EP2842231A1

EP2842231A1 - Method, apparatus and computer program for encoding a bit string

Info

Publication number: EP2842231A1
Application number: EP13728836.1A
Authority: EP
Inventors: Anna Pantelidou; Timo K Koskela; Sami-Jukka Hakola; Samuli Turtinen
Original assignee: Broadcom Corp
Current assignee: Broadcom Corp
Priority date: 2012-04-27
Filing date: 2013-04-26
Publication date: 2015-03-04
Also published as: GB2501527A; GB201207413D0; WO2013160876A1; GB2501527B

Abstract

An encoding method is selected (502) from a plurality of encoding methods based on content of a bit string (such as a traffic indication map TIM). Encoding parameters (such as codeword length) are also selected (504) based on the content, and the bit string is encoded (506) according to the selected method and parameters. In various non-limiting examples: different encoding methods are selected (508) based on whether the bit string has sequences of consecutive same-value bits interspersed with isolated opposite- value bits or interspersed with sequences of consecutive opposite-value bits; the codeword length parameter is selected (510) based on how many consecutive same-value bits are in the bit string; and encoding entails disposing a marker between codewords to indicate that one continues the other.

Description

METHOD, APPARATUS AND COMPUTER

PROGRAM FOR ENCODING A BIT STRING

Cross Reference to Related Application

This application claims the benefit under 35 U.S.C. § 119 and 37 CFR § 1.55 to UK patent application no. 1207413.4, filed on April 27, 2012, the entire content of which is incorporated herein by reference.

Technical Field

The present invention relates to a method, apparatus and computer program for encoding a bit string. The exemplary and non-limiting embodiments of this invention relate generally to wireless communication systems, methods, devices and computer programs, and in particular embodiments relate to compressing downlink signalling such as traffic indication maps in a WLAN system.

Background

The wireless local area network (WLAN) family of standards specified by the IEEE 802.1 lb/g/n radio access technologies employ an access point (AP) which aids in coordinating signalling by and to various stations (STAs). The 802.1 lah version of WLAN is being developed to support a large number of STAs attached to a single AP, such as for example the use case la Smart Grid-Meter to Pole scenario where 6000 or more STAs are associated per AP (see document IEEE 802.1 l/0457r0; Potential Compromise for 802.1 lah Use Case Document; by Rolf de Vegt; March, 2011).

Figure 1 A is reproduced from the above IEEE document and shows the smart grid scenario of the above IEEE document, where a single WLAN AP has various home-based meters attached to it from one or more homes. These may not be traditional STAs but rather sensors with WLAN communication capabilities. When the Figure 1A arrangement is extended to multiple homes such as an urban hi-rise apartment building, the AP will need to support a much larger number of STAs than traditional WLAN deployments. In conventional WLAN, the AP always indicates if it has traffic or not to the STAs through the traffic indication map (TIM) element, which is shown at Figure IB. But when a large number of STAs are associated with a single AP, sending the TIM information from the AP can be wasteful and quite long. The TIM is a vector that has as many columns as association identifiers (AIDs) and takes value 1 if the AP has traffic for the corresponding AID and value 0 if the AP does not. Since a bit is required for each station, the amount of information that must be exchanged by the AP in the TIM(s) for the 802.11ah type of deployment can be as large as 6000 (or more) bits, or equivalently 750 (or more) bytes. This is much larger than the current TIM information element for other types of 802.11 WLANs.

Document IEEE 802.1 l/1550rl entitled Extension of AID and TIM to Support 6000 STAs in 802.11 ah (by Yuan Zhou, Zander Lei, Jaya Shanker, Sumei Sun and Rongshan Li; November 2011) discusses the problem of increasing the number of AIDs so that 6000 STAs can be associated to an 802.11 ah network. The solution proposed there is to repeat the IE (e.g. of Figure IB) so that two or more TIM elements are added and used together so that the number of associated stations can be increased from 2007 to larger values.

Document IEEE 802.11-12/117r0 entitled TGah TIM Operation (by Minyoung Park, Tom Tetzlaff, Emily Qi, Young Liu, Hongyang Zhang, Raja Banerea, Matthew Fischer, Eric Wong, ChaorChun Wang, James Wang, Jianhan Liu, Vish Ponnampalam and James Yee; January 2012) proposes that the traffic indication map be transmitted in multiple partial traffic indication bitmaps over multiple beacon intervals.

If one adopts the solution of document IEEE 802.11-12/117r0 then a STA can listen to part of the TIM hoping it will find its AID there. In the best case, it will find its AID in the first TIM segment so that it does not need to listen to all of them. However, in the worst case it will need to listen to each and every TIM segment and thus effectively the whole TIM message. Alternatively, a station may know in which segment its own AID can be found and wake up just to listen to this. But this alternative would introduce more overhead because this segment number would need to be signalled to the STA. Document IEEE 802.11-12/117r0 does not provide details on how such signalling might be implemented.

Summary

According to a first aspect of the present invention, there is provided a method of encoding a bit string, the method comprising: selecting an encoding method from a plurality of encoding methods based on content of a bit string; selecting encoding parameters based on the content of the bit string; and encoding the bit string according to the selected encoding method and the selected encoding parameters.

