GB2417163A

GB2417163A - Signalling scheme for wireless communication

Info

Publication number: GB2417163A
Application number: GB0417787A
Authority: GB
Inventors: Darren Phillip Mcnamara
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2004-08-10
Filing date: 2004-08-10
Publication date: 2006-02-15
Anticipated expiration: 2024-08-10
Also published as: GB2417163B; GB0417787D0

Abstract

To transmit additional information in a packet preamble without using the data content of the preamble fields, the invention can switch the modulation scheme between fields of the preamble to communicate information. A change of modulation scheme can indicate one piece of information where no change indicates a different piece of information.

Description

M&C Folio: GBP90729 Document: 1023496

SIGNALLING SCHEME FOR WIRELESS COMMUNICATION

The present invention relates to providing signalling information over a wireless network such as a wireless local area network (WLAN). The invention is particularly suited for use with communication systems in which communication takes place over a plurality of channels such as with orthogonal frequency division multiplexing (OFDM), such as that used in the IEEE standard 802.1 la.

In wireless LAN systems, data is transmitted in packets that are formatted and transmitted by the physical layer (PHY). Figure 1 shows an example of a packet structure for 802.11 a WEAN. In 802.11 a, transmissions occur in a 20MHz bandwidth using orthogonal frequency division multiplexing (OFDM). Figure 1 shows the structure of a typical packet according to the 802.11 a standard. Such a packet is transmitted in one 20MHz channel of the available OFDM channels.

Each packet starts with a short training field (STF). The STF is used for timing acquisition, automatic gain control (AGC) and frequency offset compensation. The STF field is followed by a long training field (LTF), which is used by the receiver for estimating the channel response.

A further field is included in the preamble, known as the signal field (SIG). The SIG field is transmitted to convey the transmission rate and length of the data section of the packet. Information in the SIG field is encoded with a half-rate convolutional code and mapped to the BPSK constellation. An example of the BPSK constellation is shown in figure 2. As shown in figure 1, after the SIG field there is a DATA field carrying the data payload of the packet.

There is however a desire to build on the existing 802.11 standard. An example of a modified packet format is shown in figure 3. The new format proposes the same initial three fields as the existing format but includes new additional fields. Figure 3 shows the legacy STF, the legacy LTF and the legacy SIG, which correspond to the existing fields described above. These fields are retained so that receivers that are not adapted to operate according to the new format can still receive these headers. This allows legacy receivers to receive the STF, LTF and SIG portions of a signal according to the new format. In this way, they can use the STF and LTF to acquire the signal as normal and then receive the SIG information so that it can determine the values of the rate and length fields. These rate and length fields can be appropriately constructed by the transmitter to indicate a duration greater than that of the current packet, in order to reserve the channel for a greater length of time. This means that even though legacy devices cannot receive the rest of the packet, they can be kept from accessing the channel for a specified period without requiring them to have any new functionality.

The new format packet includes two OFDM symbols that follow on from the legacy SIG. These symbols comprise new format signal fields, new SIG1 and new SIG2.

These are followed by new format STF and LTF fields and then finally by the data field.

As indicated above, it is intended that the new format transmissions will be operated in an environment where legacy transmitters and receivers may be operating.

Consequently legacy transmissions and new-format transmissions may occur in the same channel. Therefore, a new-format receiver must be able to determine whether an incoming packet is in the legacy format or in the new format. One way in which this can be achieved is to encode the new signal fields (new SIG1 and new SIG2) using quadrature BPSK (Q-BPSK) instead of BPSK, which is used for the legacy fields. An example of the constellation for quadrature BPSK is shown in figure 4. From figure 4, it will be apparent that Q-BPSK is still a bipolar constellation but the constellation points have been rotated to lie on the quadrature rather than the in-phase axis.

