What is difference between dBi and dBd

In CDMA, GSM technology we use antenna system to transmit and receive signals from mobile and from BTS. One of the characteristic of antenna is antenna gain. dBi and dBd is a unit to measure antenna gain with respect to isotropic antenna and dipole antenna respectively.

Dipole has 2.15 dB gain compared to an isotropic antenna, hence

dBi = 2.15 dB + dBd

For example an omni antenna has antenna gain 12.15 dBi or 10 dBd

What is difference between dB, dBm and dBW

We need to express power levels in some terms weather it is CDMA technology or GSM technology. for that we use dB, dBm and dBW as unit of power levels which are differentiate with reference power level.

dB is a relative measurement of two different power levels.

dB = 10 Log (P1/P2)

If we take reference power is 1W then it is dBW

dBW = 10 log(P1/1 W)

If We take reference power is 1mw then it is dBm

dBm = 10 Log(P1/1 mw)

Normally we use dB when expressing the ratio between two power values and dBm when expressing an absolute value of power.