Multipath propagation delay determining means using...

Details Multipath propagation delay determining means using... Multipath propagation delay determining means using...

1999-05-26

2004-05-04

Trost, William (Department: 2683)

Multiplex communications

Communication over free space

Combining or distributing information via code word channels...

C455S063100, C455S065000, C455S067130, C455S501000, C370S350000

Reexamination Certificate

active

06731622

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a multipath propagation delay determining means, in particular for a CDMA base station, in which pilot symbols contained periodically in the signal radio frames are used for an efficient power delay profile calculation and an improved path selection, tracking and sector selection.
In particular, the invention relates to performing the afore-mentioned functions in connection with a so-called RAKE receiver.
BACKGROUND OF THE INVENTION
Code-division multiple access (CDMA) based on direct-sequence (DS) spread-spectrum (SS) techniques is a prospective candidate for the third generation of wideband cellular mobile telecommunication systems (e.g. in UMTS, as IMT-2000 described in reference [1]: J. E. Padgett et al: “Overview of Wireless Personal Communications”, IEEE Communications Magazine, January 1995, pages 28-41).
As shown in
FIG. 1
an area, where several mobiles stations MS
1
, MS
2
. . . MS are served by a (fixed) base station BS, can be regarded as a cell of the CDMA communication system. It has already been demonstrated that the DS-SS CDMA technique is capable of transmitting data signals of high transmission speed, for example within RACE CODIT project (reference [2]: A. Baier et al: “Design Study for the CDMA-based Third Generation Mobile Radio System”, IEEE Journal on Selected Areas and Communications Vol. 12, May 1994, pages 733-743). The potential advantages of the DS-SS CDMA technique have also been tested in the Ericsson Wideband-Testbed (WBTB) project. DS-SS-CDMA has already been used in commercial systems like systems based on IS'95 (D. P. Whipple: “The CDMA Standard”, Applied Microwave & Wireless, December 1994, pages 24-37). Also in Japan a great importance has been attributed to the DS-SS-CDMA system.
Whilst some basic properties of the CDMA receiver and the CDMA telecommunication system are implicit due to the CDMA method, special realizations of the despreaders, the searchers and path selection units have not been investigated in a great detail up to now, since a standard for the W-CDMA has so far not been established. Therefore, the present invention relates to special realizations of the individual units necessary in the CDMA-receiver. Since the inventive CDMA base station, the CDMA reception method and the CDMA system are intrinsically based on the DS-SS CDMA technique, hereinafter the basic technique of DS-SS CDMA transmission will be considered (see also the basic reference [4]: A. J. Viterbi: “CDMA: Principles of Spread Spectrum Communication, Reading, Mass.: Adison-Wesley, 1995”).
DE 19506117 C1 describes a method for estimating the impulse response of a transmission channel, over which CDMA-method-coded information is transmitted. The information is spread on the transmitter side with a spreading code and is despreaded on the receiver side with a corresponding corelation code. The temporal changes of the propagation paths are taken into account on the receiving side.
DE 19615257 A1 describes a CDMA-RAKE-receiver including a sub-chip-resolution. This receiver is adapted for use in a DS-CDMA-communication system. It includes a channel estimation means which can resolve multipath-components, which are closer than a single chip-interval.
Basic CDMA-Technique
Basically, in the CDMA technique, an input signal I having a limited bandwidth (transmission speed) is spread with a predetermined spreading sequence (PN sequence) of a much higher bandwidth and thus an output signal O is produced having a much higher bandwidth than the input signal I as is shown in
FIG. 2
a
. Since all signals considered in the CDMA technique are digital signals, the expression “bandwidth” really means the chip rate.
As is shown in
FIG. 2
b
, two bits of a digital signal constitute one symbol in a CDMA method using a QPSK modulation. Each bit of the symbol will be spread with a PN sequence, and the spread signal (the bottom curve in
FIG. 2
b
) consists of a plurality of “chips”, whereby a chip is defined as a 0→1 and 1→0 (or 1→0 and →1) portion of the despread signal.
As indicated in
FIG. 2
a
, a so-called spreading gain M equal to the ratio of the chip rate to the symbol rate is defined. M basically describes the spreading factor, i.e. how much wider the bandwidth has become due to the spreading with the PN sequence. Of course, since all signals are digital also the PN sequence is a signal which is digital (consisting of a number of bits).
If the original signal I has to be recovered in the CDMA receiver, of course a despreading process has to be carried out in a despreader DSP as shown in
FIG. 2
a
, wherein the original information is obtained by multiplying the spread signal (sequence O) with the original PN sequence that was used for the spreading process.
However, as shown in
FIG. 3
, all information in the CDMA channels are transmitted clockwise, i.e. in terms of successive radio frames RFn. This means, that the spreading and despreading must be performed also framewise. In the transmitter, each frame is spread with the spreading sequence (PN sequence) starting with the beginning of the frame and of course this means that also in the receiver there must be a time synchronized (i.e. time-aligned) despreading, i.e. the despreading sequence must be aligned to the beginning of the received frame. The PN sequence is of course a sequence which is known to the transmitter and receiver, but the time-alignment for the block-wise (M) integration (despreading) must be performed in the receiver.
A principle overview of a base station receiver is shown in FIG.
4
. As is seen in
FIG. 4
, the demodulator DEMOD receives inputs from the PN generator PN-GEN (generating the PN despreading sequence) and from a timing control unit TCU.
In principle, signals from various antennas Ant
0
, Ant
1
from various sectors
1
. . .
6
are input to an automatic gain control circuit AGC and the samples are input to a so-called searcher S (the function of which will be explained below) which calculates the (power) delay profiles. The demodulator DEMOD (comprising a so-called RAKE receiver to be explained below in more detail) outputs the demodulated and despread bit sequence to the decoder DEC. As will be seen below, the searcher S actually comprises a searching and tracking unit provided for input signals from all sectors (parts of a cell as shown in
FIGS. 1
,
12
). The output from the searcher S are the delay values and the (sector) selection information.
The reason why the searcher S also comprises a tracking unit results from the problem of multipath propagation which is an intrinsic property of any mobile communication system. Therefore, hereinafter the multipath propagation in connection with the tracking features of the CDMA system are explained.
CDMA Multipath Propagation
As shown in
FIG. 5
, between a mobile station MS and a base station BS there is not only the direct path P
1
but also indirect paths P
2
, P
3
, for example due to reflections at buildings H, cars C or mountains M. This mixture of direct and indirect paths (i.e. multipath propagation) means that the received signal energy (i.e. the power per sample of the transmitted sequence) does not have a constant time delay (corresponding to the velocity of light). This means, that a sample (bit) transmitted at t
0
arrives at the base station BS at the time t
1
and another portion of the energy arrives at the base station BS at time t
2
due to a further propagation of the energy along an indirect path P
2
or P
3
. This leads to the delay profile per sample as is illustrated in FIG.
5
. That is, each sample is spread over the delay profile, often characterized by (fading) single paths. Thus, in
FIG. 5
the time differences t
1
−t
0
, t
2
−t
0
etc. are defined as delays d
1
, d
2
etc.
In conventional DS-SS-CDMA techniques the problem of multipath propagation is usually handled by the so-called RAKE receiver as is described in the afore mentioned references [2] and [3]. The basis of th

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