introduction to ieee 802.11a wlan system

PowerPoint Presentation2004/4/29 WLAN Group2
OFDM : Orthogonal Frequency Division Multiplexing
Introduction to IEEE 802.11a Standard Algorithms for IEEE 802.11a RX
Synchronization Channel Estimation Phase Tracking
Introduction to Basic OFDM
Table of Contents Introduction
Generation of Subcarriers by Using the IFFT IFFT : Inverse Fast Fourier Transform
Guard Interval (GI) and Cyclic Prefix (CP)
2004/4/29 WLAN Group5
OFDM system structure
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 bandwidth In 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
tw0cos
tw0cos
2 where )(cos)cos(
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 : ∑ −
Table of Contents Introduction
Generation of Subcarriers by Using the IFFT IFFT : Inverse Fast Fourier Transform
One of the most important reasons to do OFDM is the efficient way it deals with multipath delay spread To 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 : (a)
τ
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 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
Introduction The 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) band The 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
PLCP : Physical Layer Convergence Protocol PMD: Physical Medium Dependent SAP : Service Access Point
Composed of 10 repetitions of a “shorting training sequence” AGC (Automatic Gain Control) convergence Timing acquisition Coarse frequency acquisition Diversity selection
Composed of 2 repetitions of a “long training sequence” Channel estimation Fine timing synchronization (timing tracking) Fine frequency tracking (phase tracking)
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 a multiple of 4 have nonzero amplitude results in a periodicity of 0.8 μs
-26 (-24)
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 convolutional encoding, interleaving, modulation mapping processes, pilot insertion, & OFDM modulation as used for a transmission of data at a 6 Mbit/s rate
Signal field (SIGNAL) Bit 4 : shall be reserved for future use Bit 17: shall be positive parity (even parity) bit for bit 0~16 Bit 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 field The 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 bits The 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 bits NSYM = Ceiling ((16 + 8 × LENGTH + 6)/NDBPS) NDATA = NSYM × NDBPS
NPAD = NDATA – (16 + 8 × LENGTH + 6)
where NSYMThe number of OFDM symbols NDATA : The number of bits in the DATA field NPAD : The number of pad bits LENGTH : The length of the PSDU
PLCP DATA scrambler and descrambler
The frame scrambler uses the generator polynomial S(x) is S(x) = x7 + x4 + 1
Convolutional encoder The DATA field shall be coded with a convolutional encoder of coding rate R = 1/2
Convolutional encoder Higher code rate: 2/3, or 3/4 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) Increasing the BW efficiency Increasing the bit error rate (BER)
Data interleaving In 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 permutation i shall be the index after the first and before the second permutation j shall be the index after the second permutation
Pilot subcarriers In 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
Pilot subcarriers The 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 1 Each 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
IFFT An OFDM symbol, rDATA, n(t), is defined as
NSD : the number of modulated data symbols NST : 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 Interval Shifting the time by TGUARD creates the “cyclic prefix” used in OFDM to avoid ISI (Inter-Symbol Interference) from the previous frame Three kinds of TGUARD are defined

introduction to ieee 802.11a wlan system

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