IEEE 802.11a无线局域网高速物理层技术详解

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"802.11a无线局域网技术文档主要涵盖了无线局域网标准802.11a的相关技术细节，包括MAC（介质访问控制）层和物理层的设计与规范。该标准由IEEE（电气和电子工程师协会）制定，允许在5GHz频段上实现高速无线通信。802.11a是IEEE 802.11标准的一个补充部分，旨在提升无线局域网的数据传输速率和性能。" 802.11a无线局域网标准是1999年由IEEE发布的一个关键规范，它扩展了802.11的基础框架，特别针对5GHz频段设计了一个高速物理层（PHY）。这个标准的目标是提供比早期的802.11标准更高的数据传输速率，以满足不断增长的无线网络需求。在MAC层，802.11a采用了CSMA/CA（载波监听多路访问/冲突避免）机制，确保多个设备能够公平且有效地共享无线信道，避免数据包碰撞。它还支持多种服务质量（QoS）策略，以适应不同类型的流量，如实时音频和视频流。在物理层，802.11a定义了在5GHz频段内的操作，提供了多个频道，每个频道有52个子载波，每个子载波可以携带1/2、1/4或1/8的数据调制符号。该标准采用了正交频分复用（OFDM）技术，将高速数据流分解成多个较低速率的子流，然后在多个并行的子频道上同时传输，大大提高了数据传输效率。802.11a支持的最大理论传输速率为54Mbps，实际应用中通常可达到20-54Mbps，远高于当时802.11b标准的11Mbps。 802.11a标准的引入也带来了一些挑战，例如5GHz频段的穿透力较弱，覆盖范围相对较小，但其高带宽优势使得802.11a适合于室内高密度环境或需要高速传输的应用场景。随着技术的发展，802.11a后续被802.11g和802.11n等更先进的标准所取代，它们在保持高速度的同时，通过使用2.4GHz频段或采用MIMO（多输入多输出）技术，改善了信号覆盖和穿透能力。 802.11a标准是无线局域网技术演进中的一个里程碑，为后来的Wi-Fi技术发展奠定了基础，并推动了无线通信行业的快速进步。虽然现在已经不再是最新的标准，但了解802.11a有助于我们理解现代无线网络的工作原理和历史背景。

IEEE

HIGH-SPEED PHYSICAL LAYER IN THE 5 GHz BAND Std 802.11a-1999

17.3 OFDM PLCP sublayer

17.3.1 Introduction

This subclause provides a convergence procedure in which PSDUs are converted to and from PPDUs. Dur-

ing transmission, the PSDU shall be provided with a PLCP preamble and header to create the PPDU. At the

receiver, the PLCP preamble and header are processed to aid in demodulation and delivery of the PSDU.

17.3.2 PLCP frame format

Figure 107 shows the format for the PPDU including the OFDM PLCP preamble, OFDM PLCP header,

PSDU, tail bits, and pad bits. The PLCP header contains the following ﬁelds: LENGTH, RATE, a reserved

bit, an even parity bit, and the SERVICE ﬁeld. In terms of modulation, the LENGTH, RATE, reserved bit,

and parity bit (with 6 “zero” tail bits appended) constitute a separate single OFDM symbol, denoted SIG-

NAL, which is transmitted with the most robust combination of BPSK modulation and a coding rate of

R = 1/2. The SERVICE ﬁeld of the PLCP header and the PSDU (with 6 “zero” tail bits and pad bits

appended), denoted as DATA, are transmitted at the data rate described in the RATE ﬁeld and may constitute

multiple OFDM symbols. The tail bits in the SIGNAL symbol enable decoding of the RATE and LENGTH

ﬁelds immediately after the reception of the tail bits. The RATE and LENGTH are required for decoding the

DATA part of the packet. In addition, the CCA mechanism can be augmented by predicting the duration of

the packet from the contents of the RATE and LENGTH ﬁelds, even if the data rate is not supported by the

station. Each of these ﬁelds is described in detail in 17.3.3, 17.3.4, and 17.3.5.

