GB2401760A

GB2401760A - Transport format selection

Info

Publication number: GB2401760A
Application number: GB0311215A
Authority: GB
Inventors: Alain Maes
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 2003-05-15
Filing date: 2003-05-15
Publication date: 2004-11-17
Anticipated expiration: 2023-05-15
Also published as: GB0311215D0; GB2401760B

Abstract

A transport format is selected by the MAC layer in a Universal Mobile Transmission Standard (UMTS) by first selecting a transport block size (eg. 360 bits) to be used in each transport channel in the next Transport Time Interval (TTI), based on the MAC Logical Priority (MLP) and Protocol Data Unit (PDU) size of the data carried by those logical channels (eg. LCH0-3) which are to be mapped to the transport channel (eg. TrCH0). An algorithm selects the block size which allows transmission of the maximum amount of highest priority data. The number of transport blocks to be included in each transport block set defining a transport format is then determined by selecting the transport format which allows transmission of a maximum number of PDU's. The entire transport format combination is then selected by checking which of these transport formats are valid. This method allows the MAC layer to select a Transport Format Combination without having to scan every possible such combination in the original allowed Transport Format Combination Set.

Description

1 2401 760

TRANSPORT FORMAT SELECTION

This invention relates to transport format selection by the Medium Access Control (MAC)layer of the Universal Mobile Transmission Standard (UMTS) protocol stack.

The UMTS protocol stack is divided into two main blocks. Firstly there is the access stratum (AS) which deals with all operations that are dependent on the radio technology used. Secondly there is the non-access stratum (HAS) which implements all the functionality that is independent to the radio technology. For example a mobile phone supporting both the GSM and UMTS technologies will have one NAS and two AS blocks (one for GSM and one for UMTS).

Within the access stratum there is provided the MAC layer. This is used to provide various services to the upper layers of the UMTS stack. It does this by providing mapping between logical and transport channels, selects its appropriate transport formats for each active transport channel to ensure efficient use of transport channels. Traffic volume monitoring also has to be provided as well as various other control functions.

In the UMTS protocol stack, the MAC layer controls the operation of transport channels, over which data is transferred to the logical layer for transmission in a radio frame. To put this another way, transport channels are the service offered by logical layer l to the MAC layer. In order to do this, the network in which a mobile telephone using UMTS operates, allocates to each transport channel a number of Transport Formats (TF). Each of these TF define amongst other things, the exact number of bits that can be transferred on the transport channel to the logical layer l during a radio frame. This set of transport formats is called a Transport Format Set (TFS).

More than one transport channel can be used by the MAC layer and it has to select one TFS for each transport 4239Sspec channel over which it wishes to transmit data. This set of transport formats is called a Transport Format Combination Set (TFCS).

Figure 1 illustrates this terminology, on the assumption that five Dedicated Transport Channels (DCH) are being used. In this, a transport format combination is illustrated at 2, a transport format set at 4, and a transport format combination set at 6. It can be seen from the diagram that there are a total of 14 different transport formats available with four being assigned to each TFS and five to each TFC. The 14 TFS which are available are known as the Allowed Transport Format Combination Set (the allowed TFCS). The allowed TFCS is determined by the network which may decide to restrict the transport format combinations that the MAC layer is allowed to use, for example by a bandwidth limitation.

When it does this, it will send to a mobile unit a list of transport format combinations that it is allowed to use.

Therefore, for every radio frame transmitted, the MAC layer has to select the most appropriate TFC within the allowed TFCS depending upon the total amount of data it has to transmit. If the size of the allowed TFCS is small enough, it is possible for the MAC layer to scan it all and pick up the best one. However, if the size of the allowed TFCS is too large this will not be possible.

Specific embodiments of the present invention provide a mechanism whereby a selection may be made from a TFCS which is too large for the MAC layer to scan it all to pick up the best TFS.

The size of the transport format combination set is potentially very large. For example, if even ten transport channels are used and ten transport formats assigned to each transport channel then the set of all TFCs is made of 101 elements. Also the network can and will prohibit the use of TFCS whose bandwidth is too high. This is explained with reference to Figure 2. The upper area of this graph 42398.spec shows TFS that when selected for a TFCS are always allowed. The second portion shows transport formats which when they are included in a TFCS may or may not be allowed. The lower portion shows TFS which when included in a TFCS are not allowed. So that the mobile is able to perform transport format selection, the network sends it a list of transport formats. In ideal conditions, the mobile could theoretically use any combination of these transport formats when it wishes to send data to the network.

