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Wednesday, November 28, 2007

Deciphering the LTE specs

Unless you are really familiar with the LTE standardization process, it is rather difficult to understand how things actually work just by reading through the specifications. Since a technical specification is merely a description of a guideline for inter-operability, it does not necessarily explain any rationale or background information for choice of the specific methodologies or parameters.

One way to find a more detailed description of the specific methodology or parameter in the specification is to go through the contribution papers (called technical documents, or tdocs) that 3GPP uploads on their meeting website. 3GPP has different work groups working on different aspects of the radio interface and network architecture. The Technical Specification Group Radio Access Networks (TSG RAN) is where all the radio air interface specifications are handled and it has five working groups (WG) as described in the following list.

- RAN WG1 (RAN1) Layer 1 (Physical layer) specification
- RAN WG2 (RAN2) Layer 2 (MAC, RLC) and layer 3 specification
- RAN WG3 (RAN3) Overall UTRAN architecture.
- RAN WG4 (RAN4) Radio performance and protocol aspects
- RAN WG5 (RAN5) Mobile terminal conformance testing

The following highlights the LTE specifications and their corresponding working groups.

-36.1xx: RAN4
-36.2xx: RAN1
-36.3xx: RAN2
-36.4xx: RAN3
-36.5xx: RAN5
-36.8xx: RAN4
-36.902: RAN3
-36.913: RAN
-36.938: RAN4
-36.942: RAN4
-36.956: RAN4

Most of the physical layer and MAC layer components are discussed in RAN1 meetings and the meeting information is at

If you go to this site, there is a column called ‘First & Last tdoc’ (tdoc = technical document). This is the paper number that identifies each contribution paper. If you click on the link, it directs you to the ftp site where you can access all the tdocs of the particular meeting. Since the tdocs are uploaded in zip files without any title (only the tdoc numbers are specified in the file name), you would have to look at the excel spreadsheet of the tdoc list (provided at the bottom of the list) to know which is which. Tdoc numbers are in the form of Rx-yyzzzz where x is the RAN work group number, yy is the year of the meeting, and zzzz is the cumulative number assigned to the document. For example, R1-081165 refers to the document 1165 of RAN1 meeting in the year 2008.

It may be daunting to go through each tdoc list spreadsheet for each meeting since it is usually a very long list (300~400) and the list is not organized by subject. What you can do is first read the meeting summary report which summarizes the discussions and decisions from the meeting by subject. You can find the meeting report by following ‘Files’ link and then ‘Report’ link. From the meeting report, you can find about the decisions made and find out the relevant tdocs. Once you the know the tdoc number, you can locate the link from the ‘First & last tdoc” column and access it from there.

For example, the channel coding scheme specified in TS 36.212 uses quadratic permutation polynomial (QPP) interleaver. Let’s say we want to find more details of the QPP interleaver. By searching through the meeting reports, we can find that there was a decision to adopt the QPP interleaver in RAN1 meeting #47b. The document that proposes this method is R1-070483 and other related documents are listed as well in the meeting report. By looking at the listed documents, we can understand more about the QPP interleaver that is new in LTE channel coding.

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