WO2003101027A1

WO2003101027A1 - Blind transport format-detection with preselection of candidate transport formats

Info

Publication number: WO2003101027A1
Application number: PCT/GB2003/002212
Authority: WO
Inventors: Timothy Fisher-Jeffes
Original assignee: Ubinetics Ltd
Current assignee: Aeroflex Cambridge Ltd
Priority date: 2002-05-23
Filing date: 2003-05-20
Publication date: 2003-12-04
Anticipated expiration: 2004-11-23
Also published as: GB0211847D0; GB2389021B; AU2003234017A1; GB2389021A

Abstract

Receiver apparatus for a transmission link, the link using any of a set of predetermined transport formats for blocks of data, and using convolutional coding for transmitting the blocks to the receiver, each block having control channels and data channels transmitted with different gain factors, the receiver having a monitor for monitoring gain factors of a received coded block, and determining a ratio of data channels to control channels from the gain factors, a transport format preselector for using the ratio to select candidate transport formats for the given block, from the set of transport formats, and a transport format detector, for carrying out speculative decodes of the block using the selected candidate transport formats, to determine which transport format was used, and to recover the data.

Description

BLIND TRANSPORT FORMAT DETECTION WITH PRESELECTION OF CANDIDATE TRANSPORT FORMATS

FIELD OF THE INVENTION

The invention relates to receivers for transmission links, in base stations or mobile terminals of radio transmission links, and to corresponding software and methods, for carrying out blind transport format detection.

BACKGROUND TO THE INVENTION

It is known to provide transport services for data over a transmission link using various telecommunication protocols. One such set of protocols, known as UMTS, is the European proposal for a third generation (3G) cellular network. It is interoperable with the existing GSM network, and is notable for providing high speed packet-switched data transmission. UMTS transmission protocols are defined in a series of standards developed by the Third Generation Protocol Partnership (3GPP). These standards define a number of layers for the transmissions, broadly in line with the well known ISO seven layer definition. There is a physical layer, L1 , a data link layer, L2, and a network layer L3. Layer 2 of this UMTS protocol stack includes procedures for handling data from or to a MAC (Medium Access Control) and higher layers. The data in the form of Transport blocks or sets of Transport blocks is encoded/decoded to offer transport services over a radio transmission link. A channel coding scheme provides a combination of error detection, error correcting, rate matching, interleaving and transport channels mapping onto/splitting from physical channels. At a receiver, it is necessary to identify a channel format and decode the overhead and payload in the data to enable these functions of the coding scheme to operate.

In any system which is based on multiple transport channels all being multiplexed onto a single channel there needs to be a method of discerning which data came from which transport channel after the data has passed through a channel. In some cases this is done by sending extra control information along with the data that signals what data belongs to which transport channel. Transport Format Combination Indication (TFCI) is an example of this where the Transport Format Combination (TFC) is the multiplexed data stream. In systems where no TFCI control information is sent another method of being able to tell transport formats apart is needed. An example described in 3GPP standard 25.212 is known as Blind Transport Format Detection (BTFD).

BTFD is a scheme for detecting the format of the transport channels and in particular detecting a finish of a block of data for one channel. It makes use of the fact that the block is terminated by an error detection code, in this case a CRC (Cyclic Redundancy Check) code. The scheme uses this to test whether a given sequence of bits in the data stream could be a CRC code, for a block of data bits preceding the CRC bits. For different channel formats, the length of the block may differ. It may be coded in various ways including convolution coding.

Explicit blind transport format detection involves performing the recursive add- compare-select (ACS) process of a trellis decode over the maximum Transport Format (TF) length, storing trace-back information as one goes. This is followed by a series of speculative trace-backs and subsequent CRC checks starting from each position where a potential transport format could have terminated. When a CRC pass is found the resulting decoded sequence has a high probability of being the correct one of the correct length and hence also the correct transport format. This implies the need for a convolutional coded data sequence that has a CRC appended to it prior to encoding.

When variable spreading factors are used in conjunction with flexible transport channel positioning within a physical channel frame (as is the case in the uplink 3GPP UMTS specification) BTFD becomes very complex. It involves completing the entire decode chain of de-spreading, both de-interleaves, de-rate-matching, channel decoding and CRC checking. This adds considerable complexity and computational load at the NodeB receiver since it has to be done for every valid transport format combination (which may have different spreading factors or numbers of channels). ADD REF TO SECOND STANDARD

The high complexity is due to the fact that spreading factor and transport channel positions are unknown. This means that in order to perform BTFD effectively one must repeat the entire bit rate processing decode process (as well as possibly de- spreading) for each possible TFC. SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved blind transport format detection method or apparatus for the same.

A first aspect of the invention provides receiver apparatus for a transmission link, the link using any of a set of predetermined transport formats for blocks of data, and using convolutional coding for transmitting the blocks to the receiver, each block having control channels and data channels transmitted with different gain factors, the receiver having: a monitor for monitoring gain factors of a received coded block, and determining a ratio of data channels to control channels from the gain factors, a transport format preselector for using the ratio to select candidate transport formats for the given block, from the set of transport formats, and a transport format detector, for carrying out speculative decodes of the block using the selected candidate transport formats, to determine which transport format was used, and to recover the data.

