WO2006030346A1

WO2006030346A1 - Method for excluding subcarriers with high bit error rates

Info

Publication number: WO2006030346A1
Application number: PCT/IB2005/052908
Authority: WO
Inventors: Hendricus T. G. M. Penning De Vries
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-09-14
Filing date: 2005-09-06
Publication date: 2006-03-23

Abstract

The invention relates to a method for the wireless transmission of data, whereby data is transmitted by using a multitude of predetermined sub carriers within a frequency range, some of the sub carriers being excluded from data transmission, and all sub carriers being used for data transmission are used at the same time. The invention also relates to a transmitter for the wireless transmission of data, comprising means (3, 4) for transmitting data by using a multitude of predetermined sub carriers within a frequency range, whereby data is transmitted in individual bursts (5), the bursts comprising at least three fields (5A, 5B, 5C). The transmitter further comprises means (6) for the insertion a piece of information in one of the fields, the piece of information informing a receiving station (2), which sub carriers, are excluded from data transmission. The means (3, 4) for transmitting data are adapted to use all predetermined sub carriers except the excluded sub carriers.

Description

METHOD FOR EXCLUDING SUBCARRIERS WITH HIGH BIT ERROR RATES

The invention relates to the wireless transmission of data, and particularly to the wireless transmission according to the IEEE 802.11 standard.

The IEEE 802.11 standard is a standard of a short-range wireless communication system targeted for wireless LAN (local area network). Currently, three versions of the IEEE 802.11 standard exist, namely 802.1 Ia, 802.1 Ib, and 802.1 Ig. In all versions data are not modulated on and transmitted by a single carrier frequency. Instead, a spread-spectrum method is used.

In IEEE 802.1 Ia and in IEEE 802.1 Ig, each frequency band is divided up into a number of frequency carriers, or sub carriers. The standards support a total of 64 sub carriers, of which 48 sub carriers can be used for data transmission. Each of the 48 sub carriers is modulated individually, for example by QAM (quadrature amplitude modulation). In this way the bit stream is divided into 48 parallel bit streams that have a much lower bit rate than the total bit stream. All sub carriers are used at the same time as they are used in parallel. This composite signal is then used to modulate the main carrier by means of orthogonal frequency division modulation (OFDM). Each frequency band allows the transmission of a single IEEE 802.1 Ia OFDM modulated signal build from its sub carriers.

The IEEE 802.11 a/g standard rules that each transmission burst consists of a preamble field, a signal field, and a data field. The receiving station uses the preamble field for frequency and clock synchronization purposes. This is necessary as the demodulation by the receiving station can only be performed when an exact carrier frequency is known.

In the signal field, the station transmitting the data informs the receiving station which type of modulation/encoding is used for the data contained in the data field. The receiving station can derive this information from the signal field because the length and the coding method for the signal field is always the same, as it is defined in the IEEE 802.11 a/g standard. Consequently, a receiving station processes the signal field, and learns from the information given in the signal field which modulation schemes were used for the data in the data field. Knowing these schemes, the receiving station can process the data field properly.

Whereas data contained in the preamble field, and data contained in the signal field each have an administrative function for the proper transmission of user data, the user data themselves are contained in the data field. The standard supports eight modulation schemes for the data field. In most cases, the modulation used for the signal field scheme will be different from the corresponding schemes used in the data field.

In general, the receiving station receives a number of reflected and attenuated signals. All sub carriers are independently subject to constructive or destructive interference. This fading effect is responsible for the fact that most sub carriers show an average signal-to- noise ratio, but a few sub carriers having a worse than average signal-to-noise ratio. After a demodulation, sub carriers with a low signal-to-noise ratio will contribute most to the system error rate that is representative for the quality of data received by the receiving station. IEEE 802.1 Ia includes a mechanism to cope with errors occurring in the process of data transmission. The transmitter adds a certain amount of redundancy to the data bits by using a convolutional code. Then the encoded data bits are transmitted by QAM modulated sub carriers. The station receiving the signal demodulates the signal, and a decoder uses the redundancy to reconstruct the data even in the case of that some parts of the data are severely degraded in the transmission process.

Bluetooth© systems and IEEE 802.11b devices implement frequency hopping methods. They transmit their signals during short intervals on different carrier frequencies. The hopping periods can be shorter than 1 ms. This time interval Δti is far shorter than the time Δt₂ needed for a complete transmission of a data packet.

US 2002/0080739 Al discloses a wireless communication system using a plurality of frequency channels, for example a Bluetooth© system or a wireless LAN system according to IEEE 802.11b. Both systems use frequency channels in the 2.4 GHz region, such that there is a risk that interference of electromagnetic signal will occur between them. To combat such an interference between an IEEE 802.1 Ib system and a Blootooth© system the authors suggest that the IEEE 802. lib system detects the contribution of each frequency channel to the total error rate, and that those channels are excluded from data transmission which have a higher contribution than a threshold value.

