Research On Transmission Of CDMA Overlaying GSM And Its Key Techniques

Based on the contradiction between the limited frequency source and the increased mobile users this paper discussed three novel scheme that CDMA and GSM share common radio-band to enhance capacity.these are N-CDMA overlay GSM, W-CDMA overlay GSM and CDMA/TDMA hybrid multiple access configuration. The N-CDMA overlay GSM means the narrowband CDMA overlay in a given sector does not use the GSM channels of that sector and neighbouring ones. The W-CDMA overlay GSM means use tuneable notch filters at the CDMA transmitters and receivers.The spectrum of notch filters is just the spectrum of GSM.The CDMA/TDMA hybrid multiple access means at the transmitter, GSM signals will be multiplied by spread spectrum codes to be transferred from TDMA mode to CDMA/TDMA mode. Then they will transmit with CDMA signals on the same bandwidths. At the receiver, CDMA/TDMA signals will be multiplied by the same spread spectrum codes to be transferred to TDMA mode, and these two signals can be separated. By analysising the multiple access interference through mathematics modeling. We can see the N-CDMA overlaying provides 4.75 times higher capacity of GSM; the W-CDMA overlaying provides 2.50 times higher capacity of GSM. This paper also showed using complete complementary code as multiple access codes and the process of single GSM user be spreading modulated in one time slot. This paper also analysised the correlation property of complete complementary code and gotted out that the multiple access interference of CDMA Based on Complete Complementary Codes equal zero .So the capacity of CDMA/TDMA systems will increase distinctness. Through analysised and compared the three scheme,we known that the N-CDMA overlay GSM do least alteration to the Um and the W-CDMA overlay GSM is easiest amalgamating to the 3G and the CDMA/TDMA hybrid multiple access configuration has the largest capacity. At last the simulation showed this three scheme of enhance capacity will be feasibility.