Introduction to Ambient BackScatter, How it works?

In spite of many technological advances in wireless communications, whats the most annoying thing we face....battery drain, in fact more advanced technologies suck more battery !! For that reason for wireless devices like sensors, mobiles, swiping machines, all need a lot of maintenance...don't worry there is a way around ABS (ABC as they like to call it), Engineers from university of washington developed a prototype and demonstrated some applications.

BackScatter is an old technology used in RFID, but Ambient BackScatter differs from RFID in many ways (See below), main USP of ABS is, it Enables wireless devices with Power and Communications Medium without requiring batteries.....amazing isn't it..?? Its works on simple (Really!!) 2 principles 

1) It gets powered on using the ambient RF signals around
2) It transmits/receives signals by reflect/absorbing ambient RF signals around

The ambient RF signals discussed were TV Signals. 

It Uses 

• FM0 Coding (Data decoded by detecting change in transitions as 0 or 1)
• Carrier Sensing
• Own Network Stack (Similar Frame format as Ethernet)
• ACK based Protocol 

Now some more technical details from the official paper submitted

What is Backscattering?

At a high level, backscattering is achieved by changing the impedance of an antenna in the presence of an incident signal. Intuitively, when a wave encounters a boundary between two media that have different impedances/densities, the wave is reflected back 

 Energy Detection?

We show that one can perform energy detection by leveraging the property of the analog comparator, in the absence of a nearby backscattering transmitter, the comparator typically outputs either a constant sequence of ones or a constant sequence of zeros. A nearby transmission, on the other hand, results in changes that are greater than the comparator’s  threshold and therefore bit transitions at the comparator’s output. 

Since the transmitted bits have an equal number of ones and zeros (due to FM0 encoding), the comparator outputs the same number of ones and zeros. Thus comparing the number of ones and zeros allows the receiver to distinguish between the presence and absence of a backscatter transmission

How it is different from RFID?

Ambient backscatter differs from RFID-style backscatter in threekey respects.  

• Firstly, it takes advantage of existing RF signals so it does not require the deployment of a special-purpose power infrastructure—like an RFID reader—to transmit a high-power (1W) signal to nearby devices 

• Second, and related, it has a very small environmental footprint because no additional energy is consumed beyond that which is already in the air.

• Finally, ambient backscatter provides device-to-device communication. This is unlike traditional RFID systems in which tags must talk exclusively to an RFID reader and are unable to even sense the transmissions of other nearby tags

References: 

http://www.theengineer.co.uk/channels/design-engineering/news/ambient-backscatter-promises-battery-free-communications/1016925.article 

Why are some of the MCS combinations invalid in 11ac?

11ac boosts the WLAN rates tremendously in to the gigabit range, but it also introduces a slight problematic area which is not really focused in 11n (because its not much of a problem) "the no of BCC encoders."

In 802.11 there are 2 types of convolutional codes (FEC) defined,

 

• BCC (binary convolutional code)

• Encodes the data as per the coding rate, if multiple encoders are used demultiplexes the scrambled bits among NES (number of BCC encoders for the Data field) BCC encoders, in a round robin manner

• Punctures the Encoded output to achieve higher coding rate, by puncturing we mean we decrease the no of output bits (the denominator of the coding rate) there by resulting in an higher coding rate.

        

                   "Higher  rates are derived from it by employing 'puncturing.' Puncturing is a procedure for omitting some of the encoded bits in the transmitter (thus reducing the number of transmitted bits and increasing the coding rate) and inserting a dummy “zero” metric into the convolutional decoder on the receive side in place of the omitted bits. Decoding by the Viterbi algorithm is recommended."

• LDPC (low-density parity check code.)

• High performance error-correcting codes, lets us discuss these some other time.

Note: The same MCS table shall be used for both BCC and LDPC coded transmissions.

Lets take a look at both 11N and 11AC with regard to BCC encoding and MCS Rates Validity.

11N:

The rules of BCC encoder in 11N are:

1. A single BCC encoder can support up to 300 Mbps of data rate at most, after that for every 300 Mbps step we need to use an additional encoder.
2. NES values were chosen to yield an integer number of punctured blocks for each BCC encoder per OFDM symbol.
3. The number of BCC encoders is not limited

So if we observe the MCS's 21, 22, 23 they all use 2 BCC encoders as the Tx Rate is > 300 Mbps. When using multiple BCC encoders the encoder parser demultiplexes the scrambled bits among NES (number of BCC encoders for the Data field) BCC encoders, in a round robin manner. 

Process of Convolutional Coding: Multiple BCC Encoders

So the total number of bits must be multiple of no of encoders, else there will be some extra bits left at the end of the distribution.

For Eg: In case of 

optional 40 MHz, NSS = 3, EQM

MCS 21: 64QAM-2/3-1944-1296-2 Encoders

The total number of coded bits 1944 are split across   648 blocks of BCC encoders with 3 bit length, each block yields 2 bits resulting in 1296 Data bits/sub carrier.

11AC:

The rules for BCC encoder in 11AC are:

1. The maximum data rate per BCC encoder is 600Mbps
2. The number of BCC encoders for a particular combination of MCS, Nsts and BW is determined by the short GI data rate and the same number of encoders are used for the corresponding normal GI rate
3. The number of BCC encoders is not limited
4. NES values were chosen to yield an integer number of punctured blocks for each BCC encoder per OFDM symbol.

