GB2413466A

GB2413466A - Variable rate OFDM system wherein rate for a data block is reduced from the channel state optimum without increasing the number of symbols required for block

Info

Publication number: GB2413466A
Application number: GB0409093A
Authority: GB
Inventors: Darren Phillip Mcnamara
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2004-04-23
Filing date: 2004-04-23
Publication date: 2005-10-26
Anticipated expiration: 2024-04-23
Also published as: GB0409093D0; GB2413466B

Abstract

OFDM systems such as those in IEEE 802.11a can operate in a number of modes which have increasing bit rates traded off for reduced robustness. Systems then select a desired mode depending upon the state of the channel. In OFDM systems it is also the case that blocks of data do not fill integer numbers of OFDM symbols and are padded out with stuff bits. This invention calculates the number of symbols required by the data block at the best rate for the channel. It then reduces the data rate to the minimum where the data block will still fit into the same number of OFDM symbols, using up the space wasted on stuff bits and increasing robustness.

Description

24 1 3466 M&C Folio GBP89615 Document 986154 Data Transmission in a

Wireless Network This invention relates to the transmission of data in a wireless communication network, and particularly, but not exclusively, to the transmission of data in a wireless local area network.

It has particular application to communication systems in which a plurality of physical transmission modes is provided for use, and in particular to orthogonal frequency division multiplexing (OFDM), such as the IEEE standard 802.11 a.

In a wireless communication network, the various hardware components of a communications device, configured by suitable software, facilitate the establishment of wireless communications with another communications device. This is achieved by means of a plurality of functional elements which are each operable to process data to prepare for transmission over an established communications channel, and to process data received from a cooperating device.

To ensure that communications devices process data for transmission so that the data can be correspondingly processed at a receiving device, communications protocols have been established. Conventionally, the functionality required for the processing of data is considered as a number of levels of functionality to be applied to data in turn before transmission of the data. The levels of functionality can thus be developed separately as long as they conform to agreed principles by which they will cooperate with each other.

A functional module developed to deliver a level of functionality is thus required only to conform to a specie cation of the data to be transferred with the functional levels immediately adjacently higher and lower in the communications protocol so described.

One generally accepted multi-layer model is the OSI model.

A communications protocol considered in accordance with the OSI model comprises seven layers of functionality, each in turn being applied to data to be transmitted by a communications device. Of these seven layers, only the two lowest level layers (levels I and 2) applied during transmission are of direct relevance to this invention. The preceding five layers are either established by a particular application executed in the communications device, or relate to the establishment of network communication for use in any distributed network such as the Internet.

Thus, the penultimate layer (layer 2) is a Data Link layer which comprises a Logic Link Control (LLC) sub-layer alongside a Sub-Network Access Protocol (SNAP), and a Medium Access control (MAC) sub-layer, each of which are conventionally operable in accordance with an established communications standard to ensure that data is presented to another communications device in a readable format. In one example, the functionality of the LLC sub-layer is defined to operate in accordance with IEEE standard 802.2, and the functionality of the MAC sub-layer is defined in accordance with IEEE 802.11.

The final layer (layer 1) is the Physical (PHY) layer, which physically establishes the means for wireless communication from the communications unit in question to another compatible device. Again, the PHY layer's function is governed by an established standard to enable the compatibility of devices made and sold separately. In a particular example, the PHY layer comprises a Physical Layer Convergence Procedure (PCLP) sub-layer and a Physical Medium Dependent (PMD) sub-layer. These sub-layers are, in this example, defined in accordance with TEEE 802.1 la.

The PHY layer employs Adaptive Modulation and Coding (AMC), in order to enable selection of a transmission mode. This means that the PHY layer presents the MAC sub-layer with a number of possible transmission modes, each of which has a different maximum possible throughput. Depending on the link (channel) quality, the transmission mode can be varied to adjust the trade-offbetween robustness and maximum throughput.

Details of the transmission modes made available by a PHY layer implemented in accordance with IEEE standard 802.1 1 a, are set out in table 1:

Table 1

Coded bits Data Bits per 802.11 aDataBits per per OFDMOFDM ModeSubcarrierssymbolCode ratesymbolsymbol 14810.54824 24810.754836 34820.59648 44820.759672 54840.519296 64840.75192144 74860.67288192.96 84860.75288216 By establishing that the PHY layer and the MAC and LLC sublayers are defined in accordance with agreed standards, efficient operation of the communications process can be maintained. This is because the MAC layer is thus not required to have detailed exposure to the operation of the PHY layer, but only to specify certain aspects of its operation, such as selecting one of the available transmission modes.