As a tangible result the encoded bit string is output to an antenna port for signalling in a downlink message, or the tangible result may be actually sending/transmitting the downlink message on a wireless link.

According to a second aspect of the present invention, there is provided apparatus for encoding a bit string, the apparatus comprising a processing system configured to cause the apparatus to perform: selecting an encoding method from a plurality of encoding methods based on content of a bit string; selecting encoding parameters based on the content of the bit string; and encoding the bit string according to the selected encoding method and the selected encoding parameters. The processing system may comprise at least one memory including computer program code and at least one processor.

According to a third aspect of the present invention, there is provided a computer program comprising a set of instructions, which when executed on a network access node, causes the access node to perform the steps of: selecting an encoding method from a plurality of encoding methods based on content of a bit string; selecting encoding parameters based on the content of the bit string; and encoding the bit string according to the selected encoding method and the selected encoding parameters. The computer program may be provided in a computer-readable memory.

According to a fourth aspect of the present invention, there is provided apparatus for encoding a bit string, the apparatus comprising: deciding means for selecting an encoding method from a plurality of encoding methods based on content of a bit string, and for selecting encoding parameters based on the content of the bit string; and encoding means for encoding the bit string according to the selected encoding method and the selected encoding parameters.

As non-limiting examples for this fourth aspect, the deciding means and the encoding means may be implemented as one or more processors executing computer program code stored on one or more memories. The encoding means may be implemented within a transmitter or a portion of a transmit chain/RF (radio frequency) front end chip, or as a baseband chip with operating (stored) software, or alternatively as the processing system noted in the second embodiment summarised above, or as the processor and memory/software that also implement the deciding means as noted immediately above.

Examples of embodiments of the present invention provide improvements over the prior art approaches for how an AP with many STAs attached might inform them whether or not they have traffic.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings. Brief Description of the Drawings

Figure 1 A shows a prior art schematic diagram illustrating a proposed use case for IEEE 802.11 ah networks and is an exemplary but non- limiting radio environment in which these teachings may be practised to advantage;

Figure IB shows the format of a traffic indication map information element which is signalled in conventional WLAN systems by the access point to its stations.;

Figure 2 shows one example of a sequence of TIM bits which are encoded according to one encoding method detailed herein to reduce the signalling overhead;

Figure 3 is similar to Figure 2 but the example is for a different sequence of TIM bits which are encoded according to another encoding method detailed herein to reduce the signalling overhead.

Figure 4 is similar to Figure 2 but the example is for a still different sequence of TIM bits which are encoded according to still another encoding method detailed herein to reduce the signalling overhead Figure 5 shows a logic flow diagram that illustrates from the perspective of the access point the operation of a method, and a result of execution of computer program instructions embodied on a computer readable memory, in accordance with certain exemplary embodiments of these teachings. Figure 6 shows a non-limiting example of exemplary electronic devices suitable for use in practising some example embodiments of these teachings.

Detailed Description

While the specific examples presented below are in the context of the ISM band and the WLAN radio access technologies (RAT), these are not limiting to the broader teachings of the invention which may be applied with similar advantages using other RATs apart from WLAN and the IEEE 802.11 family of standards. Other non-WLAN systems may use a different name for the traffic indication map, and/or for other procedures and channels that are functionally similar to those detailed below for WLAN systems, without departing from these teachings. Additionally, the encoding techniques detailed herein may be used for other than traffic maps to reduce signalling overhead for other information that is currently sent as a bitmap or similar type signalling. The TIM of an 802.11 ah system is merely an example of a specific implementation to describe the broader aspects of these teachings more clearly.

In the smart grid scenario at Figure 1A, it is quite likely that the traffic is mostly uplink traffic. It is therefore reasonable to expect that the TIM will contain multiple zeros for the AIDs for which the AP does not have traffic, and perhaps a few ones for the AIDs receiving traffic in the downlink. Furthermore, it is likely that when there is downlink traffic in that smart grid scenario, it is due to the fact that the AP is sending synchronisation messages or other information for the sensors/STAs. In the latter case the inventors herein expect that the TIM will have a large number of ones and very few (if any) zeros. Since currently the number of available AIDs is limited to take values in 0-2007, the AP cannot indicate with a single TIM message all the traffic that it may have for more than 2008 STAs, but the 802.11 ah scenario is to have the AP support up to 6000 STAs.

The approaches outlined at documents IEEE 802.1 l/1550rl and IEEE 802.11-12/117r0 (see the background section above) each result in the AP sending the entire traffic bitmap. But if the bitmap contains multiple zeros or multiple ones, the below specific teachings will show that this is a drawback in that the AP sending the entire traffic bitmap would be transmitting more bits than is necessary to communicate the needed information. This result from the perspective of the AP would be inefficient in both energy and control signalling overhead. While often the focus for energy consumption is on the STAs, energy efficiency at the AP reduces electricity usage. Furthermore, when the STAs do not know how to synchronise with the TIM segment in which they are interested, then the approach set forth in document IEEE 802.11-12/117r0 also becomes energy inefficient for the stations since they would have to wake up and listen to potentially all partial TIM bitmaps.