17.3.2.1 Overview of the PPDU encoding process

The encoding process is composed of many detailed steps, which are described fully in later subclauses, as

noted below. The following overview intends to facilitate understanding the details described in these

subclauses:

a) Produce the PLCP preamble ﬁeld, composed of 10 repetitions of a “short training sequence” (used

for AGC convergence, diversity selection, timing acquisition, and coarse frequency acquisition in the

receiver) and two repetitions of a “long training sequence” (used for channel estimation and ﬁne fre-

quency acquisition in the receiver), preceded by a guard interval (GI). Refer to 17.3.3 for details.

Figure 107—PPDU frame format

Coded/OFDM

DATA

SIGNAL

One OFDM Symbol

PSDU

Tail

Pad Bits

LENGTH

12 bits

RATE

4 bits

Parity

1 bit 6 bits

Variable Number of OFDM Symbols

PLCP Preamble

12 Symbols

Reserved

1 bit

Tail

6 bits

Coded/OFDM

(BPSK, r = 1/2)

(RATE is indicated in SIGNAL)

SERVICE

16 bits

PLCP Header

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IEEE

Std 802.11a-1999 SUPPLEMENT TO IEEE STANDARD FOR INFORMATION TECHNOLOGY—

b) Produce the PLCP header ﬁeld from the RATE, LENGTH, and SERVICE ﬁelds of the TXVECTOR

by ﬁlling the appropriate bit ﬁelds. The RATE and LENGTH ﬁelds of the PLCP header are encoded

by a convolutional code at a rate of R = 1/2, and are subsequently mapped onto a single BPSK

encoded OFDM symbol, denoted as the SIGNAL symbol. In order to facilitate a reliable and timely

detection of the RATE and LENGTH ﬁelds, 6 “zero” tail bits are inserted into the PLCP header. The

encoding of the SIGNAL ﬁeld into an OFDM symbol follows the same steps for convolutional

encoding, interleaving, BPSK modulation, pilot insertion, Fourier transform, and prepending a GI as

described subsequently for data transmission at 6 Mbit/s. The contents of the SIGNAL ﬁeld are not

scrambled. Refer to 17.3.4 for details.

c) Calculate from RATE ﬁeld of the TXVECTOR the number of data bits per OFDM symbol (N

DBPS

the coding rate (R), the number of bits in each OFDM subcarrier (N

BPSC

), and the number of coded

bits per OFDM symbol (N

CBPS

). Refer to 17.3.2.2 for details.

d) Append the PSDU to the SERVICE ﬁeld of the TXVECTOR. Extend the resulting bit string with

“zero” bits (at least 6 bits) so that the resulting length will be a multiple of N

DBPS

. The resulting bit

string constitutes the DATA part of the packet. Refer to 17.3.5.4 for details.

e) Initiate the scrambler with a pseudorandom non-zero seed, generate a scrambling sequence, and

XOR it with the extended string of data bits. Refer to 17.3.5.4 for details.

f) Replace the six scrambled “zero” bits following the “data” with six nonscrambled “zero” bits.

(Those bits return the convolutional encoder to the “zero state” and are denoted as “tail bits.”) Refer

to 17.3.5.2 for details.

g) Encode the extended, scrambled data string with a convolutional encoder (R = 1/2). Omit (puncture)

some of the encoder output string (chosen according to “puncturing pattern”) to reach the desired

“coding rate.” Refer to 17.3.5.5 for details.

h) Divide the encoded bit string into groups of N

CBPS

bits. Within each group, perform an “interleav-

ing” (reordering) of the bits according to a rule corresponding to the desired RATE. Refer to 17.3.5.6

for details.

i) Divide the resulting coded and interleaved data string into groups of N

CBPS

bits. For each of the bit

groups, convert the bit group into a complex number according to the modulation encoding tables.