When selecting the transmission mode in which the PHY layer is to operate, it is conventionally considered desirable to select the mode that will result in the highest effective throughput (determined by the amount of data in each packet and the number of packets which will be received successfully each second). In the above table, for example, transmission in mode 8 will result in higher data transmission rate than mode 1. However, selection of a mode with a high data transmission rate will result in employment of a less robust modulation and coding scheme than would be used for lower rate transmission modes. Hence, in channels with a low link quality, data transmission rate can be sacrificed for a reduction in transmission errors, in order to achieve a higher throughput.

This principle is described in further detail in "Wireless LAN Medium Access Control (MAC) and Physical Layer (PIIY) Specifications" (ANSI/IEEE Std 802.11, 1999 Edition) and "Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHZ Band" (IEEE Std 802.11 a-1 999).

A system implementing IEEE 802.1 l a is an example of an orthogonal frequency division multiplexing (OFDM) system. For each PHY transmission mode in such a system, a fixed number of coded data symbols will be transmitted in each OFDM symbol. Since it is only possible to transmit an integer number of OFDM symbols, if the coded symbol sequence does not completely fill the last OFDM symbol, the transmit data stream must be completed (or 'padded') with inserted zeros. Therefore, for a selected PHY transmission mode, as the size of the Physical layer Service Data Unit (PSDU) changes, the amount of zero padding required to fill an integer number of OFDM symbols will vary.

In the transmission of data which is contained in several OFDM symbols where the amount of data is only marginally larger than an integer multiple of the maximum number of data bits that can be carried in an OFDM symbol in the requested transmission mode, the lattermost OFDM symbol to be transmitted will contain only very few data bits and a large number of padding bits.

A particular problem arises when a system attempts to transmit small amounts of data with a high rate PHY transmission mode, since for transmissions above a certain rate, all data will be contained in a single OFDM symbol. Hence, any further increase in rate will not reduce the transmission duration, but will increase the likelihood of a packet error since a less robust modulation and coding scheme will have been employed than could have been used for the same resultant throughput of data.

One object of the invention is to provide apparatus which can ameliorate this problem, reducing the error rate and providing the means for reducing transmission duration as transmission rate increases beyond the limit described above.

In the current IEEE 802.1 la standard, the manner in which the PHY transmission mode is selected is not defined. The MAC layer passes a message to the PHY specifying the rate to be used, and the PLOP of the PHY layer then translates the specified rate to a specific modulation and coding scheme. The advantage of this arrangement is that the MAC layer can be designed to operate substantially independently of the PHY layer, and without detailed knowledge of the PHY layer but, on the other hand, the MAC layer is required to have knowledge of the modes available for use in the PHY layer.

It is a further object of the invention to provide a method of transmitting data in a multi- mode communications system which determines in a systematic manner the most appropriate transmission mode of a plurality of available modes, in prevailing circumstances.

According to a first aspect of the invention, a method of selecting a transmission mode, for a data transmission request identifying a desired transmission mode with associated transmission speed, comprises establishing the amount of data to be transmitted, noting the desired transmission speed, and, if achievable without increasing the number of OFDM symbols required to contain the data for transmission, reducing the transmission mode from the mode corresponding to the desired transmission speed.

According to another aspect of the invention, a method is provided of selecting a transmission mode in a physical layer of a communications protocol stack implemented in a communications device, said physical layer being operable to transmit data in OFDM symbols of predetermined length and in a selected one of a number of predetermined transmission modes, the method comprising receiving a request for transmission of data, said request containing an indication of a desired one of the available transmission modes for transmission of the data, determining, on the basis of the desired transmission mode and the amount of data to be transmitted, the number of OFDM symbols required for transmission of the data at the desired mode and selecting a transmission mode from said number of available modes being the mode with the slowest transmission speed capable of accommodating the data within the same number of OFDM symbols as required to transmit the data at the desired transmission mode.

