802.11a White Paper

SIGNAL. This is a 24 bits field, which contains information about the rate and length of the PSDU. The PLCP preamble is BPSK-OFDM modulated at 6 Mbps using convolutional encoding rate R=1/2. The first 4 bits (R1-R4) are used to encode the rate. The next bit is 1 reserved bit. A continuation they are 12 bits used for the length, that indicated the number of octets in the PSDU. A continuation is a parity bit and 6 tail bits.

DATA. This field contains 16 bits for the service field, the PSDU, tails bits and pad bits. A total of 6 tail bits containing 0s are appended to the PPDU to ensure that the convolutional encoder is brought back to zero state. The data portion of the packet is transmitted at the data rate indicated in the signal field.

The PLCP header of 802.11a contains 4 bits for the rate, 1 reserved bit, 12 bits for length, 1 bit for parity, 6 bits for tail and 16 bits for service.

The bits Data Rate bits (RATE) R1-R4 is set, dependent on RATE, according to the values in Table 3.

The number of bits in the DATA field in the 802.11a is a multiple of N _CBPS , the number of coded bits in an OFDM symbol (48, 96, 192, or 288 bits). To achieve that, the length of the message is extended so that it becomes a multiple of N _DBPS, the number of data bits per OFDM symbol. At least 6 bits are appended to the message, in order to accommodate the TAIL bits. The number of OFDM symbols in the 802.11a, N _SYM ; the number of bits in the DATA field in the 802.11a, N _DATA; and the number of pad bits in the 802.1a, N _PAD , are computed from the length of the PSDU (LENGTH) as follows:

Rate, reserved bit, length, parity bit and 6 "zero" tail bits appended constitute a separate single OFDM symbol, denoted signal, which is transmitted in BPSK with 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), are transmitted at the data rate described in the rate field and constitute a multiple of 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 Clear Channel Assessment (CCA) mechanism is used to predict the duration of the packet from the contents of the rate and length fields.

The encoding process is as follows:

1.       The PLCP preamble field is produced, 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 frequency acquisition in the receiver), preceded by a guard interval (GI).

2.       The PLCP header field is produced 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. 6 "zero" TAIL bits are inserted into the PLCP header in order to facilitate a reliable and timely detection of the RATE and LENGTH fields. The encoding of the SIGNAL field into an OFDM symbol follows the same steps for convolutional encoding, interleaving, BPSK modulation, pilot insertion, IFFT, and pre-pending a GI (equivalent to the prefix in a ADSL system) as described subsequently for data transmission at 6 Mbps. The contents of the SIGNAL field are not scrambled.

3.       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).

4.       When this data rate is calculated, the PSDU is appended 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.

5.       The scrambler is initialized with a pseudorandom non-zero seed, generate a scrambling sequence, and XOR it with the extended string of data bits.

6.       Replace the six scrambled "zero" bits following the "data" with six non-scrambled "zero" bits. (Those bits return the convolutional encoder to the "zero state" and are denoted as "tail bits.").

7.       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."

8.       Divide the encoded bit string into groups of N_CBPS bits. Within each group, perform an "interleaving" (reordering) of the bits according to a rule corresponding to the desired RATE.

9.       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.

10.   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.

11.   Four subcarriers are inserted as pilots into positions -21, -7, 7, and 21. The total number of the sub-carriers is 52 (48 + 4).

12.   For each group of subcarriers -26 to 26, convert the subcarriers to time domain using inverse Fast Fourier Transform. Pre-pending to the IFFF 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.

13.   Append the OFDM symbols one after another, starting after the SIGNAL symbol describing the RATE and LENGTH.

14.   Up-convert the resulting "complex baseband" waveform to an RF frequency according to the center frequency of the desired channel and transmit.

The modulation parameters dependent on the data rate used is set according to Table 4.

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

A long OFDM training symbol consists of 53 subcarriers (including a zero value at DC), which are modulated by the elements of the sequence L, given by:

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6. Data Scramber

All the bits transmitted by the 802.11a OFDM PMD in the data portion are scrambled using a frame synchronous 127 bits sequence generator. Scrambling is used to randomize the service, PSDU, pad and data patterns, which may contain long strings of binary 1s or 0s. The tail bits are not scrambled. The octets of the PSDU are placed in the transmitted serial bit stream, bit 0 first and bit 7 last. The frame synchronous scrambler uses the generator polynomial S(x) as follows:

The 127 bits sequence generated repeatedly by the scrambler is (leftmost used first), 00001110 11110010 11001001 00000010 00100110 00101110 10110110 00001100 11010100 11100111 10110100 00101010 11111010 01010001 10111000 1111111, when the "all ones" initial state is used. The same scrambler is used to scramble transmit data and to de-scramble receive data. When transmitting, the initial state of the 802.11a scrambler will be set to a pseudo random non-zero state. The seven LSBs of the SERVICE field will be set to all zeros prior to scrambling to enable estimation of the initial state of the scrambler in the receiver. The contents of the SIGNAL field of the 802.11a are not scrambled.

The PLCP length field of the 802.11a is an unsigned 12 bits integer that indicates the number of octets in the PSDU that the MAC is currently requesting the PHY to transmit. This value is used by the PHY to determine the number of octet transfers that will occur between the MAC and the PHY after receiving a request to start transmission. The transmitted value is determined from the LENGTH parameter in the TXVECTOR issued with the PHY-TXSTART.request primitive. The LSB is transmitted first in time. The PLCP length field is encoded by the convolutional encoder.

