Compressing and decompressing data about radio signals

A method comprising, performed by at least one apparatus:

• obtaining a set of radio data comprising signal strength related values for radio signals transmitted by a transmitter with an association of each signal strength related value with a representation of a geographical location;
• applying a frequency transform to the obtained set of radio data to obtain transform coefficients, each transform coefficient comprising a transform index and an associated transform value;
• selecting a subset of transform indices having more significant transform values than the remaining transform indices;
• and
• compressing the transform indices by encoding each transform index exploiting a probability of occurrence of an index value of a respective transform index, wherein encoding the transform indices exploiting a probability of occurrence of an index value of a respective transform index comprises a differential encoding, the differential encoding comprising:
• sorting the transform indices according to ordinal numbers associated with index values of transform indices, the ordinal numbers being associated with the index values of transform indices based on the probability of occurrence of each index value of transform indices;
• determining a respective distance from one ordinal number to the next to obtain a sequence of differential indices;
• and
• encoding the differential indices using shorter coding words for differential indices having index values with higher probability of occurrence than for differential indices having index values with lower probability of occurrence.

An apparatus comprising at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform:

• obtain a set of radio data comprising signal strength related values for radio signals transmitted by a transmitter with an association of each signal strength related value with a representation of a geographical location;
• apply a frequency transform to the obtained set of radio data to obtain transform coefficients, each transform coefficient comprising a transform index and an associated transform value;
• select a subset of transform indices having more significant transform values than the remaining transform indices;
• and
• compress the transform indices by encoding each transform index exploiting a probability of occurrence of an index value of a respective transform index, wherein one of:
• encoding each transform index exploiting a probability of occurrence of an index value of a respective transform index comprises using a code table that assigns different code words to different index values of transform indices based on the probability of occurrence of the respective index value of transform indices;
• and
• encoding each transform index exploiting a probability of occurrence of an index value of a respective transform index comprises using a code table that assigns different code words to different index values of transform indices based on the probability of occurrence of the respective index value of transform indices, the at least one memory and the computer program code further configured to, with the at least one processor, cause the apparatus to, in the case that the subset of transform indices comprises a transform index having an index value for which no code word is included in the code table, encode the transform index by a predetermined exception code word followed by a fixed length bit field representing the index value of the transform index.

An apparatus comprising at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform:

• obtain a set of radio data comprising signal strength related values for radio signals transmitted by a transmitter with an association of each signal strength related value with a representation of a geographical location;
• apply a frequency transform to the obtained set of radio data to obtain transform coefficients, each transform coefficient comprising a transform index and an associated transform value;
• select a subset of transform indices having more significant transform values than the remaining transform indices;
• and
• compress the transform indices by encoding each transform index exploiting a probability of occurrence of an index value of a respective transform index, wherein encoding the transform indices exploiting a probability of occurrence of an index value of a respective transform index comprises a differential encoding, the differential encoding comprising:
• sorting the transform indices according to ordinal numbers associated with index values of transform indices, the ordinal numbers being associated with the index values of transform indices based on the probability of occurrence of each index value of transform indices;
• determining a respective distance from one ordinal number to the next to obtain a sequence of differential indices;
• and
• encoding the differential indices using shorter coding words for differential indices having index values with higher probability of occurrence than for differential indices having index values with lower probability of occurrence.

A non-transitory computer readable storage medium in which computer program code is stored, the computer program code causing an apparatus to perform the following when executed by a processor:

• obtain a set of radio data comprising signal strength related values for radio signals transmitted by a transmitter with an association of each signal strength related value with a representation of a geographical location;
• apply a frequency transform to the obtained set of radio data to obtain transform coefficients, each transform coefficient comprising a transform index and an associated transform value;
• select a subset of transform indices having more significant transform values than the remaining transform indices;
• and
• compress the transform indices by encoding each transform index exploiting a probability of occurrence of an index value of a respective transform index, wherein one of:
• encoding each transform index exploiting a probability of occurrence of an index value of a respective transform index comprises using a code table that assigns different code words to different index values of transform indices based on the probability of occurrence of the respective index value of transform indices;
• or
• encoding each transform index exploiting a probability of occurrence of an index value of a respective transform index comprises using a code table that assigns different code words to different index values of transform indices based on the probability of occurrence of the respective index value of transform indices, further comprising in the case that the subset of transform indices comprises a transform index having an index value for which no code word is included in the code table, encode the transform index by a predetermined exception code word followed by a fixed length bit field representing the index value of the transform index.

A non-transitory computer readable storage medium in which computer program code is stored, the computer program code causing an apparatus to perform the following when executed by a processor:

• obtain a set of radio data comprising signal strength related values for radio signals transmitted by a transmitter with an association of each signal strength related value with a representation of a geographical location;
• apply a frequency transform to the obtained set of radio data to obtain transform coefficients, each transform coefficient comprising a transform index and an associated transform value;
• select a subset of transform indices having more significant transform values than the remaining transform indices;
• and
• compress the transform indices by encoding each transform index exploiting a probability of occurrence of an index value of a respective transform index, wherein encoding the transform indices exploiting a probability of occurrence of an index value of a respective transform index comprises a differential encoding, the differential encoding comprising:
• sorting the transform indices according to ordinal numbers associated with index values of transform indices, the ordinal numbers being associated with the index values of transform indices based on the probability of occurrence of each index value of transform indices;
• determining a respective distance from one ordinal number to the next to obtain a sequence of differential indices;
• and
• encoding the differential indices using shorter coding words for differential indices having index values with higher probability of occurrence than for differential indices having index values with lower probability of occurrence.