Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1999-09-23
2003-04-29
Le, Thanh Cong (Department: 2684)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S493000, C370S337000, C455S266000, C455S250100
Reexamination Certificate
active
06556559
ABSTRACT:
BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention relates to an ambient field level monitor device applicable suitably for a radio mobile terminal or station.
2) Description of the Related Art
In the recent years, with an explosive increase of portable telephone subscribers, communication enterprises installs a large number of base stations and increase the number of channels to be put to use, thereby attempting to stabilize communications.
Viewing a channel detection method taken in a Japanese PDC (Personal Digital Cellular) system, a radio mobile terminal, forming a mobile station, detects the level of a carrier from a base station within a peripheral cell during an idle slot time to measure a value of its received signal strength intensity (which will be referred hereinafter to as an RSSI), and reports the measurements to the base station, while the base station receives that report successively. During communications with the mobile station, if a-peripheral cell exists which shows a received level higher than that of the current cell by a given value or more, the base station refers to a base station in that peripheral cell for the presence or absence of a free channel and, if the free channel is present, transmits channel information to the mobile station (which will sometimes be referred hereinafter to as mobile equipment). Thus, the mobile station comes to use that channel.
This RSSI value detection method is made to obtain an intermediate frequency signal down-converted from a radio frequency signal. That is, while the intermediate frequency signal is inputted to a limiter to undergo amplitude limiting processing, a signal is taken out from a predetermined portion of that limiter and then processed in another circuit, thereby obtaining the RSSI value. The limiter is composed of a plurality of logarithmic amplifiers or the like cascaded, and the intermediate frequency signal taken out from the predetermined portion is smoothed through a low-pass filter in another circuit to detect a DC level so that the detection value is outputted as an RSSI value.
FIG. 18
is an illustration of a cell disposition in the Japanese PDC system and shows a base station disposition. Circles forming cells A to F represent schematic zones the base stations can cover respectively. In
FIG. 18
, in case where a mobile station
60
first stays in the zone D and then moves into the zone B, the mobile station
60
has used a channel pertaining to the zone D at first, and then gradually approaches the zones C and F and finally reaches the zone B. In this instance, in order to ensure stable communications under such an environment, there is a need for the mobile station
60
to certainly capture, of a plurality of receivable carriers from the base stations, a channel showing the highest RSSI value as occasion calls. Thus, the mobile station
60
is required to make measurements of the RSSI values in the peripheral zones with high accuracy during a period of time for an idle slot.
This idle slot is one of TDMA slots, while
FIG. 19
is an illustration of a TDMA slot configuration on the mobile station
60
side. In
FIG. 19
, the mobile station
60
side TDMA slot is made up of four types of slots: a reception slot R, an idle slot I, a transmission slot T and a control slot LM. The mobile station
60
conducts the reception through the reception slot R and receives carriers from a plurality of other base stations through the idle slot I to sense the presence or absence of a free channel the mobile station
60
itself can transmit (carrier sense), while performing the transmission through the transmission slot T and measuring a level for the antenna switching diversity control through the control slot LM. As one of the countermeasures against the fading, the mobile station
60
employs the antenna diversity for switching between usually two receiving antennas to receive a signal from one of the two antennas which exhibits the best receiving condition. That is, during this LM time period, the mobile station
60
measures the RSSI level through each of the two antennas to select one antenna indicating a higher level.
For conducting the carrier sense during the slot I time period, the mobile station
60
switches among the channels by altering the frequency value of a PLL (Phase Locked Loop; referred to as a PLL) frequency synthesizer, thereby measuring the RSSI value at every channel. Meanwhile, when the mobile station
60
makes the measurement of the RSSI value, owing to the influence of the two facts: {circle around (1)}the received level variation caused by the fading and {circle around (2)}the RSSI value sampling error occurring because the level detection is made in a non-synchronous (asynchronous) condition, the measurement result becomes unstable and its accuracy drops. The variation due to the fading stems from the fact that the received level varies momently depending on the situation of the transmission path. Further, the RSSI value sampling error originates from the fact that the amplitude of the reception signal varies at all times. With reference to
FIG. 20
, a description will be given hereinbelow of this error.
FIG. 20
is an illustration of an RSSI detection voltage waveform appearing at the application of a capacitance value of 100 pF. This waveform obtained by giving a capacitance value of 100 pF, shows the case where an “all-1” modulated wave is received, where a peak appears at an interval of symbol clock time (the inverse number of 21 kHz) along a time base and its amplitude varies at all times. Further, if the sampling is made to obtain RSSI value at the sampling timings t
1
, t
2
and t
3
indicated in
FIG. 20
, a variation of the measured level depending upon the sampling timings is large so that the RSSI value detected contains errors and becomes unstable. Accordingly, the operation of the mobile station
60
itself also becomes unstable by conducting the channel switching operation on the basis of the information on the RSSI value in a peripheral zone.
A factor to vary the signal amplitude at all times is that the transmission is made through a Nyquist filter on the transmission side. That is, the base station imposes limitation in band on a transmitting signal through the use of the Nyquist filter to eliminate the interference between codes in a receiver. Thus, at the transmission filter output in a transmitter, filter responses with symbol waveforms, each generated at every symbol, overlap with each other so that the signal amplitude of the Nyquist filter output becomes constant at the symbol point, whereas the amplitude goes high or low at the other points. For this reason, the locus of the modulated signal on a phase plane lies on a unit circle at the symbol point, but shifts from the unit circle at the other points. The shifting from this unit circle will be described hereinbelow with reference to
FIGS. 21A
to
21
C.
FIG. 21A
is an illustration of a disposition of reception signal symbol points on the receive side. In
FIG. 21A
, an amplitude locus connecting four symbol points
61
a,
61
b,
61
c
and
61
d
shifts outwardly or inwardly from a unit circle because of the effect of the interposition of a route Nyquist filter on the transmission side. Further,
FIG. 21B
is an illustration of an RSSI amplitude detection values at points in a phase synchronous condition (phase synchronized condition) In
FIG. 21B
, since the detection timings (corresponding to arrows from the origin) coincide with the aforesaid four symbol points
61
a,
61
b,
61
c
and
61
d,
the detection amplitudes are constant between each detection point of the RSSI value. On the other hand,
FIG. 21C
is an illustration of an RSSI amplitude detection values at points in a phase non-synchronous condition (not phase synchronized condition), where the detection amplitude of the RSSI value varies in accordance with the measurement timing because the detection timings (corresponding to arrows from the origin) shift from the symbol points.
Therefore, so far, a capacitor with
Mitsume Hiroyuki
Takahashi Hiroki
Cong Le Thanh
Fujitsu Limited
Katten Muchin Zavis & Rosenman
Tran Pablo N
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