In a preferred embodiment of the invention, the step of selecting the transmission mode to be used comprises checking whether a transmission mode slower than said requested transmission mode is capable of transmitting said data using the same number of OFDM symbols and, if said slower transmission mode is capable of transmitting said data using the same number of OFDM symbols, then selecting said slower mode for transmission of said data.

The step of checking may be performed iteratively until the slowest possible transmission mode is identified that does not increase the transmission duration.

A further aspect of the invention provides a method of processing a request for transmission of a packet of data in a physical layer (level 1) of a communications protocol in a communications device, said physical layer being capable of transmitting data in accordance with one of a plurality of predetermined transmission modes, comprising selecting a transmission mode by changing the mode from the requested transmission speed to a lower speed if this can be achieved without increasing the number of OFDM symbols required to transmit the data, preparing a transmission channel in accordance with the selected mode, and establishing physical transmission on said transmission channel.

A further aspect of the invention provides a method of processing a request for transmission of a packet of data, in a data link layer (level 2) for passage from said data link layer to a physical layer (level 1) of said communications protocol in a communications device, said physical layer being capable of transmitting data in accordance with one of a plurality of predetermined transmission modes and said request specifying one of said modes as a desired mode for transmission of a packet of data, comprising determining if a transmission mode with a lower transmission speed than that of the transmission mode specified in said request can be identified which will cause transmission of the data without increasing the number of OFDM symbols required to transmit the data, and, if said selected transmission mode differs from the originally requested transmission mode, revising said request accordingly.

A further embodiment of the invention provides a method for determining an appropriate transmission mode for a packet of data in a communications protocol, the packet of data including request information specifying a requested transmission mode for transmission of the data, the method including the steps of determining a slowest possible transmission mode given the requested transmission mode and the number of OFDM symbols within which said data is contained in said requested transmission mode, said slowest possible transmission mode allowing the data to be contained by the same number of OFDM symbols as in the requested transmission mode, determining a transmission mode for transmission of the data on the basis of the slowest possible transmission mode, and other network communication conditions.

In the event that the transmission mode in which the data is eventually transmitted is different from the mode originally identified in the request for transmission of the data then, in a preferred embodiment, where the transmission mode at which the data is transmitted is identified in a data packet in which the data is contained, then this identification may be appropriately adjusted.

According to another aspect of the invention, communications apparatus is provided which is configured for communication of data in accordance with an agreed protocol, and operable to transmit data in OFDM symbols of predetermined length in one of a plurality of predetermined transmission modes wherein a mode offers a transmission speed at a corresponding robustness and wherein speed and robustness are substantially inversely related, the apparatus comprising means for selecting a transmission mode, comprising means for receiving a request for transmission of data, said request containing an indication of a desired one of the available transmission modes for transmission of the data, means for determining, on the basis of the desired transmission mode and the amount of data to be transmitted, the number of OFDM symbols required for transmission of the data at the desired mode and means for selecting a transmission mode from said number of available modes being the mode with the slowest transmission speed capable of accommodating the data within the same number of OFDM symbols as required to transmit the data at the desired transmission mode.

The selecting means may comprise checking means for checking whether a transmission mode slower than said requested transmission mode is capable of transmitting said data using the same number of OFDM symbols; the selecting means may be operable to select said slower mode for transmission of said data, if said checking means determines that said slower transmission mode is capable of transmitting said data using the same number of OFDM symbols.

In a preferred embodiment, the checking means is operable iteratively until the slowest possible transmission mode is identified which does not increase the transmission duration.

The apparatus preferably further comprises means for preparing a transmission channel in accordance with the mode selected in said selecting means, and means for establishing physical transmission on said transmission channel.

A further aspect of the invention provides a communications system comprising a plurality of communications apparatus in accordance with any of the above.

Implementation of the invention may be achieved on a general purpose computer, or application specific communications apparatus, capable of being configured by a computer program. A computer program may be introduced by means of a carrier medium, which may be a computer readable medium such as an optical disk, or by means of an application specific device such as a smart card or an ASIC. Further, the computer program may be introduced, in accordance with the invention, by means of a signal.

