WO2016097688A1

WO2016097688A1 - Methods for wireless communications involving fine mcs signalling with the help of unused bits in the header of a wifi packet

Info

Publication number: WO2016097688A1
Application number: PCT/GB2015/053686
Authority: WO
Inventors: Mehul Mehta
Original assignee: Bluwireless Technology Limited
Priority date: 2014-12-19
Filing date: 2015-12-02
Publication date: 2016-06-23
Also published as: GB2533412B; GB2533412A

Abstract

An embodiment of the present invention provides a technique to enable an increased number of modulation coding combinations for wireless communications.

Description

METHODS FOR WIRELESS COMMUNICATIONS INVOLVING FINE MCS SIGNALLING WITH THE HELP OF UNUSED BITS IN THE HEADER OF A WIFI PACKET

The present invention relates to wireless communications systems. BACKGROUND OF THE INVENTION

Wireless telecommunications systems operate according to appropriate standards, set by an appropriate technical body. One such body is the Institute of Electrical and Electronics Engineers (IEEE). The IEEE 802.11 family of standards, sometime referred to as the "WiFi" standards, provides a family of modulation and coding standards, at different base frequencies, for radio frequency communications. One of these standards, known as IEEE802.1 1ad, relates to communications at a frequency of around 60GHz (the '60GHz band'), and makes use of different modulation techniques: spread spectrum for the control layer, single carrier modulation for single carrier and low power single carrier signals, and orthogonal frequency-division multiplexing (OFDM) for multiple channel transmission. Data is transmitted in the form of discrete data packets, and the well-known technique of using low-density parity-check (LDPC) codes is used for encoding data packets according to the IEEE 802.1 1 ad standard.

In order to provide enhanced data rates in wireless transmission protocols, an adaptive modulation and coding technique may be used. In such a technique, link parameters are adapted in dependence upon the link conditions. For example, different types of modulation and coding techniques can be chosen depending on the signal to noise ratio of the radio link concerned.

It is desirable to provide additional adaptation possibilities for IEEE802.1 1ad communications. Provision of enhanced data rates is particularly desirable in backhaul applications for telecommunication networks, an application for which the 60GHz band is particularly suited. SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a method of transmitting a data packet on a wireless transmission channel, the data packet comprising a header portion and a payload portion, the method comprising defining a low-density parity-check (LDPC) code for encoding data packets, the LDPC code having a first code rate, receiving transmission link quality information, receiving a data packet for transmission, determining a revised code rate for the LDPC code in dependence upon the received transmission link quality information, inserting information relating to the first code rate for the LDPC code into predetermined bits of the header portion of the data packet, inserting information relating to the revised code rate for the LDPC code into otherwise unused bits of the header portion of the data packet, encoding the header portion of the received data packet using said LDPC code at a header code rate to generate an encoded header portion for the data packet, encoding the payload portion of the received data packet using said LDPC code at said revised code rate to generate an encoded payload portion for the data packet, and transmitting the encoded data packet over a radio frequency wireless communications link.

In one example, the encoded data packet is transmitted using a transmission protocol as defined in the I EEE 802.1 1 ad standard. In one example, the transmission link quality information relates to signal to noise ratio of the link concerned.

In one example, the transmission link quality information relates to a received signal strength indicator of the link concerned.

According to another aspect of the present invention, there is provided, a method of receiving an encoded data packet over a wireless transmission channel, the data packet comprising a header portion and a payload portion, the method comprising defining a low- density parity-check (LDPC) code for decoding received data packets, the (LDPC) code having a first code rate, receiving an encoded data packet over a radio frequency wireless communications link, decoding a header portion of the received encoded data packet using the LDPC code at a header code rate to produce a decoded header portion, determining, from the decoded header portion, a revised code rate for the LDPC code, and decoding a payload portion of the received encoded data packet using the LDPC code at the revised code rate to produce a decoded payload portion.

In one example, the encoded data packet is received using a transmission protocol as defined in the I EEE 802.1 1 ad standard.

