GB2380909A

GB2380909A - Blind transport format detection via signal power changes

Info

Publication number: GB2380909A
Application number: GB0124238A
Authority: GB
Inventors: Timothy Fisher-Jeffes
Original assignee: Ubinetics Ltd
Current assignee: Aeroflex Cambridge Ltd
Priority date: 2001-10-09
Filing date: 2001-10-09
Publication date: 2003-04-16
Also published as: EP1435147A1; CN1561599A; WO2003032559A1; GB0124238D0

Abstract

A device for receiving multiplexed data signals (eg. TDMA data frames) can identify which channel or format the data originates from via detection of a change of power level of the signal. A power monitor continually monitors the magnitude of soft decisions in the data over the course of a data frame. A change in power level of greater than a given threshold value can be taken to represent a switch to a different data channel or format, so that several time slots from different channels can be present in one frame. A cyclic redundancy check (CRC) can be used to ensure that each frame is correctly synchronised.

Description

Blind Transport Format Detection The present invention relates to a method and apparatus for estimating the transport format, i. e. the start and stop of data, of a transport channel.

In mobile telecommunication systems, it is known for multiple transport channels to be multiplexed onto a single channel. However, there needs to be a method of discerning which data relates to which of the multiple transport channels after the data has passed through the single channel. In some cases, this is done by transmitting extra control information along with the data that indicates what data belongs to which of the multiple transport channels. Such control information is called Transport Format Combination Indication (TFCI) where the Transport Format Combination (TFC) is the multiplexed data stream. For example, the single channel onto which the multiple transport channels are multiplexed will be divided into frames of a particular length, and the frames are divided into time periods each relating to one of the multiple transport channels. The TFCI will be transmitted as part of the frame to indicate how that frame is divided into time periods and which time periods relate to which of the multiple transport channels.

However, in some systems, no TFCI control information is transmitted. It is, therefore, necessary to be able to identify Transport Formats in a different way. This will be referred to as Blind Transport Format Detection (BTFD). This requires each transport channel to be multiplexed onto a single channel at fixed positions or periods. In addition, at least one of the transport channels may have a cyclic redundancy check (CRC) word appended to it..

In this case, it is known to do a Viterbi trace back followed by a CRC check. This involves the lengthy analysis of the whole channel in order to identify the transport format. Such an operation is not only time consuming, but also requires considerable power consumption.

According to a first aspect of the present invention, a device for receiving blind transport format data and estimating the transport format of the data includes a soft decision magnitude/power monitor for monitoring the magnitude or power of soft decisions in the data over time; and a power or magnitude change detector for detecting changes in the magnitude of the monitored soft decisions, the position of detected changes representing characteristics of the transport format, or representing high probabilities of representing characteristics of the transport format. It is preferred that the blind transport format data includes a cyclic redundancy check (CRC).

It will be appreciated that this invention can lead to faster identification of the transport format without having to do all Viterbi trace backs followed by CRC checks. The position of the power/magnitude changes indicate which CRC positions to check first.

This also has the advantage of requiring less power consumption thereby extending battery life, or allowing the use of a smaller capacity battery for comparable battery life.

According to a second aspect of the invention, a method of estimating the transport format of received blind transport format data which comprises monitoring the magnitude (power) of soft decisions in the data over time; and detecting changes in the magnitude of the monitored soft decisions, the position of detected changes representing characteristics of the transport format or representing high probabilities of representing characteristics of the transport format.

The present invention will be described by way of example only with reference to the accompanying figure which indicates a single channel onto which multiple transport channels have been multiplexed, the single channel being divided into TrCH periods, together with associated soft decision power levels over time.

In the description of the prior art above, reference is made to the multiplexing of multiple transport channels onto a single channel. In Figure 1, the bar at the top indicates the single channel of a frame within which three transport channels are multiplexed. The single channel is divided by time into three time periods each of which is filled or part-filled by data from one of the multiple transport channels. In

each frame, the periods will be of the same size or duration. The periods are defined at the beginning and end in Figure 1 with a bold vertical line so the first period begins at the far left of Figure 1 and ends just before the second cyclic redundancy check (CRC) word. The data within a period might not fill that period. For example, in the first time period, the data fills most of the period, leaving about one sixth of the period at the end empty. At least one of the transport channel data must have a cyclic redundancy check (CRC) word appended to it. The position of the CRC word identifies the end of the transport channel since it is located at the end of the channel data. In Figure 1, the data from the first transport channel (TrCH 0) has a CRC word at the end of the data, but before the end of the time period. In the second transport channel (TrCH 1), there is no data in the transport channel and so the CRC word appears at the beginning of the time period. In the third time period, the CRC word appears before the end. In the period of time between the CRC word and the end of the time period, no data is transmitted.

Data from the transport channels (TrCHO-2) is received in the form of soft decisions.