A specific embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates a schematic diagram of a network comprising a number of communications nodes in accordance with a specific embodiment of the invention, each communications node being in wireless communication with at least one other; Figure 2 illustrates a schematic diagram of one of the communications nodes of the specific embodiment, illustrated in Figure 1; Figure 3 illustrates a schematic diagram of communications facilities, including a data link layer and a physical layer, of the communications node of the specific embodiment illustrated in Figure 2; Figure 4 illustrates a schematic diagram of a physical layer convergence protocol sub- layer, in accordance with the specific embodiment of the invention, including a request processing unit; and Figure 5 illustrates a flow diagram of a process performed by the request processing unit illustrated in Figure 4 on receipt of a transmission request from a medium access control sub-layer of the data link layer illustrated in Figure 3.

As illustrated in Figure 1 a network 10 comprises five communications nodes 20 (herein distinguished by letters A - E). The nodes 20 are capable of communication with one another by wireless links 22, and in this embodiment in accordance with IEEE standard 802.1 la.

Figure 2 illustrates schematically one of the communications nodes 20 in further detail.

The communications node 20 comprises a processor 24 operable to execute machine code instructions stored in a working memory 26 and/or retrievable from a mass storage device 28. By means of a general purpose bus 25, user operable input devices 30 are in communication with the processor 24. The user operable input devices 30 can comprise, in this example, a keyboard, a mouse or other pointing device, a writing tablet, speech recognition means, or any other means by which a user input action can be interpreted and converted into data signals.

Audio/video output hardware devices 32 are further connected to the general purpose bus 25, for the output of information to a user. Audio/video output hardware devices 32 can include a visual display unit, a speaker or any other device capable of presenting information to a user.

Communications hardware devices 34, connected to the general purpose bus 25, are connected to an antenna 36. By the communications hardware devices 34 and the antenna 36, the communications node 20 is capable of establishing wireless communication with another device.

In the illustrated embodiment in Figure 2, the working memory 26 stores user applications 40 which, when executed by the processor 24, cause the establishment of a user interface to enable communication of data to and from a user. The applications in this embodiment establish general purpose or specific computer implemented utilities that might habitually be used by a user.

Communications facilities 42 in accordance with the specific embodiment are also stored in the working memory 26, for establishing a communications protocol to enable data generated in the execution of one of the applications 40 to be processed and then passed to the communications hardware devices 34 for transmission and communication with another communications device. It will be understood that the software defining the applications 40 and the communications facilities 42 may be partly stored in the working memory 26 and the mass storage device 28, for convenience. A memory manager could optionally be provided to enable this to be managed effectively, to take account of the possible different speeds of access to data stored in the working memory 26 and the mass storage device 28.

On execution by the processor 24 of processor executable instructions corresponding with the communications facilities 42, the processor 24 is operable to establish a communications protocol in accordance with the open system interconnection (OSI) model, which is widely recognised as a standard approach to the schematic representation of a communications protocol.

In the present description of a specific embodiment of the invention, the five highest- level layers (levels 3 to 7) are not described in detail as they are of conventional construction and implementation, and their internal function is not of relevance to the understanding of the performance of the present invention.

Of the seven layers of the OSI model, the physical layer (level 1) 50 and the data link layer (level 2) 60 are illustrated in Figure 3. The physical layer 50 comprises software components required to drive the communications hardware devices to emit electromagnetic radiation via the antenna 36, and to detect electromagnetic radiation signals received in the communications devices 34 from the antenna 36. The data link layer 60 is operable to encode data packets into bits. It furnishes transmission protocol knowledge and management to the physical layer 50. The data link layer is in communication with a network layer (not illustrated) establishing a networking protocol. An example of a networking protocol would be the Internet protocol (IP).

The physical layer 50 comprises a physical layer conversion protocol (PLCP) sub-layer 52 which receives messages from the data link layer 60, in which selection will have been made of one of several transmission modes offered for selection in accordance with the 802.11 a standard. The PLCP sub-layer translates the specified rate to be used into a specific modulation and coding scheme to be used by the physical medium dependent (PMD) sub-layer 54 which will then drive the specific communications hardware device 34.

The data link layer 60 comprises a logical link control (LLC) sub-layer 62 which controls frame synchronization, flow control and error checking in data to be sent, and a media access control (MAC) layer 64 which controls how the communications device gains access to data and permission to transmit it. The MAC sub-layer 64 is the part of the data link layer 60 which specifies the transmission mode to be used by the physical layer 50.

