TurboCode LLC v. Digi Intl Inc

I. Executive Summary and Procedural Information

• Parties & Counsel:
  • Plaintiff: TurboCode LLC (Texas)
  • Defendant: Digi International Inc. (Delaware, with principal place of business in Minnesota)
  • Plaintiff’s Counsel: Robins Kaplan LLP
• Case Identification: 0:26-cv-00519, D. Minn., 01/21/2026
• Venue Allegations: Venue is alleged to be proper in the District of Minnesota because Defendant is headquartered and has its principal place of business within the district.
• Core Dispute: Plaintiff alleges that Defendant’s cellular modems and routers that comply with the 4G/LTE wireless communication standards infringe a patent related to high-speed turbo code decoder architectures.
• Technical Context: The technology concerns forward error correction, specifically the architecture of "turbo code" decoders used to ensure data integrity in high-speed, error-prone wireless communication channels.
• Key Procedural History: The patent-in-suit, U.S. Patent No. 6,813,742, was the subject of an Ex Parte Reexamination, which concluded with the issuance of a Reexamination Certificate on February 10, 2009. The asserted claim originates from this reexamination proceeding, indicating its patentability was reconsidered and confirmed by the USPTO after its initial issuance.

Case Timeline

II. Technology and Patent(s)-in-Suit Analysis

U.S. Patent No. 6,813,742 - High Speed Turbo Codes Decoder for 3G Using Pipelined SISO Log-Map Decoders Architecture

The Invention Explained

• Problem Addressed: The patent’s background section identifies the high computational complexity of prior art Maximum a Posteriori (MAP) algorithm decoders as a major obstacle for their implementation in semiconductor (ASIC) devices for mobile communications. These decoders required many multiplications, which slowed the decoding process, increased power consumption, and raised costs, making them impractical for consumer devices (’742 Patent, col. 1:46-60).
• The Patented Solution: The invention proposes a decoder architecture using two pipelined and serially connected Soft-In/Soft-Out (SISO) Log-MAP decoders operating in a feedback loop (’742 Patent, FIG. 4). By performing calculations in the logarithmic domain, the design replaces complex multiplier circuits with simpler binary adders (’742 Patent, col. 2:54-57). This pipelined structure, where one decoder processes data while the other works on the previous iteration's output, is designed to deliver high-speed data throughput, with one decoded output produced per clock cycle (’742 Patent, col. 2:45-50).
• Technical Importance: This architecture was intended to make the superior error-correction performance of turbo codes feasible for power- and cost-sensitive 3G wireless devices by simplifying the required hardware and increasing decoding speed (’742 Patent, col. 2:31-40).

Key Claims at a Glance

• The complaint asserts independent claim 6, as amended by the Ex Parte Reexamination Certificate (Compl. ¶11).
• The essential elements of method claim 6 include:
  • Providing an input buffer with at least three shift registers to generate first, second, and third shifted input signals.
  • Providing first and second soft decision decoders serially coupled in a circular circuit, with each decoder processing soft decision data from the preceding decoder's output.
  • Providing at least one memory module coupled to the output of each decoder, where the output of the second decoder's associated memory is fed back as an input to the first decoder.
  • Processing systematic and extrinsic information data using a maximum a posteriori (MAP) or logarithm approximation algorithm.
  • Generating a soft decision based on a MAP or logarithm approximation algorithm.
  • Weighing and storing the soft decision information into the corresponding memory module.
  • Performing iterative decoding for a predetermined number of times, where the output from the last decoder is fed back to the first, propagating through the decoders in a circular circuit.
• The complaint does not explicitly reserve the right to assert dependent claims.

III. The Accused Instrumentality

Product Identification

The Accused Instrumentalities include at least the Digi IX14 LTE NA, Digi Transport WR21 and 31, Digi Transport WR44 R / Digi Transport WR44 R Cellular Modem/Router, and Digi Accelerated 4G LTE Router ASB-6355-SR06-GLB (Compl. ¶10).

