【轉】   http://hxqhit.blog.163.com/blog/static/15357922200948112142918/

1. Path Loss Model, RSSI and RF Parameters A, n

      To determine the distance from the RSSI values, Chipcon applies a simple path loss model (free space path loss自由空間損耗) shown in ?gure 2.3 and equation (2.1). The location engine requires, in addition to the RSSI value two RF model parameters, A

                d1 = distance to the sender [m]
                d2 = distance to the sender [m]
                P1 = power at distance d1 [W]
                P2 = power at distance d2 [W]
                n = path loss coe?cient

One can write equation (2.1) in logarithmic notation, which is done in equation (2.2).

                 d = distance to the sender [m]
                 A = absolute value of power at one meter distance to the sender [dBm]
                 n = path loss coe?cient

  所以 測量測距離與已知RSSI、A、n 之間的關係如下所示

                                                          d=10^((ABS(RSSI)-A)/(10*n))

  其中RSSI與LQI之間的轉換關係如下：

                                  RSSI =  -(81-(LQI*91)/255)

  通過實驗，A值的最佳範圍為45—49，n值最佳範圍為3.25—4.5，N在15---25。

 如果對於靜止或者低速的運動物理的時，可用平均值濾波來提高測量精度

 或者IIR濾波執行利遞迴解得平均值處理的，典型的a去0.75--0.95 ：

 如果對於高速運動的物體可用卡曼濾波。