The examples below detail how the TIM might be compressed so as to communicate all the needed traffic map information even to a very large number of stations but with far fewer bits than the conventional (uncompressed) TIM. In some cases, the uncompressed TIM may be too large to transmit in one beacon message. For example a beacon including a full 6000 bit TIM may take several milliseconds with the lowest 802.11 ah modulation and coding scheme (MCS) on a 2 MHz bandwidth. This would generally be too large and so these teachings provide two options for segmenting a large TIM for transmission of the different segments in different beacon messages. In a first implementation, the uncompressed TIM is segmented and the separate segments are then compressed (otherwise termed encoded), and the specific way of encoding the TIM segment in one beacon may or may not be the same as the one used for encoding a different segment of the same overall TIM that is sent in the next subsequent beacon. In a second implementation, the uncompressed TIM is compressed and then the compressed TIM is segmented. This latter implementation may prove quite useful in that it would allow a single segment partition in certain special cases where the pattern in the TIM enables a very good compression. The result is that within a segment, the parts of the TIM can be encoded by using the mechanisms detailed in the below examples to further reduce the size of that TIM segment.

Alternatively, if the whole TIM can be compressed sufficiently, the TIM could be transmitted in every beacon (if the size would be smaller than a single segment) but the listening interval of the STAs could be set in the manner that their beacon listening interval is larger than the beacon interval. As an example of this alternative, the compressed TIM would be transmitted in every beacon but a STA would wake up every Nth beacon to check the TIM to see if there is any downlink traffic in the AP's buffer. Detailed below are different signalling schemes according to different embodiments of these teachings, which can be used to support different fixed length encoding methods for sending the TIM information. Specifically, instead of sending the TIM bitmap as is currently in existing 802.11 standards, these teaching encode the number of zeros and ones it contains and send this information instead. For instance, in a bit sequence with seven consecutive ones (1111111) in which each 1 -valued bit corresponds to one AID, according to these teachings these seven bits of information is instead encoded as the number 7 which requires only three bits ("111"). These three bits are referred to as a single "codeword", which encodes the underlying seven consecutive bits each having a value of one. With proper signalling explained further in the non-limiting examples below, the receiving stations will be able to reconstruct the original TIM and be able to tell whether or not the AP has downlink traffic for them. In one non- limiting embodiment of these teachings, the AP first decides whether encoding should be used or not. If encoding the TIM bit string can reduce the number of bits of the TIM bitmap, then the AP will choose to encode; if not, the AP will choose not to encode and send the TIM bitmap in the conventional manner. If the AP chooses to use encoding, then the AP next determines how many bits should be used in the encoding in order to minimise the total number of bits that need to be sent by the AP before all STAs can decode the original TIM information. That is, the AP then decides how many bits per codeword, which equivalently is deciding how many consecutive same- value bits corresponding to different AIDs are to be encoded into a single codeword. In one alternative embodiment, the number of bits for encoding can be pre-arranged, where the number is configured for different traffic profiles/traffic statistics expected, for example, for the current type of network (for example, IEEE 802.11ah case la is one "type of network"). In this alternative, both the AP and the STAs know from the type of network in which they are operating exactly what the encoding parameters are to be. This alternative saves on some of the computational overhead at the AP since each TIM for a given network type would be encoded using the same encoding parameters. Additionally, the traffic profiles/ traffic statistics may depend on the specific time of day/month/etc. or other parameters. Thus, the alternative of pre-configuring the encoding may be combined with the above per-TIM computation in that the default encoding uses the parameters that are pre-arranged for the network type, but the AP can override this via explicit signalling to the STAs if those encoding parameters are not sufficiently suitable for the actual traffic the AP is observing. In one embodiment, that explicit signalling is also in the AP's beacon frame.

In this manner, the TIM length can be variable to the extent of the encoding capability of the AP. The advantage is that encoding the TIM according to these teachings significantly reduces the amount of information that is sent in the TIM. This benefit becomes even larger when there is a large number of STAs. Even though currently in 802.11 ah, up to 6000 STAs can be associated with an AP, this requirement may increase in the future, but these teachings are readily adaptable to an even higher number of STAs, in which case sending the TIM in the encoded manner can provide even higher benefits.

Assume the AP decides that it will encode the TIM. The number of bits used per codeword (three in the above example) can vary from a minimum number min_num to a maximum number max_num. The lowest min_num can be arbitrary but in a practical deployment the meaningful values are larger than 2. The highest max_num should be chosen to be equal to (or less than) the number of bits which are required to represent the longest sequence of ones or zeros in the overall bit string. The above example had a sequence of seven same-value bits which resulted in three bits for the codeword; if in another example the longest same-value sequence (the maximum number of ones or zeros) is ten bits corresponding to ten AIDs, then the maximum number of bits max_num used per codeword would be four.

The AP will need to indicate to the STAs whether the codewords represent a sequence of 0-valued bits or a sequence of 1 -valued bits. The former is termed in the expanded examples below as case 1 and represents a first encoding example; the AP will encode sequences of 0-valued bits where there are long sequences of zeros and few ones (that are not consecutive) in the TIM. The latter is termed in the expanded examples below as case 2 and represents a second encoding example; the AP will encode sequences of 1 -valued bits where there are long sequences of ones and few zeros (that are not consecutive). This means that when the STA receives a codeword number x, it needs to know that x corresponds to a number x of 0-valued bits if the AP is following the first encoding example (case 1) or a number x of 1 -valued bits if the AP is following the second decoding example (case 2). To further reduce the size of the control signalling for the TIM, there is a rule which both the AP and all the ST As commonly understand for compressing/encoding and decompressing/decoding the TIM. Rule 1 provides that if 0- value bits are encoded as in case 1, the remaining (isolated) 1 -value bits are not signalled explicitly but only implicitly. If instead the 1 -value bits are encoded as in case 2, then Rule 2 applies which provides that the remaining (isolated) 0-value bits are not signalled explicitly but only implicitly.