Functionality and Market Context

• The accused products are industrial and enterprise-grade cellular routers and modems designed for networking applications, including for the Industrial Internet of Things (IIoT) (Compl. pp. 5-6).
• The complaint alleges that these products infringe by complying with the 4G/LTE standards as specified by the 3rd Generation Partnership Project (3GPP) (Compl. ¶10). The core of the infringement allegation is that the 3GPP standards for 4G/LTE mandate the use of a turbo coding scheme for channel coding, which the complaint alleges necessarily practices the method of the asserted patent claim (Compl. ¶¶12-14). A screenshot of a product datasheet for the Digi IX14 lists its compliance with LTE standards for North America (Compl. p. 5).

IV. Analysis of Infringement Allegations

’742 Patent Infringement Allegations

Identified Points of Contention

• Scope Questions: The patent is titled and described in the context of "3G" wireless technology. A potential issue is whether the claims, when properly construed, are limited to 3G-specific architectures or are broad enough to read on the potentially different or more advanced decoder implementations used in "4G/LTE" systems.
• Technical Questions: The complaint relies heavily on allegations of standards-compliance rather than direct evidence from the accused products. A central question will be whether Plaintiff can prove that Defendant's specific hardware and software implementations perform the claimed method steps exactly as required. For instance, what evidence demonstrates that the accused products perform "weighing and storing" in a manner that maps onto that specific claim limitation, which was added during reexamination?

V. Key Claim Terms for Construction

Term: "soft decision decoders serially coupled in a circular circuit"

• Context and Importance: This term defines the fundamental architecture of the claimed decoder system. The interpretation of "serially coupled" will be critical in determining whether the claim reads on modern, potentially more parallelized LTE decoder architectures.
• Intrinsic Evidence for Interpretation:
  • Evidence for a Broader Interpretation: The specification's focus on an iterative, feedback-based process where the output of one decoding stage informs the next could support a construction that covers any circular data flow, regardless of the precise hardware arrangement (’742 Patent, col. 4:47-53).
  • Evidence for a Narrower Interpretation: The primary embodiment shown in Figure 4 depicts a distinct Decoder A connected to a distinct Decoder B in a sequential pipeline. A defendant may argue that "serially coupled" requires this specific two-stage, non-parallel structure, distinguishing it from architectures that might process data blocks in parallel.

Term: "weighing and storing soft decision information"

• Context and Importance: This functional step was added during reexamination, suggesting it was critical for establishing patentability. The dispute will likely focus on what technical operation constitutes "weighing." The complaint asserts this covers "normalization" (Compl. ¶18).
• Intrinsic Evidence for Interpretation:
  • Evidence for a Broader Interpretation: The patent specification does not define "weighing," which may lead a party to argue it should be given its plain and ordinary meaning, potentially encompassing any form of scaling, quantization, or normalization of probabilistic values common in such algorithms.
  • Evidence for a Narrower Interpretation: A party could argue that because "weighing" was added to overcome a rejection during reexamination, it must be given a narrower meaning tied to the specific arguments or prior art presented in that proceeding. Without access to the prosecution history, the specification alone offers limited guidance, but its absence could suggest the term should not be read so broadly as to cover any and all forms of value adjustment.

VI. Other Allegations

The complaint does not provide sufficient detail for analysis of indirect or willful infringement.

VII. Analyst’s Conclusion: Key Questions for the Case

1. A central issue will be one of evidentiary proof: Can the plaintiff demonstrate, beyond mere compliance with the LTE standard, that the accused products' proprietary chipsets and software actually perform each step of the claimed method, particularly the "weighing and storing" of soft decision information as required by Claim 6?

2. The case may also turn on a question of claim scope: Does the term "serially coupled in a circular circuit," described in a 2001-era patent for 3G technology, encompass the potentially more complex and parallelized turbo decoder architectures implemented in modern 4G/LTE systems, or is it limited to the specific pipelined embodiment disclosed in the patent?