Figure 4 illustrates the PLCP sub-layer 52 of the present embodiment of the invention.

The PLCP sub-layer 52 includes a mode specification unit 56, exposed to the MAC sub- layer 64 of the data link layer 60, and thus offering a set of transmission modes available for selection by the MAC sub-layer 64. A request processing unit 57 is operable to receive and process transmission requests from the MAC sub-layer 64, and to adjust the selection of the transmission mode to be used.

The request processing unit 57 is generally operable to determine the number of OFDM symbols that a particular data transmission request will require for conveying the data contained in the request, and to determine whether the requested transmission mode is faster than is necessary in order to transmit the data within the same number of OFDM symbols. The request processing unit 57 will reduce the requested transmission mode to a lower transmission rate, but thereby increasing transmission robustness, if the data can still be transmitted in the same number of OFDM symbols, and thus with the same transmission duration. Then, the request, whether modified or otherwise, is passed to the PMD sub-layer 54.

In that way, the request processing unit selects the most robust transmission mode available to transmit data without increasing the transmission speed duration.

The process by which the PLCP sub-layer 52 operates will now be described with reference to the flow diagram illustrated in Figure 5. The process commences on receipt of a transmission request from the MAC sub-layer 64.

In Step S1-2 of Figure 5, on receipt of a packet to be sent, the request processing unit 57 performs a mode reduction process. The purpose of the mode reduction process is to determine the mode that provides the slowest possible transmission, given the number of OFDM symbols to be used in the requested mode to transmit the data.

The mode reduction process begins in step S1-2, wherein the request processing unit 57 calculates the number of OFDM symbols required to convey data in the request received from MAC sub-layer 64, at the rate indicated in that request. Then, in step S I - 4, the request processing unit 57 calculates the number of OFDM symbols required to convey the data in the request at the next slowest transmission mode available and implemented by the physical layer.

For devices operating according to the IEEE 802.1 l a standard, the number of transmitted OFDM symbols N is determined as: N = ceiling ((16 + PSDU_LENGTH + 6) / BPOS) where BPOS is the number of uncoded data bits in each transmitted OFDM symbol and PSDU_LENGTH is the number of bits of data to be transmitted, contained in what is known as the physical layer service data unit (PSDU). The addition of 16 to the PSDU length accounts for the PLCP 'Service' data, and the extra 6 bits account for the necessary tail bits.

These calculations are then considered in step S1-6, to establish if the reduction ofthe transmission has had any effect on the number of OFDM symbols required to convey the data.

if the number of OFDM symbols remains unchanged, then step S 1-4 is repeated until the check in step S 1-6 finds that the number of OFDM symbols increases. At this point, the process continues in step S 1-8 by determining that the previously considered mode is the slowest possible mode to allow transmission without increasing the number of OFDM symbols used. Unless the originally requested mode is in fact the slowest possible mode, this mode reduction procedure will increase the robustness of transmission to errors.

Following performance ofthe mode reduction procedure, in step S1-10 the packet is transmitted at the mode determined in the mode reduction procedure. The process then ends.

In general, therefore, the rate of the PHY transmission (the modulation and coding scheme employed) is chosen by the described embodiment so as to minimise the number of padding bits required to fill the last OFDM symbol, without increasing the total number of OFDM symbols.

Since this method is of particular benefit for small (e.g. control) packets, it is illustrated here for ACK, RTS and CTS packets in an 802. l 1 a system. The values in Table 2 show for each PHY transmission mode, the number of data bits conveyed in each OFDM symbol and the number of padding bits needed to fill the final OFDM symbol.

Table 2

OFDM

CodedData BitsSymbolsOFDMPadding bits perperfor anSymbolsBits inPadding 802.11 aOFDMOFDMACK orfor anACK orBits in ModesymbolsymbolCTSRTSCTSRTS 7288192.96118816 For the 802.11 a parameters shown in Table 2, it can be seen that there is no point in transmitting an RTS packet at a higher rate than mode 7, and for ACK or CTS packets there is no benefit from modes above 6. Similarly, it would always be better to choose to send ACK or CTS packets in mode 4 rather than mode 5.

It will be understood that in a system which is strictly compliant with the 802.1 la standard, not all of the modes listed above will be available for use with specific control packets, as the 802.11 a standard specifies additional constraints on PHY mode selection.

