WO2000064113A1

WO2000064113A1 - Method for channel estimation in a tdma mobile radio telephone system

Info

Publication number: WO2000064113A1
Application number: PCT/DE2000/001178
Authority: WO
Inventors: Stefan Bahrenburg; Paul Walter Baier; Dieter Emmer; Jürgen Mayer; Johannes Schlee; Tobias Weber
Original assignee: Siemens Aktiengesellschaft
Priority date: 1999-04-16
Filing date: 2000-04-14
Publication date: 2000-10-26
Also published as: DE19917334A1

Abstract

The invention relates to a method for channel estimation in TDMA mobile radio telephone systems. The receiving values of several bursts can be used for channel estimation and are used for estimating a single channel impulse response when the period (Tauf) during which the successive data bursts are transmitted is significantly smaller than the coherence time (TK).

Description

description

Channel estimation method in a TDMA mobile radio system

In TDMA-based CDMA mobile radio systems (TDMA = Time Division Multiple Access, CDMA = Code Division Multiple Access) a channel estimate is required to carry out the data estimation. The channel estimation in the TDMA-based systems is based on a training sequence, which is generally referred to as the midamble, the midamble being arranged between two data blocks. The unit consisting of the midamble and the two data blocks is called a burst. He is ^¬ from Bernd Steiner: "A contribution to the mobile channel ^¬ assessment with special reference synchronous CDMA mobile radio systems using joint detection" Progress Reports VDI, Series 10, No. 337, Dusseldorf. VDI-Verlag, 1995, that the length L _m the midamble in TD-CDMA mobile radio systems, due to the algorithm used for channel estimation, directly depends on the number of active subscribers K and the maximum expected length W of the channel impulse response

L _m «W (K +1)

is determined. The length L _{m of} the midamble is therefore a limiting factor for the number of active subscribers that can be present in the uplink per burst. If techniques such as "voice activity" or adaptive data rates are to be used, it may even be necessary to support more participants than are actually actually active per burst, in other words to include them in the channel estimation.

If, furthermore, intelligent antennas, that is to say antennas that act selectively in the direction of the mobile radio subscriber, are used in the base station, subscriber-specific middle messages are also required in the downlink, so that the midambell length becomes a limiting factor here too.

Furthermore, it may be desirable in TD-CDMA mobile radio systems with a frequency reuse factor of r = 1 to include intercell interference signals from neighboring cells in the data selection. To do this, it is necessary to estimate channels to the neighboring stations or the mobile stations of the neighboring cells.

However, this is only possible if there are sufficiently long middle ambulances available. Consequently, there is the unsatisfactory situation that the remaining, for data transfer ^¬ is the less available portion of the burst, the more channels to be estimated. Therefore, on the one hand, many channels should be estimated, but on the other hand, only the smallest possible portion of the burst should be sacrificed for the mid-amble. In principle, this problem exists not only in the above-mentioned channel estimation method of the above publication, but in all of them

Estimation method. In order to be able to achieve a sufficiently precise estimation result, a minimum number of measured values from the received midamble is required.

So far, this conflict has not been resolved satisfactorily. Rather, a compromise between mid-length and the burst portion available for data transmission has previously been found. Usually the mid-bamboo length is dimensioned such that at least as many channels can be estimated as CDMA codes can be detected simultaneously in one cell. The number of CDMA codes that can be detected simultaneously is essentially determined by the spreading factor used and the intercell interference. Therefore, a useful compromise between mid-wavelength and the number of channels that can be estimated is only possible if the spreading factor is not too large, in other words, if relatively few participants have the data transmission capacity of a burst divide. This means that the minimum data rate is too high for many applications and offers little scope for applying the abovementioned capacity-increasing measures such as voice activity, adaptive data rates, antenna diversity and elimination of intercell interference.

The invention is therefore based on the object of providing a method and a device with which an estimate of an increased number of channels is possible without increasing the proportion of the burst for the midamble.

The object is solved by the features of independent claims 1 and 10. Preferred embodiments of the invention are the subject of the dependent claims.