The new system operates by transmitting legacy signals in the normal way. However, when a new format transmission is to be made, transmission is made by transmitting a packet with the legacy STF, legacy LTF and legacy SIG fields. The content of the legacy SIG field indicates that the data part is to be transmitted using BPSK. A new format receiver having decoded the legacy SIG indicating transmission is to take place using BPSK then has to determine whether this is really a legacy format packet, which would use BPSK, or a new-format packet. This is achieved by transmitting the new SIG1 OFDM symbol, which immediately follows the legacy SIG, using Q-BPSK. In this way, a receiver that is able to receive the new-format signal can detect what constellation is being used by the OFDM symbol received immediately following the legacy SIG. If the OFDM symbol is transmitted using BPSK then the receiver can determine that the packet is in fact a legacy format packet and that it is receiving the data portion of the packet encoded using BPSK. However, if the OFDM symbol that is received is transmitted using Q-BPSK because it is a new- format packet then the receiver will determine that the packet is not a legacy format packet and that it is receiving the new SIG 1 part of a new- format packet.

The above-described system assumes that transmissions are only made through one channel. However, it may be desirable to operate in a mode that makes use of two channels. Again in order to allow for operation of legacy receivers, it is important to ensure that when operating in this mode, legacy receivers, which receive signals, are able to deal with them in a suitable manner. In order to achieve this, the transmission format used when operating in the two channel mode is similar to that shown in figure 5.

In this mode, the legacy STF, legacy LTF and legacy SIG are transmitted simultaneously in both channels. This ensures that a legacy receiver monitoring either of the channels would receive and decode the legacy SIG signal and thereby be able to determine the parameters of the packet. As indicated above, it would not be able to decode the rest of the packet but would not try to access the channel for the reserved period of time.

However, this still presents a problem to receivers that are capable of receiving new format packets but only over a single channel. The structure of the packets in each of the two channels would be as shown in figure 6. Such a receiver, would be able to decode the header information contained in the legacy SIG, new SIG1 and new SIG2.

However, it will then fail to decode the information in the data field as this information is spread across both channels. As such a receiver would attempt to decode the data signal anyway, the consequent use of processing resources and power would effectively be wasted, as there is no prospect of actually decoding the data. Consequently, it would be desirable for such a receiver to determine in advance that a signal is a two channel transmission so that it can determine that it is unable to decode the data. It could then switch to a power saving mode for the duration of the remainder of the packet.

Information about the way in which the signal is transmitted could be transmitted in the new SIGI or new SIG2 fields. However, this would use up valuable resources in terms of the transmission capacity and there is already considerable need for these resources for other signalling information. The legacy SIG field cannot be used as this would mean modifying the format of the Legacy SIG which would prevent the operation of legacy receivers.

It would therefore be desirable to provide an alternative means of signalling the type of transmission mode prior to reception of the undecodable data signal. There is therefore a need to provide additional signalling without making use of the available signalling

information in the new SIGI and new SIG2 fields.

Furthermore, it would be desirable to transmit other information without having to utilise the information capacity of the SIG1 and SIG2 signals themselves. Therefore, a means for transmitting additional information such as the number of transmission channels through some other means would be desirable.

Therefore according to the present invention there is provided a transmitter for transmitting data packets along with additional information, the transmitter comprising: a packet generator adapted to generate packets, each packet including a preamble and each preamble including at least a first, a second and a third portion; and a modulator adapted to modulate each packet for transmission, wherein the modulator modulates the first, second and third portions using first, second and third modulation schemes respectively, and wherein the first, second and third modulation schemes are selected according to the additional information.

The present invention also provides a method of transmitting data packets along with additional information, the method comprising: generating packets, each packet including a preamble and each preamble including at least a first, a second and a third portion; and modulating the first, second and third portions using first, second and third modulation schemes respectively, and wherein the first, second and third modulation schemes are selected according to the additional information.

The present invention further provides a receiver for receiving data packets, each packet including a preamble and each preamble including at least a first, a second and a third portion, wherein the first, second and third group of fields are modulated using first, second and third modulation schemes respectively, and the first, second and third modulation schemes are selected according to additional information to be transmitted with each packet, the receiver comprising: a packet analyser adapted to analyse packets to determine the modulation scheme used for each said portions of the packet; and an information decoder for processing the modulation scheme used for each portion to generate the additional information.

The present invention provides a way of sending additional information without making use of the capacity of the fields such as signal fields and without modifying the packet format, such as to enlarge it. This means that the size of the packets is not enlarged whilst still transferring more information. This allows extra information to be sent without utilising the scarce data resource of the packet itself. This provides a convenient way to enhance the functionality of an existing format without the need to modify the structure of the fields within the packet. This is convenient where a standard like the IEEE 802.11 standard is to be modified.