A third set of examples below set forth a still different type of encoding, referred to as case 3 in which the encoding starts by encoding zeros or ones. Whether the 0-value or the 1 -value bits are encoded first will determine the decoding sequence. In this case, the AP does not need to indicate to the STAs how many ones and how many zeros, but just the number of how many consecutive same-valued bits the codeword represents (e.g. 1 bit, 2 bits, 3 bits). This is referred to herein as Rule 3. This number will be mapped implicitly to a number of ones and zeros. To clarify, consider an example. Assume the whole underlying bit string is 000111000 and the codeword "111" has the meaning of "three". For case 3, the AP does not need to indicate that the first codeword "111" represents zeros and that the next codeword "111" represents ones and that the next codeword "111" represents zeros; it only needs to signal those three codewords as "111111". Since the station knows the length of the codewords and that the AP will begin this encoded TIM by sending zeros, the station can decode the AP's message. There may be an isolated 0 or 1 at the beginning or end of a TIM. In all cases above these are not encoded but instead are signalled separately from the AP, for example using 2 bits as will be detailed below in the specific examples. So for example, if from the example above for case 3 encoding the whole underlying bit string 000111000 instead had an isolated one at the start such that it were 1000111000, the AP would still signal the identical case 3 codewords as 11111 1111 and would separately signal the leading 1 using these two bits. Rule 1, Rule 2, Rule 3 have exceptions and all transitions from a zero to a one in the TIM or vice versa need to be marked for those sequences of ones or zeros in the overall bit string that can be represented by more bits than the codeword length max_num can represent. As an example of this, assume that the AP decides that the best length for the codewords is three and the following string of bits needs to be informed to the ST As: 00000000 111111 00000 111 1. The first sequence in this string is eight zeros. With a different codeword length choice, these might be encoded with only four bits but in this example, when viewing the whole bit string the AP has chosen that the codewords will be of length 3 in order to minimise its TIM signalling. The sequence of eight zeros will then be represented by two consecutive codewords, such as for example 111 and 001. However, without any more information, the receiving station may interpret these two codewords as "00000001" rather than the eight zeros that the AP is trying to communicate. To avoid this, there is an option for the AP to insert a marker between the two codewords to indicate to the station that the codeword is not finished. In this case, if we assume that the pre- arrangement understood beforehand by the AP and by the station is that the marker is to be indicated by "000", then the original length-23 (un-encoded) TIM string above can be transmitted as: "111 000 001 110 101 100", where the sequence "000" is the marker which the station recognises is not a codeword to be translated into decoded bits. There are several ways to implement the above general teachings. For example, in one embodiment the AP has the option to choose between any of cases 1, 2 or 3 for any individual TIM and signals which case is in use for a given TIM. In another non-limiting embodiment the AP does not need to signal the encoding parameters to the STAs if those parameters did not change from the previous TIM encoding.

Now consider a few examples of the above encoding cases. First, the two signalling bits mentioned above for the AP indicate whether or not the TIM sequence starts with an isolated one (or zero) and indicate whether or not it ends with an isolated one or zero. These "isolated" leading or trailing bits are not encoded; if there is a leading isolated one or zero, the first portion of the TIM that is encoded is the sequence of ones or zeros following the leading bit. In the below examples, these two signalling bits are for example as follows:

00: Isolated 0 in the beginning of TIM, isolated 0 in the end of TIM

01 : Isolated 0 in the beginning of TIM, isolated 1 in the end of TIM

10: Isolated 1 in the beginning of TIM, isolated 0 in the end of TIM

11 : Isolated 1 in the beginning of TIM, isolated 1 in the end of TIM Secondly, there is also one (non-encoded) signalling bit that indicates whether the encoding begins with a zero or a one.

Thirdly, there are two further signalling bits to indicate the encoding method. In these examples, the AP can choose between case 1, case 2, case 3, and some fourth case such as no encoding (that is, the TIM is sent as is conventional for 802.1 lb/g/n). In the below examples, these two signalling bits may have the following meanings:

00: Encode zeros (Case 1)

01 : Encode ones (Case 2)

10: Mixed scheme (Case 3)

11 : Reserved scheme (e.g. when coding is not beneficial) And fourthly and finally, there are three further bits for the AP to indicate the codeword length, as follows:

000: Codewords of length 3

001 : Codewords of length 4

010: Codewords of length 5

011 : Codewords of length 6

100: Codewords of length 7

101 : Codewords of length 8

110: Codewords of length 9

111 : Codewords of length 10

All this information can be signalled with one byte of overhead. In non- limiting embodiments, this extra byte may be implemented as part of the Partial Virtual Bitmap (see Figure IB) which has a variable length of 1-251 bytes, or it may be sent by the AP in a newly defined information element (IE).

