IEEE 802.11a s0323516@ncnu.edu.tw 1 (WLAN) [1] 1963 IEEE Institute Of Electrical and Electronics Engineers LAN MAN-Metropolitan Area Network IEEE 802 IEEE 802 Working Group 802.11 IEEE 802 802.1 LAN MAN 802.2 802.3 802.4 Token Bus 802.5 Token Ring 802.6 MAN- Metropolitan Area Network 802.7 802.8 802.9 802.10 802.11 MAC 802.12 Demand Priority Access Methods 802.14 Cable TV network 802.15 WPAN-Wireless Personal Area Network 1
802.16 MAN- Metropolitan Area Network 802.17 Resilient Packet Ring 802.11 802.11a : 5GHz 54Mbps 802.11b : 2.4GHz 11Mbps 802.11c : MAC Bridge 802.1D 802.11d : 802.11 802.11e : WhiteCAP 802.11 MAC QoS 802.11f : Access Point 802.11g : 2.4GHz 54Mbps 802.11h : 802.11a HiperLAN 2 802.11i : 802.11 MAC 802.11 a/b/g 802.11 e MAC-Media Access Control 802.11 15 ~ ~ MAC Header ~ 1Mbps, 2Mbps 802.11 a/b/g 2
802.11 a/b High Speed Physical layer in The 5GHz Band High Speed Physical layer Extension in The 2.4GHz Band 802.11 MAC 802.11 528 802.11a 91 802.11 802.11b 96 802.11 802.11a 802.11b 1999 11 IEEE 802.11a 802.11b 802.11 2.4GHz 5.5Mbps 11Mbps 802.11a 5GHz 6MBPs 54Mbps 8 2 IEEE 802.11a V.S. Hiperlan2 [4] Hiperlan2 802.11a PHY 5GHz IEEE 802 ESTI ITU-R WLAN IEEE 802.11 802.11a NTT Lucent (OFDM) WLAN OFDM (multipath channel fading effect) IEEE 802.11a ESTI WLAN Hiperlan2 OFDM 802.11a Hiperlan2 PHY PHY MAC(Medium Access Control) 3
3 IEEE 802.11a 3-1 IEEE 802.11a U-NII (Unlicensed-National Information Infrastructure) 300MHz WLAN 5GHz 6GHz Lower and Middle U-NII Band Upper U-NII Band WLAN Hiperlan2 5~6GHz 455MHz 1 1. 802.11a HiperLan2 802.11a (Channel) 20MHz 2 5GHz FCC-UNII WLAN (Lower and Middle U-NII Band) 200MHz (Upper U-NII Band) 100MHz 5GHz WLAN 300MHz Lower and Middle U-NII Band U-NII (5150MHz) (5350MHz) 30MHz Upper U-NII Band 20MHz 4
2. IEEE 802.11a 3-2 OFDM 802.11a (multi-carrier modulation) OFDM OFDM 3 ( ) FFT IFFT FEC encoder Interleaver 5
3-2.1 OFDM [3] 3. OFDM OFDM main lob spacing 2-4 ICI OFDM IFFT 4. (a) (b) 6
3-2.2 (Guard Interval or Cyclic Prefix) OFDM IFFT FFT OFDM symbol (ISI) OFDM symbol (cyclic prefix ; CP) T g delay spread ISI CP 5 5. Guard Interval 3-2.3 Windowing [2] OFDM QAM sinc OFDM (out-of-band) OFDM window main lob side lob Windowing ICI window side lob ICI 802.11a window function 7
T OFDM symbol transition time 6 6. CP window OFDM symbol Window function n OFDM sample 802.11a symbol 80 sample 3-2.4 Scrambler Scrambler 802.11a Frame Data field ( ) scrambler scrambler (Generator Polynomial) 8
802.11a generator polynomial X 7 + X 4 + 1 7 802.11a scrambler 3-2.5 Convolutional Encoder 7. Data scrambler (Forward Error Correction Code ; FEC) 802.11a convolutional code FEC convolution code generator polynomial g o = 133 8 g 1 = 171 8 code rate 1/2 Viterbi algorithm Convolutional Encoder 8 8. Convolutional Encoder ( k=7 ) 9
bit A B R = 1/2 802.11a R 1/2 2/3 3/4 code rate puncturing Puncturing convolutional encoder bits bits bits coding rate Puncturing convolution encoder Viterbi decoder code rate 2dB 2dB IEEE 802.11a puncturing pattern 9 10 9. Puncturing Pattern (R = 3/4) 10