The modulation schemes are preferably selected from a set of two. This allows a modulation scheme to be one or the other representing one bit of data to be transferred.

The modulation schemes are preferably bipolar modulation schemes, which are preferably arranged with a 90 degree phase difference. The modulation schemes are preferably selected from BPSK and Q-BPSK.

The modulation scheme used for a given portion provides one or more bits of information that may represent information about the formatting of the packet. The information may also be used to indicate the number of channels over which at least the data portion of packets is sent.

The portions of the packet may correspond to specific fields of a standard or to smaller parts of a field or to more than one field. Each portion preferably includes at least 2 subcarriers. As more subcarriers are used to code a modulation scheme, the greater the reliability of accurately detecting the modulation scheme.

The present invention is particularly applicable as part of an extension to the current IEEE 802.1 I standard. The present invention may be used in an OFDM system, particularly where the option of transmitting over 2 or more channels is desired.

The present invention can be implemented either in hardware or on software in a processor. Further the present invention can be implemented in a combination of hardware and software. The present invention can also be implemented by a single processing apparatus or a distributed network of processing apparatuses.

Since the present invention can be implemented by software, the present invention encompasses computer code provided to a processor on any suitable carrier medium.

The carrier medium can comprise any storage medium such as a floppy disk, a CD ROM, a magnetic device or a programmable memory device, or any transient medium such as any signal e.g. an electrical, optical or microwave signal.

A specific embodiment of the present invention will now be described with reference to the following drawings in which: Figure 1 shows the basic structure of a transmission packet; Figure 2 shows the BPSK constellation; Figure 3 shows a new format of transmission packet; Figure 4 shows a quadrature BPSK constellation; Figure 5 shows a new packet format for transmission using two channels; Figure 6 shows the modulation scheme for the packets of a new format transmission using two channels; Figure 7 shows part of a packet using the modulation scheme of the present invention; Figure 8 shows part of another packet using the modulation scheme of the present invention; Figure 9 shows a schematic layout of a receiver according to the present invention; and Figure 10 shows the arrangement of modulation schemes for the embodiment of the invention described below.

Figure 9 shows a schematic representation of a receiver according to an embodiment of the present invention. The receiver is adapted to detect the modulation scheme being employed by different fields in order to extract information relating to the number of channels over which the DATA field is to be transmitted.

A receiver unit 1 receives transmitted signals via an antenna 10 which are then passed to a receiver 1 1. The received signal is passed to a detector 12 for determining whether the signals are transmitted using BPSK or Q-BPSK. The detector determines which constellation, the received signal is using. Thus for BPSK, the detector would determine that the signal have no quadrature component whereas for Q-BPSK, the detector would determine that the signals had no in-phase component.

The result of the detection by the detector 12 is passed to the decoder 13 to allow decoding of the signal correctly. The output is also fed to the single/dual channel determination unit 14. The unit 14 is arranged to monitor the output of the detector 12 to obtain information concerning whether the signal is being transmitted over a single channel or over two channels. The output from the unit 14 is then used to control the decoder so that it can correctly receive the data, which in a two channel transmission will be spread over both channels. In a receiver unit which is only capable of receiving data contained in a single channel, the output from the unit 14 can be used to switch the receiver unit linto a low power or standby mode where the data transmissions are determined to be over two channels.

The structure of a transmission signal will now be described in more detail. As indicated above and as shown in Figure 1, a conventional packet according to the IEEE 802.11 WEAN is transmitted in a single 20MHz channel with short and long training fields followed by a signal field, which is in turn followed by the data payload.

Following on from that, the new format packets have the same three preamble fields as shown in figure 3 in relation to transmission over a single OFDM 20MHz channel.

When a signal is being transmitted using this format, the legacy SIG field is transmitted using BPSK. This ensures that legacy receivers can correctly receive and decode these fields. So that a new format receiver is able to determine that a received packet is in the new format, when the new SIG1 field is transmitted, it is transmitted using Q-BPSK. A new format receiver receiving this packet in the Q-BPSK format can then determined that it is receiving a new format packet rather than a legacy format packet. The receiver can then configure itself to dealing with a new format packet and decoding the new SIG1 field. A legacy receiver would not be able to decode the signal as it would be expecting a BPSK signal and so would, for example, assume that there was interference of noise preventing proper reception. After a time corresponding to the length of the packet (known from the legacy SIG field), such a legacy receiver could then resume monitoring the channel ready for the next transmission.

