In The Unicode Standard, Katakana implementation is discussed in  Chapter 18 East Asia.

Katakana was first encoded in The Unicode Standard version 1.0. The script is now encoded in the following blocks:

BlocksCharacter RangeAdded in Unicode VersionDocumentation
Katakana 30A0..30FF 1.0  U30A0.pdf
Katakana Phonetic Extensions 31F0..31FF 3.2  U31F0.pdf
Enclosed CJK Letters and Months 32D0..32FE 1.1  U3200.pdf
CJK Compatibility 3300..3357 1.1  U3300.pdf
Halfwidth Katakana FF00..FFEF 1.1  UFF00.pdf
Kana Supplement 1B000..1B0FF 6.0  U1B000.pdf

Subsequent to version 1.1, the following characters have been added to the Katakana blocks:

CharactersUnicode VersionDocumentation
30A0 3.2  n2195.doc L2/00-098
30FF 3.2  n2092.pdf,  n2195.doc L2/00-098
31F0..31FF 3.2
1B000..1B001 6.0  n3388.pdf L2/07-421

Documentation refers to  ISO Working Group and Unicode proposals

A number of documents have been submitted to the Unicode Technical Committee and WG2:

2000-03-15 Rationale for non-Kanji characters proposed by JCS committee — Japan NB ( WG2 N2092,  WG2 N2195.doc,  L2/00-098)

2000-09-20 Ad hoc Report on Japan feedback N2257 and N2258 — T. K. Sato, Michael Everson, Ken Whistler, Asmus Freytag (WG2 N2278,  L2/00-342)

2004-05-12Hiragana_or_Katakana value of Script property should die — John Cowan ( L2/04-160)

2004-06-17 Katakana_Or_Hiragana and UTR#29 — Mark Davis ( L2/04-152)

2008-01-14 Proposal to encode two Kana characters concerning YE — Nozomu Katō ( WG2 N3388,  L2/07-421)

2008-07-14 Letter from Nozomu Kato to UTC, re Kana — Nozomu Katō ( L2/08-257 - limited access to UTC members only)
2009-01-28 Background Info on "e" Letters in Kana — Nozomu Katō ( L2/09-062)

2010-04-29 Report of Phonetic Kana Usage for Lingua Latina — Nozomu Katō ( L2/10-153)

2011-02-09 Proposal to add two kana characters — Ken Lunde ( WG2 N3987,  L2/10-468)

2011-05-30 Comment on "two kana" proposal — Japan NB ( WG2 N4087,  WG2 N4087.doc,  L2/11-228)

2011-06-08 A response regarding small ko — Masahiro Sekiguchi ( WG2 N4108,  WG2 N4108.doc)

2016-05-10 Proposal for Japanese vowels with the voiced sound mark as named sequences — Hiroyuki Komatsu ( L2/16-133)

UTC #147 Minutes ( L2/16-121) (See B.15.5 for decision and action items)