With the above signalled encoding parameters now commonly understood by both the AP and by the STAs, now are detailed several very specific examples with respect to Figures 2 through 4. In Figures 2 to 4 the designation xl, x2, x3 represents a string of same-valued bits, each corresponding to one AID, which will become one or more codewords, depending on what codeword length the AP might choose. There is also an added fourth specific example below for which there is no corresponding drawing. In a first specific example, the underlying whole bit string is characterised by having long sequences of zeros and few ones (that are not consecutive). An example of this is shown at Figure 2, which is representative of where the TIM has a large number of AIDs for which there is no traffic (zeros in the bit string) and a small number of AIDs for which there is traffic (ones in the bit string). The few 1 -valued bits are not consecutive and so the AP would encode the zeros. For a bit string characterised this way, the AP would select case 1 encoding from above. Now assume the whole bit string of Figure 2 is as follows:

000000 1 0000000000 1 0000000 1 000000 1 000 1 0 1 000000

The longest sequence of same- valued bits in the overall string is ten zeros. Therefore the following possible codeword lengths are meaningful:

• 3 bit length encoding: 110 111 000 011 111 110 011 001 110 (27 bits)

• 4 bit length encoding: 0110 1010 0111 01 10 0011 0001 0110 (28 bits)

The 3 -bit codeword option saves one signalling bit over the 4-bit codeword option and so let us assume that in this case the AP selects 3 -bits as the codeword length (e.g. a 3-bit codeword length also minimises the overall signalling burden). As compared to conventional TIM signalling, these 9 codewords can be signalled with 27 signalling bits as opposed to the conventional 45 signalling bits that they encode.

For the case in which the AP uses the nine 3 -bit codewords above to encode the overall 45-length bit string, the leading six zeros of that string are encoded as the codeword 110. The subsequent ten 0-value bits in the string are encoded with the nine bits 111 000 011, where the "000" signals the receiver STA that the codeword has not finished. When decoding, the STA will decode the codeword "111" as seven zeros and the codeword "011" as three more zeros. The marker "000" is not a true codeword but indicates that the sequence of consecutive zeros indicated by codeword "111" is not the full consecutive sequence of same -valued bits.

But the AP may choose to use the 4-bit codeword length in the above case 1 example, say for instance if the current encoding parameters have case 1 and 4-bit length codewords the AP may choose that the 1-bit difference in encoding is more than offset by not having to send new encoding parameters (which in this example would increase the overall signalling burden). For the case the AP uses the seven 4-bit codewords above to encode the overall 45-length bit string, the decoder at the STA will parse the leading codeword "0110" and decode it to map to the leading 6 zeros of that bit string. The next bit, which is 1 -valued and in the seventh position of the overall 45-length bit string, will be decoded implicitly by parsing the next codeword "1010" and mapping it to 10 zeros. By continuing in this manner for each codeword, the STA can decode the whole encoded TIM to yield the original 45-length bit string.

In a second specific example, the underlying whole bit string is characterised by having long sequences of ones and few zeros (that are not consecutive). An example of this is shown at Figure 3, which is representative of where the TIM has a large number of AIDs for which there is traffic (ones in the bit string) and a small number of AIDs for which there is no traffic (zeros in the bit string). The few 0-valued bits are not consecutive and so the AP would encode the ones. For a bit string characterised this way, the AP would select case 2 encoding from above. Now assume the whole bit string of Figure 2 is as follows:

11111111111 0 1111 0 11111 0 1111111 0 1 0 1 0 1111111111

The longest sequence of same-valued bits in the overall string is eleven ones, and thus a 4-bit codeword is enough to represent that longest sequence. Following is the case 2 bit string encoded with 3-length and 4-length codewords:

3 bits: 11 1 000 100 100 101 111 001 001 111 000 011 (33 bits)

4 bits: 1011 0100 0101 0111 0001 0001 1010 (28 bits)

In this example, codewords of length 4 bits is the best encoding length.

In a third specific example, the underlying whole bit string is characterised by having long sequences of ones and zeros where these sequences are both consecutive. An example of this is shown at Figure 4, which is representative of where the TIM has a large number of AIDs with and without traffic. For a bit string characterised this way, the AP would select case 3 encoding from above. Now assume the whole bit string of Figure 3 is as follows:

1111111111 00000000000 111111111111 0000000000 111

Assume further that the AP indicates in the encoding parameters that first the 1 -value bits are encoded. The maximum number of consecutive bits in the above un- encoded string is twelve ones, and so max_num = 4. It is easy to see that for the specific bit string above, codewords of length 4 bits is the best choice for encoding:

4 bits: 1010 1011 1100 1010 0011. Thus for this example, only 20 bits are needed to signal the traffic indications given by the original 36-length bit string above.

In a fourth specific example, the underlying whole bit string is characterised by having short sequences of ones and zeros that alternate and which may or may not be consecutive. Like the third example above, in this case the TIM has a large number of AIDs with and without traffic but the overall bit string is such that there are not long sequences of same- valued bits. Encoding may not yield any signalling efficiencies in this case and therefore the TIM may be sent un-encoded. A worst case example of this type of bit string is:

10101010101010101...