After transmitting the new SIG l field, the transmitter would then transmit the new SIG2 field. If it is about to transmit over only a single 20MHz channel, it continues to transmit using Q-BPSK, as shown in figure 7. When the receiver receives the new SIG2 packet, it detects that the signal has been transmitted using Q-BPSK and so determines that the subsequent data field will be transmitted in a single 20MHz channel.

In this case, whether the receiver is one capable of receiving signals transmitted over dual channels or not, it can receive the subsequent data transmitted over a single channel.

When a transmitter wishes to transmit using two channels, it transmits using a packet structure such as that shown in Figure 5. The preamble portion of the packets is duplicated and transmitted separately in each channel. In this way all receivers are able to receive the legacy preamble fields whether they are monitoring the first or the second channel and whether they are legacy devices or not.

When a new format device receives the preamble from one of the channels, it initially determines whether the transmission is a new format or legacy format by looking at whether the field following the legacy field is BPSK or Q-BPSK. If a transmitter wishes to transmit over two channels using the full 40MHz bandwidth then when it transmits the new SIG2 field, it transmits is using BPSK, as shown in figure 8. The packet structure shown in figure 8 would be transmitted over both channels to ensure that a receiver only monitoring one of those channels would still received the information. When the receiver receives the new SIG2 signal, it determines whether the signal has been transmitted by BPSK or Q-BPSK and if it detects that it has been transmitted using BPSK then it determines that the transmitter wishes to use a dual channel transmission mode. If the receiver is not able to receive dual channel transmissions then it can simply switch to a low power standby mode. If the receiver is able to receive transmissions over two channels, then it configures the receiver accordingly.

In this way, information about the transmission bandwidth is communicated using the modulation scheme of the new SIG2 field. This allows this information to be transmitted without using the available capacity of the new SIGI or new SIG2 fields themselves.

Figure 10 summarises the arrangement of packets for use with the above embodiment.

The field after the legacy SIG is monitored to determine whether or not the packet is a legacy format or a new format packet. Having determined that the packet is a new format packet because it is transmitted using QBPSK, the new SIG2 signal is monitored to determine whether transmission is to be made using BPSK or Q-BPSK and hence whether the DA 1TA field will be sent over a single channel or a dual channel.

Of course, the modulation of the new SIG2 field could be reversed so that BPSK represents a single channel transmission and Q-BPSK represents dual channel transmission.

In the embodiment above, the information communicated by selecting the modulation scheme for the new SIG2 field is used to indicate to a receiver whether the remainder of the packet is to be transmitted using two channels or one. However, the system can equally be applied to transmitting other kinds of information. This system provides a simple way of communicating a one bit piece of information.

Multiple bits of information can be transmitted in this way by selecting the modulation of further parts of the packet. For example, the new SIG1 and new SIG2 fields contain data bearing subcarriers. These fields could be broken into smaller fields or groups of subcarriers and the modulation scheme varied for each of these. In this way, a bit of additional information can be transmitted using the modulation scheme for each field.

For example, if the new SIG1 and new SIG2 field each contain 48 data subcarriers, these could be modulated in groups of 24 subcarriers to transmit 4 bits of information data.

Furthermore, data bits may be transferred over multiple packets. Thus an 8 bit information signal may be sent by passing one bit per packet on 8 separate packets.

The above embodiments have been described in the context of transmitting using the 802.11 WEAN standard. However the invention may be utilised in different systems using similar kinds of packet formatting.

The above embodiment was described in relation to a modulation scheme that switches between BPSK and Q-BPSK. However, the invention may also be applied to a system where the signal switches between different modulation schemes having more than 2 constellation points.

