蜂巢環境下正交分頻多工與分碼多工系統效能之比較

Abstract

今日的行動通訊系統都是根據蜂巢式架構所建構的。蜂巢式行動通訊系統可以在有限的頻寬下提供多個使用者與基地台做連結。分頻多工(FDMA)、分時多工(TDMA)、以及分碼多工(CDMA)之技術皆可以應用在蜂巢式系統中。在這篇論文裡，我們將兩種調變技術－正交分頻多工(OFDM)與直接序列展頻分碼多工(DSSS－CDMA)，應用在蜂巢式系統中。針對OFDM系統，我們使用強制歸零等化器來還原訊號。針對CDMA系統，我們使用四種在頻域上的等化器來降低使用者間的干擾。蜂巢式系統可以藉由在空間上重覆使用相同的頻道來達到較高的系統容量，但這也導致了同頻干擾問題的產生。我們使用電腦模擬來比較以上兩種調變方式在蜂巢式環境下的訊雜比(SIR)和錯誤率。由於我們的系統是模擬是在蜂巢式環境下進行，所以我們也考慮了無線電傳輸通道的能量衰減(path loss)與遮蔽效應(shadowing effect)。

Most of today’s mobile radio communication systems are based on the cellular structure. Cellular mobile radio communication systems allow a large number of mobile users to communicate to the base stations using a limited amount of radio frequency spectrum. Cellular systems use either frequency division multiple access (FDMA), time division multiple access (TDMA), or code division multiple access (CDMA). This paper addresses cellular systems based on orthogonal frequency division multiplexing (OFDM modulation) and direct sequence spread spectrum modulation with code division multiple access (DSSS-CDMA). For the OFDM system, we use a zero-forcing equalizer for data detection. For the CDMA system, we use four kinds of frequency domain equalizers to suppress both the multipath interference and multi-code interference. Cellular systems achieves high capacity by allowing nearby cells to reuse frequency channels spatially, but this also leads to co-channel interference among users sharing the same channel. We use computer simulation to investigate and compare the signal-to-interference ratio (SIR) distribution and the bit error performance of the two systems in a cellular environment. As the systems are simulated in a cellular environment, we take into account of both the path loss and shadowing effect.