Closed Loop Transmission via Uplink Sounding Zone
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Solution Overview
Problem
In mobile broadband cellular communication systems, closed-loop transmission techniques face challenges due to the lack of uplink channel information, especially in asymmetric traffic scenarios where reciprocity-based methods are ineffective, leading to degraded performance.
Innovation Solution
A signaling method is introduced to dynamically reserve a Sounding Zone in the uplink for channel sounding, allowing the base station to measure the uplink channel response, and specifying the characteristics of this zone through information elements in the control channel, enabling efficient closed-loop transmit antenna array processing and optimal modulation/coding scheme selection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of information
If reciprocity-based channel estimation is used in TDD systems, then downlink channel response can be derived from uplink data transmission, but performance degrades in asymmetric traffic scenarios where uplink transmission bandwidth is smaller than downlink bandwidth or no uplink transmission exists
Solution Approach 1:
The patent introduces a sounding zone where subscriber stations transmit sounding signals before actual data transmission. This preliminary action allows the base station to obtain uplink channel response information in advance, which can then be used for downlink closed-loop transmission even when no uplink data transmission occurs or when uplink bandwidth is smaller than downlink bandwidth.
Solution Approach 2:
The patent uses the uplink channel response obtained from sounding signals as a copy or proxy for the downlink channel response. By assuming channel reciprocity in TDD systems, the base station can derive downlink channel characteristics from uplink sounding measurements, enabling closed-loop transmission without requiring actual downlink channel measurements or feedback from subscriber stations.
2Productivity
If multiple subscriber stations perform channel sounding simultaneously, then signaling efficiency is improved, but interference between sounding signals may increase
Solution Approach 1:
The patent divides the sounding zone into multiple orthogonal resources (time slots, frequency subcarriers, or code sequences) that can be assigned to different subscriber stations. This segmentation allows multiple stations to transmit sounding signals simultaneously without interference, as each station uses a distinct orthogonal resource. The base station can then separately measure the channel response for each subscriber station based on these orthogonal resources.
Solution Approach 2:
The patent employs orthogonal resources that are periodically assigned to different subscriber stations within the sounding zone. By using orthogonal time-frequency-code resources in a periodic or structured manner, multiple subscriber stations can perform channel sounding simultaneously while maintaining signal separability at the base station, thus achieving both high signaling efficiency and low interference.
3Measurement precision
If channel sounding is performed in low SNR environments, then channel estimation accuracy is improved, but the power consumption and signal distortion increase due to high peak-to-average power ratio waveforms
Solution Approach 1:
The patent changes the waveform parameter from traditional high peak-to-average power ratio signals to constant envelope waveforms with low peak-to-average power ratio. This parameter change allows the sounding signals to be transmitted more efficiently in low SNR environments, reducing power consumption and avoiding signal distortion while maintaining adequate channel estimation accuracy. The constant envelope property ensures that the signals can be transmitted without excessive peak power requirements.
Data Source
AI summary
In a wireless communication system, a method and apparatus for closed loop transmission is disclosed. In accordance with the preferred embodiment of the present invention, a time frequency portion of an uplink frame is dynamically reserved as a sounding zone for uplink channel sounding. A first message is transmitted to a first subscriber station in a downlink frame assigning a time-frequency resource within the sounding zone, and a sounding waveform. Furthermore, a signal is received from the subscriber station within the assigned time-frequency resource, a partial channel response is determined from the received sounding signal, and the subsequent transmission to the subscriber station is tailored based on the at least partial channel response.