Refer to 17.3.5.7 for details.

j) Divide the complex number string into groups of 48 complex numbers. Each such group will be

associated with one OFDM symbol. In each group, the complex numbers will be numbered 0 to 47

and mapped hereafter into OFDM subcarriers numbered –26 to –22, –20 to –8, –6 to –1, 1 to 6,

8 to 20, and 22 to 26. The subcarriers –21, –7, 7, and 21 are skipped and, subsequently, used for

inserting pilot subcarriers. The “0” subcarrier, associated with center frequency, is omitted and ﬁlled

with zero value. Refer to 17.3.5.9 for details.

k) Four subcarriers are inserted as pilots into positions –21, –7, 7, and 21. The total number of the sub-

carriers is 52 (48 + 4). Refer to 17.3.5.8 for details.

l) For each group of subcarriers –26 to 26, convert the subcarriers to time domain using inverse Fourier

transform. Prepend to the Fourier-transformed waveform a circular extension of itself thus forming a

GI, and truncate the resulting periodic waveform to a single OFDM symbol length by applying time

domain windowing. Refer to 17.3.5.9 for details.

m) Append the OFDM symbols one after another, starting after the SIGNAL symbol describing the

RATE and LENGTH. Refer to 17.3.5.9 for details.

n) Up-convert the resulting “complex baseband” waveform to an RF frequency according to the center

frequency of the desired channel and transmit. Refer to 17.3.2.4 and 17.3.8.1 for details.

An illustration of the transmitted frame and its parts appears in Figure 110 of 17.3.3.

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IEEE

Std 802.11a-1999 SUPPLEMENT TO IEEE STANDARD FOR INFORMATION TECHNOLOGY—

17.3.2.4 Mathematical conventions in the signal descriptions

The transmitted signals will be described in a complex baseband signal notation. The actual transmitted sig-

nal is related to the complex baseband signal by the following relation:

(1)

where

(.) represents the real part of a complex variable;

denotes the carrier center frequency.

The transmitted baseband signal is composed of contributions from several OFDM symbols.

(2)

The subframes of which Equation (2) are composed are described in 17.3.3, 17.3.4, and 17.3.5.9. The time

offsets t

SUBFRAME

determine the starting time of the corresponding subframe;

SIGNAL

is equal to 16

s, and

DATA

is equal to 20

All the subframes of the signal are constructed as an inverse Fourier transform of a set of coefﬁcients,

with C

deﬁned later as data, pilots, or training symbols in 17.3.3 through 17.3.5.

(3)

The parameters ∆

and N

are described in Table 79. The resulting waveform is periodic with a period of

FFT

= 1/∆

. Shifting the time by T

GUARD

creates the “circular preﬁx” used in OFDM to avoid ISI from the

previous frame. Three kinds of T

GUARD

are deﬁned:

for the short training sequence (= 0 µs), for the long

training sequence (= T

GI2

), and for data OFDM symbols (= T

). (Refer to Table 79.) The boundaries of the

subframe are set by a multiplication by a time-windowing function, w

TSUBFRAME

(t), which is deﬁned as a

rectangular pulse, w

(t), of duration T, accepting the value T

SUBFRAME

. The time-windowing function,

(t), depending on the value of the duration parameter, T, may extend over more than one period, T

FFT

. In

particular, window functions that extend over multiple periods of the Fast Fourier Transform (FFT) are uti-

lized in the deﬁnition of the preamble. Figure 108 illustrates the possibility of extending the windowing

function over more than one period, T

FFT

, and additionally shows smoothed transitions by application of a

windowing function, as exempliﬁed in Equation (4). In particular, window functions that extend over multi-

ple periods of the FFT are utilized in the deﬁnition of the preamble.

(4)

RF()

t〈〉

Re{rt〈〉exp j2π f

t〈〉}=

PACKET

t() r

PREAMBLE

t() r

SIGNAL

–() + r

DATA

–()+=

SUBFRAME

t() w

TSUBFRAME

t() C

exp j2πk∆

()tT

GUARD

–()

kN–

2⁄=

2⁄

∑

t()

---

0.5 tT

⁄+()





2⁄– tT

2⁄<<()

sin

1 T

2⁄ tTT

2⁄–<≤()

---

0.5 tT–()T⁄

–()





2⁄ tTT

2⁄+<≤–()sin











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