However, depending on dynamic traffic conditions (numbers of users in a cell, total bandwidth consumed), the network may decide to restrict temporarily the bandwidth available to any single user.

IS Assuming that the transport formats are sorted on increasing bandwidth - i.e. we always have Bandwidth (TFij) ≥bandwidth (Tfik) if junk, Figure 1 illustrates such a restriction.

A preferred embodiment of the invention makes a selection of the best transport format combinations available in dependence upon parameters such as the amount of data to transfer on each transport channel, regardless of whether or not this TFC is allowed or not.

If the best TFC is not allowed, a number of other TFCs are selected and tested, progressively reducing their bandwidth with a non-restrictive one is found.

Embodiments of the invention will now be described in detail by way of example with reference to the accompanying figures in which: Figure 1 shows schematically the transport format of terminology described above; Figure 2 shows the range of allowed and not allowed TFS described above; Figure 3 shows a schematic diagram of the interaction between the transport selection in the MAC layer, the RLC 42398.spec layer which supplies data to it, and the logical layer 1 which provides data for transmission; Figure 4 is a flow diagram showing a transport format selection algorithm; Figure 5 shows a possible configuration of a transport format selection unit; Figure 6 is a graph showing allowed TFCs.

The MAC layer shown in Figure 3 can receive data from the RLC entities at any time. It has no knowledge of when data will arrive and therefore it is supplied with a buffer and multiplexing unit 10 which receives the data and which can subsequently determine where it will be sent. Buffer and multiplexing unit 10 is connected to a transmission handler 12 which includes a transport format selection unit 14.

The transmission handler has to send data to the logical layer 1 every radio frame. Therefore, it does this every 10 milliseconds. The transport format selection unit 14 determines the exact amount of data which is to be transmitted to logical level 1 during the radio frame every 10 milliseconds. The transmission handler 12 then fills the transport blocks sets 16 with the data for logical level 1 and passes them to logical level 1 for transmission. The process then repeats.

The transport format selection unit 14 therefore executes every radio frame so that it can request the data to be transmitted during the next TTI (Transmission Time Interval) from the Radio Link Control (RLC). The algorithm is illustrated in Figure 4 and consists primarily of four steps as follows: (a) at 20 in Figure 4, the MAC layer first selects the transport block size it will use for transmission on each transport channel. This depends upon the mapping between the logical and transport channels and will 42398 spec be described in more detail below under the heading Transport Block Size Selection.

(b) at 22 in Figure 4, having chosen the transport block size for each transport channel, the MAC layer then determines the number of transport blocks that will make up each transport block set 16. Its value depends upon the number of RLPCDUs (where PDU is Protocol data unit) which are cued in the MAC buffers 10. This will be described in more detail below under the sub-heading Transport Block Set Size Selection.

(c) The MAC layer has now determined the optimum transport format combination set for transmission of the data. However, the network may have explicitly ruled out the use of the set of transport formats selected. This may happen when the network wishes to reduce the bandwidth of a user equipment (UE). The MAC layer therefore checks whether or not the preferred combination is allowed or not at 24 in Figure 4. If it is not allowed, then it will select a valid combination 26, preferably the best valid transport format combination available. This will be described under the sub-heading Transport Format Combination Selection below. If the selected format combination is already a valid one, then it is passed to 28 in Figure 4 which allocates the bandwidth to the logical channels and will be described below under the sub-heading Allocation of Bandwidth to Logical Channels.

(d) Following the selection described above, the MAC layer knows the total amount of data it is allowed to transmit during the current TTI and can start transmitting the PDUs cued in its buffers.

42398.spec Transport Block Size Selection The MAC logical channel priority (MLP) parameter of each logical channel defines the priority of the data carried by the channel. It is an integer value which ranges from l, the highest priority to 8 the lowest priority. For a given PDU size and per transport channel all data with the highest priority must be transmitted before lower priorities can be transmitted. Therefore, for each transport channel the MAC layer must first determine the priority of the data it is going to transmit. As high priority data is to be transmitted first, it must first choose the minimum value of MLP for all logical channels mapped into that transport channel. Whilst the MAC layer knows the MLP of the data it is going to transmit, it must determine the size of the transport blocks it is going to use. As a transport block carries a single RLCPDU and the size of the RLCPDU may vary from one logical channel to another, the MAC layer must use the size which allows the transmission of as much data as possible on that transport channel. The algorithm for transport block size selection is as follows: (a) for each transport channel, determine which logical channels mapped onto the transport channel carry the highest priority data (lowest value of MLP).

