IEEE 802.11a-1999标准：5GHz频段无线局域网物理层规范

802.11a-1999

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"IEEE Std 802.11a-1999 是一份由电气和电子工程师协会(IEEE)发布的标准，旨在定义无线局域网(WLAN)的媒体访问控制(MAC)和物理层(PHY)规范。该标准是 IEEE 802.11 家族的一部分，于1999年出版，主要关注在5GHz频段的高速物理层规格。" IEEE Std 802.11a-1999 是一个关键的无线通信标准，它扩展了基本的 IEEE 802.11 标准，提供了一种在5GHz频段工作的高速无线连接方式。这个标准的发布对于推动无线网络技术的发展起到了重要作用，特别是在家庭、办公室和公共场所的无线网络覆盖方面。标准的主要内容包括： 1. **物理层(PHY)规范**：802.11a 标准引入了OFDM(正交频分复用)技术，这允许在5GHz频段的多个子信道上同时传输数据，从而实现了高达54Mbps的数据速率，远高于之前802.11b标准的11Mbps。OFDM技术通过将高数据速率信号分解为多个较低速率的信号，在不同的子载波上进行传输，降低了干扰和多径衰落的影响。 2. **频道分配**：802.11a 标准在5GHz频段定义了多个非重叠频道，每个频道宽度为20MHz，这样可以同时支持多个并发连接，避免了频道之间的干扰。 3. **媒体访问控制(MAC)层**：虽然802.11a主要关注物理层的改进，但同时也对MAC层进行了调整以配合新的PHY层，确保数据的高效传输和网络资源的公平分配。 4. **安全性**：尽管802.11a标准本身并未包含WPA或WPA2等高级安全协议，但它是与这些后续安全标准兼容的，能够通过后续的升级和更新来提升无线网络的安全性。 5. **互操作性**：作为802.11家族的一员，802.11a与其他802.11标准（如802.11b和后来的802.11g）在设计时考虑到了一定程度的互操作性，允许不同标准的设备在一定条件下协同工作。 6. **应用领域**：802.11a 标准广泛应用于企业网络、热点区域、家庭网络以及无线回程连接等场景，提供了高速且稳定的无线连接。 IEEE Std 802.11a-1999 的发布标志着无线局域网技术的一个重要里程碑，为后来的802.11系列标准，如802.11g和802.11n，奠定了基础，并显著提升了无线通信的性能和用户体验。

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

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.

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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