Pulse or digital communications – Spread spectrum – Frequency hopping
Reexamination Certificate
1999-10-08
2004-08-17
Tse, Young T. (Department: 2634)
Pulse or digital communications
Spread spectrum
Frequency hopping
C375S354000
Reexamination Certificate
active
06778589
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a symbol synchronous device and a frequency hopping receiver, suitable to demodulate digital-modulated signals, particularly, preferable to use in radio LAN systems.
In the spread spectrum (SS) communication technique, there are various methods including Direct Sequence (DS), Frequency Hopping (FH), DS/FH hybrid, Chirp modulation, and others.
FIG. 7
is a block diagram illustrating a conventional frequency hopping communication system. Referring to
FIG. 7
, the frequency hopping communication system consists of an encoder
41
, a digital modulator
42
, a mixer
43
, a hopping pattern generator
44
, a frequency synthesizer
45
, a high-frequency amplifier
46
, a transmission antenna
47
, a receiving antenna
48
, a high-frequency amplifier
49
, a mixer
50
, a hopping pattern generator
51
, a frequency synthesizer
52
, a digital demodulator
53
, and a demodulator
54
.
On the transmission side, the encoder
41
converts transmission data into error detectable correctable transmission data. The digital modulator
42
digital-modulates the transmission data over an intermediate frequency band. Then, the mixer
43
frequency-converts the digital-modulated signal based on the output signal of the frequency synthesizer
45
. The frequency synthesizer
45
varies over time the frequency to be frequency-converted according to the hopping pattern produced by the hopping pattern generator
44
, thus switching the transmission frequency channel. Thus, the digital-modulated signal is transmitted with the frequency channel according to the hopping pattern. As a result, the digital-modulated signal becomes the spread spectrum signal with a broad frequency band. The spread spectrum signal is amplified by the high-frequency amplifier
46
and then radiated from the transmission antenna
47
.
On the receiving side, the spread spectrum signal is received with the receiving antenna
48
and then is amplified by the high-frequency amplifier
49
. The mixer
50
inverse-spreads the amplified spread spectrum signal. The hopping pattern generator
51
generates the same hopping pattern in synchronous with the hopping pattern generator
44
on the transmission side. The frequency synthesizer
52
outputs the reference oscillation signal with the same frequency as that of the frequency synthesizer
45
on the transmission side. The frequency synthesizer
52
selectively receives the signal with the same frequency as the transmitted signal and then inverse spreads the transmitted spread spectrum signal to convert into a signal in an intermediate frequency band.
Bandpass fitlers (not shown) pass the reversed spread signal and then transmit the signal components thereof over the received frequency band in each frequency channel to the digital demodulator
53
. The digital demodulator
53
performs digital demodulation corresponding to the digital demodulator on the transmission side to obtain demodulated data. The demodulator
54
subjects the demodulated data to the error and correction corresponding to the encoder
41
on the transmission side and then outputs received data.
The digital demodulator
53
multiplies the digital-modulated signal by a regenerated carrier to convert it into the baseband signal. Then, demodulated data is extracted by performing the level decision with the timing of the clock signal in synchronous with the symbol (bit) of the digital-modulated signal.
As the digital modulation method in the current mainstream are listed Frequency Shift Keying (FSK), Phase Shift keying (PSK) such as Quadrature Phase Shift keying (QPSK) and Quadrature Amplitude Modulation (QAM). In the frequency hopping communication system, the FSK digital modulation has been mainly used because of easiness of designing. However, where QPSK, QAM, or the like, performing the so-called IQ modulation, is adopted in the frequency hot communication system, the conventional carrier regenerative method and the symbol synchronous method cannot be applied without any change.
In the frequency hopping communication system, the procedure including symbol synchronization (bit synchronization), frame synchronization and data reception must be carried out every time frequency channel is changed. However, this system has a slow response characteristic to phase changes because the frequency of a carrier jitters in the initial state after a change in frequency and the reference frequency oscillator within the carrier generation circuit is based on the analog PLL system. In the digital modulation such as QPSK and QAM where the phase with respect to a carrier is modulated according to transmission data, it is difficult to regenerate the clock signal symbol-synchronized. Moreover, the symbol synchronization requires a long time. It is required to reduce the time for symbol synchronization as short as possible, in view of an improved throughput of transmission data.
The above-mentioned problem will be specifically described below by referring to the QPSK demodulation circuit in the frequency hopping system.
FIG. 8
is an IQ phase plane coordinate diagram illustrating signal points in 4-phase modulation (QPSK). Referring to
FIG. 8
, the X-axis represents I-phase in phase with a carrier and the Y-axis represents Q-phase perpendicular to the carrier. Before data is transmitted, the synchronous signals, the signal point in 0° phase and the signal point in 270° phase, for instance, are alternately repeated, so that the continuous synchronous signal where the phase changes ±90° is transmitted. Thus, the carrier regeneration as well as the clock regeneration synchronized with symbols are carried out according to the synchronous signal.
FIG. 9
is a block diagram illustrating the digital demodulator of FIG.
8
. Referring to
FIG. 9
, the digital demodulator consists of a carrier generation circuit
61
, a 90° phase shifter
2
, demodulation multipliers
3
and
4
, low-pass filters
5
and
6
, a comparator
62
, a ½ symbol-length delay circuit
63
, an exclusive OR logic
64
, decision units
65
and
66
and a decoder
67
.
FIG. 10
is a diagram illustrating waveforms at various portions in the digital demodulator of FIG.
9
.
FIG. 10
schematically illustrates waveforms at synchronous signal reception.
Generally, carrier regeneration is carried out for the QWPSK demodulation. However, it is impossible to perfectly match the frequency of a carrier of a received digital-modulated signal with the frequency of the reference frequency oscillator on the receiving side. For that reason, the carrier regeneration is carried out based on the received signal to create a copy (replica) of a carrier. The demodulation is carried out using the replica as a reference frequency signal. First, the synchronous signal shown in
FIG. 8
is received as a received signal. The carrier regeneration circuit
61
detects the carrier frequency and phase of the received signal based on the synchronous signal so that the replica of the carrier is created.
The carrier regeneration is realized by various methods. Basically, in order to achieve the frequency synchronization, a frequency-multiplied received signal is phase-compared with the output signal from the phase locked-loop (PLL). In the case of the BPSK, the frequency of the received signal is multiplied by 2. In the case of the QPSK, the frequency of the received signal is multiplied by 4.
Each of the demodulation multipliers 3 and 4 receives the created carrier replica and the oscillation signal obtained by phase-shifting the replica by 90° with the phase shifter
2
so that the balanced demodulation can be carried out. The low-pass filter
5
extracts as a baseband component the I-phase baseband signal I, shown in FIG.
10
(
a
), from the demodulation signal. The low-pass filter
6
extracts as a demodulation signal component the Q-phase baseband signal Q, shown in FIG.
10
(
b
), from the demodulation signal. The decision unit
65
decides the level of the baseband signal I with the timing of the clock signal shown in FIG
Ahn Sam K.
Futaba Denshi Kogyo Kabushiki Kaisha
Tse Young T.
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