Apple is being sued by a patent troll that has acquired a BT patent.

Steelhead Licensing now owns the "Mobile Radio Handover Initiation Determination" first filed by BT in 1993.

Steelhead has also filed patent lawsuits against Sprint Nextel, RIM, and Motorola.

According to BT the company has a patent portfolio of more than 5,000. BT is one of the world's largest telecoms companies and one of the largest investors in R&D in the UK. It employees 10,000 scientists worldwide.

The patent in question, number 5,491,834, is described thus: "An intelligent mobile radio unit is served by one of a network of macrocells and microcells. The unit is arranged to monitor, for example, the signal strength or bit error ratio of signals transmitted from each of a set of candidate base stations of cells surrounding the serving base based on the assessment of a running average of the rise/fall in signal quality, the unit is arranged to determine from a look-up table of stored templates of conditions for handover whether a handover between base stations and between macro- and microcells."

The patent, now owned by the 'troll', describes the way the quality of a signal is maintained when transmitted between base stations. 

The patent was first filed on 27 August 1993 and the inventor is listed as Chia; Si Tak S, based in Suffolk, where BT's R&D labs is based.

Back in 2011 BT sued Google for patent infringement claiming that six of its patents have been infringed. Those patents related to location-based services, navigation and guidance information and personalised access to services and content, according to a BBC report.

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