# AN INTRODUCTION TO OFDM

## Presentation on theme: "AN INTRODUCTION TO OFDM"— Presentation transcript:

AN INTRODUCTION TO OFDM

Outlines Multi path delay spread Serial to parallel
How to create OFDM signal Implementation by FFT Guard interval Synchronization Distortion Conclusion

FDM VS OFDM The main concept of OFDM is orthogonality of the sub-carriers The orthogonality allows simultaneous transmission on a lot of sub-carriers in a tight frequency space without interference from each other Advantage：High spectral efficiency FDM與OFDM差別在於OFDM每個子載波間具有正交性 當我們現在有六組資料要送以FDM的角度來看我們會將可用來傳輸的頻寬劃分為6個Channel來傳送六組Data 頻譜彼此之間互不重疊 而OFDM因為正交性的關係他可以在一個更小的頻寬範圍內傳送這六組Data 即使頻譜之間有重疊也不會互相干擾 因此會有多餘的頻寬可以做使用 具有較高的頻寬效益 Fig From [4]

Example of multi path delay spread

Inter Symbol Interference (ISI)

Serial to parallel Longer symbol duration Advantage :
Reduce ISI caused by multi-path delay spread OFDM為了解決Multipath delay spread的問題 採用STP元件，將原有的資料傳輸序列分配在不同的子載波上平行傳送，因此每一個子載波的傳輸間隔變為原來的數倍 使得子載波上的Symbol duration增加，所以降低了Multipath delay spread 引起的ISI問題 Fig From [3]

Data sequence : 1 1 -1 -1 1 1 1 -1 1 -1 -1 -1 -1 1 -1 -1 -1 1 1 -1 -1 -1 1 1
Fig From [2]

Sub – Carriers use BPSK modulation
Fig From [2]

Sub – Carriers use BPSK modulation
Fig From [2]

Fig From [2]

Disadvantage: High peak to average power ratio

OFDM 可以用 FFT 方式實現 Fig From [4]

Use inverse FFT to create OFDM signal
Fig From [2]

Functional diagram of an OFDM signal creation the outlined part is often called an IFFT block
Fig From [2]

The incoming block of bits can be seen as a four bin spectrum
Fig From [2]

Fig From [2]

At the receiver proper choice of FFT window position is needed
Guard interval must be inserted between neighboring OFDM symbols to avoid IBI At the receiver proper choice of FFT window position is needed 保護區間長度必須大於所預期之最大多重路徑延遲擴散，使得OFDM區塊不會受到上一個區塊的干擾 Fig From [4]

ZP 保護區間內如果不送信號會造成ICI問題 載波之間不再具有正交性 Fig From [3]

Fig From https://goo.gl/hqkl0R

Fig From [3]

Fig From [3] 三載波兩路徑傳輸 實線為主路徑 虛線為延遲擴散路徑
OFDM符元邊界處由於不同符元之間的不連續而會有相位跳躍產生的問題，但是對於虛線信號而言相位跳躍不是發生在符元邊界處 而是在第一路徑之後的特定延遲 當此延遲小於保護區間 則完整的FFT區段中不會有相位跳躍產生，因此雖然OFDM信號有相位的變化但因為有週期延伸的保護區間存在 使得FFT區段中的載波之間仍難維持正交性，但如果最大傳輸延遲大於保護區間，相位跳躍會發生在FFT區段而破壞了正交性而產生ICI問題 所以說保護區間長度需要大於預測的最大延遲擴散 Fig From [3]

Symbol timing offset Sampling clock offset Carrier phase offset Carrier frequency offset STO 當接收訊號進入fft時，要找到適當起點從起點後選取多點作離散傅立葉變換，將訊號從時域轉回頻域，若選取太早或太晚都會產生ISI SCO 由於傳送端及接收端的取樣速率不一樣，會造成取樣點的誤差，而且越後面的子載波SCO誤差會越大。 CPO 傳送端在傳送端最後會乘上一載波f1使基頻訊號載至旁頻，在接收端要將旁頻降回基頻會再乘上一載波f2，由於f1 f2兩載波相位的不同在升降頻之間，會造成carrier phase offset。 CFO 傳送升頻及接收端降頻載波的頻率不同步，會造成carrier frequency offset。進而破壞正交性產生嚴重的ICI問題

Addition of Cyclic prefix to the OFDM signal further improves its ability to deal with fading and interference Fig From [2]

Conclusion Fig From [4] 為了以數位的方式作FFT 我們必須對類比信號取樣，而且取樣時需滿足取樣定理，

High spectral efficiency as compared to other modulation schemes. Can easily adapt to severe channel conditions without complex time-domain equalization. Robust against narrow-band co-channel interference. Robust against inter symbol interference (ISI) and fading caused by multipath propagation. Efficient implementation using Fast Fourier Transform (FFT). 2. 不需要複雜的時域等化器 (One tap 就夠) 所以可以簡單的克制多重路徑干擾 3. OFDM具有頻率分集的效果，所以比單載波機制更能對抗窄頻干擾，因為窄頻干擾只會影響很少比例的載波 4. Guard interval 保護區域的加入以及 Symbol duration 上升 5. 可用FFT 實作