Basic idea: divide spectrum into several 528 MHz bands.

Transcription

1 IEEE a Wireless Information Transmission System Lab. Institute of Communications Engineering g National Sun Yat-sen University

2 Overview of Multi-band OFDM Basic idea: divide spectrum into several 528 MHz bands. Information is transmitted using OFDM modulation on each band. OFDM carriers are efficiently generated using an 128-point IFFT/FFT. A MultiBand Orthogonal Frequency Division Modulation (MB-OFDM) scheme is used to transmit information. Frequency-domain spreading, time-domain spreading, and forward error correction (FEC) coding are provided for optimum performance under a variety of channel conditions ns prefix provides robustness against multi-path even in the worst channel environments. 9.5 ns guard interval provides sufficient time for switching between bands. 2

3 UWB Physical Layer UWB system utilizes the unlicensed GHz UWB band. Provides a wireless PAN with data payload communication capabilities of 53.3, 80, 106.7, 160, 200, 320,and 480 Mb/s. Forward error correction coding (convolution coding) is used with a coding rate of 1/3, 1/2, 5/8, and 3/4. UWB system employs orthogonal frequency division multiplexing (OFDM). The system uses a total of 122 modulated and pilot subcarriers out of a total 128 subcarriers. A total of 110 sub-carriers (100 data carriers and 10 guard carriers) are used per band. 12 pilot sub-carriers allow for coherent detection. 3

4 Band Group Allocation 4

5 Tx Block Diagram 5

6 Mathematical Description of the Signal The actual RF transmitted signal is related to the complex baseband signal as follows: r k (t) is the complex baseband signal of the k th OFDM symbol and dis nonzero over the interval lfrom 0to T SYM. N is the number of OFDM symbols. T SYM is the symbol linterval. f k is the center frequency for the k th band. 6

7 Mathematical Description of the Signal All of the OFDM symbols r k (t) can be constructed using an inverse Fourier transform with a certain set of coefficient C n, where the coefficients are defined as either data, pilots, or training symbols: bl T FFT : IFFT / FFT period T CP : Cyclic prefix duration T GI : Guard interval duration f: Sub-carrier frequency spacing N ST : Number of total sub-carriers used 7

8 Discrete-time time Implementation Considerations 8

9 Frequency Domain 9

10 Discrete-time time Implementation Considerations After performing the IFFT, a zero-padding gprefix of length 32 is pre-appended to the IFFT output and a guard interval (5 samples) is added at the end of the IFFT output to generate an output with the desired length of 165 samples. 10

11 Packet Structure 11

12 Rate-dependent Parameters 12

13 Time-related Parameters 13

14 PLCP Preamble 14

15 PLCP Preamble A PLCP preamble shall be added prior to the PLCP header to aid receiver algorithms: Synchronization Carrier-offset recovery Channel estimation. The preamble can be subdivided into two distinct portions: A packet/frame synchronization sequence A channel estimation sequence. 15

16 Time-Frequency Code (TFC) 16

17 Time-Frequency Code (TFC) 17

18 Zero-Padded Prefix Using a zero-padded (ZP) prefix instead of a cyclic prefix is a well-known and well-analyzed technique. Recall that the OFDM with cyclic prefix is created by preappending the last 32 samples of IFFT output: A ZP-OFDM signal is created by pre-appending 32 zeroes to the IFFT output. 18

19 Zero-Padded Prefix A Zero-Padded Multi-band OFDM has the same multi-path robustness as a system that uses a cyclic prefix (60.6 6ns of protection). The receiver architecture for a zero-padded multi-band OFDM system requires ONLY a minor modification. 19

20 PLCP Header 20

21 PLCP Header 21

22 Bit Interleaving The coded bit stream is interleaved prior to modulation. Bit interleaving provides robustness against burst errors. The bit interleaving operation is performed in three stages: Symbol interleaving, which permutes the bits across 6 consecutive OFDM symbols, enables the PHY to exploit frequency diversity within a band group. Intra-symbol tone interleaving, which permutes the bits across the data subcarriers within an OFDM symbol, exploits frequency diversity it across subcarriers and provides robustness against narrowband interferers. Intra-symbol cyclic shifts,, which cyclically yshift the bits in successive OFDM symbols by deterministic amounts, enables modes that employ time-domain spreading and the fixed frequency interleaving (FFI) modes to better exploit frequency diversity. 22

