Spread spectrum receiver

Details Spread spectrum receiver Spread spectrum receiver

1999-07-29

2002-08-20

Corrielus, Jean (Department: 2631)

Pulse or digital communications

Spread spectrum

Direct sequence

Reexamination Certificate

active

06438157

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a spread spectrum receiver having a synchronous tracking section for synchronously tracking a spread signal by correlating a phase-lead spread signal and a phase-lag spread signal by use of time division to thereby produce an error signal, and more particularly, to a spread spectrum receiver which enables reliable and highly accurate synchronous tracking and synchronous holding by simplified control of timing of a synchronous tracking loop and which affords high flexibility in terms of system design.
Rapid leads have been made in mobile communications technology, typified by a portable cellular phone compliant with a spread spectrum communication (hereinafter often abbreviated as “SS communication”) system or a code division multiple access (i.e., CDMA) communications system. In such a communications system, synchronization used for extracting and holding a required timing signal from a received signal is indispensable for correctly reproducing received data through demodulation and despreading of a spectrum spread (SS) signal.
Synchronization of a carrier wave and data is required for receiving the spectrum spread signal, as in the case with receipt of another digital signal. Synchronization of a spread signal is critical, thus imposing a unique problem. Since a spread spectrum signal whose energy is spread over a wide frequency range is used in the spread spectrum communications system, a signal-to-noise (S/N) ratio is considerably low. Particularly, in the CDMA communications system, another channel signal is transmitted at the same frequency as is a desired signal, and hence a signal-to-interference (S/I) ratio is considerably low. For this reason, the S/N ratio or the S/I characteristic must be improved by correctly despreading a received spread spectrum signal. As a result of correct despreading of the received spread spectrum signal, a conventional technique used for demodulating an ordinary signal can be applied to synchronization of a carrier wave and data.
For these reasons, in the spread spectrum communications system, synchronization is carried out in two steps by use of two independent circuit means. One step is a “synchronization acquisition” step in which timing information is narrowed down to predetermined uncertainty, e.g., an extent substantially equal to the time duration of a chip, by observation of a received signal, and the timing at which the receiver activates a spread system is matched to the timing of the received signal. Another step is “synchronization tracking” for more accurately holding the timing that is obtained through synchronization acquisition so as to prevent the spread system of the receiver from causing a timing lag.
To correctly carry out synchronization tracking, a timing error of the received signal must be restrained within a certain extent through synchronization acquisition. Since synchronization acquisition requires high-speed characteristics, an open loop is primarily employed. In contrast, since synchronization tracking imposes special emphasis on accuracy, observation must be performed for a comparatively long period of time in order to reduce the influence of noise. For this reason, a closed loop is primarily used.
FIG. 4
is a circuit configuration of a synchronization tracking section in a RAKE receiver, which is one example of a conventional spread spectrum receiver. A &tgr;-dither loop is used as the synchronization tracking section of the RAKE receiver, and this &tgr;-dither loop constitutes a closed loop employing one set of correlators (despreading sections). Taking note that an autocorrelation function assumes a triangular shape in the vicinity of a correct point of synchronization, the &tgr;-dither loop causes the phase of a spread signal (or a pseudo noise system code) produced by the receiver to slightly lead the point of synchronization and slightly lag the point of synchronization, extracts variations in a correlation value caused by the phase lead and phase lag of the spread signal relative to the point of synchronization (i.e., variations in the amplitude of the signal that has been despread), and controls the phase of oscillation of the entire synchronization tracking section so as to reduce the variation to zero.
In
FIG. 4
, the synchronization tracking section of the conventional spread spectrum receiver comprises a sampling section
101
, a despread section
102
, an integration section
103
, a lead/lag phase level difference output section
409
, a pseudo noise (PN) code generator
111
, and a timing control section
412
.
The sampling section
101
samples a received signal RxI[i-
1
:
0
], which is received byway of an unillustrated antenna and is in phase with a received carrier wave, as well as a received quadrature signal ExQ[i-
1
:
0
], which differs in phase from the carrier wave by 90°, at the timing of an ELCLK signal output from the timing control section
412
. Each of the received signals RxI and RxQ has a duration of i-bits. To clarify the bit configuration of a signal, the received signals are represented as RxI[i-
1
:
0
] and RxQ[i-
1
:
0
].
Signals ELI[i-
1
:
0
] and ELQ[i-
1
:
0
] resulting from sampling of the received signals RxI[i-
1
:
0
] and RxQ[i-
1
:
0
] by the sampling section
101
are despread by the despread section
102
through correlation of the PN system codes PnI and PnQ that are output from the pseudo noise code generation section
111
and serve as spread signals. The despread section
102
outputs to the integration section
103
correlation data PELI[j-
1
:
0
] and PELQ[j-
1
:
0
], each correlation data set comprising j-bits. According to a signal PNc which is output from the timing control section
412
and represents whether the spread signal is in a lead phase or a lag phase, the PN code generation section
111
supplies the PN system codes PnI and PnQ, each of which is in a lead phase or a lag phase, to the despread section
102
by use of time division.
On the basis of a signal EL
1
which is supplied from the timing control section
412
and indicates whether the lead-phase or lag-phase correlation data is computed, as well as on the basis of an Integ signal representing an integration number, the integration section
103
integrates the correlation data PELI[j-
1
:
0
] and PELQ[j-
1
:
0
] thereby outputting integration data PELICc[k-
1
:
0
] and PELQCc[k-
1
:
0
], each integration data set comprising k-bits, to the delay circuit
108
which delays the lead/lag phase level difference output section
409
and the integration data by one symbol. More specifically, the delay circuit
108
outputs delayed integration data PELId[k-
1
:
0
] and PELQd[k-
1
:
0
] each comprising k-bits, that are formed by delaying the integration data PELICc[k-
1
:
0
] and PELQCc[k-
1
:
0
] by one symbol.
The lead/lag level difference output section
409
outputs a level difference (i.e., an error signal) between the lead phase and the lag phase to the timing control section
412
, on the basis of the integration data PELICc[k-
1
:
0
] and PELQCc[k-
1
:
0
], the delayed integration data PELId[k-
1
:
0
] and PELQd[k-
1
:
0
), and the signal EL
2
representing whether the current symbol corresponds to lead phase or lag phase.
The timing control section
412
controls the phase of the entire synchronization tracking section by controlling sampling timing (i.e., the timing of the ELCLK signal) and the timing at which each of the signals PNc, EL
1
, and EL
2
is switched between lead phase and lag phase, so as to reduce the level difference TAC to zero.
However, in the synchronization tracking section of the foregoing conventional spread spectrum receiver, the signal EL
1
—which represents whether or not the integration section
103
integrates a lead-phase or lag-phase correction value—and the signal EL
2
—which represen

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