Mentone sued Digi for alleged infringement of Mentone's U.S. Patent No. 6,952,413. The U.S. District Court for the District of Delaware found the claims of the patent to be ineligible under 35 U.S.C. § 101. Mentone appealed.

The invention of the '413 patent is directed to improvements in the allocation of wireless resources for wireless packet data channels (PDCHs).[1] In the relevant technology, the wireless air interface resources are shared using a technique called time-division multiplexing. Each PDCH is represented by a repeating set of paired time slots in the uplink and downlink directions. The uplinks carry information from a mobile device to a base station and the downlinks carry information from the base station to one or more mobile devices.

Figure 1 of the '413 patent, shown above, provides an example, with time flowing from left to right. Each uplink and downlink consists of repeating frames of 8 slots, numbered 0 to 7. Thus, Figure 1 depicts two frames of slots in both the uplink and downlink directions.

A PDCH is the set of slots, from both the uplink and the downlink, that have the same number. Thus, PDCH 0 consists of all slots numbered 0, PDCH 1 consists of all slots numbered 1, and so on. These PDCHs are shared amongst the mobile devices that communicate by way of the base station. Thus, a mechanism to allocate them dynamically is needed to allow for efficient packet data communication, which is often bursty. A particular mobile device may be allocated n contiguous PDCHs at any given point (where n is no more than 8 in this example).

To facilitate uplink communication, the base station transmits an uplink status flag (USF) in a downlink PDCH slot. The USF identifies a particular mobile device and informs the mobile device that it may transmit in up to n uplink slots in the subsequent frame, beginning with the uplink slot of the same PDCH. In other words, if n=4 and a USF in downlink slot 2 identifies the mobile device, then the mobile device may transmit in uplink slots 2, 3, 4, and 5 of the next frame.

For example, in Figure 2 of the '413 patent, n=4 and the USFs in each downlink slot 0 indicates that a particular mobile device may transmit in uplink slots 0, 1, 2, and 3 (indicated by the shaded uplink slots). Thus, there is a fixed relationship between the downlink slot in which a USF is received and the first uplink slot in which the mobile device can transmit.

Such a system is limited by the ability of the mobile device to switch between receiving and transmitting modes. In particular, a mobile device might not be able to receive a USF in a downlink slot that immediately follows an uplink slot in which the mobile device was transmitting. Figure 3 of the '413 patent illustrates this problem -- the second (rightmost) USF immediately follows the mobile device's last uplink transmission of the first frame. The mobile device might not be able to transition quickly enough from transmitting mode to receiving mode in order to be able to receive this USF.

The '413 patent purports to overcome the limitations of these standardized techniques by allowing the first uplink slot associated with a USF to be "shifted" so that there is enough time between downlink and uplink slots assigned to the mobile device for the mobile device to change its mode. The resulting advantages, as stated in the patent's specification, involve elimination of rigid slot assignment restrictions thereby allowing increased data transmission rates.

Applying the invention to the arrangement of Figure 3 could result in the arrangement of Figure 4, in which the USF slots and the transmission slots are at least one slot apart from one another.

Independent claim 5 of the '413 patent was at issue, and recites:

A multiple access communication method in a mobile station, comprising the steps of:
receiving an assignment of at least a first PDCH (packet data channel) and a second PDCH;
monitoring an assigned PDCH to detect a USF; and
transmitting on an assigned PDCH corresponding to the USF,
wherein (i) if shifted USF operation is not used then a first assigned PDCH is monitored to detect a USF corresponding to the first assigned PDCH and (ii) if the shifted USF operation is used then a second assigned PDCH is monitored to detect the USF corresponding to the first assigned PDCH and a USF corresponding to the second assigned PDCH.

Digi moved to dismiss claim 5 and the other asserted claims under Rule 12(b)(6) on grounds of patent-ineligibility.

In Alice Corp. v. CLS Bank Int'l, the Supreme Court set forth a two-part test to determine whether claims are directed to patent-eligible subject matter under § 101. One must first decide whether the claim at hand involves a judicially-excluded law of nature, a natural phenomenon, or an abstract idea. If so, then one must further decide whether any element or combination of elements in the claim is sufficient to ensure that the claim includes an inventive concept amounting to significantly more than the judicial exclusion. But elements or combinations of elements that are well-understood, routine, and conventional will not lift the claim over the § 101 hurdle. While this inquiry is generally carried out as a matter of law, factual issues can come into play when determining whether something is well-understood, routine, and conventional.

Applying the Alice test, the Court rapidly determined that part one was met. Rather than an abstract idea, the Court found that claim 5 "is directed to a patent-eligible improvement to computer functionality, namely permitting additional multislot configurations for certain classes of mobile stations using extended bandwidth allocation. Further, "[i]t adds this capability through using a shifted USF that breaks the fixed relationship in the timing of downlink allocation signaling (i.e., receipt of a USF on a timeslot) and subsequent uplink transmission."

In coming to this conclusion, the Court leaned heavily on the teachings of the specification with regard to the improvements over prior art techniques. Particularly, the differences between the prior art arrangement of Figure 3 and that of Figure 4, as well as the resulting technical advantages, were emphasized. Indeed, the Court emphasized that the term "shifted USF" was likely coined by the inventor and therefore one must look to the specification in order to be able to understand its meaning.

In the District Court, Digi had contended that claim 5 lacked specificity and that it should have recited "when, how, or why one would . . . shift the USF or how a shifted USF would specifically improve the functioning of a prior art system." But the Federal Circuit dismissed this notion, indicating that the claimed shifted USF operation was particular enough and that the specification contained a detailed flow chart supporting the claimed operation. Further, the Court noted that an explicit recital of the improvement provided by the invention was not necessary.

The District Court had found that claim 5 was directed to the abstract idea of "receiving a USF and transmitting data during the appropriate timeslots." But the Federal Circuit shot down this interpretation, pointing to how the District Court had relied on "a high-level description of how USFs operate in mobile stations using extended bandwidth allocation generally" and was therefore improperly "untethered to the invention as claimed."

Concluding that claim 5 was not abstract, the Federal Circuit reversed the District Court's dismissal.

[1] The opinion does not do a fantastic job of describing the underlying technology. A clearer explanation is found in the ‘413 patent, on which this description was based.

Mentone Solutions LLC v. Digi International Inc. (Fed. Cir. 2021)
Nonprecedential disposition
Panel: Chief Judge Moore and Circuit Judges Lourie and Dyk
Opinion by Chief Judge Moore