IEEE n Clause 20

IEEE n (Clause 20) This is a technological portent resulting from the application of the most advanced telecommunication theory

802.11n This is a technological portent that delivers high data bit rate by making use of: 1.Multiple streams, multiple antennas 2.Complex digital signal processing DSP 3.Orthogonal carriers in frequency division OFDM 4.Channel bandwidth aggregation

PHYSICAL PHY LAYER IEEE n

802.11n PHY Layer Summary High Throughput (HT) PHY layer is based on the same Clause 17 OFDM PHY Layer. However, Clause 20 (11n) adds up to four (4) spatial streams. The operating bandwidth is 20 MHz per channel. Additionally, n allows aggregation of two contiguous channels to yield 40 MHz bandwidth channels. When n operates with 40 MHz bandwidth with four spatial streams, it can reach 600 Mbps.

802.11n PHY Layer Modulators The HT PHY data subcarriers are modulated using: – BPSK – QPSK – 16-QAM – 64-QAM

802.11n PHY Layer FEC Forward error correction (FEC) coding (convolutional coding) with a Coding rate of:  ½  2/3  ¾  5/6

802.11n Other optional features at both transmit and receive sides are: – 400 ns short guard interval (GI) – Transmit beamforming – HT- greenfield format – STBC.

802.11n PHY Layer Transmitter Blocks

802.11n PHY Layer Formats n has the following PHY layer compatibility formats: Non-HT format: – Structured according to the Clause 17 (OFDM) or Clause 19 (ERP) specification. – Support for non-HT format is mandatory. HT-mixed format (HT_MF): – Contains a preamble compatible with Clause 17 and Clause 19 receivers. – The rest of the frame cannot be decoded by Clause 17 or Clause 19 STAs. – Support for HT-mixed format is mandatory. HT-greenfield format (HT_GF): – Frames of this format assume only Clause 20 devices operate in the BSS. – Support for HT-greenfield format is optional.

802.11n PHY Layer HT Preamble The HT-mixed format ensures compatibility with non-HT devices by using fields that the non-HT devices can read. The HT-greenfield format does not have any field that could be read by non-HT devices.

THE PHASE DIFFERENCE Making use of

Multipath Effect Wireless station STA transmits RF waves which are received by the wireless access point WAP Both antennas Left and Right pick up the signals

Multipath Effect The antennas are separated by certain distance Typically, the wavelength size Suppose that the STA is separated from the left antenna by a distance of 1.2 metres and that the wavelength is 0.12 metres (for 2.4 GHz Band) What is the distance to the right antenna?

Multipath Effect What is the distance to the right antenna? The difference between the left and right distances is merely metres (5.9 mm) However, the WAP is designed to perceive such slight difference by detecting the phase difference The WAP can detect which of the two path is the shortest Consequently, the WAP can choose the strongest and most direct signal path

Multipath Effect The WAP can detect the position of the client depending of the combination of phases that appear in the two antennas

Extending this Concept Multipath has been a traditional problem for wireless communications. However, by implementing a very clever disposition of antennas, and signal processors, multipath can become an ally.

MIMO IEEE n

Spatial Multiplexing Spatial Multiplexing is a wireless technology based on the separation of the main data stream into several, separate streams. Each separate mini-stream is transmitted by a separated antenna. However, all the separated mini-stream are modulated in the same frequency channel. In the receiving side, all the mini-streams are reassembled together using a special signal processing technology (MIMO).

MIMO Principles The idea of MIMO is actually very old. But it was difficult to implement because it required very precise phase locking technologies. Nowadays, this is easier to implement with modern electronics digital processors and phase locking integrated circuits.

For Example A radio link is designed so that two streams are transmitted to a destination with two receiving antennas. The two streams carry different information however their carrier frequency is the same. Problem: How to accomplish this setting without interference? How to recover each information stream at the receiving site?

For Example Frequency = 300 MHz Lambda = 1 m Distance D = 100 metres Hypotenuse = 100 metres plus ¼ lambda What is height h 2 ?

MIMO Frequency = 300 MHz Lambda = 1 m Distance D = 100 metres Hypotenuse = 100 metres plus ¼ lambda What is height h 2 ? Or where to place the bottom antenna so that the previous condition happens.

Pythagoras Frequency = 300 MHz Lambda = 1 m Distance D = 100 metres Hypotenuse = 100 metres plus ¼ lambda What is height h?

