1 2
DIGITAL RADIO COMMUNICATION end instruction, the base station judges the quality of the
APPARATUS WITH A RSSI INFORMATION radio channel now being used, on the basis of the measure
MEASURING FUNCTION ment data. If the quality of the radio channel currently being
used has deteriorated below a specific level, the base station
BACKGROUND OF THE INVENTION 5 will instruct the mobile station to hand off the channel to
• another empty channel. In response to the hand-off
1. Field of the Invention instruction, the mobile station hands off the channel to a new This invention relates to a radio communication apparatus radi0 channel and thereafter communicates with the base
used in a cellular mobile radio communication system, such station through the new radio channel. Therefore, for
as a mobile telephone system, a hand held phone system, or 10 example, even if the mobile station has moved to another
a cordless telephone system, and more particularly to a cell during communication, it can continue communication,
digital radio communication apparatus having the function when MAHQ is formed; however; conventional radio
of measuring the reception signal strength indicator (RSSI) communication apparatuses have caused the following prob
information of radio channels. lem ^ a radio communication apparatus used in a digitai
2. Description of the Related Art 15 mobile radio communication system, an AGC circuit is In recent years, a digital scheme has been replacing an provided in the reception system. The AGC circuit functions
analog scheme and getting dominant in the field of cellular so that the received signal may not be saturated even when
mobile communication systems. a high-level modulated signal has been received, and
The digital scheme is such that in the transmission thereby enables the amplitude information component of the
apparatus, the audio signal and data are coded and the carrier 20 QPSK modulated signal to be demodulated accurately. For
is modulated digitally by, for example, a QPSK (quadrature this reasons the time constant of AGC is set at a value (e.g.,
phase shift keying) scheme using the coded signal, and then 400 msec) large enough to prevent the amplitude informa
the modulated carrier sent from the transmission apparatus tion component of the QPSK modulated signal from being
is received by the reception apparatus, in which the received followed up.
signal is demodulated digitally, and thereafter the demodu- 25 In a radio communication apparatus using the TDMA
lated signal is decoded to reproduce the audio signal and scheme, the AGC circuit is brought into an operating state
data. only in the reception slot periods SRI and SR4 of its own
Furthermore, many digital cellular mobile radio commu- station and is out of operation in the other periods as shown
nication systems have used a time-division multiple access in FIG. 7. Since the AGC circuit has a small time constant
(TDMA) scheme as a radio channel accessing method. The as indicated above, it holds AGC value at the end of the
TDMA scheme is such that a plurality of stations transmit reception slot and it starts operating with this AGC value
signals using the same carrier in such a manner that the when the next reception slot assigned to the station is
signals may not overlap each other in time. FIG. 7 shows an supplied to the station. During any other channel period, the
example of a frame format in the TDMA scheme. In each of AGC circuit remains inoperative. This is because the AGC
an up carrier and a down carrier, a single frame consists of 1°°P cannot operate as fast as the AGC circuit during this
six time slots. When a mobile station gets into communica- period since the RSSI-measuring time is much shorter
tion with a base station, a pair of empty time slots is selected (about 2 msec) than the AGC time constant. Because of this,
from the six pairs of time slots in the half rate transmission the conditions for the measurement of RSSI in the reception
mode, whereas two pairs of empty time slots are selected slot periods SRI, SR4 differ from those for the measurement
from the six pairs of time slots in the full rate transmission of RSSI in the idle period I, depending on the on/off
mode. Then, the selected slot pairs are assigned to the operation of the AGC circuit, with the result that RSSI
mobile station as radio channels. FIG. 7 shows an example cannot be measured accurately.
SUMMARY OF THE INVENTION
, 45
assigned slots. The object of the present invention is to make it possible
This type of system provides what is called MAHO to always measure the quality of radio channels accurately
(mobile assisted hand-off), which is such that when the by eliminating the effect of changes in the reception condi
quality of the radio channel has deteriorated during tions.
communication, the radio channel is changed to another 50 To achieve the object, a radio communication apparatus of
radio channel. FIG. 8 illustrates the operation sequence of the present invention is provided with first and second
MAHO. correction tables for correcting the measurement values of
The mobile station always measures the RSSI information RSSI information by reception condition. Then, the appa
of the radio channel now in use and the other radio channels ratus measures not only the RSSI information of the radio
and the bit error rate (BER). The measuring of the RSSI and 55 channel in a reception period of the radio channel assigned
BER of the radio channel now in use is done in a reception to itself by a TDMA scheme but also the RSSI information
slot period SRI. The measuring of the RSSI and BER of the of the other radio channels in an idle period, and then
other radio channels is effected in an idle period I excluding corrects the RSSI information measurement values on the
reception slot period SRI and transmission slot period ST1. basis of the first and second correction tables.
It is assumed that in this state, the base station has sent a 60 Therefore, with the present invention, by previously pre
measurement instruction to the mobile station now in com- paring the first and second correction tables so as to corre
munication. Then, the mobile station returns a response to spond to, for example, the reception conditions that change,
the instruction and then reports the latest measurement data depending on whether the AGC circuit is on or off, both of
on the RSSI and BER obtained at this time to the base the measurement values of RSSI information obtained when
station. Receiving the report of the measurement data, the 65 the AGC circuit was on and the measurement values of RSSI
base station instructs the mobile station to end the measure- information obtained when the AGC circuit was off are
ment. After the mobile station has returned a response to the corrected on the basis of the most suitable correction tables.
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