In one example, the step of determining the revised code rate includes determining a lookup value from otherwise unused bits in the header portion, and using the determined lookup value to retrieve a revised code rate from an alternative code rate table. Such an example may further comprise determining if the lookup value is a valid value, and, if the lookup value is not a valid value, decoding the payload portion using the first code rate. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a wireless communications network;

Figure 2 illustrates part of a wireless communications device embodying one aspect of the present invention; Figure 3 illustrates a data packet structure; and

Figures 4 and 5 are flowcharts illustrating steps for transmitting and receiving respectively data packets in accordance with another aspect of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An exemplary wireless communications network 1 is illustrated in Figure 1 of the accompanying drawings. The network 1 comprises a number, in this example four, wireless communications devices 2, 4, 6 and 8. These devices communicate with one another over radio frequency wireless links 3, 5, 7 and 9. In the example shown, a first device 2 operates to communicate with a second device 4 and a fourth device 8. The second device 4 communicates with the first device 2, and a third device 6. The third device 6 communicates with the second device 4 and the fourth device 8, and the fourth device 8 communicates with the first device 2 and the third device 6. These pairs of devices communicates bi- directionally over the respective RF links 3, 5, 7 and 9. It will be readily appreciated that the network shown in Figure 1 is merely an example, and a network embodying the principles of the present invention may have any number of radio frequency devices arranged to communicate with one another in any suitable pattern. The network of Figure 1 is a simplified mesh network.

Each RF device 2, 4, 6, and 8 includes, in this example, a plurality of radio transmission and reception units 2a, 2b, 2c, and 2d (in the case of the first device 2). The RF device may have any number of radio units 2a, 2b, 2c and 2d, depending on the specific application in which the RF device is being used. An example of a radio unit 20 of an RF device for use in a network embodying the principles of the present invention is illustrated in Figure 2.

The radio unit 20 includes a data reception interface 22 connected to receive data 21 for transmission. The data reception interface 22 operates to buffer data for transmission, and transfers this data to an encoder 24. The encoder 24 operates to encode the received data in accordance with encoding parameters 25 received from a controller (not shown for clarity). The encoding parameters determine the modulation and coding techniques used to encode the data for transmission over the radio link 28 served by the radio unit 20. The modulation and coding techniques are determined in an adaptive manner, and are determined by reference to the quality of the radio link 28 served by the radio unit 20. Operation of the encoder 24 will be described in more detail below. A radio frequency (RF) transmitter 26 receives the encoded data form the encoder 24, and transmits the data over the radio link 28 to a receiving unit.

The radio unit 20 also includes a radio frequency receiver 32 which receives RF signals over an incoming radio link 30. The received RF signals contain encoded data, as will be described in more detail below. The RF receiver 32 supplies the received data to a decoder 34 which operates to decode the data according to parameters received with the encoded data. The decided data is then supplied to an output interface 36 for output 37 from the radio unit 20.

According to the IEEE802.1 1 ad standard, data is transferred between devices in the form of encoded data packets. A simplified data packet structure is shown in Figure 3, and each data packet comprises a header portion and a payload portion. The header portion has a predetermined structure that defines the function of the specific bits in the header. Some of the header bits signal the modulation and coding scheme (MCS) used for the payload portion. Other bits in the header are used for signalling the payload length and type. Further bits of the header portion are left unused. The payload portion contains the encoded data being transferred. The I EEE802.1 1 ad standard sets out various modulation and coding combinations that can be used for the payload portion of the data packet. The combination of modulation and coding used for a particular packet is determined by the radio link quality, and the encoder is instructed to use the chosen combination prior to transmission of the data packet. The header then includes information for the receiver to enable a look up to a modulation-coding scheme table to determine the scheme for decoding of the payload of the received data packet. In I EEE802.1 1 ad, the coding techniques are primarily low-density parity-check (LDPC) codes at various rates. In the I EEE802.1 1 ad standard, the transmitted data is sent in an encoded word length of 672 bits. The LDPC code rate is chosen dependent upon link conditions. For example, a half rate encoding scheme would transmit 336 information bits encoded over 672 transmitted bits. The fewer information bits transmitted, the stronger the code rate, which leads to lower bit-error rate at a given signal to noise ratio. However, it is desirable to encode as many information bits as possible in order to maximise the data transfer rate. The modulations schemes used in the IEEE802.1 1 ad standard include the well-known and understood binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), and quadrature amplitude modulation (QAM), particularly 16QAM and 32QAM.