It is also important to appreciate that it may, under some circumstances, be appropriate to employ a lower rate than the slowest possible, such as for reasons of transmission quality problems detected in past transmissions.

It will be appreciated that the present invention as described above is applicable to any communications protocol, irrespective of the specific protocols used in establishing the top 5 layers of the OSI model protocol stack. The invention relates to the interaction between the level 2 (MAC sub-layer) functionality and the level 1 (PHY layer) functionality. As such, it will further be appreciated that the invention could be embodied in a MAC layer implementing unit, rather than in the PHY layer. Though it is often considered that providing the MAC layer with further functionality relating to the selection of criteria of operation of thePHY layer is undesirable, in certain circumstances this could be a convenient implementation.

Further, in other circumstances, variation of the functionality of the MAC layer, as opposed to the PHY layer, may be necessary. It will be appreciated that this could be the case in systems such as those employing the IEEE 802.11 standard, where for instance the MAC layer must observe specific constraints on the PHY mode at which control packets are transmitted. Additionally, for the transmission of data packets there may also be constraints governing which PHY modes are permissible for a particular transmission, depending on knowledge of which PHY modes are supported by the destination device or devices. In such a case, a system would need to be devised which combined the present invention with any such additional rules governing PHY rate selection.

It will further be appreciated that implementation of the present invention is not limited to the IEEE 802.1 la standard. The invention can also be applied, with appropriate modification, to any devices and systems employing multiple PHY modes and multi- carrier modulation, such as OFDM or multi-carrier code division multiple access communication. This may be in accordance with the IEEE 802.1 l standard and its amendments, or any other relevant governing standards.

The 802.1 l a standard also specifies that the MAC header contained in each packet has a Duration/lD field specifying the duration for which the medium is to be considered as being busy, and which is calculated based on the transmission duration. Implementation of the present invention, for example by means of the illustrated embodiment, would not affect this calculation since the transmission duration of the packet is not changed.

An alternative embodiment may entail the use of adaptive bit loading, wherein a different PHY transmission mode may be selected for each OFDM subcarrier. In respect of the present invention, all transmission mode combinations offered by the adaptive bit loading scheme may be considered as providing a larger set of transmission modes from which the rate selection algorithm may choose. The method by which the present invention applies to such a scheme will be apparent to those skilled in the art.

While the present invention has been described by way of a specific embodiment involving provision of software, comprising processor executable instructions configured to cause operation of a protocol stack on a computer or other communications unit, it will be appreciated that the invention can also be implemented using an application specific device, and also by means of hardware configured to operate in a comparable and equivalent manner.

Other networking protocols, such as protocols used in mobile telephony to establish networking, are also capable of being used in accordance with the invention.

Introduction of software into the communications unit can, in a specific embodiment of the invention, be by means of a software product supplied on a carrier medium such as a computer readable optical disk (e.g. CD-ROM or DVD format), by means of a computer receivable signal, such as via the Internet and wireless communication, from a server unit, or, in the case of a mobile telephony application, by means of a short message (SMS) or other messaging technology in which executable instructions can be bundled in data for automatic configuration of a remotely located device.

Other embodiments and variations of the described specific embodiments will also be apparent to the reader, and no feature in the specific embodiment described herein is intended to form a limitation on the scope of protection sought hereby; the scope of protection to be determined from the accompanying claims.

