IEEE 802.11a: 5GHz频段的高速物理层标准

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"802.11a 高速物理层在5 GHz频段" 802.11a 是一个重要的无线局域网（WLAN）标准，由IEEE（电气和电子工程师协会）制定，是802.11标准的一个补充，主要关注在5 GHz频段实现高速无线数据传输。这个标准于1999年首次发布，并在2003年进行了重新确认，被ISO/IEC采纳并命名为ISO/IEC 8802-11:1999/Amd1:2000(E)。802.11a标准的推出，为无线通信领域带来了显著的性能提升，特别是在家庭、办公室和公共场所的无线网络应用中。 802.11a 标准的主要特性包括： 1. **频段**：与802.11b/g工作在2.4 GHz频段不同，802.11a工作在5 GHz频段。5 GHz频段提供了更多的非授权频谱，因此可以减少干扰并支持更高的数据速率。 2. **数据速率**：802.11a最初定义了五个不同的数据速率：6, 9, 12, 18, 24, 36, 48 和 54 Mbps。这些速率远高于同时期的802.11b（最高11 Mbps），为用户提供了更快的网络连接速度。 3. **调制技术**：802.11a采用了正交频分复用（OFDM）技术，这是一种高效的数据传输方法，能够将信号分解成多个子载波，每个子载波独立传输数据，提高了频谱利用率和抗干扰能力。 4. **频道划分**：在5 GHz频段，802.11a标准定义了14个互不重叠的20 MHz频道，这允许在同一地理区域内部署多个独立的无线网络，而不会相互干扰。 5. **覆盖范围**：由于5 GHz频段的波长较短，802.11a的覆盖范围通常小于2.4 GHz的802.11b/g标准。然而，较高的数据速率可以在短距离内提供更优质的无线连接。 6. **兼容性**：802.11a与早期的802.11b/g标准不兼容，因为它们工作在不同的频率上。随着时间的推移，802.11g的出现解决了这个问题，它在2.4 GHz频段提供了与802.11a相近的数据速率，并且与802.11b兼容。 7. **应用**：802.11a适用于需要高速无线连接的场合，如企业级网络、无线回传、高清视频流等，但由于覆盖范围相对较小，它通常用于室内环境或短距离传输。 8. **标准化过程**：802.11a标准是由IEEE的局域/城域网标准委员会（LAN/MAN Standards Committee）开发的，这个过程涉及大量志愿者的参与，他们可能不是IEEE的成员，但他们在该领域的专业知识和兴趣推动了标准的形成。 802.11a标准为无线通信带来了革命性的变化，它开启了5 GHz频段的广泛应用，为后来的802.11n、802.11ac和802.11ax等更高速标准奠定了基础。尽管它的覆盖范围有限，但802.11a在高密度数据传输和避免2.4 GHz频段拥堵方面具有显著优势。

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











IEEE

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

In the case of vanishing T

, the windowing function degenerates into a rectangular pulse of duration T. The

normative speciﬁcations of generating the transmitted waveforms shall utilize the rectangular pulse shape. In

implementation, higher T

is typically implemented in order to smooth the transitions between the consec-

utive subsections. This creates a small overlap between them, of duration T

, as shown in Figure 108. The

transition time, T

, is about 100 ns. Smoothing the transition is required in order to reduce the spectral side-

lobes of the transmitted waveform. However, the binding requirements are the spectral mask and modulation

accuracy requirements, as detailed in 17.3.9.2 and 17.3.9.6. Time domain windowing, as described here, is

just one way to achieve those objectives. The implementor may use other methods to achieve the same goal,

such as frequency domain ﬁltering. Therefore, the transition shape and duration of the transition are infor-

mative parameters.

17.3.2.5 Discrete time implementation considerations

The following descriptions of the discrete time implementation are informational.

In a typical implementation, the windowing function will be represented in discrete time. As an example,

when a windowing function with parameters T = 4.0 µs and a T

= 100 ns is applied, and the signal is sam-

pled at 20 Msamples/s, it becomes

(5)

The common way to implement the inverse Fourier transform, as shown in Equation (3), is by an inverse

Fast Fourier Transform (IFFT) algorithm. If, for example, a 64-point IFFT is used, the coefﬁcients 1 to 26

are mapped to the same numbered IFFT inputs, while the coefﬁcients –26 to –1 are copied into IFFT inputs

Figure 108—Illustration of OFDM frame with cyclic extension and windowing for (a) single

reception or (b) two receptions of the FFT period

T = T

FFT

GUARD

FFT

T = T

GI2

+2T

FFT

GI2

(a)

(b)

GUARD

n[] w

()

11n 79≤≤

0.5 0 80,

0 otherwise











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