LTE Pilot Sequence Orthogonality via CAZAC-Hadamard Multiplexing
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Solution Overview
Problem
In 3GPP LTE communication systems, there is a challenge in achieving sufficient orthogonality between reference signals from multiple user equipment (UEs) to minimize intra- and inter-cell interference, particularly due to the limited number of CAZAC sequences available, which complicates re-use planning and affects channel estimation accuracy.
Innovation Solution
The solution involves generating pilot sequences as a combination of existing CAZAC codes and Hadamard codes for block-wise spreading, ensuring each user device has a dedicated pilot sequence with a unique cyclic shift or frequency pin allocation, thereby improving orthogonality through code division multiplexing and frequency division multiplexing, and using a combination of CDM and FDM to separate pilot signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If CAZAC sequences are used for pilot signals, then channel estimation can be performed, but the limited number of sequences causes insufficient orthogonality between multiple user equipments
Solution Approach 1:
The patent combines two different coding schemes (CAZAC codes and Hadamard codes) into a hybrid structure. The pilot sequence is formed by multiplying a CAZAC sequence with a Hadamard code, creating a composite sequence that leverages the constant amplitude property of CAZAC codes and the orthogonality property of Hadamard codes to achieve both reliable channel estimation and sufficient orthogonality among multiple UEs.
Solution Approach 2:
The patent introduces an additional dimension of orthogonality by using Hadamard codes in the time domain while CAZAC sequences operate in the frequency domain. This multi-dimensional approach (combining frequency-domain CAZAC properties with time-domain Hadamard orthogonality) expands the available orthogonal resources beyond what single-domain sequences can provide.
2Object-affected harmful factors
If more CAZAC sequences are allocated to increase orthogonality, then interference between UEs is reduced, but the sequence length and system complexity increase
Solution Approach 1:
The patent segments the orthogonality requirement into two independent components: CAZAC sequences handle frequency-domain orthogonality while Hadamard codes handle time-domain orthogonality. This segmentation allows each component to be optimized independently, reducing the overall sequence length and simplifying re-use planning compared to relying solely on longer CAZAC sequences.
Solution Approach 2:
The patent creates a composite pilot sequence structure by combining CAZAC codes and Hadamard codes, similar to how composite materials combine different materials to achieve properties that neither material alone can provide. This composite structure achieves superior orthogonality and interference resistance without requiring excessive sequence length.
3Adaptability or versatility
If cyclic shifts are used to generate orthogonal sequences, then more UEs can be supported, but orthogonality degrades in frequency selective channels
Solution Approach 1:
The patent introduces Hadamard codes as an intermediary layer between the CAZAC sequence and the channel. The Hadamard code provides an additional orthogonality mechanism that is less sensitive to frequency selective fading, acting as a mediator that preserves orthogonality even when cyclic shift-based orthogonality degrades in multipath environments.
Data Source
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AI summary
A network element may provide a plurality of user equipments with a dedicated pilot sequence for uplink reference signal transmission. A user equipment may, after receipt of a dedicated pilot sequence, spread the pilot sequences using a block spreading method. The main embodiments considers a CAZAC sequence scrambled by a Walsh code for use as a pilot sequence for SC-FDMA systems.