The use of soft decisions signifies probabilities of data bits being +1 or-1 instead of 1's

and 0'S. For example, the signal for a particular bit might be +0. 82 which represents a high probability of that bit being +1. Alternatively, a bit value of-0. 71 shows a high probability of the bit being-1. During the reception of real data, and not DTX, the probabilities will be high, and so the power associated with these signals will also be high, as is shown in the graph in Figure 1 during the period indicated"inferred TF for TrCH 0". The power is also high during the transmission of the second (CRC) word at the end of the second time period. Again, the power is high during transmission of the data and CRC word associated with the third transmission channel (TrCH 2). However, during the remainder of the time, after the CRC word has been transmitted at the end of each transmission channel, but before the next transmission channel starts, DTX bits are transmitted at zero power and hence should be received at a lower power level, as can be seen from Figure 1. By detecting the power during the frame, the position of the end of each transport channel can be identified or at least narrowed down. Identifying a drop in power will not necessarily indicate the exact transport format length, but will identify a number of likely positions for the ends of transport format lengths. Of

course, the beginning of transport format lengths are always known because they are fixed.

In high signal to noise ratio conditions, this will be sufficient on its own to identify the exact sizes and hence transport format whereby the transport format combination can be inferred without having to check the CRC words. In such an instance, if the CRC word is used merely to identify transport format combinations, they may be omitted entirely from the signal since this invention will identify the TFCs on their own.

In lower signal to noise ratio conditions, the exact positions of the beginning and end of transport formats may become more difficult to detect because of the noise. More jumps in power might be detected than actually exist. This is no problem since each of those positions can then be tested for the presence of the end or beginning of a period. This method significantly narrows down the positions of the beginning and end of periods, still resulting in a very significant reduction in the amount of work which needs to be done to identify the transport format.

The detection of jumps in the power is not difficult to achieve. The power is measured and monitored, and can be fed into an edge detector. The edge detector can work in any one of a number of ways. It could detect jumps in the power of greater than a certain size, or could be arranged to identify when the power crosses a pre-set threshold value, where the threshold value is located between the normal power value during transmission of data, and the normal power value of the DTX soft decisions during the time in a period between the transmission of the CRC word and the end of the period.

The detection of such edges can be carried out using an efficient algorithm, a number of which already exist.

The speed at which blind transport format detection takes place is significantly increased by use of this invention either by immediately identifying the transport formats from the jumps in power, or by prioritising which transport format to decode by identifying the most likely positions of the ends of periods or by prioritising which

CRC word to check first if the channel decoding was used to encode the data for error correction purposes.

This invention applies to any system that relies on blind transport format detection where the multiplexed data is positioned in fixed positioned periods, such as in the 3G UMTS specification or GSM specification, thereby correctly demultiplexing the received data.

Claims

1. A device for receiving blind transport format data and estimating the transport format of the data, the device including : a soft decision power monitor for monitoring the magnitude of soft decisions in the data over time; and a power change detector for detecting changes in the magnitude of the monitored soft decisions, the detected changes representing characteristics of the transport format, or representing high probabilities of representing characteristics of the transport format.

2. A device according to claim 1, wherein the detector is arranged to detect decreases in the magnitude of the monitored soft decisions.

3. A device according to Claim 1 or 2, wherein the detector is an edge detector arranged to detect changes in the magnitude of the monitored soft decisions of greater than a predetermined amount.

4. A device according to Claim 1 or 2, wherein the detector is a threshold detector that detects the change in magnitude of the monitored soft decisions as it crosses a predetermined power threshold.

5. A device according to any one of the preceding claims, in which the blind transport format data includes a cyclic redundancy check (CRC).

6. A device according to claim 5, further comprising a CRC checker for checking whether the data at any point is the correct CRC.

7. A device according to any one of the preceding claims, further comprising a prioritising element which prioritises each of the detected changes according to its likelihood of representing characteristics of the transport format.

8. A method of estimating the transport format of received blind transport format data comprising: monitoring the magnitude (power) of soft decisions in the data over time; and detecting changes in the magnitude of the monitored soft decisions, the detected changes representing characteristics of the transport format or representing high probabilities of representing characteristics of the transport format.

9. A method according to claim 8, further comprising the step of estimating the transport format from the detected changes.

10. A method according to Claim 8 or 9, wherein the detecting of changes in the magnitude of the monitored soft decisions is the detection of decreases in the magnitude.

11. A method according to any one of claims 8 to 10, wherein the detecting of changes in the magnitude of the monitored soft decisions is the detecting of an edge of more than a predetermined size.

12. A method according to any one of claims 8 to 10, wherein the detecting of changes in the magnitude of the monitored soft decisions is the detecting of a change which crosses a predetermined power threshold.

13. A method according to any one of claims 8 to 12, further comprising the checking of data at any point to identify if that data is a cyclic redundancy check (CRC) word.

14. A method according to any one of claims 8 to 13, further comprising the prioritising of each of the detected changes according to its likelihood of representing characteristics of the transport format.