多載波傳輸及其在分碼多工之應用

Abstract

多載波 (Multi-Carrier, or Multi-Tone) 傳輸, 或垂直分頻多工( Orthogonal Frequency Division Multiplexing, OFDM),是一種可有效抵 抗選擇性衰退的方法。 而近年來受到注意的分碼多重存取 (CDMA)，可 以提供較傳統多工更多的容量。 因此 結合 MC 與 OFDM 應可提供更好的 抗多重路徑與抗衰退的多重存取。 在本論文中，我們分析了具保護時間 (Guard Time) 的OFDM在時變頻率選擇雷式(Rayleigh)衰退中的時域與頻 域的反應。我們討論兩種OFDM/CDMA，即分碼在不同的時間傳送與分碼在 不同的子頻道傳送。OFDM 在各種系統參數，如保護時間、載波個數，使 用者人數，及通道狀態，如最大延遲時間、最大都普勒頻移 (Doppler Shift)等，之下的表現也都有探討。 如果我們在信號前加入一段大於最 大延遲時間的保護時間，則可消除符際干擾 (ISI)。在此保護時間內可加 入原信號的部份周期擴展，或保持空白。然而前者可能能引起環狀的頻際 干擾。因此，應針對不同的通道，選擇不同的系統參數，以達最佳的效果 。我們也提出了一種適合OFDM/CDMA的頻域等化器，模擬的結果顯示其改 善了系統的表現。 Multi-carrier (MC) or multi-tone (MT) transmission, which is often referred to asOrthogonal Frequency Division Multiplexing (OFDM), is an effective signalingmethod against frequency- selective fading. On the other hand, the code divisionmultiple access (CDMA) technique has attracted a great deal of interest recently,because its spatial diversity combining capability and its potential to offer a systemcapacity larger than those achieved by other more conventional multiple access (MA) techniques. The combination of MC and CDMA techniques can therefore provide enhancedanti-multipath and anti-fading capability while accommodating a large number of activesystem users.In this thesis, we analyze the frequency and time response of OFDM systems. We discusstwo classes of OFDM/CDMA systems: those spread across time and those spread acrossfrequency (subcarriers). Time- and frequency-selective Rayleigh fading channels are considered.etc.--and channel conditions--delay spread, maximum Doppler shift, etc.--on the systemperformance are examined. Guard time insertion can eliminate intersymbol interference (ISI)if its length is longer than the maximum delay spread. In a guard interval, we can eitherinsert a periodic extension of the desired message or leave it empty. The former alternative,however, may incurs cyclic interchannel interference. Hence, an accurate estimation of thechannel condition and a judicious selection of the system parameters are needed to achievethe optimal performance.We also propose a new equalization method of OFDM/CDMA using different updatealgorithms in training mode and tracking mode. Simulation results show thatthis new equalizer is effective.