By this preselection of likely transport formats, the number of speculative decodes can be reduced. As this decoding is computationally intensive, any reduction is valuable, to increase the speed, or reduce the power consumption. Particularly for applications in mobile handsets, power consumption affects battery life, which is a critical limitation. In summary, it can reduce complexity by providing an algorithm for reducing the number of possible TFCs to test during BTFD.

The predetermined transport formats may differ in various ways such as variable spreading factor (i.e. variable combined data rates) and/or flexible transport channel positioning for a (set of) data channel(s) code separated - from another control channel. Each channel should be supplied with its own gain factor that varies based on its data content.

Preferred additional features include monitoring the number of physical channels, or averaging the ratio over a frame of chips. The block may be a TrCH in a 3GPP system, the control and data channels being DPCCH and DPDCH respectively in a 3GPP system. According to other aspects, the receiver apparatus may be incorporated in a mobile terminal, or in a base station. Other aspects include corresponding methods and software. Other aspects provide for methods of offering a data transmission service to subscribers over a link using the receiver apparatus, or methods of using a data transmission service to transmit data using the receiver apparatus. These reflect how the commercial value of the data transmission services enabled or improved by the use of the apparatus may be much greater than the initial sale value of the apparatus. Other advantages will be apparent to those skilled in the art. Preferred features may be combined with other aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example with reference to the drawings in which:-

Figure 1 shows DPCCH and DPDCH gain factors and multiplexing arrangement, and

Figure 2 shows a procedure for preselection with blind transport format detection, according to an embodiment of the invention.

DETAILED DESCRIPTION

For ease of explanation of the embodiments of the invention, the implementation of BTFD known from the above referenced 3GPP standard will be described in more detail before describing embodiments of the invention. Yet further details are set out in the standard.

At the transmitter, the data stream with variable number of bits from higher layers is block-encoded using a cyclic redundancy check (CRC) and then convolutionally encoded. CRC parity bits are attached just after the data stream with variable number of bits as shown in figure 1. The size of the CRC is 24, 16, 12, 8 or 0 bits and it is signalled from higher layers what CRC size that should be used for each channel. Convolutional codes with constraint length 9 and coding rates 1/3 and 1/2 are defined. The receiver knows only the possible transport formats (or the possible end bit position {n_enci}) by Layer-3 negotiation. The receiver performs Viterbi-decoding on the soft decision sample sequence. The correct trellis path of the Viterbi-decoder ends at the zero state at the correct end bit position. The blind transport format detection method using CRC traces back the surviving trellis path ending at the zero state (hypothetical trellis path) at each possible end bit position to recover the data sequence. For each recovered data sequence error-detection is performed by checking the CRC, and if there is no error, the recovered sequence is declared to be correct.

The 3GPP standard sets out that for the uplink, blind transport format detection is a network controlled option. For the downlink, the receiver shall be capable of performing blind transport format detection, if certain restrictions on the configured transport channels are fulfilled.

A UMTS uplink DPCH consists of a dedicated physical control channel (DPCCH) and any number between zero and 6 dedicated physical data channels (DPDCHs). Optionally, for a dual transport format case (the possible data rates are 0 and full rate, and CRC is only transmitted for full rate), blind transport format detection using received power ratio is known from the standard. The transport format detection is then done using average received power ratio of DPDCH to DPCCH. If Pd/Pc >T then full rate transport format is detected, otherwise zero rate transport format is detected, where:

- Pc: Received power per bit of DPCCH calculated from all pilot and TPC bits per slot over a radio frame;

Pd: Received power per bit of DPDCH calculated from bits per slot over a radio frame;

X: the number of DPDCH bits per slot when transport format corresponds to full rate;

T: Threshold of average received power ratio of DPDCH to DPCCH for transport format detection.

The transmission of these DPCCH and DPDCH channels is split between an I- channel and a Q-channel. Each of these DPCCH and DPDCH channels on either of a I- or Q-channel are transmitted with a separate orthogonal variable spreading factor (OVSF) code. All the DPDCHs are transmitted at a certain linear gain factor, β_d , while the DPCCH is transmitted at a different linear gain factor, β_c . This is shown in Figure 1 as will now be explained further.

Spreading is applied to the physical channels. It consists of two operations. The first is the channelization operation, which transforms every data symbol into a number of chips, thus increasing the bandwidth of the signal. The number of chips per data symbol is called the Spreading Factor (SF). The second operation is the scrambling operation, where a scrambling code is applied to the spread signal. With the channelization, data symbol on so-called I- and Q-branches are independently multiplied with an OVSF code. With the scrambling operation, the resultant signals on the I- and Q-branches are further multiplied by complex-valued scrambling code, where I and Q denote real and imaginary parts, respectively.

Figure 1 illustrates the principle of the uplink spreading of DPCCH and DPDCHs. The binary DPCCH and DPDCHs to be spread are represented by real-valued sequences, i.e. the binary value "0" is mapped to the real value +1, while the binary value "1" is mapped to the real value -1. The DPCCH is spread to the chip rate by the channelization code C_chι0, while the rr.th DPDCH called DPDCH_n is spread to the chip rate by the channelization code C_ch,_n. One DPCCH and up to six parallel DPDCHs can be transmitted simultaneously, i.e. 0 < n ≤ 6.