(b) determine the different values of transport block size which could be used to transfer the RLCPDUs carried by those logical channels.

(c) for each possible transport block size compute the highest amount of priority data to be transferred using that transfer block size.

(d) select the transport block size which allows the transmission of the maximum amount of highest priority data.

42398.spec Figure 5 shows a possible configuration of logical channels being mapped to transport channels by the transport format selection unit 12. All channels are provided as inputs into the transport format selection unit and the transport handler 12 and their priorities are given in Figure 5 along with their PDU sizes and the number of PDUs in each one. From this it can be seen that LCHo and LCH3 carry the highest priority data and this therefore has to be transmitted first. The size of the PDUs carried by LCHo is 360 and that of LCH3 is 168 bits.

Both of these sizes are amongst the available transport format sizes in TRCHO. The transport block size (TBS) has to be selected. A TBS with 168 bits will allow the MAC layer to transmit the 3 PDUs of LCH3 as a total of three times 168 bits. Selecting a TBS with 360 bits will allow the MAC layer to transmit the 3 PDUs of LCHo as a total of 3 times 360 bits. Therefore, selection of 360 bits as a block size allows the largest amount of data to be transmitted and a transport block size of 360 bits on TRCHO will therefore be selected.

Transport Block Set Size Selection

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Once the size of the transport blocks to be transmitted during the next TTI has been determined, the MAC layer must determine how many blocks it will transmit.

To do this it takes into account the total number of RLCPDUs of that size to be transmitted, regardless of their priority. In other words, the MAC layer will attempt to transmit as many PDUs of the selected size as possible.

To do this, for each transport channel the total number of RLCPDUs of the selected size waiting for transmission on this transport channel is determined.

Then, from the list of possible transport formats for this channel the transport format which will allow it to transmit as many PDUs as possible will be selected without 42398.spec exceeding the total number of RLCPDUs waiting for transmission.

Looking again at Figure 5, it can be seen that the transport block size for TRCH0 selected by transport block size selection is 360 bits. It is now necessary to choose between three possible formats for TRCH0 which are l, 2 or 4 times 360 bits. LCHo and LCH1 both have three 360 bits PDUs to transmit. Therefore, the total amount of data to transmit is 6 360 bits PDUs. Therefore, the MAC layer decides to transmit a total of 4 PDUs as this is the largest allowed value which does not exceed the total number of PDUs waiting for transmission.

The optimum transport format to use on TRCH0 is therefore 4 times 360 bits.

Transport Format Combination Selection The MAC layer has now determined the optimum transport formats to use on each of the available channels, the best transport format combination. The use of this particular transport format combination may, however, be restricted by the network. The MAC layer therefore has to check whether or not the TFC has selected is a valid one or not. If it is, it may proceed to the last step of the algorithm and start transmission.

Otherwise, it will have to pick a valid TFC. This is done by trying a series of TFCs progressively reducing the amount of data it is trying to transmit, i.e. the transport channels carrying the data of the lowest priority. The amount of data is successively reduced until an acceptable TFC is found or until the bandwidth is reduced to zero. In the latter case, no data will be transmitted during the next TTI.

The algorithm uses the notion of Transport Channel Priority, defined as the lowest lend of MLP mapped into that channel. The algorithm operates by searching on the 42398.spec transport channel TFS to find a TFC belonging to the sub- set allowed of TFCs, a sort is done first by increasing transport block sizes and then increasing transport block set sizes. A restricted transport format combination set is taken along with the ID of a transport channel and the transport format combination is modified to return a boolean value indicating whether or not the new TFC is a valid one or not. This is illustrated in Figure 6 which assumes that the initial TFC was four times 360 bits and two times 240 bits and TRCH1 and TRCH2 take MLPs of priorities 1 and 2 respectively.

For each transport channel, which is sorted on decreasing values of MLP and then on increasing amount of data to transmit, TRCH2 has the highest MLP and therefore the lowest priority. A searched allowable TFC will therefore select different transport formats for TRCH2.

The current transport format of TRCH2 is 2 by 240 bits whose index in the TF array is 2 and there is therefore no need to try to use TFS with a higher index since their data rate is higher. The current transport block size is 2 by 240 bits. 0 by 240 bits transport format is entry 0 in the TF table so min is 0. There will be no point in using a TF for the lower index. If the TFC first tested with the lowest bandwidth, i.e. 4 by 360, 0 by 240 and as TFC O is allowed the algorithm now checks if the higher bandwidth TFC could be used. This is done by checking the TF halfway between the minimum TF and the maximum TF in the index of TFS. If this gives a TFC which is valid then the new value is set to be the minimum position in the TF index and the process restarts again looking between the new minimum and the maximum position of the transport format in the index.