Pythagoras Distance D = 100 metres Hypotenuse = metres What is height h? Height is 7.07 metres

Two TXs, Two RXs Now, let’s add a second transmitter B with the same separation of 7 metres and the same wavelength as before. TX A TX B Receiver RX A gets the signals from both TX A and TX B. Receiver RX B gets the same signals than RX A

Two TXs, Two RXs Let’s call: TX A ƛ A – The signal transmitted from TX A lambda A (ƛ A ) and... TX B ƛ B – The signal transmitted from TX B lambda B (ƛ B ) Both Receivers RX A and RX B get the signals transmitted from SITE A.

Two TXs, Two RXs All together: ƛ A ƛ B delayed ¼ ƛ B Receiver RX A gets ƛ A and ƛ B delayed ¼ ƛ B at the same time. ƛ B ƛ A delayed ¼ ƛ A Receiver RX B gets ƛ B and ƛ A delayed ¼ ƛ A at the same time.

RX A ƛ A ƛ B delayed ¼ ƛ B Receiver RX A gets ƛ A and ƛ B delayed ¼ ƛ B at the same time. ƛ B delayed ¼ ƛ B ƛAƛAƛAƛA ¼ wave

RX B ƛ B ƛ A delayed ¼ ƛ A Receiver RX B gets ƛ B and ƛ A delayed ¼ ƛ A at the same time. ƛBƛBƛBƛB ƛ A delayed ¼ ƛ A ¼ wave

Two TXs, Two RXs Let’s take the received signals at Receiver RX A ƛ A ƛ B delayed by ¼ ƛ B That is ƛ A and ƛ B delayed by ¼ ƛ B combined. ƛ B delayed ¼ ƛ B ƛAƛAƛAƛA ¼ wave

Two TXs, Two RXs Let’s place a device, in series, that further delays the signals by ¼ wavelength.

The Effect of a new delay Let’s place a device that delays the signals by another ¼ wavelength. This happens: Input Signals Reference Line ƛ B delayed ¼ ƛ B ƛAƛA ½ wave After the added delay (see how the signals have shifted to the right with respect the input signals above)

Delay Effect The signals are delayed ¼ of a wave again causing: A A to become a negative cosine and B B to become a negative sine Additional Delay of ¼ wave

All together Now, let’s add the output of the delay device to the signals present in RX B What is the result?

Adding the Signals What is the result? This is the result of received signals at RX A after being delayed again by ¼ wavelength These are the signals as they are received at RX B This is the result of adding the previous signals: The + sine cancels out with the – sine. The two – cosine signals add up to have a stronger signal - sine - cosine + sine - cosine This is exactly the desired effect. The antenna RX A receives the two signals but only one is obtained at the end of the process. Even better, the signal is doubled in amplitude.

The Complete Scheme What is the point of all this complication?

The Complete Scheme What is the point of all this complication? The point is that both “sources of information A and B” can be transmitted using the same licensed carrier frequency. Just one carrier frequency for two different radio links. The signals interfere with each other in the air, but it does not matter, because they are recovered at the destination by this ingenuous system.

The Complete Scheme This is the fundamental principle of “spatial multiplexing” A type of spatial mux are MIMO radios or Multiple Inputs Multiple Outputs radios. It is a more efficient way to use the radio spectrum. However, do not see this technology as a “license to print money” as there are practical limitations like in any other technology.

Just to be clear The previous example used a difference of phase of ¼ wavelength between signal A and B because it is easier to demonstrate. By no means, it is required that the difference of phase be ¼ wavelength. In principle, any phase difference, that can be detected, could be used. This is where the Digital Signal Processor DSP has a central role. As long as the DSP can detect the phase differences, it will process the received signals accordingly.

SPATIAL MULTIPLEXING Implementation in IEEE n

Spatial Multiplexing Spatial Multiplexing is a wireless technology based on the separation of the main data stream into several, separate streams. Each separate mini-stream is transmitted by a separated antenna. However, all the separated mini-stream are modulated in the same frequency channel. In the receiving side, all the mini-streams are reassembled together using a special signal processing technology (MIMO).

Spatial Multiplexing The main data bit stream is separated into several streams Each separate mini- stream is transmitted by a separated antenna All the separated mini-streams are modulated in the same frequency channel

Spatial Multiplexing The problem is: how does the receiving-end differentiate and detect the several streams and put the data back together?