For each transport format combination (TFC) in an uplink transport format combination set (TFCS) there is a corresponding pair of β_cand β_d values. There is not necessarily a one-to-one mapping between these, however, it is unlikely that all TFCs in a TFCS will share the same β_c and β_d values since these β_c and β_d values are chosen based on the amount of data the DPDCHs are carrying. By definition TFCs will have different amounts of data within them. The valid ratios of DPCCH : DPDCH are given in table 1 below:

Table 1

The embodiment of the invention shown in figure 2 includes the provision of circuitry or software at the NodeB receiver to monitor the number of DPDCHs and the received magnitude levels on the DPDCHs and the DPCCH over a frame. Since these channels are transmitted time aligned down the same DPCH channel as shown in figure 1 , the number of DPDCHs and ratios between the DPDCHs and the DPCCH will most likely be preserved to a high degree of accuracy. This can be used to advantage by picking out the TFCs (Transport Format Combinations) from the TFCS that have a β_c value, a β_d value and a number of DPDCHs corresponding to those measured at the NodeB. In doing so the set of possible TFCs to try during BTFD will be reduced to a sub-set of the TFCS.

Measured DPCCH : DPDCH magnitude ratios can be used to quickly and accurately narrow down the likely transport formats to check during explicit blind transport format detection, particularly in cases where variable spreading factor and flexible transport channel positions are used.

Figure 2 shows the steps of monitoring gain factors of a received coded block, determining a ratio of data channels to control channels from the gain factors, using the ratio to select candidate transport formats for the given block, from the set of transport formats, and carrying out speculative decodes of the block using the selected candidate transport formats, to determine which transport format was used, and to recover the data. These can be applied to UMTS or 3GPP compatible equipment, or to other transmission links.

Monitoring a) the number of physical channels being sent, and b) the relative magnitude ratios(s) of the DPCCH : DPDCH, can considerably narrow down the selection of transport formats to check, simply by finding the mapping between transport format combinations (TFCs) and DPCCH : DPDCH scaling factors (β_c and β_d values). If these are signalled then this would involve a straight forward look-up and if they were calculated based on a reference TFC or in the case during compressed mode, then only a range of transport format combinations would fit this criterion. As shown in table 1 above, there are 30 different valid magnitude ratios in the 3GPP UMTS specification. Assuming reasonable phase estimation this magnitude ratio will not be altered significantly if averaged over an entire frame of chips, one could quite accurately estimate the transmitted magnitude ratios(s) and hence infer a sub-set of the total number of TFCs in the transport format combination set (TFCS) to check.

Claims

1. Receiver apparatus for a transmission link, the link using any of a set of predetermined transport formats for blocks of data, and using convolutional coding for transmitting the blocks to the receiver, each block having control channels and data channels transmitted with different gain factors, the receiver having: a monitor for monitoring gain factors of a received coded block, and determining a ratio of data channels to control channels from the gain factors, a transport format preselector for using the ratio to select candidate transport formats for the given block, from the set of transport formats, and a transport format detector, for carrying out speculative decodes of the block using the selected candidate transport formats, to determine which transport format was used, and to recover the data.

2. The receiver apparatus of claim 1, wherein the predetermined transport formats have different spreading factors.

3. The receiver apparatus of claim 1 or claim 2, the transport formats differing in transport channel position within the block.

4. The receiver apparatus of any preceding claim, the monitor being arranged to monitor a number of physical channels.

5. The receiver apparatus of any preceding claim, the monitor being arranged to determine the ratio by averaging over a frame of chips.

6. The receiver apparatus of any preceding claim, the block being a transport channel in a 3GPP system.

7. The receiver apparatus of any preceding claim, the control and data channels being a dedicated control channel and a dedicated physical channel of a 3GPP system.

8. The receiver apparatus of any preceding claim, the data and control channels being transmitted on either of a l- or Q-channel and multiplied by a separate orthogonal variable spreading factor (OVSF) code.

9. The apparatus of any preceding claim embodied in software.

10. A mobile terminal for use with a radio base station, having: receiver apparatus as set out in any of claims 1 to 9.

11. A base station having receiver apparatus as set out in any of claims 1 to 9.

12. A method of receiving data transmitted over a transmission link using any of a set of predetermined transport formats for blocks of the data, and using convolutional coding for transmitting the blocks, each block having control channels and data channels transmitted with different gain factors, the method having the steps of: monitoring gain factors of a received coded block, determining a ratio of data channels to control channels from the gain factors, using the ratio to select candidate transport formats for the given block, from the set of transport formats, and carrying out speculative decodes of the block using the selected candidate transport formats, to determine which transport format was used, and to recover the data.

13. A method of offering a data transmission service to subscribers over a link using the receiver apparatus as set out in any of claims 1 to 9.

14. A method of using a data transmission service to transmit data using the receiver apparatus as set out in any of claims 1 to 9.