The combination of a coding scheme with a modulation scheme determines the data transmission rate for the data packet across the radio link. It is preferable to choose the combination that provides the highest reliable data rate across any given radio link.

The available combinations of coding scheme and modulation scheme give a fixed number of predefined modulation-coding schemes (MCSs) for use in data transmission under the I EEE802.1 1 ad standard. Table 1 below shows the available MCS combinations for orthogonal frequency division multiplex (OFDM) transmissions, for each modulation scheme, as defined by the I EEE 802.1 1 ad standard.

Table 1 :

There are situations where it is necessary, due to radio link conditions, to move to a more reliable, but slower, modulation-coding scheme in order to preserve transmission quality. However, the available combinations set out in the standard MCS table are restrictive. There are circumstances in which it would be necessary to change only one of the coding scheme or modulation scheme, and not both. Such possibilities are not available in the I EEE802.1 1 ad standard MCS table. Accordingly, in embodiments of the present invention, additional MCS combinations are enabled. In particular, the coding scheme can be changed independently of the modulation scheme, and additional coding schemes can be provided for each modulation scheme. Table 2 below illustrates some examples of additional coding schemes. It will be appreciated that the table illustrates examples only and is not to be considered limiting: Table 2:

For example, the MCS could move from a high speed 13/16 LDPC in combination with 64QAM, to 3/4 LDPC with 64 QAM, or to 1/2 LDPC with 64 QAM. Such a technique enables additional coding rates to be supported for each type of modulation scheme, so that it is not necessary to drop to a slower modulation scheme in the case of lower radio link quality, but rather a shift to a more robust coding scheme. The additional coding gain then improves transmission quality without sacrificing too much in terms of data rate. The additional coding rates shown in Figure 2 are exemplary, and any suitable coding rates can be employed However, since such additional combinations are outside of the standard MCS combinations, it is not possible to use the specified header bits to allow for this amended scheme. As such, in embodiments of the present invention, the unused bits in the header of the standard IEEE802.1 1 ad data packet are used to signal that an alternative MCS is used for the payload. In this manner, there is no change to the standard specified header information for devices that do not use the alternative MCS combinations.

In an embodiment of the present invention, a data packet is received by the RF receiver 32, and the data packet is transferred to the decoder 34. The decoder 34 determines from the header which MCS is used for the payload data. If an alternative MCS is indicated as being used, by the presence of data in the otherwise unused bits of the header, then the decoder 34 determines the MCS combination by reference to an alternative MCS table look up based the data in those bits. The decoder 34 then decodes the payload, and outputs the decoded data to the output interface 36 for output 37. If an alternative MCS combination is not indicated in the header, then one of the standard MCS combinations is used to decode the payload, by reference to the specified MCS table. The predetermined header bits are used for the MCS table look up, in accordance with the standard I EEE802.1 1 ad definition.

In this manner, additional coding rates can be provided for a given modulation scheme, without the need to extend or amend the MCS table defined by the IEEE802.1 1 ad standard. Thus, data rates can be maintained, or at least degraded only minimally, by avoiding a change in modulation scheme. As mentioned, changing the coding scheme to a more robust scheme enables the maintenance of a desirable modulation scheme for a given radio link condition. Figure 4 is a flowchart illustrating steps in a method for transmitting a data packet on a wireless transmission channel in accordance with the principles described above. The method comprises, at step A, defining a low-density parity-check (LDPC) code for encoding data packets. The LDPC code having a first code rate. At step B, transmission link quality information is determined. At step C, a data packet for transmission is received. At step D, a revised code rate for the LDPC code is determined in dependence upon the received transmission link quality information.

