MBOK架構之無線系統基頻傳收器設計

Abstract

本篇論文主要在根據IEEE 802.11標準設計適用於無線區域網路 (wireless local area network, WLAN) 的基頻傳收器。我們利用SPW (Signal Processing Workstations) 這套電腦輔助系統設計軟體模擬並驗證系統的性能。 我們的傳送器是根據IEEE 802.11標準所規範直接序列展頻 (direct sequence spread spectrum, DSSS) 傳輸的方法設計。這個傳收器可以操作於2MBps or 11MBps 資料傳輸率兩種模式。在這兩種模式下皆利用差分四元相移鍵調變 (differential quadrature phase shift keying, DQPSK)。兩種模式之不同點在於展頻方式的不同。當操作於低速率模式時，我們是利用巴可碼 (Barker code) 將訊號展頻；而當操作於高速率模式時，我們則是利用M維二相正交鍵 (M-ary bi-orthogonal keying, MBOK) 同時將訊號展頻及調變。 我們的接收器利用差分式偵測法去免除載波相位的追蹤。我們採用有回授的無限響應濾波器 (infinite impulse response, IIR) 架構之偵測器作為資料解調器。M維二相正交鍵碼的粗調及解調是利用M維二相正交鍵相關器完成。數位資料轉換追蹤迴路 (data transition tracking loop, DTTL) 則是微調接收碼之間的相位差。而自動頻率控制迴路 (automatic frequency control loop, AFC loop) 改正可能頻率誤差殘留。同時資料擾亂器及解擾亂器亦考慮於系統之中。

This thesis presents a candidate baseband transceiver design suitable for use in a wireless local area network (WLAN) following the IEEE 802.11 standard. We use a computer-aided system design tool called SPW (Signal Processing Workstations) to simulate and verify the performance of the proposed system. Our transmitter uses the direct sequence spread spectrum (DSSS) transmission technique as specified in IEEE 802.11. This transceiver can either operates at the $2$ MBps mode or the 11 MBps mode. Differential quadrature phase shift keying (DQPSK) modulation is used for both modes. The difference of the two modes lies in the spreading sequence. While the lower speed mode uses Barker code for the purpose of spectrum spreading the higher speed mode takes the advantage of the sequence using $M$-ary bi-orthogonal keying (MBOK) to serve the dual purposes of spectrum spreading and modulation. Our receiver employs differential detection thus eliminates the need for carrier phase tracking. The structure of our detector consists of a conventional differential detection circuit equipped with an infinite impulse response (IIR) filter combined with decision feedback. MBOK code acquisition and demodulation are accomplished by using MBOK correlators. Digital data transition tracking loop (DTTL) is used to fine turning the phase difference between the local code and the received bi-orthogonal code. An automatic frequency control (AFC) loop is in place to correct possible residual frequency error. Data scarmabler/descrambler are included and the functions of carrier-sense, signal recognition, unique word detection are considered as well.