Claims

CLAIMS: 1. A transmitter for transmitting data packets along with

additional information, the transmitter comprising: a packet generator adapted to generate packets, each packet including a preamble and each preamble including at least a first, a second and a third portion; and a modulator adapted to modulate each packet for transmission, wherein the modulator modulates the first, second and third portions using first, second and third modulation schemes respectively, and wherein the first, second and third modulation schemes are selected according to the additional information.
2. A transmitter according to claim 1, wherein the first, second and third modulation schemes are selected from a set comprising two modulation schemes having the same number of constellation points and with the constellation points of one being orthogonal to those of the other.
3. A transmitter according to claim 1 or 2, wherein the modulation schemes are bipolar.
4. A transmitter according to claim 1, 2 or 3, wherein the modulation schemes are selected from BPSK and Q-BPSK.
5. A transmitter according to any one of the preceding claims, wherein the additional information includes information regarding the format of the packet and whether data forming part of the packet is transmitted in one or two channels.
6. A transmitter according to any one of the preceding claims, wherein packets are transmitted using OFDM.
7. A transmitter according to any one of the preceding claims, wherein each of the first, second and third portions include at least 2 data subcarriers.
8. A transmitter according to any one of the preceding claims, wherein each of the first, second and third portions includes one or more fields.
9. A transmitter according to any one of the preceding claims, wherein said first portion is a SIGNAL field according to the IEEE standard 802.1 la.
10. A method of transmitting data packets along with additional information, the method comprising: generating packets, each packet including a preamble and each preamble including at least a first, a second and a third portion; and modulating the first, second and third portions using first, second and third modulation schemes respectively, and wherein the first, second and third modulation schemes are selected according to the additional information.
1 1. A method according to claim 10, wherein the first, second and third modulation schemes are selected from a set comprising two modulation schemes having the same number of constellation points and with the constellation points of one being orthogonal to those of the other.
12. A method according to claim 10 or 11, wherein the modulation schemes are bipolar.
13. A method according to claim 10, 1 1 or 12, wherein the modulation schemes are selected from BPSK and Q-BPSK.
14. A method according to any one of claims 10 to 13, wherein the additional information includes information regarding the format of the packet and whether the data associated with the packet is transmitted in one or two channels.
15. A method according to any one of claims 10 to 14, wherein packets are transmitted using OFDM.
16. A method according to any one of claims 10 to 15, wherein each of the first, second and third portions includes at least 2 data subcarriers.
17. A method according to any one of claims 10 to 16, wherein each of the first, second and third portions includes one or more fields.
18. A method according to any one of claims 10 to 17, wherein said first portion is a SIGNAL field according to the IEEE standard 802.11a.
19. A receiver for receiving data packets, each packet including a preamble and each preamble including at least a first, a second and a third portion, wherein the first, second and third group of fields are modulated using first, second and third modulation schemes respectively, and the first, second and third modulation schemes are selected according to additional information to be transmitted with each packet, the receiver comprising: a packet analyser adapted to analyse packets to determine the modulation scheme used for each said portions of the packet; and an information decoder for processing the modulation scheme used for each portion to generate the additional information.
20. A receiver according to claim 19, wherein the first, second and third modulation schemes are selected from a set comprising two modulation schemes having the same number of constellation points and with the constellation points of one being orthogonal to those of the other and the information decoder determines which of the two modulation schemes are used to provide the additional information.
21. A receiver according to claims 19 or 20, wherein the additional information includes information regarding the format of the packet and whether the data associated with the packet is transmitted in one or two channels, the receiver further comprising for a controller to decode each packet according to the information regarding the format of the packet and whether the data associated with the packet is transmitted in one or two channels.
22. A transmitter for transmitting data packets and an additional information bit for each packet, the transmitter comprising: a packet generator adapted to generate packets, each packet including a preamble and each preamble including at least a first, a second and a third portion, a modulator adapted to modulate each packet for transmission, wherein the modulator modulates the first, second and third portions using first, second and third modulation schemes respectively, wherein the first modulation scheme is BPSK, the second modulation scheme is Q- BPSK and the third modulation scheme is selected to be BPSK or Q-BPSK depending on said information bit.
23. A transmitter according to claim 22, wherein the packet includes a data portion and said information bit indicates whether data portion is transmitted over one or two channels.
24. A carrier medium carrying computer readable instructions for controlling a computer to carry out the method of any one claims 10 to 18.
25. A transmitter substantially as described herein with reference to the attached drawings.
26. A method of transmitting a data packet substantially as described herein with reference to the attached drawings.
27. A receiver substantially as described herein with reference to the attached drawings.