It is an object of the invention to provide a method for the wireless transmission of data of the kind mentioned above, whose average bit error rate has a comparatively low value, and which is implemented in an easy fashion. This object is solved by the features of the independent claims. Further embodiments of the invention are described by the features of the dependent claims. It should be emphasised that any reference signs in the claims shall not be construed as limiting the scope of the invention. According to the invention, this object is achieved by a method for the wireless transmission of data, whereby data is transmitted by using a multitude of predetermined sub carriers within a frequency channel. Not all of the maximum number of (allowed) sub carriers within a given frequency band are used however. Instead, some of the sub carriers are excluded from data transmission and are not used at all for transmitting data. All sub carriers which are used for data transmission are used at the same time.

The method described in this specification uses a spread-spectrum method for transmitting data, namely uses spread spectrum OFDM (orthogonal frequency division modulation). Identical to the situation in the IEEE 802.1 la/g standard, the method makes use of a frequency range having a predetermined number of predetermined sub carriers. A sub carrier for the purposes of this specification should be understood as an information carrying wave which in turn modulates the main carrier. Preferably, the invention makes use of 48 sub carriers which carry data. More preferably, the data carrying sub carriers are identical to those of the IEEE 802.1 Ia and g standard. Data transmission can be done by using OFDM. In IEEE 802.1 Ia and g, all sub carriers within a 20 MHz frequency band must be used, whereby all sub carriers are used at the same time. Contrary to the standard, it is suggested to exclude a predetermined number of sub carriers from data transmission, and to use all of the remaining sub carriers at the same time. As an example, in a method which would have 48 sub carriers at its disposal, 3 sub carriers could be excluded from data transmission. All of the remaining 45 sub carriers would then be used at the same time. Those sub carriers having a high contribution to the total bit error rate will be excluded, whereby this step improves the average bit error rate.

The bit error rate is improved when one or more sub carriers show worse than average performance over a time interval Δti which is longer than the time interval Δt₂ needed to transmit a data packet, or in other words when Δti > Δt₂, The worse sub carrier performance can be caused either by channels that have severe attenuation at some sub carriers, or by external interference sources that radiate significant energy at some sub carrier frequencies. The time interval for packet transmission should include the time needed to estimate the channel response, and to make this information available to the transmitter. In order to realize the above mentioned method, a communication system, comprising a transmitter and a receiver which are adapted to carry out the method, has to be used.

The transmitter includes means for transmitting data by using a number of predetermined sub carriers within a frequency range, whereby data is transmitted in individual bursts, the bursts comprising at least three fields. Furthermore, the transmitter has means for the insertion of a piece of information in one of the fields, the piece of information informing the receiving station which sub carriers are excluded from data transmission. To take the list of excluded sub carriers into account, the means for transmitting data are adapted to use all predetermined sub carriers for data transmission except the excluded sub carriers. Correspondingly, a receiver which carries out the method as described above has means for receiving data from a number of predetermined sub carriers within a frequency range, whereby data is received in individual bursts, the bursts comprising at least three fields. In addition, the receiver has means for extracting a piece of information from one of the fields, the piece of information informing the receiver which sub carrier are excluded from data transmission. To take the list of excluded sub carriers into account, the means for receiving data being adapted to receive data from all predetermined sub carriers except the excluded sub carriers.

The adaptation of the transmitter and the receiver can be realized in hardware and/or can be realized by software. The transmitter or the receiver has a functionality, either in hardware, software, or firmware, which provides the additional means for inserting or extracting the above mentioned piece of information from one of the fields, and the other means mentioned below.

According to the invention, data is transmitted in a sequence of bursts. Each burst comprises at least three fields, whereby one of the fields contains the information which sub carriers are excluded from data transmission. Preferably, there are three fields identical to the IEEE 802.1 la/g standard, namely a preamble field, a signal field, and a data field. In this case the signal field can contain the information which sub carriers are excluded from data transmission. In order to improve the throughput an adaptive modulation technique can be used, in which each sub carrier has its own modulation scheme. This is called water filling. The modulation scheme which is chosen for the individual sub carrier depends on its individual bit error rate. This makes it necessary to describe the modulation scheme for each sub carrier in the signal field. Projecting the water filling method to IEEE 802.1 Ia, this would require a maximum of 48 (number of sub carriers)* 3 bits (for describing the modulation scheme) = 144 bits in the signal field. In practice, only a few sub carriers will contribute significantly to the total error rate. This is why a far smaller number of sub carriers will be excluded in practice. If 4 out of 48 sub carriers were excluded, an application of the water filling approach to the method according to the invention would require only 3 (bits specifying the modulation for the sub carriers)+ 4 (number of sub carriers) * 6 bits (for describing the index of the sub carrier)= 27 bits.