The maximum time interval between two channel impulse responses within which the channel impulse responses can be regarded as correlated is defined as the coherence time T _κ . Furthermore, the coherence time T _{% is} inversely proportional to the speed of a participant.

Now, the channel impulse responses of successive bursts almost the same, that is, the period T _au f in which the successive data bursts are transmitted, is essential lent smaller than the above-defined coherence time TR, as a single channel impulse response to estimate the usable for channel estimation received values multiple bursts used. In particular, participants with low speed meet the condition

Baptism « ^T K-

Short midambles (also called training sequences) can preferably be transmitted within the burst, the channel estimate then determining a current channel impulse response from the received values e (x) of several successive bursts x originating from the midambles. The Received values of so many bursts are taken into account in channel estimation that the quality required for channel estimation is achieved. This procedure is exactly possible if the channel impulse responses within the time period, m which the benotigten reception values e (x), x = 1 ... X are received, are largely identical, is satisfied, in other words the condition T _au f "T . Furthermore, the midamble can be the shorter, ηe the lower the speed of a mobile station, since this makes the coherence time T the greater, and the higher the data rate can be achieved.

The principle of gradually sending the midamble can consequently increase the number of participants. However, if you leave the length of the midamble within the bursts and interpret them as sections of a long midamble, it is possible to accommodate significantly more participants. The problem of the number of subscribers in TD-CDMA mobile radio systems, which is normally hard limited by the channel estimation, can therefore be regarded as solved.

Furthermore, given the validity of the estimate T _au f «T _jζ within a burst, it is not absolutely necessary to use midambles, but the use of preambles is generally possible. Because of their property that they are not disturbed by the interference of previous symbols, preambles can advantageously be chosen to be shorter than midambles, which, for example, increases the data rate. If the preamble and the midamble are of the same length, the use of preambles enables a higher number of participants.

Preferred embodiments of the invention are described below with reference to the drawings.

Fig. 1 shows the division of a midamble m several shorter midambles, and Fig. 2 shows the use of periodic midamble codes.

1 shows the division of a midamble m into a plurality of midamble m (x), where x runs from 1 to X. By transmitting the midamble in sections in successive bursts, the midamble of a burst is shortened, so that the number of channels which can be estimated is increased due to the short midamble.

In the case described here by way of example, the midamble m consists of X = 5 blocks of length W. The received signal, which originates only from the not yet divided midamble m, is calculated for any subscriber k to n = W- \., = ∑ m _{1 + w} - _n ^• k, ^ι = ι K ^{W (i)}

The total received signal originating from the middle tones of the participants results from the superimposition of the signals of the K participants

k = K

(*)

X 1, KW k = 0

Furthermore, the reception sequence ^ is shorter by W elements than the midamble, since reception values with interference from the data blocks cannot be used for channel estimation.

So that the same algorithm can be used for channel estimation after splitting the midamble over several bursts as previously, which is based on the reception sequence ^ of length KW, the signal e ^ must also arise through the transmission of the midamble in sections. In the example in FIG. 1, the signal β _j - _{j is} composed of four sub-signals ey, = [β _j - _j U) ... eι-ι (4)] if the midamble m distributes X = 4 bursts becomes. The received values of e ^ l) result from the transmission sequence of the midamble part m (l) and, since the channel impulse response is of length W, from the W-1 previous samples from block m (0). The received values of en (2) result from the transmission sequence m (2) and, since the channel impulse response has the length W, from the W-1 previous sample values from block m (l). The same applies to the reception values em) and £ _2. ().

After the transmission of the four bursts, as shown in FIG. 1, the identical signal £ ₁ -, = [β _j π (1) ... ₃ , (4)] can be composed of the four reception sequences if the Channel impulse response is constant. The channel estimation can be done inexpensively according to a method described in the article mentioned above.

The estimation of the channel impulse response can be carried out in the steady state after the reception of each burst and thus of each section e _πι (x), for example if a temporally older £, “(1) is replaced by a section of the same name that has just been received. Therefore, the result of the channel estimation is adapted to the slowly changing channel.

