By Hikmet Sari (auth.), Khaled Fazel, Stefan Kaiser (eds.)
Stefan Kaiser Khaled Fazel Radio procedure layout German Aerospace heart (DLR) Marconi Communications lnstitute of Communications and Navigation D-71522 Backnang, Germany D-82234 Wessling, Germany we're at present looking at that frequency spectrum is a restricted and worthwhile source for instant cellular communications. a superb instance should be saw between ecu community operators for the costs to pay for frequency bands for UMTS/IMT2000. conserving this in brain, the 1st target whilst designing destiny instant verbal exchange structures should be the rise in spectral potency by means of permitting greater flexibility for the method layout and deployment. the advance in electronic communications some time past years has enabled effective modulation and coding thoughts for strong and spectral effective information, speech, audio and video transmission. the following, we must always point out attention-grabbing and profitable strategies. those are the multi-carrier modulation (e. g. OFDM) and the unfold spectrum approach (e. g. DS-CDMA). over the last decade of this millennium the means of multi-carrier transmission for instant broadband multimedia purposes has bought broad curiosity. Its fIrst nice good fortune used to be in 1990 because it used to be chosen for the eu electronic Audio Broadcasting (DAB) common. Its extra favourite good fortune was once in 1995 and 1998 because it used to be chosen as modulation scheme for the ecu electronic Video Broadcasting (DVB-T) common and in 3 broadband instant indoor criteria, specifically ecu ETSI-HIPERLAN/2, American IEEE-802. 11a and jap MMAC, respectively.
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Additional info for Multi-Carrier Spread-Spectrum & Related Topics: Third International Workshop, September 26–28, 2001, Oberpfafenhofen, Germany
Equation (2) is referred to as the user specific frequency mapping. Notice that as we do not assume that the subcarriers of a given user are grouped together, the indexes p(k) (I), 0 ~ I ~ L -1, can be anywhere the interval 0 ~ n ~ N -1. m (k) (l)IN (3) O~n~N-l 'V JV 1=0 and are arranged in a vector b;::). To eliminate any interference between adjacent OFDM blocks, an NG-point prefix is appended to b;::) such that b;::)(l)=b;::)(l+N) for -NG ~l~-l. The resulting vector is fed to a linear modulator with impulse response g(t) and signaling interval T.
At low SNR values it is seen that increasing N G makes the system more robust against timing errors. Vice versa, as the SNR grows large, the lSI level becomes insensitive to the length of the cyclic prefix. To see why this is so recall that we have assumed P=15. Now, from (25)-(26) it can be shown that: (i) for jJ - Jl = -1 there is no lSI with either NG =16 or NG =32; (iz) for jJ - Jl =+1 the same amount of lSI occurs with either N G =16 or N G =32. It follows that, since jJ - Jl is generally ±t at high SNR, the average lSI power is independent of N G .
1 strongly restricts the choice of the time delays b'n, b'm. In the next section cyclic delay diversity is introduced, which overcomes this problem. 2. Cyclic Delay Diversity Fig. 2 illustrates the difference between DD and CDD in the time domain and shows the transmission of 2 consecutive OFDM symbols with their cyclic prefixes as guard intervals. For clarity, the 1st subcarrier is plotted as a sine wave. The reference signal is undelayed and transmitted (resp. received) for both DD and CDD. In the case ofDD it can be seen, that the DD signal is a simple copy of the reference signal, but delayed by b'.