Bit 4 reserved for future use. Bit 17 is a positive parity (even parity) bit for bits 0-16. The bits 18-23 constitute the SIGNAL TAIL field, and all 6 bits is set to zero.

The DATA field contains the SERVICE field, the PSDU, the TAIL bits, and the PAD bits, if needed. All bits in the DATA field are scrambled.

The IEEE 802.11 SERVICE field has 16 bits, which is denoted as bits 0-15. The bit 0 is transmitted first in time. The bits from 0-6 of the SERVICE field, which are transmitted first, are set to zeros and are used to synchronize the descrambler in the receiver. The remaining 9 bits (7-15) of the SERVICE field is reserved for future use. All reserved bits is set to zero.

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7. Convolutional Encoding

The PPDU tail bit field is six bits of "0", which are 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 be not be available past the end of the message. The PLCP tail bit field is produced by replacing six scrambled "zero" bits following the message end with six non-scrambled "zero" bits.

The appended bits ("pad bits") are set to "zeros" and are subsequently scrambled with the rest of the bits in the DATA field.

The DATA field of the 802.11a, composed of SERVICE, PSDU, tail, and pad parts, is coded with a convolutional encoder of coding rate R = 1/2, 2/3, or 3/4, corresponding to the desired data rate. The convolutional encoder used the generator polynomials, g0 = (133)o and g1 = (171)o , of rate R = 1/2. Higher rates are derived from it by employing "puncturing" 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) and inserting a dummy "zero" metric into the convolutional decoder on the receive side in place of the omitted bits.

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8. Data Interleaving

In the 802.11a standard, a block interleaver interleaves all encoded data bits. The block size corresponds to the number of bits in a single OFDM symbol, N _CBPS. The interleaver is defined by a two steps permutation. The first permutation ensures that adjacent coded bits are mapped onto nonadjacent subcarriers. The second ensures that adjacent coded bits are mapped alternately onto less and more significant bits of the constellation and, thereby, long runs of low reliability (LSB) bits are avoided.

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9. Modulation and Mapping

The OFDM subcarriers are modulated by using BPSK, QPSK, 16 QAM, or 64 QAM modulation, depending on the RATE requested. In the 802.11a the encoded and interleaved binary serial input data are divided into groups of N _BPSC (1, 2, 4, or 6) bits and converted into complex numbers representing BPSK, QPSK, 16 QAM, or 64 QAM constellation points. The conversion is performed according to Gray coded constellation mappings.

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 are in subcarriers -21, -7, 7 and 21. The pilots are BPSK modulated by a pseudo binary sequence to prevent the generation of spectral lines.

The stream of complex numbers is divided into groups of N _SD = 48 complex numbers. The complex number d _(k,n), corresponds to subcarrier k of OFDM symbol n, is:

PLCP provides the capability to perform CCA and report the result to the MAC. The CCA mechanism detects a "medium busy" condition. The primitive PHY_CCA.indicate indicates the medium status report.

The PLCP preamble of the 802.11a is transmitted using an OFDM modulated fixed waveform. The 802.11 SIGNAL field, BPSK OFDM modulated at 6 Mbps, indicates the modulation and coding rate that is used to transmit the MPDU. The transmitter (receiver) initiates the modulation (demodulation) constellation and the coding rate according to the RATE indicated in the SIGNAL field. The MPDU transmission rate is set by the DATARATE parameter in the TXVECTOR, issued with the PHY-TXSTART.

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10. OFDM Operating Channels and Transmit Power Requirements

For the 802.11a standard the 5 GHz U-NII frequency bans is segmented into three 100 MHz bands for operation in the US. The lower band ranges from 5.15 -5.25 GHz, the middle band ranges from 5.25-5.35 GHz and the upper band ranges from 5.725-5.825 GHz. The lower and middle band, accommodate 8 channels in a total bandwidth of 200 MHz and the upper band accommodates 4 channels in a 100 MHz bandwidth. The frequency channel center frequencies are spaced 20 MHz apart. The outermost channels of the lower and middle bands are centered 30 MHz from the outer edges. In the upper band the outermost channel centers are 20 MHz from the outer edges.

In addition to the frequency and channel allocations, transmit power is a key parameter regulated in the 5 GHz U-NII band. Three transmit power levels are specified: 40 mW, 200 mW and 800 mW. The upper band defines RF transmit power levels suitable for bridging applications while the lower band specifies a transmit power level suitable for short-range indoor home and small office environments.

Table 6 shows the operating frequency and maximum power of the 802.11a standard.

Table 6. OFDM Operating Bands and channels

Band	Channel numbers	Frequency (MHz)	Maximum output power
U-NII lower band 95.15 to 5.25 MHz	36	5180	40mW (2.5mW/MHz)
	40	5200
	44	5220
	48	5240
U-NII lower band 95.15 to 5.25 MHz	52	5260	200mW (12.5mW/MHz)
	56	5280
	60	5300
	64	5320
U-NII lower band 95.15 to 5.25 MHz	149	5745	800mW (50mW/MHz)
	153	5765
	157	5785
	161	5805

Enhanced GPRS are already specified. The more important difference is the modulation scheme used, 8 PSK that provides Spectral Efficiency of 3 bit/s/Hz. The modulation used in GPRS is the same used in GSM, the GSMK that provides Spectral Efficiency of 1 bit/s/Hz.

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11. 802.11a System Description

Figure 2 shows a block diagram of the transmitter.

Figure 2. 802.11a Transmitter Block Diagram

Figure 3 shows a block diagram of the receiver.

Figure 3. 802.11a Receiver Block Diagram

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