Cross Modulation in CDMA Mobile Phone Transceivers phần 3

IS-95 định nghĩa đáp ứng xung của các bộ lọc này với 4 mẫu / con chip, giả định rằng tôi và Q dữ liệu đầu vào là một dòng xung. Sau khi các bộ lọc FIR, Q kênh bị chậm trễ do Tchip / 2 tức là 2 mẫu, cho điều chế QPSK bù đắp. Các tín hiệu I và Q được chuyển đổi sang miền thời gian và điều chế trên một tàu sân bay ở tần số MHz "ftx" QAM. | 4 17 01 H Variation of c t and d t with PT t Large signal instantaneous power PT t in dB below IP3 Agilent Technologies Page 21 Simulation results for gain compresion of a large signal and simultaneous desensitization of a single tone small signal are shown above for a 3rd order nonlinearity. The x-axis is the large signal input level relative to the input IP3 in dB. The y-axis shows the change in gain for the large and the small signals in dB. 21 4 17 01 H Linearization by Gain Modulation Remove AM modulation by making d t constant. Modulate LNA gain as a1 t . Total LNA gain for jammer is a1 t d t . Equating a1 t d t 1 the equation for a1 t is . gain changes with signal power Investigate envelope gain E versus Cross Mod noise power ỊƠ iq r P3j E 1 for cancellation of AM Agilent Technologies Page 22 The time varying desensitization of the single tone jammer produces spectral spread of its power resulting in significant increase in the receiver channel cross mod noise. To greately reduce cross modulation the total gain of the jammer should be made time independent. This can be done by linearily varying the LNA gain by the instantaneous power PT t according to the equation above. By doing this linear gain modulation a1 t d t 1 for the optimum case when E 1 and there is negligible AM modulation of the jammer and so there is very little spectral spreading. 22 4 17 01 H Gain Modulation of LNA ị- Agilent Technologies Page 23 The proposed block diagram for the LNA linearization is shown above. In this figure the constant E represents the gain of the envelope power detector combined with the proportionality constant of the gain control of the LNA. Ideally the value of the envelope gain E should be one. .