23 Bit Interleaving 23

24 Parameters for Interleaving 24

25 Bit Interleaving Example: First stage (symbol interleaver) for a data rate of 53.3 Mbps. 25

26 Bit Interleaving Second stage (tone interleaver) for a data rate of 53.3 Mbps Third stage (cyclic shift) for a data rate of 53.3 Mbps 26

27 Subcarrier Constellation Mapping The OFDM sub-carrier shall be modulated using gqpsk modulation (except for the data rate modes using dualcarrier modulation (DCM)) 27

28 Dual Carrier Modulation The coded and interleaved binary serial input data, b[i] [ ] where i = 0, 1, 2,, shall be divided into groups of 200 bits and converted into 100 complex numbers using a technique called dual-carrier modulation. 28

29 Dual Carrier Modulation 29

30 Dual-Carrier Modulation Encoding Table 30

31 OFDM Modulation For information data rates of 53.3, and 80 Mb/s, the stream of complex number is divided into groups of 50 complex numbers. These complex numbers c n,k, which corresponds to sub-carrier n of OFDM symbol k, as follows: cnk = d n + n= 0,1,..., 49, k = 0,1,..., N 1 50k SYM, c = d Conjugate Symmetric Input to IFFT * n+ 50, k 49 n + 50k ( ) For information data rates of 106.7, 160, 200, 320, 400 and 480 Mb/s, the stream of complex number is divided into groups of 100 complex numbers. c nk, = d n k n= 0,1,...,99,,,,, k = 0,1,...,,,, N SYM 1 31

32 Example For 53.3, and 80 Mb/s, c c = d nk, n+ 50k = d * * n+ 50, k 49 n + 50k , ,0 ( ) c = d 1,0 1 c = d = c c0,0 c1,0 c2,0 c97,0 c98,0 c99,0 d0 d1 d2 * d 2 * d 1 * d 0 32

33 Simplified TX Section For rates 80 Mb/s, the input to the IFFT is forced to be conjugate symmetric (for spreading gain = 4) Output of the IFFT is REAL. The analog section of TX can be simplified when the input is real: Need to only implement the I portion of DAC and mixer. Only requires half the analog die size of a complete I/Q transmitter. For rates 80Mb/s, need to implement full I/Q transmitter. 33

34 Pilot Sub-carriers In each OFDM symbol, 12 of the sub-carriers are dedicated to pilot signals in order to make coherent detection robust against frequency offsets and phase noise. These pilot signals shall be put in sub-carriers -55, -45, -35, -25, - 15, -5, 5, 15, 25, 35, 45, and 55. The pilot signals shall be BPSK modulated d by a pseudorandom d binary sequence, generated using a linear feedback shift register (LFSR), to prevent the generation of spectral lines. 34

35 Pilot Sub-carriers The contribution due to the pilot sub-carriers for k-th OFDM symbol is given by the inverse Fourier Transform of the sequence P n,k : 1+ j n = 25, j Pnk, = pmod (,127) n 5,15,35, 45 k = 2 0 otherwise For modes with data rates less than Mbps: P * nk, = P nk, For Mbps and all higher rate modes: P nk, = P nk, 35

36 Guard Sub-carriers 36

37 Time-domain Spreading For data rates of 53.3, 80, 106.7, 160 and 200 Mbps a timedomain spreading operation shall be performed with a spreading factor TSF =2, in order to improve frequency diversity. ' The repeated version of this OFDM symbol, represented as Sk ( n ), shall be obtained in the time domain as follows: S ' k ( n ) { { Sk( n) } j { Sk( n) }} pmod ( k+ 6,127) Sk ( n ) p ( k+ ) Im + Re no conjugate symmetry = with conjugate symmetry mod 6,127 where k=0 shall hllcorrespond to the first OFDM symbol blfll following the PLCP preambl and the values of the index k are OFDM symbol numbers before time spreading. 37