In order to accommodate the explosive growth of wireless communication in recent years, network operators are trying to enhance the system capacity by dividing the conventional cell into small microcells. However, for the practical implementation of such systems, it is essential to reduce the size and cost of remote base stations (RBS's). Fiber optic network offers many advantages for this purpose. In these networks, it would be possible to process the complicated operations including modulation/demodulation of radio signals and channel control in the central base station (CBS), which, in turn, allows each RBS to be implemented simply by using an optical source and a photodetector. Nevertheless, the previous fiber optic networks were based on the single-star architecture since the upstream wireless signals requires a dynamic range of >60 dB. Thus, these networks required a lot of optical fibers and high performance optical components for deployment.
In this dissertation, I propose and demonstrate passive optical networks for CDMA wireless communication systems. The proposed networks are based on either double star or ring architecture. The fiber optic link transporting CDMA signals requires dynamic ranges of about 29 and 30 dB for downstream and upstream signals, respectively. These relaxed requirements, compared with >60 dB of TDMA/FDMA systems, are due to the stringent power control included in the CDMA mobile handsets and enable us to use either double-star or ring architecture.
First, I have demonstrated a passive double-star optical network for CDMA PCS. The CBS was connected to 8 RBS's via 1×8 star coupler and each RBS could accommodate up to 120 subscribers with four CDMA signals. Thus, the CBS transmitted 32 intermediate frequencies (=4 signals × 8 RBS's) to identify each RBS. The proposed network was implemented with various optical sources (DFB laser, FP laser, and LED's) to examine the possibility of using inexpensive components. The experimental results showed that the proposed network could be implemented by using a FP laser and LED's for downstream and upstream signals, respectively. These networks were also demonstrated over a single strand of optical fiber. For the bidirectional transmission, either WDM or SCM technique was used to avoid the crosstalk between the downstream and upstream signals. The measured results showed that both techniques satisfied the required dynamic range of CDMA signals.
Passive optical network based on double-star is vulnerable to the fiber failure. It could be resolved by using ring architecture. I demonstrated a bidirectional SCM self healing ring network for CDMA PCS. This network utilizes two transceivers at the CBS and RBS. Thus, it can not only restore the fiber failure but also be robust against equipment failures in the RBS's. Because of the fast switching time of the RF switches, the restoration time of the proposed network was measured to be less than 50 ㎲ when the circumference was 9.3 km.
The cost effectiveness of the proposed network could be further improved by using unisolated optical sources at both CBS and RBS's. In this case, however, the high back reflection of unisolated optical sources might degrade the performance of bidirectional transmission systems. Thus, I have measured the reflection characteristics of commercial unisolated FP lasers and LED's, and analyzed their effects on the performance of bidirectional SCM transmission systems. When the FP lasers were used for both downstream and upstream signals, the system's performance was mainly limited by optical beat intereference (OBI). The dominant source of the OBI was the Rayleigh back-scattering when the fiber is longer than ~10 km. However, when the fiber is short (<10 km), the OBI was mainly caused by the reflection from the optical source at the opposite end. The effect of OBI could be suppressed by replacing the upstream FP lasers with LED's. In this case, the system's performance was limited by the destabilization of the downstream laser caused by optical reflection from the upstream LED when the fiber length is less than 10 km.
