Section A.3. CDMA-Based Cellular Systems

A.3. CDMA-Based Cellular Systems

In CDMA, the uplink is generally the limiting link in terms of capacity [2]. Consider N mobiles communicating to a single isolated base station. Assuming perfect power control, the signal from each mobile is received at the base station at its target power, S. Therefore, the interference from N 1 interfering mobile stations is

Equation A-21

Now the interference due to the UWB transmitters can be introduced to modify I_CDMA to

Equation A-22

The signal-to-interference ratio, SIR, for a particular user's signal arriving at the base station is

Equation A-23

The total interference power can be expressed as

Equation A-24

where J₀ is the combined PSD of the interference and receiver noise. The power, S, received from each mobile station is the product of the energy per bit, E_b, and the transmission bit rate, R_b:

Equation A-25

Therefore, we can calculate E_b / J₀ as

Equation A-26

where G is the processing gain, which is

Equation A-26 is based on the analysis of a single isolated cell. Due to the limited time scales associated with this study, factors such as imperfect power control, voice activity, and inter-cell interference have not been considered in this analysis.

Equation A-26 can be rearranged to calculate the target received power, S, which we now denote as S_UWB:

Equation A-27

A similar expression can be written for the target received power, S_{NO_UWB}, when the CDMA base station receives no interference from UWB transmitters:

Equation A-28

The received powers, S_UWB and S_{NO_UWB}, required to maintain a target

can be calculated, and the UWB interference that makes S_UWB greater than S_{NO_UWB} by M dB can be evaluated. In this way the UWB PSD that increases the received power by a particular amount, M dB, can be identified. The increase in received power is

Equation A-29

Equations A-19 and A-29 can be combined to derive the UWB PSD, G_p, that increases the mobile station transmit power in a CDMA system by M dB:

Equation A-30