10. Puncturing Pattern (R = 2/3) 3-2.6 Interleaver Convolutional encoder bits interleaving Interleaver Interleaver bits bits bits (independent) burst error IEEE 802.11a Interleaver (R 1/2 2/3 3/4) (BPSK QPSK 16~64QAM) OFDM (coded bits per OFDM symbol) interleaver coded bit coded bits mapping QAM less and more significant bit bit (reliability) 11
IEEE 802.11a 24Mbps Interleaver 24Mbps 2 OFDM coded bits 192 interleaving 192 bits 12x16 interleaving table 11 11. Interleaving Table 12. Table 12
13. Interleaving Table 12x16 Table 12 Interleaving Table 13 24Mbps 48Mbps Interleaving Table 18x16 Table 4 802.11a 802.11a OFDM 64 IFFT (modulator) 64 (subcarrier) 802.11a 20MHz (subcarrier spacing) 20MHz 64 = 312.5KHz subcarrier spacing OFDM (symbol period) 802.11a OFDM (frequency offset) ICI(Intercarrier Interference) (frequency selective fading) 13
64 IFFT 64 subcarrier 64 subcarrier 6 5 (virtual carrier) (guard band) DC 64-5-6-1=52 52 4 pilot 802.11a 48 1 48 BPSK QPSK 16-QAM 64-QAM convolutional encoder coding rate 6 ~ 54 Mbits/s 802.11a 6 12 24Mbits/s 2 1. 802.11a 14
2. 802.11a 5 IEEE 802.11a Frame [2] OSI PHY PHY 0 1 PHY RF PHY MAC layer (Medium Access Control Layer) PHY MAC PHY MAC PHY MAC PHY IEEE 802.11a PHY (1). PMD system function PMD (physical medium dependent) system 15
OFDM (transceiver) (2). PHY convergence function physical layer convergence procedure (PLCP) IEEE 802.11 PHY service data units (PSDU ; ) frame frame OFDM PMD system MAC PMD PLCP PLCP 802.11a 5-1 IEEE 802.11a frame format 802.11a (PSDU) PSDU MAC 802.11 802.11a frame format frame format 14 (1). PLCP Preamble (2). Signal Field (3). Data Field 16
14. 802.11a frame format PLCP preamble Signal field PLCP header PLCP header MAC PMD PLCP header LENGTH (12 bits) RATE (4 bits) Reserved bit (1 bit) Parity bit (1 bit) Tail bit (6 bits) SERVICE (16 bits) SERVICE PLCP header Signal field OFDM symbol SERVICE bits Signal field SERVICE DATA field 14 Data field SERVICE (16 bits) Tail (6 bits) Pad bits PSDU ( ) 5-2 Preamble OFDM 802.11a (frequency offset) (oscillator) mobile (Doppler Effect) OFDM 17
ICI 802.11a OFDM 802.11a training sequence training sequence preamble Training sequence 802.11a training sequence short sequence 10 symbol 10 symbol burst mode frame synchronization (coarse synchronization) long sequence 2 symbol long sequence (channel estimation) (fine synchronization) 15 802.11a frame 10 short 2 long sequence 15. 802.11a training sequence frame short sequence (frequency estimation) long sequence symbol 1/T short sequence 18
f 1 2 T 1 2 800 ns = ± = ± = ± short short 625 KHz long sequence f 1 2 T 1 2 3.2us = ± = ± = ± long long 156.25 KHz 802.11a subcarrier spacing 312.5KHz short sequence 2 subcarrier spacing long sequence 0.5 subcarrier spacing 5-2.1 Short preamble Short training sequence 10 symbol 802.11a 64 IFFT 64 QAM mapping 64 subcarrier 802.11a 11 virtual carrier (guard band) 53 subcarrier short sequence 53 subcarrier IFFT S -26,26 S -26,26 13/6 OFDM (average power) (normalize) IFFT 19