Spatial Multiplexing Multipath signals are actually used to recover the data stream

Spatial Multiplexing A MIMO (multiple input multiple output) digital signal processor recovers and reassembles the data

Spatial Multiplexing A processor extracts the desired signals by performing math operations (S 1 +S 2 )+(S 1 -S 2 )=2S 1 (S 1 +S 2 )-(S 1 -S 2 )=2S 2

Spatial Multiplexing n defines Multiple Input Multiple Output in configurations of MxN:S M is the number of transmitters N is the number of antennas S is the number of spatial streams For example, 3x3:2 means 3 transmitters, 3 antennas and 2 spatial streams

CHANNEL UTILIZATION IEEE n

Channel Allocation in IEEE n IEEE n makes use of the channels in both the 2.4 GHz band and the 5 GHz band. However, the usage is different since 2.4 GHz channels main lobes interfere with the adjacent channels. On the other hand, the 5 GHz channels have a clear separation of their main lobes so they are not interfering channels. Consequently, adjacent, available channels can be aggregate into just one larger channel.

Channel in the 5 GHz Band This is a channel in the 5 GHz band. The bandwidth, around the carrier frequency, is 22 MHz n makes use of 20 MHz channels

Channel Bonding This is a technique employed by n to aggregate the bandwidth of two channels of 20 MHz each in the ISM band of 5 GHz. The channels must be adjacent. The total bandwidth is 40 MHz with bonding. There are 11 non-overlapping 40 MHz (bondable) channels possible in the 5 GHz band.

Channel Bonding The bonded channel is described as (N primary, secondary) The N primary is the channel number The secondary might be +1 or -1 depending if it is bonding above or below the primary channel For example (36, +1) means that the primary is 36 and the secondary is one channel above or the next channel (40) For example, (40, -1) means that the primary is 40 and the secondary is one channel below (or channel 36)

SPREAD SPECTRUM IEEE

High Throughput HT Orthogonal Frequency Division Multiplexing OFDM is the spread spectrum technique employed

High Throughput HT 20 MHz channels Each channel of 20 MHz can have 56 orthogonal sub-carriers 52 of those sub-carriers are for data transmission 4 sub-carriers are pilots or test tones for calibration and reference

High Throughput HT 40 MHz (bonded) channels Each channel of 40 MHz can have 114 orthogonal sub-carriers 108 of those sub- carriers are for data transmission 6 sub-carriers are pilots for calibration and reference

802.11n PHY Layer For OFDM implementation, the channel of 20 MHz is divided into 64 possible frequency slots. 20 MHz / 64 = MHz. In the non-HT format, the subcarriers go from –26 to –1 and from 1 to 26, with 0 being the center (dc) carrier. Four sub-carriers are reference pilots. 48 sub-carriers carry data.

802.11n PHY Layer For OFDM implementation, the channel of 20 MHz is divided into 64 possible frequency slots. 20 MHz / 64 = MHz. In the HT format, the subcarriers go from –28 to –1 and from 1 to 28 A total of 56 sub-carriers. Four sub-carriers are reference pilots. 52 sub-carriers carry data.

802.11n PHY Layer Aggregated Channels When two channels are aggregated, the resulting 40 MHz bandwidth is divided into 128 subcarriers. Data is transmitted on subcarriers –58 to –6 and 6 to 58.

Guard Interval WLAN systems transmit Symbols (the smallest unit of data transmitted at one time) Guard Interval is the period of time in between symbols, used to reduce the Inter-Symbol Interference which occurs when multiple copies of a signal arrive at the receiver at different times, due to multipath a/b/g devices use an 800 nanoseconds guard interval n devices can use (optional) a shorter Guard Interval of 400 nanoseconds. The Short Guard Interval provides an 11% data rate improvement with enough Inter-Symbol Interference protection for most typical environments.

MCS Modulation Coding Schemes

Modulation and Coding Scheme MCS n defines 77 Modulation and Coding Schemes for HT. Each MCS index defines: – A modulation system (BPSK, QPSK, and QAM) – A number of spatial streams – The inter-symbol guard interval. – The channel bandwidth Consequently, each MCS results in a different data bit rate.

802.11n PHY Layer MCS Modulation and coding scheme (MCS) The MCS is a value that determines the modulation, coding, and number of spatial channels. MCSs with indices 0 to 7 and 32 have a single spatial stream. MCSs with indices 8 to 31 have multiple spatial streams using equal modulation (EQM) on all the streams. MCSs with indices 33 to 76 have multiple spatial streams using unequal modulation (UEQM) on the spatial streams. MCS indices 77 to 127 are reserved.