One best case example which saves the most signalling overhead as compared to conventional 802.1 1 b/g/n TIM signalling is where the AP has traffic for all 2007 AIDs. In this case, the AP may select case 2 encoding with length- 10 codewords, and would need only 10 bits to encode the 2007 length bit string. As detailed above, there are 8 extra bits of overhead for signalling the encoding parameters. This one extra byte of overhead will become insignificant as the number of stations associated with an AP increases. Certain of the above non-limiting embodiments provide the technical effect of energy savings at the stations, since they can sleep longer when they do not need to listen to the conventional long bitmaps of TIM which may be indicated by 6000 bits or more. These teachings also provide for a more efficient transmission of the TIM, and the resulting technical effect is less signalling overhead. Signalling overhead is reduced because the overhead of the 8 extra signalling bits for conveying the encoding parameters can be negligible as compared to TIM bit sequences of the order of 2008 bits long. Another technical effect is energy savings at the AP since according to these teachings the AP is not sending unnecessary information.

Figure 5 is a logic flow diagram which summarises some example embodiments of the invention. Figure 5 summarises some of the above teachings from the perspective of the AP, which may be more generally referred to as a wireless network access node. An apparatus implementing the summary shown at Figure 5 may be the entire device/system 20 shown at Figure 6, or may be one or more components thereof such as a modem, chipset, or the like. Figure 5 may be considered to illustrate the operation of a method for operating a device, and a result of execution of a computer program tangibly stored in a computer readable memory, and a specific manner in which components of an electronic device are configured to cause that electronic device/system to operate. The blocks of Figure 5 and the functions they represent are non-limiting examples, and may be practised in various components such as integrated circuit chips and modules. Exemplary embodiments of this invention may be realised in an apparatus that is embodied as an integrated circuit. The integrated circuit, or circuits, may comprise circuitry (as well as possibly firmware) for embodying at least one or more of a data processor or data processors, a digital signal processor or processors, baseband circuitry and radio frequency circuitry that are configurable so as to operate in accordance with the exemplary embodiments of this invention.

Such circuit/circuitry embodiments include any of the following: (a) hardware-only circuit implementations (such as implementations in only analog and/or digital circuitry) and (b) combinations of circuits and software (and/or firmware), such as: (i) a combination of processor(s) or (ii) portions of processor(s)/software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a network access node/AP, to perform the various functions summarised at Figure 5) and (c) circuits, such as a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even if the software or firmware is not physically present. This definition of "circuitry" applies to all uses of this term in this specification, including in any claims. As a further example, as used in this specification, the term "circuitry" would also cover an implementation of merely a processor (or multiple processors) or portion of a processor and its (or their) accompanying software and/or firmware. The term "circuitry" also covers, for example, a baseband integrated circuit or applications processor integrated circuit for a station/mobile phone or terminal/user equipment or a similar integrated circuit in a server, a cellular/WLAN network device, or other network device.

Specifically, the AP at block 502 of Figure 5 selects an encoding method from a plurality of encoding methods based on content of a bit string. Then at block 504 the AP selects encoding parameters based on the content of the bit string. While one parameter may indicate the selected encoding method, there is at least one additional parameter beyond that one, such as for example the codeword length. The AP may in an embodiment select the codeword length by trying to encode the bit string using different codeword lengths and testing which length yields the greatest compression. If for example there is only a marginal improvement in compression by changing the codeword length or other encoding parameters as compared to the encoding method used most recently, the AP may also consider whether it would need to re-signal new encoding parameters when deciding from a signalling perspective whether the marginal compression improvement sufficiently offsets the additional parameter signalling to justify changing from that most recently used encoding method. At block 506, the AP encodes the bit string according to the selected encoding method and the selected encoding parameters. The following reviews some of the non-limiting examples that are detailed with more particularity in the above specific examples. In one specific embodiment summarised at block 508, the AP selects different ones of the plurality of encoding methods based on whether the bit string has sequences of consecutive same-value (1 or 0 in the examples) bits interspersed with isolated opposite-value bits (the first or second examples above), or whether the bit string has sequences of consecutive same- value bits interspersed with sequences of opposite-value bits (the third example above). In this context, isolated means singular bits having the opposite value, not pairs or longer sequences. Block 510 gives further detail in that selecting the encoding parameters as in block 504 means at least selecting a codeword length from a plurality of codeword lengths based on how many consecutive same-value bits are in the bit string. In the above examples, the set of encoding parameters included an indication of the selected codeword length. In some of the above examples, the bit string was encoded by disposing a marker between a first and a subsequent second codeword to indicate that the second codeword continues the first codeword. Block 512 of Figure 5 details also from the above examples all of the four different kinds of encoding parameters: a first indication of whether the bit string begins with a one or a zero; a second indication of whether a first codeword of the encoded bit string encodes 1 -value bits or 0-value bits; a third indication of the selected encoding method; and a fourth indication of a selected codeword length.