As shown in the 11N example, lets take the case of 

mandatory 20 MHz, NSS = 1, 

MCS-8: 256QAM-3/4-416-312

Here the total no of coded bits 416 should be split across 104 blocks BCC encoders with 4 bit length each block yielding 3 data bits resulting in 312 data bits/sub carrier.

MCS-9: 256QAM-5/6-416-??

Here the total no of coded bits 416 cannot be split across a interger number of encoders violating the above rule #4, hence this MCS combination is invalid.

mandatory 20 MHz, NSS = 3, 

MCS-9: 256QAM-5/6-1248-1040

1248/6 = 208 Blocks of 6 bits each yielding 5 bits output resulting in 1040 data bits, which is valid.

Similarly we can do for other MCS holes (Missing MCS rates which are thrown because they are considered invalid).

Conclusion:

So if we convert the above example in to a mathematical forumala...it sumps as below

The reminder of the division of Total no of coded bits/denominator of coding rate (the length of each convolutional code block) should be 0.

         "For BCC encoding, some of the MCS-N_SS combinations are excluded from the MCS table to avoid additional padding symbols. Allowed MCSs are selected such that the number of coded bits in each OFDM symbol contains an integer number of punctured blocks from all encoders, i.e., mathematically every allowed MCS-N_SS satisfies:"

Condition for a MCS to be valid: For definitions refer the IEEE spec

References:

1) IEEE 802.11ac-draft 5.0

2) IEEE 802.11-2012

3) 802.11ac: Survival Guide

4) IEEE Discussions-11ac spec

5) IEEE Discussion-11ac-MCS Holes

802.11 AC Primer: Whats all the fuzz about?

802.11ac is the upcoming big standard with tremendous increase in the data rates and throughput if properly utilized. Lets take a Brief/Raw look at the all the new features and how they help to achieve greater data rates and spectrum utilization.

PHY Features

• 256 QAM

• Very high order modulation scheme which increases the spectral efficiency only when used with beamforming technology, as high order modulation schemes are susceptible to noise and interference.

• Compared to 11n 64QAM, spectrum efficiency improves by 33% 

• Require about 30dB increase in SNR and coverage area is reduced (beam forming can solve this)

• Sub-carriers
• The maximum subcarriers that can be used with OFDM in WLAN is 64/20MHz, as of now 11ac uses this limit most efficiently, the next standard will run out the this limit, its time to increase it to 128/20MHz #IEEE :-)
• 11a/11g ==> 52
• 11n        ==> 57
• 11ac      ==> ~59
• 80 MHz

• 160MHz

• 80+80 MHz

• Single continuous 160MHz and 2 discrete 80MHz can also be combined as 160MHz channel, increases the throuhgput but not the spctral efficiency.

• 8 Spatial Streams

• Sounds high, yeah for a single user it doesn't make sense, but with Multi User MIMO we can exploit this to increase the overall spectrum efficiency.

• MU-MIMO

• Instead of using all the Antennae for a single user (Even though some of them are not really used for some MCSes) we can use each antenna for a single user (max up to 4) and serve all of them in parallel.  This poses few issues like how do we identify each STA? What about group frames? How can a STA for which the data is not destined can ignore the frames? 

• The answer to these questions is the additional features introduced in MAC as explained below. 

• AES-256

• GMAC and GCMP

• Present in 11n in most of the enterprise AP's, now its official along with few other extra algorithms.

MAC Features

PHY ID's (Included in the VHT SIG field) 

Basic motivation is determine if the packet is not destined for you at the earliest possible stage (PHY instead of MAC) and go to micro sleep. (Most likely the case when MU-MIMO is in use)

GroupID

"An AP determines the possible combinations of STAs that can be addressed by an MU PPDU by assigning
STAs to groups and to specific user positions within those groups. (through a new GroupID management Frame).
So after decoding the TXVECTOR the STA can decided whether the frame is for itself (or) not."

Note: Group ID 0 is reserved for transmissions to AP and Group ID 63 is reserved for downlink SU transmissions

Partial AID: The partial AID is a non-unique identifier of a STA and is 9 bits conveyed in the TXVECTOR To identify whther the transmissions are destined to a STA/not, used in conjunction with GroupID.

PHY Power-saving with PHY ID's

TXOP Sharing 

In the TXOP won for a particular AC, we can also send frames destined for others AC's to other STA's as well.

"This mode only applies to an AP that supports DL-MU-MIMO. 

The AC associated with the EDCAF that gains an EDCA TXOP becomes the primary AC. TXOP sharing is allowed when primary AC traffic is transmitted in a VHT MU PPDU and resources permit traffic from secondary ACs to be included, targeting up to four STAs."

TXOP power save

Sounds weird but is a good feature. Basically in a TXOP for MU-MIMO, if the frame is not destined for the STA it can doze off for that TXOP duration.

"If the AP allows non-AP VHT STAs to enter Doze state during a TXOP, then a non-AP VHT STA that is in VHT TXOP power save mode may enter the Doze state till the end of that TXOP when one of the following

conditions is met:

— On receipt of a VHT MU PPDU, the STA determines that it is not a member of the group indicated by the RXVECTOR parameter GROUP_ID. 

— On receipt of an SU PPDU, the STA determines that the RXVECTOR parameter PARTIAL_AID is neither equal to 0 nor does it match the STA’s partial AID. 

— The STA finds that the PARTIAL_AID in the RXVECTOR matches its partial AID but the RA in the MAC header of the corresponding frame that is received correctly does not match the MAC address of the STA."

 References:

1. IEEE Discussions: Spectrum Efficiency 
2. IEEE Discussion: PHY Powersave
3. 802.11ac-draft 5.0