Evolution of 802.11 (Physical Layer)

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20 May 2007

Denis Bakin

Evolution of 802.11 (physical layer)

Introduction

The purpose of this document is to explain the basic ideas laying in the foundation

of the technologies adopted by IEEE 802.11 standards for wirelesscommunications at the physical layer. It is designed for audience working with or

administrating the devices complying to the named standards, and willing to know

their principles of operation believing that such knowledge can help to makeeducated decisions regarding the related equipment, choose and utilize the

available hardware more efficiently.

Using Radio Waves For Data Transmission

Designing a wireless high speed data exchange system is not a trivial task to do.Neither is the development of the standard for wireless local area networks. Themajor problems at the physical layer here caused by the nature of the chosen media

are:

bandwidth allocation; external interference; reflection.

The 802.11 standards had to address them all.

802.11 First Standard For

Wireless LANs

The Institute of Electronic andElectrical Engineers (IEEE) has

released IEEE 802.11 in June1997. The standard definedphysical and MAC layers of

wireless local area networks(WLANs).

The physical layer of theoriginal 802.11 standardized

three wireless data exchange techniques:

Infrared (IR); Frequency hopping spread spectrum (FHSS);

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Direct sequence spread spectrum (DSSS).The 802.11 radio WLANs operate in the 2.4GHz (2.4 to 2.483 GHz) unlicensedRadio Frequency (RF) band. The maximum isotropic transmission power in this

band allowed by FCC in US is 1Wt, but 802.11 devices are usually limited to the

100mWt value.

The physical layer in 802.11 is split into Physical Layer Convergence Protocol

(PLCP) and the Physical Medium Dependent (PMD) sub layers. The PLCP

prepares/parses data units transmitted/received using various 802.11 media accesstechniques. The PMD performs the data transmission/reception and

modulation/demodulation directly accessing air under the guidance of the PLCP.

The 802.11 MAC layer to the great extend is affected by the nature of the media.For example, it implements a relatively complex for the second layer

fragmentation of PDUs.

Infrared

The use of infrared for WLAN has not been accepted by public. There was no

successful commercial implementations of 802.11 IR technology.

Frequency Hopping

The frequency hopping was the first step in the evolution to the

DSSS and more complex data transmission techniques. Theidea is to transmit on a given frequency for a very short time

and switch to another frequency according to a pre-defined

frequency hopping pattern known to both transmitter andreceiver. This allows to deal with high energy interference in anarrow band as well mutual interference of two FHSS

transmitters positioned close to each other.

802.11 frequency hopping separates the whole 2.4Ghz ISM

band into channels spaced 1MHz. The transmitter has to

change channels at least 2.5 times per second (every 400msecor less). The hopping patterns are described by 3 setscontaining 26 hopping sequences each. The sets are defined in

such way that the sequences in each set, when set up ondifferent access points, provide minimum mutual interference.

The idea was patented by

Hedy Lamarr, a famous

actress, in 1942. The

story is that she overheardand understood the FHSS

idea during one of the

meetings her husband (an

Austrian businessman

selling weapons to

Hitler) had with his

customers. She managed

to patent it after leaving

her husband and escaping

to US right before the

WWII.

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The frequency picked up according to the hopping pattern is

then modulated usingtwo-level GFSK(Gaussian Frequency

Shift Keying) for 1Mbps andfour-level GFSKmodulation for2Mbps data rate.

The FHSS is quite stable to interference, cost effective andsimple data transmission technique but it is not widely used for

WLANs nowadays. Mostly because of its growingrequirements to bandwidth when data transmission rates are

increased.

Direct Spread Spectrum Sequencing

This is one of the most successful data transmission technique for today. The

DSSS is used in cellular networks (CDMA systems), Global Positioning Systems

(GPS) and of course, Wireless LANs.

The idea is to multiply the data being transmitted to a pseudo random binarysequence of a higher bit rate.

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The pseudo random binary sequence (PRN or PN) is called the chipping sequenceand the bit rate of the sequence is called the chipping rate. The data isunrecoverable from the result of such multiplication unless the chipping sequence

is known.

In order to be used by a DSSS system a PRN sequence should posses followingproperties:

the sequence should be balanced (the difference between number of onesand zeros in the sequence is less or equal 1) ;

the autocorrelation function of the sequence should be close to 0 everywhereexcept at the shift 0.

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The mentioned above properties are the properties of the white noise. Thespectrum of the white noise is a flat line, and the multiplication of the data to thePRN have the effect of flattening the spectrum of the resulted signal. The

Rc=1/Tc in the picture below corresponds to the chipping rate. The conventional

modulation of the original data stream (without the chipping sequence) wouldresult in the shoulders of the spectrum being placed at the distance of the data rate

1/Ts (i.e. symbol rate) from the carrier frequency.

