跳時超寬頻系統發射分送技術及跳頻階層式分碼多工存取系統頻帶共用之性能分析研究

Abstract

在這篇論文中，我們研究了跳頻 (frequency hopping) 和跳時 (time hopping) 系統的系統效能。在跳頻系統方面，我們研究了階層式 (hierarchical) 跳頻碼分多址 (frequency hopped code division multiple access, FH-CDMA) 系統的使用者容量。而在跳時系統方面，我們則是研究脈衝位置調制 (pulse position modulation, PPM ) 跳時超寬帶 (time-hopping ultra-wideband, TH-UWB) 系統下，如何結合分送發射 (transmit diversity) 來改善誤碼率 (bit error rate, BER)。以下，我們說明我們是如何研究上述兩個系統的系統效能。 在跳頻系統方面，我們使用了分析的方式來找出在跳頻碼分多址系統中大小細胞共用頻帶的情況下其上鏈 (uplink) 使用者容量。我們研究了如何利用跳率及功率控制 (power control) 來讓階層式大小細胞共用頻帶。我們考慮了隨機跳頻 (random frequency hopping, RFH) 和動態跳頻 (dynamic frequency hopping, DFH) 結合以下三種功率管理的方式： 1) 以信號強度為基準的動態功率控制 (power control, PC)， 2) 不同最大傳輸功率限制的靜態功率設定 (static power setting. SPS)，和 3) 不做功率控制 (no power control, NPC)。經由分析，我們發現在不使用功率管理的情況下，只有利用動態跳頻共用頻帶的系統仍然可以有比頻帶分割系統擁有更多的使用者容量。在沒有功率控制的情況下，利用隨機跳頻來共用頻帶的系統其使用者容量有可能會比頻帶分割系統來的少。即使有使用功率控制，利用隨機跳頻來共用頻帶的系統只有在大小細胞基地台離得夠遠的情況下才會有比頻帶分割系統擁有更多的使用者容量。而且我們發現在跳頻系統系統中靜態功率設定可以和動態功率控制有一樣好的效果，此兩種能量管理的方式皆能讓階層式跳頻碼分多址系統共用頻帶。 在跳時系統方面，我們利用模擬和分析的方式去研究一個可以使用在脈衝位置調制跳時超寬帶系統中的分送發射技術。其實，找到一個可以使用在脈衝位置調制的分送發射技術是一個很有趣的研究課題，因為脈衝位置調制系統是把資訊調變在脈衝位置上，所以大部份的分送發射技術，包括空時區碼 (space-time block code, STBC) 和空時格碼 (space-time trellis code, STTC)，都不能使用在脈衝位置調制跳時超寬帶系統中。然而，利用脈衝聯合檢測，我們發現在跳時超寬帶系統中使用開環時間切換分送發射 (time-switched transmit diversity, TSTD) 技術可以達到最大的分送等級。可是因為脈衝聯合檢測的限制，我們只能利用此一技術於跳時超寬帶系統把資料調變在兩個脈衝以上的時候。在我們的模擬中，我們考慮了脈衝位置調制和脈衝振幅調制、發射及接收天線的個數、多路徑接收其路徑數 (RAKE finger number)、跟脈衝位置調制有關的延遲時間。在我們的分析中，我們則是觀察未經硬式決策 (hard decision) 已處理資料信噪比和誤碼率的關係。總結，在脈衝位置調制和脈衝振幅調制的跳時超寬帶系統中，我們發現使用開環時間切換分送發射技術皆可達到最大的分送等級。 在這篇論文中，我們考慮兩個頻帶共用的系統： 1) 跳時超寬帶系統， 2) 階層式跳頻碼分多址系統。我們發現可以為階層式跳頻碼分多址系統設計比較好的跳頻樣式及為跳時超寬帶系統設計可以使用在此系統的分送發射技術來改善此二個頻帶共用系統之性能。因此使用這些設計可以促進跳時超寬帶系統及階層式跳頻碼分多址系統其頻帶共用的可能性。

In this thesis, we study the performance of the two spread spectrum systems: one is the hierarchical frequency hopped code division multiple access (FH-CDMA) system, and the other is the time-hopping ultra-wideband (TH-UWB) system with transmit diversity. For the frequency hopping system, we analyzes the uplink capacity of a FH-CDMA system with spectrum sharing between a hot spot microcell and an overlaying macrocell. We investigate how frequency hopping can function together with power control in order to allow the same frequency spectrum to be shared between the two tiers in the hierarchical CDMA cellular system. We consider random frequency hopping (RFH) and dynamic frequency hopping (DFH) combined with three power management methods: 1) signal strength based dynamic power control (PC), 2) static power setting (SPS) with different maximum transmit power constraints, and 3) no power control (NPC). Through analysis, we demonstrate that without using any power management methods, only frequency sharing with DFH can still perform better than frequency splitting. Without power control, frequency sharing with RFH may even have lower capacity than frequency splitting in some cases. Even with power control, frequency sharing with RFH can improve system capacity over frequency splitting only if the embedded microcell is separated with enough distance from the overlaying macrocell. Furthermore, we find the simple SPS method is as effective as dynamic power control in the frequency hopped system, both of which can facilitate frequency sharing in the hierarchical CDMA system. This thesis studies a feasible transmit diversity scheme for the pulse position modulation (PPM) time-hopping ultra-wideband (TH-UWB) system through simulation and analysis. Because the PPM scheme modulates an information bit into pulse position, most of the transmit diversity schemes, including space-time block code and space-time trellis code, can not be straightforwardly adopted to the PPM TH-UWB system directly. However, we find that we can achieve the maximum diversity gain by employing open loop time-switched transmit diversity (TSTD) on the TH-UWB system. In our study, we evaluate the impacts of the PPM and the PAM schemes, the numbers of transmit and receive antennas, the number of RAKE fingers, and the delay time associated with PPM on the performance of the UWB system. In our analysis, we observe the relation between bit error rate and the signal-to-noise ratio for the different diversity schemes. In summary, we show that one can achieve the maximum transmit diversity gain on both the PPM and PAM TH-UWB by employing the open loop TSTD scheme with pulse joint detection. To summarize, in this thesis, we have investigated two frequency sharing systems: 1) the TH-UWB system, and 2) the hierarchical FH-CDMA system. To improve these two frequency sharing system performance, we propose to utilize a good frequency hopping pattern or power management methods for the hierarchical FH-CDMA system and a feasible transmit diversity scheme for the PPM-based TH-UWB system. These simple but effective methods can resolve challenging interference problem in the hierarchical FH-CDMA system and the UWB signal detection issue, thereby facilitating frequency sharing in the hierarchical FH-CDMA systems and further improving the performance for the TH-UWB system.