Orthogonal Frequency Division Multiplexing (OFDM) is a modulation scheme which is typically used in transmission systems exhibiting a time dispersion which is much greater than the bit period. OFDM is already specified in Digital Audio Broadcasting (DAB) and Digital Video Broadcasting (DVB). Currently, OFDM is also contemplated for use in Wireless Local Area Networks (WLAN) in the 5 GHz band as specified in Europe, USA and Japan. The European standard is the so-called HIgh PErformance Radio Local Area Network type 2 (HIPERLAN/2). This standard is currently being developed by the ETSI (European Telecommunication Standard Institute) Project BRAN (Broadband Radio Access Network). Furthermore, it may be noted that the North American and the Japanese standards for the OFDM systems have very similar physical layers as the aforementioned HIPERLAN/2 standard.
An overview of a typical OFDM transmission system SYS showing the blocks relevant for the invention is shown in FIG. 1a. Typically, an OFDM transmission TR and an OFDM receiver RC, e.g. a mobile terminal (MT), communicate over a transmission link TL. The transmitter TR comprises conventional modulation circuitry MODCRT for modulating some source information into a plurality of OFDM symbols on a plurality of subcarriers. As is known to the skilled person, this is performed by using essentially a coder and an inverse discrete Fourier transform process.
The receiver RC comprises some conventional demodulation circuitry DEMCRT for demodulating the OFDM symbols on the plurality of subcarriers back into the source information.
As known to the skilled person such demodulated circuitry DEMCRT comprises as main components a decoder and a discrete Fourier transform.
The transmission system SYS may be a fixed network or a mobile radio communication network where for example access points AP are used in order to provide an access to a receiver RC, e.g. a mobile terminal MT. However, in principle the system architecture also applies to other types of transmission systems in addition to a mobile radio communication network.
Whilst it is common to all OFDM systems that the OFDM modulation takes place at a transmitter TR and an OFDM demodulation takes place at the receiver RC, the specific manner in which the transmission takes place over the transmission link depends on the used protocol for the data exchange. FIG. 1a shows an example of the data transmission in accordance with the HIPERLAN/2 standard according to which a transmission in terms of MAC (Multiple Access Control) frames FR having a duration of e.g. 2 ms takes place. In accordance with the HIPERLAN/2 standard, each transmission frame FR consists of Broadcast Control Channel BCCH information followed by Frame Control Channel FCC information after which the respective downlink traffic and uplink traffic and information of Random Access Channels RAC follows. The actual OFDM symbols are contained in individual bursts BST. Each burst BST contains a preamble part PRE and some protocol data units PDU. The preamble part PRE is necessary in each burst in order to allow error correction and receiver training.
Typically, as shown in FIG. 1b, each preamble part PRE comprises two training symbols TS and a cyclic prefix CP. There are several different preambles for the different burst types for downlink, uplink and random access. However, every preamble includes the same part of the cyclic prefix CP and training symbols TS to allow a channel estimation. At a 20 MHz sampling frequency and a duration of 1.6 μs for the cyclic prefix CP and a duration of 3.2 μs for the training symbol TS, each preamble PRE contains 32 samples for the cyclic prefix CP and 64 samples for each training symbol.
Of course, the transmission via the transmission link TL (wireless or over wire) suffers from noise, distortion or other interferences such that the receiver RC can make wrong decisions regarding the assumed sent OFDM information. One of the reasons to include the known training symbols (i.e. also known on the receiver side) into the respective preamble part is to allow a receiver training, i.e. to compare the received training symbols with the known training symbols for example to estimate the channel coefficients in the receiver RC in order to avoid wrong decisions as much as possible.
Since the extent of interference, noise or distortion is dynamic, i.e. it may depend on the number of interfering users, the received signal power, the transmission conditions, etc., the transmitter TR is typically equipped with a transmission link property adjustment unit ADP which can dynamically adapt or adjust the transmission characteristics of the communication on a transmission link TL. Thus, the adjustment unit ADP performs a function which is usually referred to as link adaptation (LA), i.e. a setting of predetermined transmission properties which are assumed to increase the transmission link quality. For example, according to the HIPERLAN/2 standard, various physical layer modes can be set by the adjustment unit ADP. FIG. 2a shows the key parameters of the HIPERLAN/2 physical layer modes. FIG. 2b shows a table of the key parameters of the HIPERLAN/2 physical layer. It should be noted that the physical layer mode setting is performed on the basis of the available physical layer modes as shown in FIG. 2a. LA should be understood as a general term for methods to select transmitter parameters. This, for instance, includes the setting of output power, which is also referred to as power control. For example, in a conventional transmitter the transmission power may be controlled on the basis of the received power at the receiver and/or on the basis of the measured SNR.
As shown in FIG. 2a, HIPERLAN/2 provides six mandatory modes with bit rates of 6, 9, 12, 18, 27, 36 Mbps and one further optional mode with a bit rate of 54 Mbps. By switching between the different physical layer modes, the transmission quality on the transmission link TL can be dynamically adapted to the prevailing transmission conditions.
However, the adjustment unit ADP needs an indication from a transmission link property selector TL-SEL in order to know which physical layer mode needs to be selected. Typically, the transmission link property selector TL-SEL is formed by a link quality measurement unit LQ-DET which performs link quality measurements (LQMs) on the transmission link TL and which outputs a link quality measure Q to a transmission property decider TR-DEC. On the basis of the link quality measurements carried out by the link quality measurement unit LQ-DET, the transmission property decider TR-DEC decides the physical layer mode and provides an indication with respect to the selected physical layer mode to the adjustment unit ADP which then sets the selected physical layer mode.
Link quality measurements can in principle be carried out by a link quality measurement unit LQ-DET on the transmitter TR or elsewhere in the access point AP site (the transmitter is part of the access point AP), on the receiver RC site or even within another unit AU of the transmission system SYS involved in the communication and being arranged elsewhere, i.e. neither in the access point AP or the transmitter TR or the receiver RC. Likewise, the transmission property decider TR-DEC may be provided in the transmitter TR or elsewhere in the access point AP, in the receiver RC or in any other unit AU. If the transmission property decider and the link quality measurement unit are provided outside the transmitter TR, the adjustment unit ADP will eventually receive a corresponding signal from the outside provided transmission property decider TR-DEC. Therefore, the transmission link property selector TL-SEL constituted by the transmission property decider TR-DEC and the link quality measurement unit LQ-DET should not be seen as situated exclusively in the transmitter TR or receiver RC since the particular arrangement will depend on the system implementation. A common aspect is that LQMs must be carried out and a corresponding selection signal with respect to the selected physical layer mode must be provided to the adjustment unit ADP. Link adaptation (LA) schemes may use a variety of link quality measurements which may be derived either on the data link control (DLC) layer or the physical layer.
Of course, it is very important how the link quality measurements are carried out and how the link quality measure is determined since the link quality measure is the very criterion which will be used as decision criterion for selecting the appropriate transmission mode. For example, if the link quality measure is not accurate, an over compensation, i.e. a lower bit rate than would actually be possible, may be selected. Likewise, if the link quality measure is incorrect, i.e. predicts a better transmission quality than it is actually present, then a too high bit rate may be selected than would actually be appropriate. Therefore, the determination of the link quality measure, i.e. how the link quality measurements are carried out and what parameters are used for deriving the link quality measure, is of essential importance for an accurate link adaptation.