Multi-Carrier Systems

Transcription

1 Wireless Information Transmission System Lab. Multi-Carrier Systems 2006/3/9 王森弘 Institute of Communications Engineering National Sun Yat-sen University

2 Outline Multi-Carrier Systems Overview Multi-Carrier System Models MC-CDMA MC-DS-CDMA MT-CDMA OFDMA Combining Equal Gain Combining (EGC) Maximum Ratio Combining (MRC) Orthogonality Restoring Combining (ORC) 2

3 Multi-Carrier Systems Overview Recently, CDMA technique has been considered to be a candidate to support multimedia services in mobile radio communication. On the other hand, the multicarrier modulation scheme, often called orthogonal frequency-division multiplexing (OFDM), has drawn a lot of attention in the field of radio communications. 3

4 Multi-Carrier Systems Overview It was in 1993, an epoch of CDMA application, that three types of new multiple access schemes based on a combination of code division and OFDM techniques were proposed, such as Multi-carrier (MC-) CDMA Multi-carrier (MC-) DS-CDMA Multi-tone (MT-) CDMA The new multiple access scheme based on a combination of frequency division and OFDM techniques were proposed, such as Orthogonal Frequency-Division Multiple Access (OFDMA) 4

5 Multi-Carrier Systems Overview OFDM scheme is robust to frequency selective fading, however, it has sever disadvantages such as sensitivity to frequency offset. Therefore, The multicarrier CDMA schemes inevitably have the same drawbacks. However, the combination of OFDM signaling and CDMA scheme has one maor advantage that it can lower the symbol duration makes it easier to quasisynchronize the transmissions. 5

6 Multi-Carrier Systems Overview The multicarrier CDMA schemes are categorized mainly into two groups. One spreads the original data stream using a given spreading code, and then modulates a different subcarrier with each chip (in a sense, the spreading operation in the frequency domain), and other spreads the serial-to-parallel (S/P) converted data streams using a given spreading code, and then modulates a different subcarrier with each of the data stream (the spreading operation in the time domain), similar to a normal DS-CDMA scheme. 6

7 MC-CDMA MC-DS-CDMA MT-CDMA OFDMA Multi-Carrier System Models 7

8 MC-CDMA Transmitter c 2 c 1 c 3 a T s c 1 c N Frequency c2 ( ) s t MC a c N T s 8

9 MC-CDMA Receiver c 1 q 1 c 2 q 2 Received Signal Serial to Parallel Converter FFT D c N q N 9

10 MC-CDMA The MC-CDMA transmitter spreads the original data stream over different subcarriers using a given spreading code in the frequency domain. We can use the Hadamard Walsh codes as the given spreading code. The capability of suppressing multiuser interference is determined by the cross-correlation characteristic of the spreading codes. Therefore, the capability of distinguishing one component from other components in the composite received signal is determined by the auto-correlation characteristic of the spreading codes. 10

11 Hadamard Code Hadamard code is obtained by selecting the rows of a Hadamard matrix. A Hadamard matrix M n is an n x n matrix that any row differs from any other row in exactly n/2 positions. 11

12 Hadamard code of length 8 Hadamard Code 12

13 MC-CDMA MC-DS-CDMA MT-CDMA OFDMA Multi-Carrier System Models 13

14 MC-DS-CDMA transmitter c 1 c3 NT s Time 0 c 2 c M c ( t) c ( t) Data Stream a Serial to Parallel Converter c ( t) IFFT Parallel to Serial Converter ( ) s MD t 0 T s Time a 0 NTs Time 14

15 MC-DS-CDMA Receiver c ( t) c ( t) Received Signal Serial to Parallel Converter FFT c ( t) Parallel to Serial Converter D ( t) 15

16 MC-DS-CDMA The Multicarrier DS-CDMA transmitter spreads the Serial-to-Parallel converted data streams using a given spreading code in the time domain so that the resulting spectrum of each subcarrier can satisfy the orthogonality condition with the minimum frequency separation. This scheme is originally proposed for a uplink communication channel, because the introduction of OFDM signaling into DS-CDMA scheme is effective for the establishment of a quasi-synchronous channel. 16

