Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
2001-07-26
2004-09-28
Trinh, Sonny (Department: 2685)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S132000, C455S137000
Reexamination Certificate
active
06799055
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method and a receiver for receiving a spread spectrum signal in a radio system employing the code division multiple access method.
BACKGROUND OF THE INVENTION
In mobile systems, information is transferred between a mobile network and a mobile station by means of radio resources. The radio resources of a mobile network are defined in different ways depending on the multiple access method of the system. Those using the frequency division multiple access (FDMA) method are distinguished from each other on the basis of the frequency used. In radio networks using the time division multiple access (TDMA) method, several users are able to communicate in the same frequency band, in which users are distinguished from each other in time by dividing the information transmitted or received into timeslots. In radio systems using the code division multiple access (CDMA) system, several transmitting and receiving stations communicate simultaneously in the same frequency band of the radio spectrum. A spreading code for spreading the information in a base band signal is reserved for each user for the duration of a connection. The receiver of the signal, in turn, is able to identify the information transmitted by the user by decoding is using a corresponding despreading code. Compared with other systems using multiple access methods, the advantages of CDMA include efficient utilization of a frequency band and system data security. A disadvantage is that users operating in the same frequency band interfere with each other's transmissions because spreading codes and despreading codes are not orthogonal and transmitters are not mutually synchronized.
In addition to the interference caused by users to each other, for example the shapes of the surrounding terrain interfere with transmissions of information on the radio path. Multipath propagation means that, when propagating, a user signal is reflected from various points causing components from the same signal to arrive at a receiver at various delays. The multipath propagated signal components may cause fading, for example, in a situation when a signal is reflected from two adjacent objects. When the fading is significant, the reception of the signal fails altogether. This problem can be avoided for example by implementing the transmission on a radio channel by means of frequency hopping, whereby the transmission frequency is changed for each burst. In channel coding, the data to be transmitted are coded with a coding algorithm, allowing the authenticity of the coding and, consequently, of the received data to be verified at the receiving end. When transmitted interleaved, the data are divided into several bursts, whereby the loss of one burst is not fatal to the transmission of speech, for example. Furthermore, the receiver can be implemented with more than one antenna to gain antenna diversity, whereby the loss of a signal in one antenna is not fatal to the reception of the signal; instead, another antenna can receive the signal. The implementation of antenna diversity sets high requirements on the apparatus used, wherefore in practice it is preferably implemented in a base station of a mobile network.
In a RAKE type of CDMA receiver, multipath propagation may be utilized such that components that are delayed in various ways are received and combined to achieve optimal user signal identification. In a RAKE receiver, a delay profile, i.e. impulse response, is typically generated for a radio channel. A matched filter (MF), for example, can be used to generate the impulse response. A matched filter is used for example for information received on a pilot channel or in the pilot sequence of a radio burst. Pilot symbols are a group of symbols known to the receiver and transmitter, whereby the receiver of the information, being aware of what the received information should have contained, is able to make an estimate of the quality of the radio channel. A matched filter is shifted over the received information for example one half of a spreading code unit (chip) at a time, and the reception power is measured always when the matched filter synchronized with the received information. This allows an impulse response graph containing information on the signal strengths and delays of the multipath propagated components to be formed for the multipath propagated components of the received radio channel.
Let us assume that the measuring period of the impulse response is divided into N sequences of the length of L chips, the length of the measuring period being NLT
c
, wherein T
c
is the duration of a chip. An impulse response estimate P
i
(a)
(t) is generated for each sequence i=0, . . . , N−1 of length L, and antenna 0, . . . , N
a
−1 in accordance with formula (1):
P
i
(
a
)
(
t
)
=
&LeftBracketingBar;
1
L
∑
j
=
0
L
-
1
γ
(
a
)
(
(
iL
+
j
)
T
c
+
t
)
c
iL
+
j
*
&RightBracketingBar;
α
,
wherein
(
1
)
&ggr;
(a)
(t) is a signal received from antenna a, c
i
* is a spreading code and a is either 1, corresponding to an absolute value or 2, corresponding to squaring. The spreading code may be a real or complex value. The denotation c
i
* refers to the complex conjugate of a complex spreading code. The final power intensity for antenna a as a function of the propagation delay t is given as an incoherent mean over N measuring periods according to formula (2):
P
(
a
)
(
t
)
=
1
N
∑
i
=
0
N
-
1
P
i
(
a
)
(
t
)
.
(
2
)
In a prior art receiver based on antenna diversity, each antenna branch has a matched filter, on the basis of which symbol estimates are separately and independently generated for a user signal irrespective of the other antenna branches. By combining the generated symbol estimates, better estimates for the symbols transmitted in the user signal are obtained than by generating the symbol estimates on the basis of a signal received in one antenna only. Said matched filter can be implemented for example time dividedly, allowing the matched filter's processing capacity to be shared by several users. The processing capacity of a matched filter may also be divided time dividedly between antennas, allowing, in the case of for example two antennas, a user to be received during a first reception timeslot in a first antenna, antennas to be changed and the user received during the next reception time-slot in another antenna. Other users would be received in the following time-slots, until it is again said user's turn, who is first received in antenna one and then in antenna two. The code phases of the RAKE branches are selected using the impulse response P
(a)
(t)
measured for the antenna or the mutual impulse response of antennas directed to the same sector in accordance with formula (3).
P
(
a
)
(
t
)
=
1
N
a
∑
a
=
0
N
a
-
1
P
(
a
)
(
t
)
.
(
3
)
The prior art solution for the reception along several antennas in a receiver involves significant drawbacks. A reception solution where the antennas used for receiving a user signal are changed at given intervals is not well applicable to the reception of a radio channel containing fading. For example in the case of a time-divided matched filter, when antennas are changed for the user at given reception timeslots, the problem is that the reception time for each antenna becomes short. Accordingly, when hitting said short timeslot, a fading gap significantly disturbs the reception of the signal. In this case the averaging time of the impulse response becomes short, emphasizing the harmful effect of a fading gap in the generation of the impulse response. Furthermore, in a matched filter where reception antennas are changed at intervals of reception timeslots, a processing delay is caused during change of antennas when the matched filter synchronizes with the new antenna.
BRIEF DESCRIPTION OF THE INVENTION
The object of the invention is thus to provide an improved method and apparatus for receiving a signal in a radio system. This is achieved by
Nokia Networks Oy
Pillsbury & Winthrop LLP
Trinh Sonny
LandOfFree
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