Adaptive QPSK Power Control for Legacy GSM Compatibility
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Solution Overview
Problem
Existing cellular radio systems face challenges with backward compatibility and power control when implementing Orthogonal Sub Channels (OSC), leading to poor performance in legacy receivers and increased handovers, which degrade Key Performance Indicators such as dropped calls, and none of the proposed solutions for Multi-User Reusing One Slot (MUROS) have fulfilled backward compatibility requirements.
Innovation Solution
The method involves using adaptive α-QPSK modulation, where the data is transmitted with a variation of quadrature phase shift keying that allows rectangular symbol constellations, and the transmission power is adjusted based on the relative gain of the two branches, enabling individual power control loops for each sub-channel and reducing total system interference by dynamically allocating energy between the I and Q branches.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If Orthogonal Sub Channels (OSC) are implemented to allow two users to share the same frequency and time slot, then the subscriber base capacity increases, but legacy Gaussian minimum shift keying (GMSK) mobile stations exhibit very poor performance due to QPSK modulation
Solution Approach 1:
The patent applies dynamics by making the modulation scheme adaptive rather than fixed. The system dynamically switches between QPSK modulation (for OSC compatibility) and GMSK modulation (for legacy compatibility) based on the receiver type. This allows the system to maintain OSC functionality for modern devices while ensuring backward compatibility with legacy GMSK mobile stations, resolving the contradiction between increased capacity and legacy performance
Solution Approach 2:
The patent changes the modulation parameter from fixed QPSK to variable modulation types. By introducing the ability to switch between QPSK and GMSK modulation schemes, the system can adapt to different receiver capabilities. This parameter change enables legacy GMSK mobile stations to receive signals in their native modulation format, eliminating performance degradation while maintaining OSC benefits for QPSK-capable devices
2Device complexity
If OSC is implemented with fixed power control, then the system structure simplifies, but the Key Performance Indicators such as dropped calls degrade due to additional handovers and cell subdivision requirements
Solution Approach 1:
The patent applies segmentation by separating the power control into two independent loops: one for the I-branch and another for the Q-branch. This segmentation allows each sub-channel to have its own optimized power control parameters and thresholds, enabling fine-grained power management that reduces interference and improves call reliability without requiring cell subdivision or additional handovers
Solution Approach 2:
The patent introduces dynamic power control where the transmit power is independently adjusted for each branch based on real-time channel conditions and interference levels. This dynamic adaptation allows the system to optimize performance for each user independently, reducing the need for handovers and improving dropped call metrics while maintaining a relatively simple overall power control architecture
3Device complexity
If equal power is allocated to both I and Q branches, then the power control implementation simplifies, but the total system interference increases
Solution Approach 1:
The patent applies local quality by allocating different power levels to different branches based on their specific channel conditions and interference characteristics. Instead of uniform power allocation, each branch (I and Q) receives power optimized for its particular usage pattern and interference environment. This localized power optimization reduces overall system interference while the differential power control mechanisms keep implementation complexity manageable
Data Source
AI summary
In a method and a transmitter data is transmitted to two mobile stations sharing the same frequency band and the same timeslot. The data is modulated using a quaternary symbol constellation. Further, the data is transmitted to two mobile stations multiplexed on a shared channel comprising two branches and the transmission power for the transmitted data is set in response to the relative gain of the two branches. Hereby, the total system interference is reduced. The method and transmitter further allows for a cellular radio system individual power control loops for the two sub-channels when the system uses MUROS.