In the above examples the radio environment was assumed to be WLAN and the encoding was for a TIM, such that each bit of the bit string comprises a traffic indicator corresponding to an AID, each of which uniquely identifies a mobile device/station and where each traffic indicator bit indicates whether or not the AP performing the Figure 5 process has wireless traffic for the mobile device identified by the corresponding AID. In this case, the AP also compiles into a downlink message (e.g. the beacon frame) the encoded bit string and also indications for the corresponding association identifiers. Additionally, the bit string operated on at Figure 5 may not be the whole and complete TIM bit string. As detailed early in the description above, the bit string operated on at Figure 5 may be segmented from that larger whole bit string. In this case, the process illustrated at Figure 5 is preceded by the AP determining that every one of the plurality of encoding methods would result in the encoded whole bit string being too large for a beacon frame (not in and of itself, but too large for the space allowed in the beacon frame for the TIM). In response to this determination, the AP then segments the whole bit string such that the bit string operated on at Figure 5 is one of multiple portions of the whole bit string. Then the AP will perform the steps of Figure 5 on those other bit strings also, and transmit the encoded bit strings in different beacons.

Reference is now made to Figure 6 for illustrating a simplified block diagram of various electronic devices and apparatus that are suitable for use in practising some example embodiments of this invention. In Figure 6 there is an AP 20, or more generally a wireless access node if these teachings are implemented in other than a WLAN environment. The AP 20 is adapted for communication over a wireless link 15 with an apparatus such as the illustrated STA, which may be more generically referred to as a radio device 10 which may or may not be portable (in the Figure 1A diagram the signalling is wireless but the meters and corresponding radios may be operating on AC power). The AP may also provide connectivity with a broader network (e.g. a cellular network and/or a publicly switched telephone network PSTN and/or a data communications network/Internet), but shown in Figure 6 is the data collector and control 22 from Figure 1 A. The data collector and control entity 22 may have connectivity to the APs via a wired (electric, optical, etc.) connection or via a wireless backhaul connection.

The station 10 includes processing means such as at least one data processor (DP) 10A, storing means such as at least one computer-readable memory (MEM) 10B storing at least one computer program (PROG) IOC, communicating means such as a transmitter TX 10D and a receiver RX 10E for bidirectional wireless communications with the network access node/AP 20 via one or more antennas 10F. Also stored in the MEM 10B at reference number 10G is the station's rules for figuring out from the AP's signalled encoding parameters and the signalled (encoded) TIM just what encoding method to use to decode that encoded TIM and the parameters such as codeword length to use for the decoding, as is detailed further above.

The AP 20 also includes processing means such as at least one data processor (DP) 20A, storing means such as at least one computer-readable memory (MEM) 20B storing at least one computer program (PROG) 20C, and communicating means such as a transmitter TX 20D and a receiver RX 20E for bidirectional wireless communications with the station 10 via one or more antennas 20F. The AP 20 may also have software at 20G for deciding the encoding method from several encoding methods, and for deciding the particular encoding parameters for the chosen encoding method as detailed more fully above.

For completeness there is also shown the data collector and control system 22 which has its own processing means such as at least one data processor (DP), storing means such as at least one computer-readable memory (MEM) 22B storing at least one computer program (PROG) 22C, and communicating means such as a modem 22D for bidirectional communications with the AP 20 via a data/control path 30.

While not particularly illustrated for the station 10 or for the access point 20, those devices are also assumed to include as part of their wireless communicating means a modem which may be inbuilt on an RF front end chip within those devices 10, 20 and which also carries the TX 10D/20D and the RX 10E/20E.

At least one of the PROGs lOC/lOG in the station 10 is assumed to include program instructions that, when executed by the associated DP 10A, enable the device to operate in accordance with the exemplary embodiments of this invention for proper decoding of the TIM it receives from the AP 20. The AP 20 also has software stored in its MEM 20B to implement certain aspects of these teachings as detailed above particularly with respect to Figure 5. In this regard, the exemplary embodiments of this invention may be implemented at least in part by computer software stored on the MEM 10B, 20B which is executable by the DP 10A of the station 10 and/or by the DP 20A of the access node/AP 20; or by hardware, or by a combination of tangibly stored software and hardware (and tangibly stored firmware). Electronic devices implementing these aspects of the invention may not be the entire station 10 or the access node/AP 20, but exemplary embodiments may be implemented by one or more components of same such as the above described tangibly stored software, hardware, firmware and DP, modem, system on a chip SOC or an application specific integrated circuit ASIC.

In general, the various embodiments of the station 10 can include, but are not limited to personal portable digital devices having wireless communication capabilities, including but not limited to user equipments, cellular telephones, navigation devices, laptop/palmtop/tablet computers, digital cameras and Internet appliances, as well as machine-to-machine devices such as those implied by Figure 1 A which operate without direct user action. Various embodiments of the computer readable MEMs 10B, 20B, 22B include any data storage technology type which is suitable to the local technical environment, including but not limited to semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory, removable memory, disc memory, flash memory, DRAM, SRAM, EEPROM and the like. Depending on the implementation the database system memory 22B may be a disk array. Various embodiments of the DPs 10A, 20A, 22A include but are not limited to general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs), and multi-core processors. Some of the various features of the above non-limiting embodiments may be used to advantage without the corresponding use of other described features. The foregoing description should therefore be considered as merely illustrative of the principles, teachings and exemplary embodiments of this invention, and not in limitation thereof. The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

1. A method of encoding a bit string, the method comprising:

selecting an encoding method from a plurality of encoding methods based on content of a bit string;

selecting encoding parameters based on the content of the bit string; and encoding the bit string according to the selected encoding method and the selected encoding parameters.