The result of the multiplication (the sequence of chips) is transmitted to the

receiver using any of the standard modulation techniques. At the receiver's side,the signal is demodulated and the received bit stream is multiplied to the PRN

sequence generated by the receiver. When the transmitter's and the receiver'ssequences match and are synchronized in time the correlation function will

generate a spike of size n if the data bit was 1, and -n if the data bit was 0 (signal-1). If the sequences do not match or not synchronized in time the correlation will

stay close to 0. The receiver, when synchronized can use the output of its

correlator to determine the quality of the reception and react to the synchronizationdrift.

The use of different uncorrelated PRN sequences is exploited in CDMA systems toallow multiple simultaneously operating data channels in the same frequency band.

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The DSSS transmission is also secure if the PRN is long enough and known totransmitter and receiver only. Basically the DSSS signal can be below the noise

level and therefore undetectable.

Besides the mentioned above useful properties DSSS is self synchronizing and

immune to the narrowband interference. These are the properties that justified itsuse in 802.11.

The 802.11 DSSS separates 2.4 ISM band into 11 overlapping channels spaced

5MHz. The 802.11 transmitter always sends symbols (1 or several chips) at the rate

of 11Mbps, which requires 22MHz bandwidth. The 802.11 does not use thebenefits of the multiple coding sequences. All 802.11 nodes transmit using the

same PRN, the 11 bit long Barker code (+1,1, +1, +1,1, +1, +1, +1,1,1,1),

and therefore only three non-overlapped channels can simultaneously operate inthe ISM band without interference.

The data being transmitted should pass several stages and be prepended by PLCP

(Physical Layer Convergence Procedure) before transmission. The DSSStransmission starts by sending PLCP preamble and header.

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The PLCP Preamble:

- Synchronization (Sync) field consists of 128 bits. It is filled in with predefinednumbers and allows receiver to synchronize to the transmission.

- 16-bit Start Frame Delimiter (SFD) field is used to mark the start of every frame.

The PLCP Header:- 8-bit Signal or Data rate (DR) field indicates how fast the data will be

transmitted. The only two possible values for the June 1997 version of 802.11 are00001010 for 1 Mbps DSSS and 00010100 for 2 Mbps DSSS.

- 8-bit Service field is reserved for future use.- 16-bit Length field indicates the length of the ensuing MAC PDU.

- 16-bit Cyclic Redundancy Code (CRC) field is used for error detecting.

The preamble and the PLCP header are transmitted at 1Mbps regardless of the

current data transmission speed. After the preamble the payload prepared by theMAC layer is sent to the receiver at the rate specified in the services field.

The transmitter usesDBPSKandDQPSKmodulation, which results in 1Mbps and

2Mbps data transmission rates correspondingly.

802.11b Need For Speed

In 1998, Lucent Technologies and Harris Semiconductor (later owned by Intersil)

proposed a standard called Complementary Code Keying (CCK) to achieve

5.5Mbps and 11Mbps transmit rates. The IEEE adopted the CCK and released the802.11b in 1999.

The standard included a new option to transmit PLCP header with a short (56 bits)preamble. In the short preamble mode only the Synchronization and Start Frame

Delimiter fields are transmitted at 1Mbps. The rest of the PLCP header istransmitted at 2Mbps (using DSSSDQPSK) and the data payload at either the

same 2Mbps, or using CCK at 5.5Mbps or 11Mbps.

The 802.11b also introduced the auto rate fallback mechanism missed in the

original 802.11, thus standardizing the procedure of adjusting the data transmissionrate depending on the link quality.

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Complementary Code Keying

The CCK modulation is based on the use of the polyphase complementarycodes. The codes posses nearly orthogonal (close to zero autocorrelation if shift is

is not 0) properties. The polyphase complementary codes are not binary, they are

complex codes. The picture below shows a polyphase code with its real componentplaced in the vertical plane and the complex component in the horizontal plane.

Assuming the data transmission rate is set to 11Mbps, the CCK modulator is fed bybytes of data at the rate of 1.375MBytes/sec. The modulator uses 6 bits of each

byte to pick one of 64 unique orthogonal eight chips long polyphase

complementary codes (like the one on the picture). The other two bits of the byteare used to rotate the whole code word (0, 90, 180 or 270 degrees). Finally, 11

million times per second, the real and complex parts of the resulted code go to the

I(in-phase) and Q(quadrature) channels of the IQ modulator. The resulted symbol

rate is 11Mbps, the bandwidth occupied by the channel is 22MHz andconsequently the CCK modulation may coexist with original 802.11 DSSS.

802.11a Even Faster

The 802.11a introduced to the WLAN world a new modulation technique called

Orthogonal Frequency Division Multiplexing (OFDM). The similar to the OFDM

modulation techniques have been used in the modems world, primarily since theapproach allows higher data transmission rates in the smaller bandwidth. Besidesproposing the new modulation method, 802.11a also switches from the rapidly

getting overused 2.4GHz ISM band to 5GHz ISM band. The 5GHz ISM bandwidth

is not continuous. There are two areas 5.15GHz - 5.35GHz and 5.725GHz -5.825Ghz. Both areas are separated by 802.11a into 12 overlapping carriers(similar to 802.11 channels) spaced 20MHz.