MCS Table shows the first 22 MCS Indexes. Notice the line velocities.

LAYER 2 IEEE n

Improved MAC sub-layer operation Multiple data frames can be aggregated into just one. The size of the aggregate frames can go from 2304 bytes, 7935, and bytes! (depending of MCS). The acknowledgements are issued per blocks, so the “air-time” is used more efficiently.

Coexistence with 11a/b/g Non-HT Legacy – Fully compatible with legacy HT Mixed – Legacy can read non-HT fields with short and long symbols – The rest of the fields can not be read by legacy equipment HT Greenfield – This format assumes that only 11n devices are present. – No backward compatibility with legacy a/b/g

Coexistence with 11a/b/g APs operating a 40 MHz BSS must continuously monitor the environment for legacy or non-40 MHz capable HT STAs in both the primary and secondary channels APs and STAs exchange information about what channel widths are supported using HT Information Element and HT Capabilities Element frame fields.

CONTINUATION Appendix Additional Data to be completed

Airmagnet

PHY Non-HT or Legacy Compatible The n frame is preceded by a PHY layer PLCP Protocol Data Unit PPDU. The main function of this PHY header is to provide information to non-HT devices about how long the medium will be busy. This is accomplished by the Legacy Signal L-SIG field which contains the time needed to transmit the frame. Non-HT devices can read this field, but not the rest of the data. The training fields carry symbols that are used for synchronization. LegacyShortTrainingFieldLegacyLongTrainingFieldLegacySignal PHY Layer Preamble + PHY Layer header Frame

PHY HT-Mixed Mode Mixed mode format contains information for both legacy and HT-only devices. Legacy non-HT devices read the corresponding fields to detect the duration of the frame so that they can set their NAVs timers. HT devices read the HT-mode or Greenfield part of the PHY header. LegacyShortTrainingFieldLegacyLongTrainingFieldLegacySignal PHY Layer Preamble + PHY Layer header Frame HT PHY Header HT-mode or Greenfield (details in next slide)

PHY HT-Only or Greenfield Mode Only HT devices can read this PHY header. Non-HT devices will detect the pure n frame as strong background noise. HT GF ShortTrainingField LongTraining Field 1 HT GF Signal PHY Layer Preamble + PHY Layer header Frame HT GF Long Training Field

PHY HT-Only or Greenfield Mode The High Throughput Long Training field provides means for the receiver to estimate the channel between each spatial mapper input (or spatial stream transmitter if no STBC is applied) and receive chain; the number of training symbols is equal or greater than the number of space time streams (with an exception in the case of 3 space time streams). In the 20 MHz transmission mode: the HT-LTF is assigned to sub-carriers –28 to –1 and 1 to 28; the training sequence is an extension of the L-LTF where the 4 extra sub-carriers are filled with +1 for negative frequencies and -1 for positive frequencies In the 40 MHz transmission mode: the HT-LTF is assigned to sub-carriers –58 to –2 and 2 to 58, the training sequence is also constructed by extending the L-LTF in the following way: first of all, the legacy LTF is shifted and duplicated for the duplicate legacy mode; then the missing sub-carriers are filled: sub-carriers [ ] are filled with the values [ ] respectively. HT GF ShortTrainingField LongTraining Field 1 HT GF Signal PHY Layer Preamble + PHY Layer header Frame HT GF Long Training Field

High Throughput Short Training Field (HT-STF) The purpose of the High Through Short Training Field is to improve AGC (Automatic Gain Control) training in a multi-transmit and multi-receive system. The duration of the HT-STF is 4μsec. In the 20 MHz transmission mode: the HT-STF is assigned to sub-carriers –28 to –1 and 1 to 28; the frequency sequence used to construct the HT- STF is identical to L-STF. In the 40 MHz transmission mode: the HT-STF is assigned to sub-carriers –58 to –2 and 2 to 58, and constructed from the 20MHz version by frequency shifting and duplicating, and rotating the upper sub-carriers by 90°. HT GF ShortTrainingField LongTraining Field 1 HT GF Signal PHY Layer Preamble + PHY Layer header Frame HT GF Long Training Field

PHY HT-Only or Greenfield Mode HT GF ShortTrainingField LongTraining Field 1 HT GF Signal PHY Layer Preamble + PHY Layer header Frame HT GF Long Training Field