2 shows how blocks of the midamble emerge from a periodic midamble basic code described in the above-mentioned publication. If the supported number of subscribers K is a multiple of the number X of bursts to which the midamble is divided, the midamble distribution at the channel estimator manifests itself as a cyclical interchanging of the midambles used by the subscribers. Exactly the same channel estimator can be used for each burst, which uses the midamble reception signals of the last X bursts. The estimated value h of the channel impulse responses is given by the constant, right-circulating matrix G ^~ ^, which depends only on the midamble codes used

m Since only a part of the reception sequence β ^ is newly acquired per burst, it is not absolutely necessary to carry out a complete channel estimation for each burst. Since it is a linear treasure algorithm, instead of the difference between the old and new reception sequence e _{m, one} can directly determine the difference between the old and new estimate of the channel impulse response and update the treasure result h.

In the example of Fig. 2, X = K = 4. The basic periodic ^¬ code PGC m is four components divided, starting in each case with mι_, 5JW ₊ i / 52 _W +1 'and 5 ^ ₃ ₊ 1 In this case, ends of the peri ^¬ odic basic code PGC with the character πf - the basic code PGC is always periodically shifted by one block between the subscribers Tl, T2, T3 and T4, where m the midamble Ml of the subscriber Tl sends the components m_ and mw ₊ 1 , while the subscriber T2 sends the components ∞ ₊ 1 and 52 _W +1 ', the subscriber T3 the components ∞2 +1 ^and 5.3 _{W +} 1 and the subscriber T4 the components m2 _{W + l} ^{and <} ^ ™ 1 ^{ι in} this order . Correspondingly, the 8 components of the periodic basic code PGC which are arranged above it in each case in the middle messages M2 to M4 are sent. The midamble Ml is sent in the first burst B1, while the midambles M2 to M4 m are sent to the corresponding subsequent second to fourth bursts. In FIG. 2, the components of the first midamble Ml sent in the first burst Bl are shown in the partial image at the bottom right.

The invention can be applied to any mobile radio system with a TDMA component, that is to say it is not restricted to the use of m TD-CDMA systems.

Claims

claims

1. Method for channel estimation in a TDMA mobile radio system, in which bursts transmitted between stations of the mobile radio system have training sequences for carrying out the channel estimation,

- and the one necessary for channel estimation

Training sequence m is divided into several partial training sequences πj (x) with x = 1, ..., X, each partial training sequence m (x) being transmitted in a burst of a sequence of X bursts, so that the total Training sequence m is composed of the partial training sequences m (x) of the successive bursts.

2. The method of claim 1, wherein a period T _au f, in which the successive bursts are transmitted, than the coherence time T is substantially smaller.

3. The method according to claim 2, in which the condition T _au f «TJJ applies to participants at low speed.

4. The method according to any one of the preceding claims,

in which the initial values £ π, (x), x = l, ..., X of several bursts that can be used for the channel estimation are used to estimate a single channel impulse response h,

- And in which the partial reception values £ ₂₂ , (x), x = l, ..., X of the received signal θ _j ^ are taken into account in the burst estimation in the channel estimation that the quality required for the channel estimation is achieved.

5. The method according to any one of the preceding claims, in which the estimation of the channel impulse response h in the steady state is carried out after the start of each burst and each section (θ _jj lx).

6. The method according to any one of the preceding claims, in which midambles or preambles are used as training sequences.

7. The method according to any one of the preceding claims, in which the channel is adaptively tracked by replacing a temporally older section - ₂ , (x) of the received signal es -, a burst by the section just received.

8. The method according to any one of the preceding claims, in which a periodic basic code is used to generate the training sequences of the participants.

9. The method according to claim 8, wherein the training sequence distribution at the channel estimator manifests itself as a cyclical interchanging of the training sequences used by the participants if the supported number of participants K is a multiple of the proportions X of the bursts over which the training sequence is divided.

10. TD-CDMA mobile radio system for using the method according to one of the preceding claims.