Information relating to the first code rate, and to the revised code rate for the LDPC code is inserted into the header portion of the data packet (step E). The first code rate information is inserted at the predetermined locations in the header specified by the standard, whilst the revised code rate information is inserted into otherwise unused bits of the header portion. The data packet is then encoded; the header portion of the received data packet is encoded using a header code rate to generate an encoded header portion for the data packet (step F), and the payload portion of the received data packet is encoded using at the revised code rate to generate an encoded payload portion for the data packet (step G). The encoded data packet is then transmitted over a radio frequency wireless communications link (step H).

Figure 5 is a flowchart illustrating steps in a method for receiving a data packet according to another aspect of the present invention. The method includes, at step I, defining a low- density parity-check (LDPC) code for decoding received data packets, the (LDPC) code having a first code rate. At step J, an encoded data packet is received over a radio frequency wireless communications link. The header portion of the received encoded data packet is decoded using the LDPC code at a header code rate to produce a decoded header portion (step K). From the decoded header portion, a lookup value is determined from the otherwise unused bits in the header portion (step L), and the determined lookup value is used to retrieve a revised code rate from an alternative code rate table (step M). In this example, the lookup value is examined to determine whether it is a valid value, and, if the lookup value is not a valid value, the payload portion is decoded using the first code rate. If the lookup value is valid, then a revised code rate for the LDPC code is determined by reference to an alternative code rate table (step N). The payload portion of the received encoded data packet is decoded using the LDPC code at the revised code rate to produce a decoded payload portion (step O).

Claims

A method of transmitting a data packet on a wireless transmission channel, the data packet comprising a header portion and a payload portion, the method comprising: defining a low-density parity-check (LDPC) code for encoding data packets, the LDPC code having a first code rate; receiving transmission link quality information; receiving a data packet for transmission; determining a revised code rate for the LDPC code in dependence upon the received transmission link quality information; inserting information relating to the first code rate for the LDPC code into predetermined bits of the header portion of the data packet; inserting information relating to the revised code rate for the LDPC code into otherwise unused bits of the header portion of the data packet; encoding the header portion of the received data packet using said LDPC code at a header code rate to generate an encoded header portion for the data packet; encoding the payload portion of the received data packet using said LDPC code at said revised code rate to generate an encoded payload portion for the data packet; and transmitting the encoded data packet over a radio frequency wireless communications link.

A method as claimed in claim 1 , wherein the encoded data packet is transmitted using a transmission protocol as defined in the IEEE 802.11 ad standard.

A method as claimed in claim 1 , wherein the transmission link quality information relates to signal to noise ratio of the link concerned.

A method as claimed in claim 1 or 2, wherein the transmission link quality information relates to a received signal strength indicator of the link concerned.

5. A method of receiving an encoded data packet over a wireless transmission channel, the data packet comprising a header portion and a payload portion, the method comprising: defining a low-density parity-check (LDPC) code for decoding received data packets, the (LDPC) code having a first code rate; receiving an encoded data packet over a radio frequency wireless communications link; decoding a header portion of the received encoded data packet using the LDPC code at a header code rate to produce a decoded header portion; determining, from the decoded header portion, a revised code rate for the LDPC code; and decoding a payload portion of the received encoded data packet using the LDPC code at the revised code rate to produce a decoded payload portion.

6. A method as claimed in claim 5, wherein the encoded data packet is received using a transmission protocol as defined in the IEEE 802.1 1ad standard.

7. A method as claimed in claim 5 or 6, wherein the step of determining the revised code rate includes: determining a lookup value from otherwise unused bits in the header portion; and using the determined lookup value to retrieve a revised code rate from an alternative code rate table.

8. A method as claimed in claim 7, further comprising determining if the lookup value is a valid value, and, if the lookup value is not a valid value, decoding the payload portion using the first code rate.