signal is further down-converted into an output signal by mixing the intermediate frequency signal and the local oscillator signal of a second intermediate frequency phase. The signal is detected from the output frequency. The first intermediate frequency is constant and the local oscillator of the second intermediate frequency stage operates at a constant frequency.
In the basic solution of a Superheterodyne transmitter, a local oscillator is used in a first intermediate frequency stage, the frequency of a local oscillator signal produced by it being constant. An input signal is up-converted by a first local oscillator signal into an intermediate frequency, the frequency of which is also constant. The intermediate frequency is further mixed by the local oscillator signal of a second intermediate frequency stage to a desired transmitting frequency. A transmit signal with the desired frequency is produced by adjusting the frequency of the second local oscillator signal and by mixing the second local oscillator signal and the intermediate frequency signal. The frequency of the local oscillator signal is the difference or sum of the transmit signal frequency and the intermediate frequency.
The problem with the basic solutions is the length of the settling time with the required frequency resolutions. For instance, in the GSM system (GSM, Global System for Mobile communications), channel spacing is 200 kHz and so the settling time of conventional frequency synthesisers is far too slow. On the other hand, the phase-comparison frequency of the currently available fractional-n synthesisers is normally 5- or 8-fold compared to the frequency resolution, and frequency locking takes about 40 phase-comparison periods of a phase-locked loop. This being the case, a phase-comparison frequency of 1.6 MHz (modulo 8) and 25 microsecond (=1/1.6 MHz*40 periods) settling times at best are achieved by fractional-n synthesisers, the frequency resolution of which is 200 kHz.
In a GSM network that uses frequency hopping, the frequency is changed after each burst. In between the bursts, there is a guard period, which is 8.25 bits long, i.e., which lasts for 30 microseconds. For example, in a base transceiver station, the changing and locking of a frequency should take place during a period less than half a guard period of a burst. Thus, the base transceiver station's settling time should be distinctly less than 15 microseconds. In this case, the phase-comparison period can be 0.375 m
Chin Vivian
Dao Minh D.
Harrington & Smith ,LLP
Nokia Corporation
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