The preferred system assigns one modulation method for all the sub carriers, for example 16QAM or 64QAM, and those carriers with strong negative impact on the error rate are excluded from the transmission. Identical to the situation in the last paragraph, only a few sub carriers need to be excluded. A receiving station (also referred to as a receiver) would process the signal field first, and would extract the information which sub carriers are excluded from data transmission. The other way round the receiving station learns which sub carriers are actually used for data transmission.

Accordingly, a transmitter carrying out the method as described in the last paragraph has means for transmitting data which are adapted to transmit data in bursts, comprising a preamble field, a signal field, and a data field. Such a transmitter is common for IEEE 802.11 communication, and has, so far not known in the prior art, additional means for inserting or placing the above mentioned piece of information in the signal field. ' : • Correspondingly, the receiver which communicates with such a transmitter has means¹ for extracting the piece of information concerning the excluded sub carriers from the signal field.

When a transmitting station (also referred to as a transmitter) transfers data to a receiving station according to the invention, a decision has to be taken which sub carriers should be excluded from data transmission. The decision can be taken by the transmitter, who communicates the decision to the receiving station. In this case, the transmitter may send a transmission request or channel information request to the receiver. Preferably, the receiver has a measurement system for determining the bit error rate of individual sub carriers, measures the channel response with this measurement system, and sends this information back to the transmitter. The transmitter uses the information as a serious advice, and makes the final decision to which carriers are excluded. The information is put in the signal field, so that the receiver can process the data packet correctly.

The data format that the receiver uses to send the channel information to the transmitter could also be based on a number of carriers to exclude. For performing the above mentioned tasks, the receiver is provided with a logic for determining the bit error rates of all the individual sub carriers within a frequency band. With this information a logic can determine which sub carrier has a high bit error rate, or which sub carrier has a bit error rate higher than average. The logic may be adapted to determine the contribution of the bit error rate of the individual sub carrier to the total bit error rate. For simplicity, the sub carriers having the largest contribution to the total bit error rate will be excluded from data transmission. This decision can be taken by a logic which may be identical or different from the logic mentioned above, which chooses sub carriers to be excluded from data transmission to optimise the throughput and the total bit error rate simultaneously.

Whereas excluding sub carriers will in most cases reduce the throughput, not excluding sub carriers will yield a higher bit error rate. In this way a compromise has to be found between to achieve both goals at the same time. In many cases excluding less than 10% of the maximum number of allowed sub carriers, for example roughly 4 out of 48 in the IEEE 802.11 a/g standard, will be sufficient.

Transmission of data is preferably performed in the frequency ranges of the IEEE 802.11 standard, namely in the frequency ranges of about 5.150 GHz to about 5.850 GHz (IEEE 802.11a), or of about 2.4 GHz to about 2.4835 GHz (IEEE 802.11 g).

Depending on the circumstances the number of sub carriers which are excluded from data transmission can be constant, or is variable. In the first case a constant bit rate can be achieved more easily which is useful for the transfer of audio signals, or of video signals. If the number of sub carriers which are excluded from data transmission is variable, the total number of sub carriers being used for data transmission can be chosen to be very high. At the same time the bit error rate can be acceptably good if the transmission conditions are good. A variable number of excluded sub carriers is thus a good choice for many applications where throughput should be high, for example in computer applications.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described thereafter.

Fig. 1 shows a communication system with a transmitter and a receiver, Fig. 2 shows a signal burst with three signal fields. Fig. 1 shows a system for the wireless transmission of data having a transmitting station 1, also called a transmitter, and a receiving station 2, also called a receiver. Transmitter 1 has a generator 3 for electromagnetic radiation in the frequency range from about 2.4 GHz to about 2.4385 GHz. The radiation is emitted by means of an antenna 4. Receiver 2 has a corresponding antenna 4' which absorbs the radiation, and converts it into an electric current by means of a converter 3'.

Transmitter 1 transmits coded signals in a sequence of bursts 5 and uses a total of 48 QAM modulated OFDM sub carriers for the transmission of data. Each burst contains three fields, namely a preamble field (5A), a signal field (5B), and a data field (5C) corresponding to the IEEE 802.11 a/g standard, see Fig. 2.

Transmitter 1 has means 6 for inserting a list 8 of sub carriers into the signal field 5B. This list 8 contains the indexes of 4 sub carriers which are excluded from data transmission. In this way the sub carriers which are not used for data transmission can be unambiguously identified. Correspondingly, receiver 2 has means 7 for extracting the indexes of these sub carriers which are not used for data transmission.

