To obtain higher transfer speeds than GPRS uses GMSK modulation (Minimum Shift Keying gausian), EDGE uses a different modulation technique with GPRS is 8PSK (8-Phase Shif Keying). Figure below shows the visualization of GMSK modulation on GPRS and EDGE are described 8PSSK Pasa on a diagram of I / Q, where I is the real axis and Q is the imaginary axis.
By using 8PSK modulation, a symbol is encoded using three bits, while in GMSK a symbol is encoded with one bit. Because GMSK and 8PSK symbols have the same level, that is equal to 270 ksimbol / s, then the overall level of 8PSK modulation will be three times larger than GMSK, amounting to 810 kb / s.
Based on the explanation above, the distance between the symbols on the 8PSK is shorter than the distance between the symbols on the GMSK, because in ad 8 8PSK symbol in GMSK sedengkan there are only two symbols. Shorter the distance between the symbol lead to greater levels of signals between one symbol to another symbol is more difficult to distinguish. So that the risk of error is greater.
On the condition of the radio signal is good enough, the difference in the distances between symbols is not very influential on the quality of the data sent. At the time of the radio signal conditions are bad, then the addition of extra bits needed to be used as a correction of errors, so that any data received could be improved. So that the quality of data on EDGE is not inferior to the quality of data on GPRS using MPSK. Besides, the EDGE is also used MPSK modulation used in CS1 to CS4 -, and also in EDGE have a process of "adjustment package" which can change the type of CS used in case of errors in the data being transmitted.
Source : http://id.wikipedia.org/wiki/Enhanced_Data_Rates_for_GSM_Evolution#Modulasi_Pada_EDGE