Femtocell Time-Frequency Spreading for Interference Reduction

Resolve Bottlenecks,
Find Innovative SolutionsGenerate Solutions

Solution Overview

Problem

Femtocells face interference issues with macrocells and neighboring femtocells, particularly due to low user loads and idle spectrum, which affects capacity and requires inefficient coordination for orthogonal resource allocation.

Innovation Solution

Implementing a time or frequency spreading mechanism in femtocell networks using the LTE standard, where transmission power is reduced by spreading resource blocks across the available spectrum, reducing interference without the need for coordination among femtocells.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If orthogonal resource allocation is used between macrocell and femtocell, then interference between the two networks is avoided, but the number of users that can be supported decreases

Engineering Contradiction:

Improveinterference between macrocell and femtocellVSAvoidnumber of users supported

Core Design Contradiction:

Object-affected harmful factorsVSProductivity

Solution Approach 1:

The patent segments the frequency spectrum into different sets of subcarriers for macrocell and femtocell operations. By dividing the available frequency resources into orthogonal subcarrier sets, the system avoids interference between macrocell and femtocell while maintaining the ability to support multiple users through time-division and code-division multiplexing within each cell type.

Inventive Principle:

Principle #1Segmentation

2Productivity

If shared spectrum is used for macrocell and femtocell, then the number of users supported increases, but interference between the networks increases

Engineering Contradiction:

Improvenumber of users supportedVSAvoidinterference between macrocell and femtocell

Core Design Contradiction:

ProductivityVSObject-affected harmful factors

Solution Approach 1:

The patent applies local quality by assigning different time slots and code sequences to macrocell and femtocell users sharing the same frequency resources. Within the femtocell, users are further differentiated by unique spreading codes. This localized differentiation allows multiple users to share spectrum efficiently while maintaining low interference through orthogonal time and code domains.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The system employs periodic time slots for macrocell and femtocell transmissions. By organizing communications into periodic time divisions, the network can support shared spectrum operations while maintaining structured interference avoidance patterns that enable multiple users to access the spectrum efficiently.

Inventive Principle:

Principle #19Periodic action

3Reliability

If time-hopping is used to reduce interference, then outage probability decreases, but the system is designed specifically for CDMA which does not use frequency-hopping

Engineering Contradiction:

Improveoutage probabilityVSAvoidfrequency-hopping capability

Core Design Contradiction:

ReliabilityVSAdaptability or versatility

Solution Approach 1:

The patent creates a universal communication framework that integrates both time-division and frequency-division multiplexing capabilities. The system can operate in CDMA mode with time-hopping for interference avoidance, or in OFDMA mode with frequency-hopping for spectrum utilization, or combine both approaches. This multi-functional design enables the system to adapt to different operational requirements and network conditions.

Inventive Principle:

Principle #6Universality (Multi-functionality)

4Reliability

If spreading is applied to reduce interference, then robustness against interference increases, but transmission power must be reduced

Engineering Contradiction:

Improverobustness against interferenceVSAvoidtransmission power

Core Design Contradiction:

ReliabilityVSPower

Solution Approach 1:

The patent applies spreading by mapping transmitted symbols across multiple frequency subcarriers and time slots, effectively adding frequency and time dimensions to the transmission. This dimensional expansion allows the system to achieve interference robustness through diversity gain while maintaining acceptable power levels, as the energy is distributed across multiple dimensions rather than concentrated in a single time slot.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

Data Source

PatentUS8340038B2Method for time frequency spreading in a femtocell network for interference reduction

Publication Date: 2012.12.25 NTT DOCOMO INC

AI summary

A femtocell network uses idle resource blocks of a data frame to reduce interference by spreading the resource blocks of the users over the available spectrum. Spreading may be achieved by repeating the transmission using a number of the resource block groups. As a result, (a) more robustness is obtained against interference; (b) transmission power levels may be decreased because of the spreading, resulting in reducing interference between nearby femtocells and between a macrocell and a femtocell. Other methods of spreading such a frequency or time slot hopping may also be used.