Introduction to IEEE 802.11a WLAN System140.117.160.140/.../9222/Introduction_to_OFDM.pdf · 2004/4/29 6 WLAN Group Introduction The basic principle of OFDM is to split a high-rate

Introduction to IEEE 802.11a WLAN System

Advisor : 李志鵬博士Presenter : WLAN Group

2004/4/29 WLAN Group2

Table of contents Introduction to Basic OFDM

OFDM : Orthogonal Frequency Division Multiplexing

Introduction to IEEE 802.11a StandardAlgorithms for IEEE 802.11a RX

SynchronizationChannel Estimation Phase Tracking

Introduction to Basic OFDM

Ming-Li Wang (王鳴立)


Table of ContentsIntroduction

Generation of Subcarriers by Using the IFFTIFFT : Inverse Fast Fourier Transform

Guard Interval (GI) and Cyclic Prefix (CP)


OFDM system structure

Baseband RF


Introduction The basic principle of OFDM is to split a high-rate data stream into a number of lower rate streams that are transmitted simultaneously over a number of subcarriers.It eliminates or alleviates the problems of inter-symbol interference (ISI), low spectrum efficiency, and frequency selective fading.OFDM transmission is a very useful transmission technique for a frequency selective fading channel. (τd > Ts)


The use of transmission bandwidthIn a classical parallel system, the channel is divided into N non-overlapping sub-channels to avoid inter-carrier interference (ICI).

For bandwidth efficiency, the frequency of each sub-carrier is orthogonal to one another. (i.e. each sub-carrier has zero contribution on other sub-carrier frequencies.)

Frequency

Ch.1 Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10

Frequency

Saving of bandwidth


The basic OFDM system : Transmitter

tw0cos

twN 1cos −

twN 1sin −

tw0sin

t0T

+1

-1

a(k)

tT

+1

a(0)

a(1) a(N-1)

a(0)

( ∑ )


The basic OFDM system : Receiver

tw0cos

twN 1cos −

twN 1sin −

tw0sin


Proof

0

])1sin[()1(

1])1sin[()1(

121

|])1sin[()1(

1|])1sin[()1(

121

])1[(cos])1cos[(21

2 where )(cos)cos(

00

0

0

=

⎥⎦

⎤⎢⎣

⎡−

−++

+=

⎥⎦

⎤⎢⎣

⎡−

−++

+=

−++=

=⋅

∫

∫

TNN

TNN

tNN

tNN

dttNtN

TdttNt

TT

T

T

ωω

ωω

ωω

ωω

ωω

πωωω


Computational load [1]N-point DFT : N2 (complex multiplications )

FFT : N · log(N) ( if N = 2x, N-point DFT FFT )

FFT (radix-2 butterfly) : (N/2) · log(N)

FFT (radix-4 butterfly) : (3/8) · N · log2(N-2)

IFFT algorithm :∑−

=

=1

0)2exp(1)(

N

ii N

injDN

ns π


Table of ContentsIntroduction

Generation of Subcarriers by Using the IFFTIFFT : Inverse Fast Fourier Transform




One of the most important reasons to do OFDM is the efficient way it deals with multipath delay spreadTo eliminate inter-symbol interference (ISI) almost completely, a guard time is introduced for each OFDM symbol(The guard time is chosen larger than the delay spread)

Direct Wave

Delayed Wave

∆τ : (b)

∆ UTOne OFDM Symbol

Sampling Window



The figure shows the effect of multipath with zero signal in the guard time; the delayed subcarrier 2 causes ICI on subcarrier 1 and vice versa.



Guard interval definition :

OFDM symbol with cyclic prefix :guard interval. useful sym bol.

copy



16-QAM constellation for a 48-subcarrier OFDM link with a two-ray multipath channel, the second ray being 6 dB lower than the first one.


The FFT-based OFDM system


The characteristics of the OFDM system

The advantage of the FFT-based OFDM system :The use of FFT can reduce the computation complexity.The orthogonality between the adjacent subcarriers will make the use of transmission bandwidth more efficient.The guard interval is used to resist the inter-symbol interference (ISI).The main advantage of the OFDM transmission technique is its high performance even in frequency selective channels.

The drawbacks of the OFDM system :It is highly vulnerable to synchronization errors.Peak to Average Power Ratio (PAPR) problems.


References[1] Blahut, R. E., “ Fast Algorithms for Digital Signal Processing.” MA: Addison-Wesley, 1985[2] R.V. Nee and R. Prasad, “OFDM for Wireless Multimedia Communications.” Artech House, 2000[3] J. Terry and J. Heiskala, “OFDM Wireless LANs: A Theoretical and Practical Guide.” Sams, 2002

WLAN IEEE 802.11a SPEC.(Physical Layer)


802.11a PHY SPEC. for the 5GHz band

IntroductionThe radio frequency (RF) WLAN system is initially aimed for the 5.15-5.25, 5.25-5.35, & 5.725-5.825 GHz unlicensed national information infrastructure (U-NII) bandThe support of transmitting & receiving at data rates of 6, 12, 24 Mbit/s is mandatory (9, 18, 36, 48, 54Mbit/s may be supported) The system uses 52 subcarriers that are modulated using

binary or quadrature phase shift keying (BPSK/QPSK)16-quadrature amplitude modulation (QAM), or 64-QAM

Forward error correction coding (convolutional coding) is used with a coding rate of 1/2, 2/3, or 3/4


PHY layer

PLCP : Physical Layer Convergence Protocol PMD: Physical Medium DependentSAP : Service Access Point


