Pulse or digital communications – Transceivers
Reexamination Certificate
1998-03-31
2001-10-23
Vo, Don N. (Department: 2631)
Pulse or digital communications
Transceivers
C375S224000, C375S344000, C375S345000, C375S350000
Reexamination Certificate
active
06307879
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention generally relates to digital radio communication apparatuses and, more particularly, to a digital mobile radio communication apparatus provided with an analog filter and a digital filter.
Recently, with the depletion of radio wave resources foreseen, the communication standards stipulate increasingly tight restriction of the use of a channel band width. Conventionally, such a restriction has been met by improving hardware elements and circuit technology. More specifically, the performance of an analog filter is improved for that purpose. As the requirements stipulated by the communication standards become more strict, it is demanded that a software approach be introduced to implement a digital filter or to complement the performance of an analog filter.
FIGS. 1-5
illustrate the technology used in a digital radio communication apparatus according to the related art.
FIG. 1A
shows a model of a digital radio transmission system. Referring to
FIG. 1A
, T
b
(&ohgr;) indicates a low-pass filter characteristic of a transmission unit, T
r
(&ohgr;) indicates a band-pass filter characteristic, F
r
(&ohgr;) indicates a transfer characteristic of a transmission path (air), R
r
(&ohgr;) indicates a band-pass filter characteristic of a receiver, and R
b
(&ohgr;) indicates a low-pass filter characteristic of the receiver. An overall transfer characteristic H(&ohgr;) is given by
H(&ohgr;)=T
b
(&ohgr;)T
rb
(&ohgr;)F
rb
(&ohgr;)R
rb
(&ohgr;)R
b
(&ohgr;),
where T
rb
(&ohgr;) indicates an equivalent low-pass filter characteristic of T
r
(&ohgr;), F
rb
(&ohgr;) indicates an equivalent low-pass filter characteristic of F
r
(&ohgr;) and R
rb
(&ohgr;) indicates an equivalent low-pass filter characteristic of R
r
(&ohgr;)
When such a transmission system is to transmit a pulse signal G(&ohgr;) from a signal source, an input waveform for a discrimination circuit is given by
r(t)={fraction (1/2&pgr;)}∫
−∞
∞
G(&ohgr;)H(&ohgr;)e
j&ohgr;t
d&ohgr; (1)
FIG. 1B
shows an eye pattern of an input waveform for the discrimination circuit. Assuming that the signal source transmits a &pgr;/4-shifted PSK modulated signal, there is no intersymbol interference occurring in the input waveform of the discrimination circuit if H(&ohgr;) satisfies the Nyquist condition. The eye aperture is open ((a) of FIG.
1
B). However, if the Nyquist condition fails to be satisfied due to a variation of the performance of filter elements that has occurred in the process of fabrication, or due to a variation in the operating conditions (temperature, power-supply voltage, etc.), intersymbol interference occurs so that the eye aperture begins to close ((b) of FIG.
1
B).
FIG. 1C
shows a constellation (arrangement of codes) that illustrates the above-described relation. Generally, code points on the transmitting side ((a) of
FIG. 1C
) vary (are displaced) in the air as shown in (b) of
FIG. 1C
before arriving at the receiving side. If the combination of filters on the receiving side satisfies the Nyquist condition, the variation in the air settles to a state as shown in (c) of
FIG. 1C
at a discrimination point. That is, the intersymbol distance H at the discrimination point is relatively large. However, if there is a deviation in the filter characteristic on the reception side, intersymbol interference occurs so that it is impossible to properly restore the code points ((d) of FIG.
1
C). That is, the intersymbol distance H at the discrimination point is relatively small.
FIG. 2
shows a relation between a cosine roll-off factor a and the constellation in the air.
FIG. 2A
shows the relation that occurs when &agr;=0.8;
FIG. 2B
shows the relation that occurs when &agr;=0.5; and
FIG. 2C
shows the relation that occurs when &agr;=0.2. The smaller the factor &agr;, the smaller the occupied bandwidth so that the more preferable it is in terms of efficient use of the bandwidth. Accordingly, &agr; tends to be controlled to maintain it at low level in current digital communication systems. However, the constellation in the air deviates from that of the point of origination as the level of &agr; is lowered, requiring precise control of the receiver filter in order to restore the constellation.
Conventionally, in order to construct a receiver with a strict requirement for selectivity between adjacent channels, a high-performance analog filter formed of crystal or ceramic is used.
FIG. 3A
shows a characteristic of attenuation of an analog filter with respect to frequency. Generally, in order to obtain a large attenuation, a plurality of analog filters are connected in multiple stages so as to produce a high performance (large attenuation). Such an approach causes the number of required elements to increase, and increases the size and cost of the resultant apparatus.
FIG. 3B
shows a group delay characteristic of an analog filter with respect to frequency. The delay time of a signal varies with respect to the frequency. Therefore, connecting a plurality of analog filters to form multiple stages in an attempt to obtain a high-attenuation characteristic causes degradation in the group delay characteristic.
Further, a characteristic of analog elements is subject to a variation that occurs in the process of production. The characteristic also varies significantly with time and due to a variation in the operating conditions (temperature, power-supply voltage, etc.). Thus, it is difficult to implement and maintain the precise Nyquist characteristic.
According to one approach, an analog filter designed to eliminate out-of-band noise is used in the first stage, several stages of the receiving system are linearized, and the majority of the filter performance (the Nyquist characteristic, the attenuation characteristic, etc.) is implemented (covered) by the digital filter in a subsequent stage.
FIG. 4
shows a construction of a digital radio communication apparatus (portable terminal) according to the related art. The digital radio communication apparatus comprises an antenna
1
; a transmission/reception branching switch
2
(C); a transmitter
3
, a frequency synthesizer
4
(SYN), a receiver
5
, including an RF amplifier (RFA)
6
, a first mixer (x)
7
, a second mixer
9
(x), analog band-pass filters (BPF)
8
,
10
,
12
formed of crystal or ceramic, IF amplifiers (IFA)
11
,
13
, a quadrature detecting unit (QDT)
14
using the QPSK system, an A/D converter (A/D)
15
, adaptive transversal filters
16
,
17
using a digital system, a discriminating circuit (DSC)
18
, a clock generator (CG)
19
, an automatic frequency controller (AFC)
20
, a voltage controlled oscillator (VCO)
21
, and an automatic gain controller (AGC)
25
.
CG
19
generates (reproduces) a sampling clock signal SK and a data clock signal DK based on the edges of demodulated I/Q signals. AFC
20
detects frequency deflection of the IF signal based on the edges of the demodulated I/Q signals. An output of AFC
20
is input to DSC
18
and used in control of a discriminated phase (phase rotation by &pgr;/4-shifted QPSK and the like). The output of AFC
20
is input to VCO
21
and used to maintain the frequency of the IF signal at a regular level.
Further, the digital radio communication apparatus comprises a TDMA synchronization controller
31
for controlling timings according to the TDMA system; a codec (CODEC)
32
for converting a sound signal into codes; a baseband processor (BBP)
33
of the sound signal; a microphone (MIC)
34
; a speaker (SPK)
35
; a CPU
41
for performing main control (console control and call control including location registration, standby, call origination, call incoming, and handover) of the apparatus; a main memory (MM)
42
embodied by a RAM, a ROM and an EEPROM or the like for storing control programs executed by the CPU
42
and associated data; a console unit (CSL)
43
operated by a user, including a display unit
44
embodied by a liquid crystal or the like for displaying dial numbers an
Fujitsu Limited
Helfgott & Karas P.C.
Vo Don N.
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