Claims

CLAIMS: 1. A method of generating a transmission mode selection for

transmission of data, said data being associated with a desired one of a number of predetermined transmission modes, for configuration of a physical layer of a communications protocol stack implemented in a communications device, said physical layer being operable to transmit data in OFDM symbols of predetermined length and in a selected one of said predetermined transmission modes, comprising: determining, on the basis of the desired transmission mode and the amount of data to be transmitted, the number of OFDM symbols required for transmission of the data at the desired mode and generating a transmission mode selection from said number of available modes, the mode selection identifying the mode with the slowest possible transmission speed capable of accommodating the data within the same number of OFDM symbols as required to transmit the data at the desired transmission mode.
2. A method in accordance with claim 1 wherein the step of selecting the transmission mode to be used comprises checking whether a transmission mode slower than said requested transmission mode is capable of transmitting said data using the same number of OFDM symbols and, if said slower transmission mode is capable of transmitting said data using the same number of OFDM symbols, then selecting said slower mode as said transmission mode selection.
3. A method in accordance with claim 2 wherein the step of checking is performed iteratively until the slowest possible transmission mode is identified.
4. A method of processing a request for transmission of a packet of data in a physical layer (Ievel 1) of a communications protocol in a communications device, said physical layer being capable of transmitting data in accordance with one of a plurality of predetermined transmission modes, comprising: selecting a transmission mode by means of a method in accordance with any preceding claim, preparing a transmission channel in accordance with the selected mode, and establishing physical transmission on said transmission channel.
5. A method of processing a request for transmission of a packet of data, in a data link layer (level 2) for passage from said data link layer to a physical layer (level 1) of a communications protocol in a communications device, said physical layer being capable of transmitting data in accordance with one of a plurality of predetermined transmission modes and said request specifying one of said modes as a desired mode for transmission of a packet of data, comprising selecting a transmission mode by means of a method in accordance with any one of claims 1 to 4 and if said selected transmission mode differs from the originally requested transmission mode, revising said request accordingly.
6. A method of processing a request for transmission of a packet of data, in a data link layer (level 2) for passage from said data link layer to a physical layer (level 1) of a communications protocol in a communications device, said physical layer being capable of transmitting data in accordance with one of a plurality of predetermined transmission modes and said request specifying one of said modes as a desired mode for transmission of a packet of data, comprising determining a transmission mode selection by means of a method in accordance with any one of claims I to 4, using said transmission mode selection in determining, in said data link layer, which mode, of said predetermined transmission modes, is most appropriate for use given the requested mode and prevailing system conditions, and transmitting said data in accordance with said most appropriate transmission mode.
7. Communications apparatus configured for communication of data in accordance with an agreed protocol, operable to transmit data in OFDM symbols of predetermined length in one of a plurality of predetermined transmission modes wherein a mode offers a transmission speed at a corresponding robustness and wherein speed and robustness are substantially inversely related, the apparatus comprising means for selecting a transmission mode, comprising: request receiving means for receiving a request for transmission of data, said request containing an indication of a desired one of the available transmission modes for transmission of the data, determining means for determining, on the basis of the desired transmission mode and the amount of data to be transmitted, the number of OFDM symbols required for transmission of the data at the desired mode and transmission mode selection generating means for generating a transmission mode selection from said number of available modes, the mode selection identifying the mode with the slowest possible transmission speed capable of accommodating the data within the same number of OFDM symbols as required to transmit the data at the desired transmission mode.
8. Apparatus in accordance with claim 7 wherein the transmission mode selection generating means comprises checking means for checking whether a transmission mode slower than said requested transmission mode is capable of transmitting said data using the same number of OFDM symbols and wherein the transmission mode selection generating means is operable to select said slower mode as said transmission mode selection if said checking means determines that said slower transmission mode is capable of transmitting said data using the same number of OFDM symbols.
9. Apparatus in accordance with claim 8 wherein the checking means is operable iteratively until the slowest possible transmission mode is identified.
10. Apparatus in accordance with any of claims 7 to 9 and further comprising means for preparing a transmission channel in accordance with the mode identified in said transmission mode selection, and means for establishing physical transmission on said transmission channel.
11. A communications system comprising a plurality of communications apparatus in accordance with any one of claims 7 to 10.
12. A computer readable carrier medium storing computer executable instructions which, when loaded into a general purpose computer, configure the computer to perform the method of any of claims 1 to 6.
13. A computer readable carrier medium storing computer executable instructions which, when loaded into a general purpose computer, configure the computer to as communications apparatus in accordance with any of claims 7 to 12.
14. A computer receivable signal bearing computer executable instructions which, when loaded into a general purpose computer, configure the computer to perform the method of any of claims 1 to 6.
15. A computer receivable signal bearing computer executable instructions which, when loaded into a general purpose computer, configure the computer to as communications apparatus in accordance with any of claims 7 to 12.
16. A method of determining a transmission mode to be used in relation to a request to transmit data, substantially as described herein with reference to the accompanying drawings.
17. Apparatus for determining a transmission mode to be used in relation to a request to transmit data, substantially as described herein with reference to the accompanying drawings.
18. A communications network substantially as described herein with reference to the accompanying drawings.