PLCP sublayer


PLCP sublayerIn the PLCP preamble field

Composed of 10 repetitions of a “shorting training sequence”AGC (Automatic Gain Control) convergence Timing acquisitionCoarse frequency acquisitionDiversity selection

Composed of 2 repetitions of a “long training sequence”Channel estimation Fine timing synchronization (timing tracking)Fine frequency tracking (phase tracking)


RATE-dependent parameters


Timing related parameters


PLCP preamble (SYNC.)It consists of 10 short symbols & 2 long symbols

TFFT (IFFT/FFT period)


Mathematical conventions in the signal descriptions

All the subframes of the signal are constructed as an inverse Fourier transform of a set of coefficients, Ck.where Ck defined later as data, pilot, or training symbols

Note : The resulting waveform is periodic with a period of TFFT = 1/Δf


Short training symbol

A short OFDM training symbol consists of 12 subcarriers, which are modulated by the elements of the sequence S

is in order to normalize the average power of the resulting OFDM symbol, which utilizes 12 out of 52 subcarrier (see p.56 table G.2)The fact that only spectral lines of S-26:26 with indices that are amultiple of 4 have nonzero amplitude results in a periodicity of 0.8 μs

-26 (-24)

(4) (8) (12)

(0)


Long training symbol

A long training symbol consists of 53 subcarriers (including a zero value at dc) which are modulated by the elements of the sequence L Two periods of the long sequence are transmitted


Inverse Fourier transform implementation considerations

The common way to implement the inverse Fourier transform is by an Inverse Fast Fourier Transform (IFFT)algorithm


Signal field (SIGNAL)


Signal field (SIGNAL)The SIGNAL field contains the RATE & the LENGTH field of the TXVECTOR The RATE field conveys information about the type of modulation & the coding rate as used in the rest of the packet The encoding procedure, which includes convolutionalencoding, interleaving, modulation mapping processes, pilot insertion, & OFDM modulation as used for a transmission of data at a 6 Mbit/s rate


Block diagram


Signal field (SIGNAL)Bit 4 : shall be reserved for future useBit 17: shall be positive parity (even parity) bit for bit 0~16Bit 18~23 : all 6 bits shall be set to zero (in order to facilitate a reliable & timely detection of the RATE and LENGTH fields)Data rate


Service field

The 0 ~ 6 bits are set to zeros and are used to synchronize the descrambler in the receiver


PPDU tail bit fieldThe tail bit field shall be six bits of “0,” required to return the convolutional encoder to the “zero state.”

This procedure improves the error probability of the convolutional decoder, which relies on future bits when decoding and which may not be available past the end of the message.


Pad bitsThe number of bits in the DATA field shall be a multiple of NCBPS( the number of coded bits in an OFDM symbol )

To achieve that, the length of the message is extended so that it becomes a multiple of NDBPS ( the number of data bits per OFDM symbol )


Pad bitsNSYM = Ceiling ((16 + 8 × LENGTH + 6)/NDBPS)NDATA = NSYM × NDBPSNPAD = NDATA – (16 + 8 × LENGTH + 6)

whereNSYM：The number of OFDM symbolsNDATA : The number of bits in the DATA field NPAD : The number of pad bitsLENGTH : The length of the PSDU


PLCP DATA scrambler and descrambler


PLCP DATA scrambler and descrambler

The frame scrambler uses the generator polynomial S(x) is S(x) = x7 + x4 + 1


Convolutional encoderThe DATA field shall be coded with a convolutional encoder of coding rate R = 1/2


Convolutional encoderHigher code rate: 2/3, or 3/4Puncturing 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)Increasing the BW efficiency Increasing the bit error rate (BER)


Puncturing


Data interleavingIn order to avoid the presence of burst error.The interleaver is defined by a two-step permutation.The first permutation ensures that adjacent coded bits are mapped onto nonadjacent subcarriers.The deinterleaver performs the inverse relation.We shall denote by

k the index of the coded bit before the first permutationi shall be the index after the first and before the secondpermutationj shall be the index after the second permutation


Data interleavingFirst permtation

Senond permutation

where


Modulator


Pilot subcarriersIn each OFDM symbol, four of the subcarriers are dedicated to pilot signals in order to make the coherent detection robust against frequency offsets and phase noise

These pilot signals shall be put in subcarriers –21, –7,7, and 21


Before adding pilot subcarriers


After adding pilot subcarriers


Pilot subcarriersThe polarity of the pilot subcarriers is controlled by the PN sequence ( i.e. the output sequence of the scrambler )Replacing all 1’s with -1 and all 0’s with 1Each sequence element is used for one OFDM symbol In the sequence of the pilot polarity, the first element, p0, multiplies the pilot subcarriers of the SIGNAL symbol, the others pn are used for the DATA symbols


Pilot subcarriers

0 11 -1


Pilot subcarriersPolarity of the pilot subcarriers


IFFT An OFDM symbol, rDATA, n(t), is defined as

NSD : the number of modulated data symbolsNST : the number of pilot symbols


Windowing function

where TTR : Transition time (about 100ns), smooth the transition is required in order to reduce the spectral sidelobes of the transmitted waveform


Guard IntervalShifting the time by TGUARD creates the “cyclic prefix” used in OFDM to avoid ISI (Inter-Symbol Interference) from the previous frameThree kinds of TGUARD are defined

For the short training sequence (= 0μs)For the long training sequence (= TGI2 = 1.6μs)For the data OFDM symbols (= TGI = 0.8μs)

Introduction to IEEE 802.11a WLAN System140.117.160.140/.../9222/Introduction_to_OFDM.pdf · 2004/4/29 6 WLAN Group Introduction The basic principle of OFDM is to split a high-rate

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