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Each carrier separated into 52 subcarriers (also called tones) positioned accordingto the picture below. Four of the subcarriers are called pilot and have to transmit a

sequence that can be used by the receiver for synchronization control.

The subcarriers of the OFDM signal are modulated in such a way that adjacentchannels even though their shoulders overlap, do not interfere with each other. Thechapter "Theory of OFDM Operation" in online article [5] gives a very good

explanation (exactly at the level targeted in this document) of how the desired

orthogonality of modulated OFDM tones is reached and at what cost.

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Similarly to the DSSS the OFDM transmission prefixes each data packet with

PLCP synchronization sequence (PLCP preamble).

The PLCP preamble in OFDM is sometimes called training sequence. It consists of10 short OFDM symbols used by the receiver to tune AGC (automatic gain

control), select antenna and do coarse timing synchronization estimate. Thefollowing two long OFDM symbols allow the receiver to fine tune for the data

transmission.

The picture below shows the OFDM packet data layout. It starts with training

sequence (PLCP preamble), followed by the SIGNAL field and data. The data is

followed by 6 tail bits and padding (not shown on the picture). Both the trainingsequence and the 24 bit SIGNAL field are transmitted at 6Mbps rate. The SIGNAL

field tells the receiver at what rate the following data will be transmitted and

indirectly defines the subcarriers' modulation technique employed. The BPSK,

QPSK, 16-QAM and 64-QAM are the available choices. The SIGNAL field alsodelivers the length (12 bit) of the following data and includes a zero bit sequencefor the data scrambler synchronization. The total training sequence and SIGNAL

field transmission times add up to about 20 s, which is an overhead equivalent to

approximately 140 bytes transmission at the maximum transmission rate of54Mbps defined by the standard.

On the transmitter side the actual data to be sent get padded, scrambled and

distributed among the subcarriers according to the modulation technique/rate

chosen in the fashion that assures equal among the subcarriers' transmissionenergy. The signal (in the time domain) is digitally calculated from the

combination of all the modulated subcarriers. The resulted numbers are used

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directly to form the signal on the air. The whole process is reversed at the receiverside.

802.11g Faster But Compatible

Although the 5GHz band is not as crowded as 2.4GHz, one may expect decrease in

the operational range when upgrading from 802.11b to 802.11a. Besides, theproblem of upgrading the whole network may seem to be scary enough. The new

802.11g standard is called to solve all the problems. The standard is backward

compatible with 802.11b and 802.11. The support for the old modulation method ismandatory. For the high transmission rates it introduces the OFDM to the 2.4GHz

ISM band.

802.11n Chasing The Ethernet

The emerging new 802.11n standard addresses the need for wireless networks toprovide data transfer rates similar to those of the Fast Ethernet. The idea that

allows 802.11n compatible equipment to transfer even more data within the same

bandwidth is relatively simple. Lets consider a conventional 802.11g adapter radio.One of the major problems that radio receiver has to deal with is the multipath

propagation. Essentially the receiver has to filter various echo of the main signal itis tuned to, which is similar to having multiple transmitters of that signal in the

environment with no echo. The receivers successfully do the job, so why not to

install two transmitters, then install two receivers and tune each of them to the

individual transmitter. If that works, data can be sent through two channels and the54Mbs rate of the standard 802.11g equipment is doubled.

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Why would it work in the real environment where the additional transmitterswould generate more echo and would make it more difficult to filter the useful

signals? The intuitive answer is that another dimension, "space" is now used for the

data transmission and therefore increasing the bandwidth use efficiency. Inaddition, since the receivers work together and each one is synchronized to its ownsignal, one receiver's reception can be used to couterphase or nullify its component

of the signal for the opposite receiver and therefore improve the overall quality ofthe reception.

The 801.11n is not yet approved standard, however pre-standard equipment

utilizing concepts of MIMO (Multiple Input Multiple Output) is already availablefor purchase and is relatively inexpansive. We probably can expect the standard tobe ratified in the nearest future.

Conclusion

In the recent years we have witnessed as the demand and availability of theinexpensive technology made wireless network access real for everyone. The802.11 standards have been growing like mushrooms after the rain in order to

address the needs of the consumers. Since the consumers of the WiFi services areplaced on the edge of the network, with the introduction of the transmission rates

of 100Mbs and over the immediate attention of the "802.11 community" is likely to

shift even more from speed and the physical layer to wireless network structure,reliability, control, manageability, ease of use and security issues. In the future, as

the demand for the speed grows, hopefully we will have a "quantum leap" similarto the one that took Internet end users from telephone modems to Cable and DSL.

References:

1. "High Rate" Wireless Local Area Networks by Kanoksri Sarinnapakorn -March 15, 2001

2. ANSI/IEEE Std 802.11, 1999 Edition

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3. A beautiful mind: how Hollywood starlet Hedy Lamarr invented spreadspectrum technology - and transformed the wireless world by Rick

Mathieson4. Spread Spectrum (SS) applications by ir.J.Mill5.

"OFDM for Mobile Data Communications", IEC Tutorials(http://www.iec.org/online/tutorials)