Code Cycle Modulation in Optical CDMA Systems
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Solution Overview
Problem
Conventional 1-D OCDMA systems face challenges in accommodating a sufficient number of users due to high chip rates required for current data rates, limiting spectral efficiency and user capacity.
Innovation Solution
The implementation of code-cycle modulation (CCM) in OCDMA systems, which selects and transmits distinct cyclic shifts of spreading codes to enhance spectral efficiency, allowing for increased user capacity by encoding data with unique sequences of time chips and utilizing phase to convey information.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional 1-D OCDMA systems use high chip rates to achieve current data rates, then data transmission capability is improved, but the number of users that can be accommodated is limited
Solution Approach 1:
The patent transitions from conventional 1-D time-domain spreading to 2-D code-cycle modulation that utilizes both time and phase dimensions. By modulating the phase of the spreading code across T distinct cyclic shifts, the system achieves an additional degree of freedom that allows more users to be accommodated without increasing chip rate, thereby resolving the contradiction between data transmission capability and user capacity.
2Productivity
If 2-D OCDMA architectures spread codes over time and wavelength, then spectral efficiency is improved, but the number of active users that can be accommodated is still insufficient
Solution Approach 1:
The patent changes the modulation parameter from binary OOK (On-Off Keying) to T-ary code-cycle modulation where T distinct cyclic shifts of the spreading code are used. Each cyclic shift represents a different phase state, enabling the system to accommodate T times more users for the same spectral resources. This parameter change in the modulation scheme directly increases user capacity while maintaining spectral efficiency.
3Adaptability or versatility
If code-cycle modulation selects one out of T distinct cyclic shifts of the spreading code, then user capacity is increased, but system complexity increases
Solution Approach 1:
The patent uses cyclic shifts of the same spreading code rather than requiring completely different codes for each user. By generating T distinct cyclic shifts from a single base spreading code, the system achieves high user capacity while reducing the overall code storage requirements. The receiver simply needs to correlate with the original spreading code and detect which cyclic shift was transmitted, avoiding the need to store and manage T separate unique codes.
Data Source
AI summary
An optical code division multiple access (OCDMA) system includes an optical transmitter system configured to transmit data from a plurality of users through a shared optical channel by encoding the data from each user with a spreading code assigned to that user. Each spreading code includes a unique sequence of T time chips along a time axis. The data from each user is representable by T symbols. The OCDMA system further includes an optical receiver system configured to demodulate the data from each user by correlating signals received from the transmitter system with the spreading code assigned to that user. When transmitting a data symbol for each user, the transmitter system selects one out of T distinct cyclic shifts of the spreading code assigned to that user, and transmits the selected cyclic shift of the assigned spreading code along the time axis.