17 MC-CDMA MC-DS-CDMA MT-CDMA OFDMA Multi-Carrier System Models 17

18 MT-CDMA Transmitter c c 1 3 Time c 2 c M c ( t) cos 2 ( π f t ) 1 c ( t) cos 2 ( π f t ) 2 Data Stream a Serial to Parallel Converter c ( t) cos 2 ( π f t ) N smt ( t) Time a Time 18

19 MT-CDMA Receiver cos 2 ( π f t ) 1 cos 2 ( π f t ) 2 Rake Combiner 1 Received Signal cos 2 ( π f t ) N Rake Combiner 2 Parallel to Serial Converter D ( t ) Rake Combiner N 19

20 Conclusion System features comparison DS-CDMA MC-CDMA Multicarrier DS- CDMA MT-CDMA Symbol duration at subcarrier T S N C T S /G MC N C T S N C T S The number of subcarriers 1 N C N C N C Processing gain G DS G MC G DS G MD = G DS G MT = N C G DS Chip duration T S / G DS N C T S / G MD N C T S / G MT Subcarrier separation G MC /N C T S G MD /N C T S 1/N C T S Required bandwidth (main lobe) G DS /T S Nyquist filter with rolloff factor = 0 (N C +1)/ N C. G DS /T S (N C +1)/ N C. G MD /T S (N C 1+2G MT ) /(N C T S ) 20

21 MC-CDMA MC-DS-CDMA MT-CDMA OFDMA Multi-Carrier System Models 21

22 OFDMA Transmitter User 1's Data 1 X i Subcarriers Placement IDFT Add Guard Interval x 1 n DAC RF 1 h n User K's Data K X i Subcarriers Placement IDFT Add Guard Interval K x n DAC RF K h n RF ADC Remove Guard Interval r 22

23 OFDMA In OFDMA, closely spaced and overlapped subcarriers are divided into groups and assigned to multiple users for simultaneous transmissions. The N subcarriers are divided into K groups and assigned to K users. Since one subcarrier is only allocated to one user, each user has a group of subcarriers. 23

24 Combining Equal Gain Combining (EGC) Maximum Ratio Combining (MRC) Orthogonality Restoring Combining (ORC) 24

25 Combining Overview In an MC-CDMA receiver the received signal is combined, in a sense, in the frequency domain, therefore, the receiver can always employ all the received signal scattered in the frequency domain. Through a frequency selective fading channel, all the subcarriers have different amplitude level and different phase shift. In this section, we will introduce three different combining techniques. Equal Gain Combining (EGC) Maximum Ratio Combining (MRC) Orthogonality Restoring Combining (ORC) 25

26 MC-CDMA Receiver c 1 q 1 c 2 q 2 Received Signal Serial to Parallel Converter FFT D c N q N 26

27 Combining Equal Gain Combining (EGC) The equal gain combining only compensates the channel phase shift. m The gain for the EGC is given by = e θ. Maximum Ratio Combining (MRC) The maximum ratio combining compensates the channel phase shift and given different weights to each subcarrier. m The gain for the MRC is given by q. m = Ame θ In the case of one user, the maximum ratio combining method can maximum the SNR. qm 27

28 Combining Orthogonality Restoring Combining (ORC) The orthogonality restoring combining compensates the channel phase shift and the channel amplitude fading. 1 θ m The gain for the ORC is given by q. m = e Am However, low level subcarriers tend to be multiplied by high gains, and the noise components are amplified at weaker subcarriers. The noise amplification effect degrades the BER performance. 28

29 Reference S. Hara and R. Prasad, Overview of Multicarrier CDMA, IEEE Communications Magazine, pp , December S. Hara and R. Prasad, DS-CDMA, MC-CDMA and MT-CDMA for Mobile Multi-Media Communications, Proc. of IEEE VTC 96, Atlanta, USA, April 1996, pp