42398 spec Allocation of Bandwidth to Logical Channels Once the TFC has been selected, the MAC layer must share the available bandwidth between different logical channels depending on their priorities. It does this as follows: for each transport channel and for each logical priority channel (sorted out on increasing MLP), if the logical channel PDU size added to the size of the appropriate MAC header is equal to the transport block size and any transport blocks are allocated to the current logical channel in proportion to the ratio between the number of RLC PDUs it has to transmit and the total number of RLC PDUs of the same priority. The first RLC PDUs IS buffered on this logical channel is then flagged as ready for transmission.

If this approach is applied to the example of figure 5, it gives the following results for transport channel 0.

The transport format previously selected is 4x360 bits and there are therefore 4 transport blocks to allocate between the logical channels. As many blocks as needed will be first allocated to the highest priority channels. A total number of 360 bit PDUs with a logical priority of 1 is 3. Therefore, LCHo has 3 PDUs to transmit, and the total number of PDUs to transmit is 3, therefore all available transport blocks could be used by LCHo. As 4 blocks are available and LCHo only needs 3, the MAC requests 3 PDUs from LCHo and 1 transport block is still available.

On LCH3, as the sizes of blocks do not match with LCHo, the MAC requests 0 PDUs from LCH3 thereby leaving 1 transport block still available on LCHo.

There are a total of 3x360 bits PDUs with a logical priority 2. LCH1 has 3 PDUs to transmit and the total number of PDUs to transmit is 3. All available transport blocks can be used by LCH1. Only 1 block is available and 42398 spec the MAC therefore requests l PDU from LCHl. No more transport blocks are available and the MAC therefore requests O PDUs from the last logical channel LCH2.

The Transport Format Selection discussed above is implemented in both a mobile unit and in the network.

As far at the mobile is concerned, before it is able to transmit anything it has to be configured by the network. This will send its list of Transport Format Combinations that it is free to use as it wishes. In UMTS, a mobile has the opportunity of transmitting data every lo, 20, 40, or 80 mS. This will depend on the configuration it has received from the network. At each transmit opportunity (which could be as frequently as every lo mS) the mobile or more precisely the mobile MAC layer will choose one the Transport Format Combinations to use for transmission from the set it has received from the network.

As far as the network is concerned, a UMTS network will implement a similar mechanism to transmit data. The UTRAN also has the opportunity of transmitting data every lO, 20, 40, or 80 mS. It also implements a MAC layer very similar to the MAC layer on the mobile cycle. This will select a Transport Form Combination at each transmit opportunity.

Therefore, it will be apparent that both the mobile and the network implement a Transport Format Selection of the present invention but do this independently of each other.

Using the methods described above, a much more efficient selection of transport channels in the MAC layer can be made thereby improving the usage of the available bandwidth in transmissions in a MAC based communication system.

4239Sspec

Claims

1. A method for transport format selection by the MAC layer in the Universal Mobile Transmission Standard, comprising the steps of: selecting a transport block size to be used in each transport channel; determining the number of transport blocks to include in each transport block set defining a transport format (TF); and applying data in the defined TF for transmission.

2. A method according to claim l including the step of determining whether or not the defined transport format is a permitted format and if it is not permitted, searching for a valid TF.

3. A method according to claim 2 in which the searching step comprises progressively reducing the amount of data to be transmitted until a valid TF is selected.

4. A method according to any preceding claim in which the steps are executed every Transmission Time Interval (TTI).

5. A method according to claim l in which the steps of selecting the transparent block size comprises: determining which logical channel supplied on each transport channel has the highest priority data; determining the different block sizes which can be used for this data; deriving the amount of highest priority data that can be transmitted using each block size; and 42398.spec selecting the block size that allows transmission of the maximum amount of highest priority data.

6. A method according to claim 1 in which the step of determining the number of transport blocks to include in each transport block set comprises: determining how much data of the selected block - size is waiting for transmission; and selecting a block size that permits transmission of as much data as possible without exceeding the total number of blocks of data waiting for transmission.

7. A method for transport format selection in the MAC layer of the Universal Mobile Transmission Standard substantially as herein described with reference to the drawings.

42398.spec