When a communication between the transmitter 1 and the receiver 2 is established, the transmitter 1 sends a channel information request to the receiver 2. The receiver 2 uses its measurement system 12 for determining the bit error rate of all 48 sub carriers within a 20 MHz frequency channel. Furthermore, receiver 2 determines the contribution of each sub carrier to the total bit error rate by means of a logic 9. On the basis of the measured contribution of the individual sub carrier to the total bit error rate logic 10 determines a constant number of 4 sub carriers which have the highest contribution to the total bit error rate.

Having determined the worst 4 sub carriers, the receiver 2 transmits the indices of the 4 sub carriers to the transmitter 1 as a suggestion, that these 4 sub carriers should be excluded. If this suggestion is accepted by the transmitter 1, the transmitter 1 will only use the remaining 44 sub carriers of the frequency channel for data transfer. Transmitter 1 uses its insertion means 6 to insert the information, which sub carriers are excluded from data transmission, in the signal field 5B of a data packet. Correspondingly, receiver 2 has an extraction means 7 to extract this information from the signal field 5B. LIST OF REFERENCE NUMERALS:

01 transmitter

02 receiver

03 generator for electromagnetic radiation 03' converter 04 antenna

04' antenna

05 burst

5A preamble field

5B signal field 5C data field

06 insertion means

07 extraction means

08 list of indexes

09 logic (of the receiver) for determining the bit error rate 10 logic (of the receiver) for suggesting which sub carriers can be excluded from data transmission

11 logic (of the transmitter) for deciding which sub carriers are excluded from data transmission

12 measurement system

Claims

CLAIMS:

1. Method for the wireless transmission of data, whereby a) data is transmitted by using a multitude of predetermined sub carriers within a frequency range, b) whereby some of the sub carriers are excluded from data transmission, and c) whereby all sub carriers being used for data transmission are used at the same time.

2. Method according to claim 1, whereby data is transmitted in bursts, each burst comprising at least three fields (5 A, 5B, 5C), whereby one of the fields (5B) contains the information which sub carriers are excluded from data transmission.

3. Method according to claim 1, whereby data is transmitted in individual bursts (5), each burst comprising a preamble field (5A), a signal field (5B), and a data field (5C), and whereby the signal field (5B) contains the information which sub carriers are excluded from data transmission.

4. Method according to claim 1, whereby the transmission is performed in the frequency range of about 5.150 GHz to about 5.850 GHz, or of about 2.4 GHz to about 2.4835 GHz.

5. Method according to claim 1, whereby the number of sub carriers excluded from data transmission is kept constant.

6. Method according to claim 1, whereby the number of sub carriers excluded from data transmission is variable.

7. Method according to claim 1, whereby all sub carriers use the same modulation scheme.

8. Transmitter for the wireless transmission of data, comprising: a) means (3, 4) for transmitting data by using a multitude of predetermined sub carriers within a frequency range, whereby data is transmitted in individual bursts (5), the bursts comprising at least three fields (5A, 5B, 5C), b) means (6) for the insertion a piece of information in one of the fields, the piece of information informing a receiving station (2) which sub carriers are excluded from data transmission, and c) the means (3, 4) for transmitting data being adapted to use all predetermined sub carriers except the excluded sub carriers.

9. Transmitter according to claim 8, whereby the means (3, 4) for transmitting data are adapted to transmit data in bursts (5), the bursts comprising a preamble field (5A), a signal field (5B), and a data field (5C), and whereby the insertion means (6) place the piece of information in the signal field (5B).

10. Transmitter according to claim 8, with a logic (9) for determining the bit error rate of each sub carrier within a frequency band.

11. Transmitter according to claim 8, with a logic (11) for deciding which sub carriers are to be excluded from data transmission.

12. Receiver for the wireless transmission of data, a) with means (3', 4') for receiving data from a multitude of predetermined sub carriers within a frequency range, b) whereby data is received in individual bursts (5), the bursts comprising at least three fields (5A, 5B, 5C), c) with means (7) for extracting a piece of information from one of the fields, the piece of information informing the receiving station (2) which sub carrier are excluded from data transmission. d) the means (3', 4') for receiving data being adapted to receive data from all predetermined sub carriers except the excluded sub carriers.

13. Receiver, according to claim 12, whereby the means (3', 4') for receiving data are adapted to transmit data in bursts (5) the bursts comprising a preamble field (5A), a signal field (5B), and a data field (5C), and whereby the extraction means (7) extract the piece of information from the signal field (5B).

14. Receiver according to claim 12, with a measurement system (12) for determining the bit error rate of individual sub carriers.

15. Receiver according to claim 12, with a logic (11) adapted to transmit a piece of information to a corresponding transmitter, whereby the piece of information identifies those sub carriers of a given frequency band which have the highest contribution to the total bit error rate.

16. System for the wireless transmission of data, comprising a transmitter (1) and a receiver (2), with a transmitter according to one of the claims 8 to 11, and a receiver according to one of the claims 12 or 15.