2. A method according to claim 1 , in which different ones of the plurality of encoding methods are selected based on whether the bit string has sequences of consecutive same- value bits interspersed with isolated opposite-value bits or interspersed with sequences of consecutive opposite-value bits.

3. A method according to claim 2, wherein selecting the encoding parameters comprises at least:

selecting a codeword length from a plurality of codeword lengths based on how many consecutive same- value bits are in the bit string;

and wherein the encoding parameters comprise an indication of the selected codeword length.

4. A method according to any of claims 1 to 3, in which encoding the bit string comprises disposing a marker between a first and a subsequent second codeword to indicate that the second codeword continues the first codeword.

5. A method according to any of claims 1 to 4, in which the encoding parameters comprise:

a first indication of whether the bit string begins with a one or a zero;

a second indication of whether a first codeword of the encoded bit string encodes 1 -value bits or 0-value bits;

a third indication of the selected encoding method; and a fourth indication of a selected codeword length.

6. A method according to any of claims 1 to 5, in which each bit of the bit string comprises a traffic indicator corresponding to an association identifier, wherein:

each association identifier uniquely identifies a mobile device,

each traffic indicator indicates whether or not a network access node executing the method has wireless traffic for the mobile device identified by the corresponding association identifier, and

the method comprises compiling into a downlink message the encoded bit string and indications for the corresponding association identifiers.

7. A method according to any of claims 1 to 6, wherein the said bit string is segmented from a larger whole bit string; the method comprising the initial steps of: determining that every one of the plurality of encoding methods would result in the encoded whole bit string being too large for a beacon frame; and

segmenting the whole bit string such that the said bit string is one of multiple portions of the whole bit string.

8. Apparatus for encoding a bit string, the apparatus comprising a processing system configured to cause the apparatus to perform:

9. Apparatus according to claim 8, in which different ones of the plurality of encoding methods are selected based on whether the bit string has sequences of consecutive same- value bits interspersed with isolated opposite-value bits or interspersed with sequences of consecutive opposite-value bits.

10. Apparatus according to claim 9, wherein the processing system is configured to cause the apparatus to perform:

11. Apparatus according to any of claims 8 to 10, in which encoding the bit string comprises disposing a marker between a first and a subsequent second codeword to indicate that the second codeword continues the first codeword.

12. Apparatus according to any of claims 8 to 11 , in which the encoding parameters comprise:

a first indication of whether the bit string begins with a one or a zero;

a third indication of the selected encoding method; and

a fourth indication of a selected codeword length.

13. Apparatus according to any of claims 8 to 12, in which each bit of the bit string comprises a traffic indicator corresponding to an association identifier, wherein: each association identifier uniquely identifies a mobile device,

the processing system is configured to cause the apparatus to perform:

compiling into a downlink message the encoded bit string and indications for the corresponding association identifiers.

14. Apparatus according to any of claims 8 to 13, wherein the said bit string is segmented from a larger whole bit string; and the processing system is configured to cause the apparatus to perform the initial steps of:

determining that every one of the plurality of encoding methods would result in the encoded whole bit string being too large for a beacon frame; and

15. A computer program comprising a set of instructions, which when executed on a network access node causes the access node to perform:

selecting encoding parameters based on the content of the bit string; and encoding the bit string according to the selected encoding method and the selected encoding parameters .

16. A computer program according to claim 15, in which different ones of the plurality of encoding methods are selected based on whether the bit string has sequences of consecutive same- value bits interspersed with isolated opposite-value bits or interspersed with sequences of consecutive opposite-value bits.

17. A computer program according to claim 16, wherein the set of instructions when executed causes the access node to perform:

wherein the encoding parameters comprise an indication of the selected codeword length.

18. A computer program according to any of claims 15 to 17, in which encoding the bit string comprises disposing a marker between a first and a subsequent second codeword to indicate that the second codeword continues the first codeword.

19. A computer program according to any of claims 15 to 18, in which the encoding parameters comprise:

a first indication of whether the bit string begins with a one or a zero;

a third indication of the selected encoding method; and

a fourth indication of a selected codeword length.

20. A computer program according to any of claims 15 to 19, in which each bit of the bit string comprises a traffic indicator corresponding to an association identifier, wherein:

each association identifier uniquely identifies a mobile device,

each traffic indicator indicates whether or not a network access node executing the method has wireless traffic for the mobile device identified by the corresponding association identifier,

and the set of instructions when executed causes the access node to perform: compiling into a downlink message the encoded bit string and indications for the corresponding association identifiers.

21. A computer program according to any of claims 15 to 20, wherein the said bit string is segmented from a larger whole bit string; and the set of instructions when executed causes the access node to perform the initial steps of:

22. Apparatus for encoding a bit string, the apparatus comprising: deciding means for selecting an encoding method from a plurality of encoding methods based on content of a bit string, and for selecting encoding parameters based on the content of the bit string; and

encoding means for encoding the bit string according to the selected encoding method and the selected encoding parameters.

23. A method of wireless downlink signalling from a wireless access point, the method comprising the wireless access point transmitting a downlink signal comprising an encoded bit string that has been encoded by a method according to any of claims 1 to 7.