Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2001-01-03
2004-11-02
Tran, Khai (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S130000
Reexamination Certificate
active
06813309
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a reception method and reception circuit in a wireless communication system employing CDMA (Code Division Multiple Access), and in particular, to a reception method and reception circuit that employ a RAKE combining receiver.
BACKGROUND ART
CDMA is one multiple access technique in wireless communication systems. The application of CDMA in a mobile communication system made up by a base station and a plurality of mobile communication terminals has the advantages of both allowing an increase in the number of terminals that can be accommodated by the system and a reduction of the transmission power.
Spread-spectrum modulation is generally used in communication systems that employ CDMA. In communication by spread-spectrum modulation, the transmission signals are subjected to spreading modulation using a PN (Pseudo-random Number) code as a spreading code on the transmitting side to spread the spectrum of transmission signals, and these signals are then sent to the receiving side. On the receiving side, the demodulated transmission signals are obtained by establishing synchronization and then subjecting the received signals that have a spread spectrum to despreading using the same PN code as was used on the transmission side. This PN code that is the same as was used on the transmission side is referred to as the “spreading code replica.”
Although direct sequence is used as the method of spread-spectrum modulation in the following explanation, the same discussion holds true if a frequency-hopping spread modulation or the like is employed.
Radio waves that arrive at a receiving station from a transmitting station include, in addition to direct waves, i.e., the component that is propagated directly from the transmitting station to the receiving station, components that are reflected by, for example, mountains, the ground surface, and buildings, and reach the receiver by different propagation paths. The number of radiowave components that pass over different paths are equal in number to the different propagation paths, and the time for each radiowave component to arrive at the receiving station differs according to the length of path taken to arrive at the receiving station. These radiowave components that pass over different paths are referred to as the multipath components. Collecting and combining these different radiowaves having different arrival times while giving each a delay time according to the time to arrive would allow addition of these signals to the received signal of the direct wave, thereby enabling a larger received signal than for the direct wave alone, an improvement in S/N (signal-to-noise ratio), and with this improvement in S/N ratio, a reduction in transmission power.
The direction of arrival of a radiowave component as seen from the receiver differs for each propagation path, and the technique of increasing reception sensitivity by receiving and combining radiowaves that arrive from many directions is called RAKE combining because a visual representation would be similar to a rake.
FIG. 1
is a block diagram showing the configuration of a conventional receiver used in a CDMA communication system that employs RAKE combining, i.e., a RAKE receiver. Although here referred to as a receiver, this is actually a mobile terminal that communicates with a base station in a CDMA mobile communication system.
This receiver is provided with: antenna
222
, transmission/reception filter
221
connected to antenna
222
for separating transmission signals and received signals; reception radio-frequency unit
201
connected to the receiving-side port of transmission/reception filter
221
for amplifying and frequency-converting radio-frequency received signals received at antenna
222
and converting it to received signals on the baseband; a plurality of baseband reception units
202
that receive baseband received signals in parallel from reception radio-frequency unit
201
and perform despreading of received signals using a prescribed PN code; power combiner
204
that combines signals after despreading that are outputted from each of baseband reception units
202
; reception speech processor
206
for decoding the combined signal to a speech signal; ear receiver
207
for outputting speech after processing; and arrived radiowave search circuit
203
that performs a search of arrived radiowaves of each propagation component for carrying out RAKE combination and that reports the timing of despreading for each of baseband reception units
202
. The example shown in the figure is provided with six baseband reception units
202
. In this case, reception radio-frequency unit
201
, baseband reception units
202
, arrived radiowave search circuit
203
, power combiner
204
, reception speech processor
206
, and ear receiver
207
constitute a reception unit. This receiver is further provided with a transmission unit that is made up of: microphone
210
that converts input speech to an electronic signal (speech signal), transmission speech processor
211
that encodes the speech signal that is outputted from microphone
210
, baseband transmission processor
213
that spread-modulates the encoded signal by a prescribed PN code and converts it into baseband transmission signals; and transmission radio-frequency unit
214
that converts baseband transmission signals to radio-frequency transmission signals. The output of transmission radio-frequency unit
214
, i.e., radio-frequency transmission signals, are applied to the transmission-side port of transmission/reception filter
221
.
Since the arrival times of radiowaves differ for differing propagation paths, the receiver shown in
FIG. 1
not only has a plurality of baseband reception units
202
for despreading the baseband signals, but uses arrived radiowave search circuit
203
to find the arrival time of each component having a different propagation path. Arrived radiowave search circuit
203
then reports the arrival time (delay time) of the radiowave component of each propagation path to a respective baseband reception unit
202
, and each baseband reception unit
202
carries out despreading of the received radiowaves while shifting the timing of the PN code in accordance with the reported arrival time. Since the timing of the PN code is shifted according to the arrival times, the signals that are outputted from each of baseband reception units
202
after despreading have matched phase, and combination of the power of these signals at power combiner
204
enables a larger received signal. In a RAKE receiver, the component that carries out despreading for each arrival time is called a “finger.” The above-described RAKE receiver has six baseband reception units
202
, and thus has six fingers.
The chip rate of the PN code that is used for spread modulation on the transmission side and despreading on the receiving side may be, for example, 4 MHz, in which case the time per chip is 0.25 &mgr;s. In contrast, the difference in arrival times of the multipath components may reach several tens of microseconds.
FIG. 2
is a graph showing the relation between the delay times (the amount of shift in chip phase) and the received power after despreading if the chip phase of the PN code used in despreading is shifted by degrees, for example, by ¼ chip, in an environment in which the radiowaves on different propagation paths are received at the same time. This graph is called a delay profile.
Since radiowaves that arrive from a transmission station by different propagation paths will have different arrival times, peaks will occur in the received power at different delay times. The example shown in the figure has three peaks, peak #
1
, peak #
2
, and peak #
3
, and the spacing of these three peaks corresponds to the difference in arrival times. Arrived radiowave search circuit
203
searches for the positions of these peaks and assigns one peak to each of baseband reception units
202
. The search for the positions of peaks in this case is equivalent to finding differenc
NEC Corporation
Tran Khai
Young & Thompson
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