r SHORT (t) = w TSHORT (t) N ST k = N / 2 ST S k / 2 exp( j2πk F t) W(t) window function IFFT 4 symbol 16 IFFT CP IFFT 5 short sequence symbol short sequence IFFT 10 short symbol IFFT 3.2 sec CP 1/4 IFFT 5-2.2 Long preamble Long training sequence short sequence Long sequence 2 long symbol 1.6 sec guard interval (GI2) short sequence S -26,26 long sequence L -26,26 IFFT r LONG (t) = w TLONG (t) N ST k= N / 2 ST L k / 2 exp( j2πk F (t T G12 )) long sequence 2 IFFT CP 1/4 IFFT long sequence CP 20
IFFT OFDM symbol 1/4 1.6 sec CP 2 long symbol long sequence IEEE 802.11a Standard Annex G 5-2.3 802.11a IFFT OFDM IFFT subcarrier QAM mapping 802.11a subcarrier QAM mapping 802.11a IFFT subcarrier QAM mapping 16 16. 802.11a IFFT Null 0+0j QAM symbol subcarrier 0 DC 802.11a 27 37 11 subcarrier 802.11a (guard band) 64 subcarrier 6 5 guard band (channel) DC subcarrier 52 subcarrier 52 21
4 pilot (channel estimation) 48 subcarrier 17 802.11a subcarrier -32 31 30-29 28 27 26 25 24-2 1 0 1 2 24 25 26 27 28 29 30 31 π 6 virtual carrier 5 virtual carrier 17. 802.11a DC π DC 802.11a standard To avoid difficulties in D/A and A/D converter offsets and carrier feed through in the RF system, the subcarrier falling at DC (0 th subcarrier) is not used. short long sequence IFFT 802.11a IFFT 5-3 Signal field signal field signal field MAC PMD PLCP header PLCP header signal field LENGTH RATE Reserved bit Parity bit Tail bit SERVICE bit 18 22
18. Signal field bit 5-3.1 Data rate (RATE) 802.11a 6 9 12 18 24 36 48 54Mbits/sec MAC PHY RATE PHY RATE 4 bits 3-1 RATE 3-1. RATE field 23
5-3.2 PLCP length field (LENGTH) 802.11a 5-1 802.11a frame PSDU PLCP length field (LENGTH) 12 bits PSDU PHY MAC LENGTH PSDU 5-3.3 Parity (P), Reserved (R), Signal tail (SIGNAL TAIL) signal field 24 bits Bit 4 ( 5 bit) reserved bit default 0 Parity (P) Bit 17 ( 18 bit) 16 bits even parity check bit SIGNAL TAIL bits 18 23 6 bits 0 convolutional encoder (states) 0 5-3.4 SIGNAL field signal field MAC PHY 24 bits bits 802.11a signal field BPSK convolutional encoder code rate 1/2 Signal field convolutional encoding interleaving BPSK mapping pilot insertion OFDM modulation 19 signal field (Signal field OFDM symbol 24 bits) 24
24 Singal field bits Convolutional Encoding (R=1/2) 48 bits Interlevaing 48 bits BPSK Mapping 48 symbol Pilot Insertion (4 pilot) 52 symbol OFDM Modulation (64 points) Add CP (16 points) Windowing 19. Signal Field 5-4 Data field Data field PLCP header service field (SERVICE) PSDU TAIL bits PAD bits 25
5-4.1 Service field (SERVICE) IEEE 802.11 SERVICE field 16 bits 20 Bits 0 6 0 descrambler 9 bits default 20. SERVICE field bit 5-4.2 PPDU tail bit field (TAIL) TAIL 6 bits 6 bits 0 convolutional encoder 6 ( register ) 0 convolutional decoder 5-4.3 Pad bits (PAD) Data field bit OFDM symbol PSDU TAIL bits bit PAD bits PAD bits 802.11a standard Annex G 26
References [1] 2002 3 192 ; http://www.mem.com.tw/serial/2002-3-192/m19214.html [2] Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification, IEEE standard, supplement to standard 802 part 11: wireless LAN, New York, NY, 1999. [3] Duan Matiae, OFDM as a possible modulation technique for multimedia applications in the range of mm waves [4] Anibal Luis Intini, Orthogonal Frequency Division Multiplexing for Wireless Networks 27