Decoding the UMTS/UTRAN Specifications

The purpose of this wiki area is to provide a clear and concise presentation of the information given in the 3GPP UMTS/UTRAN specifications, specifically on the "Uu" interface. Because UMTS/UTRAN inherit many concepts and subprotocols from GSM and GPRS (" Um"), some understanding of those technologies is probably a prerequisite for understanding this wiki.

General Comments on the Wiki

One of the problem with UMTS complexity is that the standard supports a lot of deployment variations. For simplicity, this wiki should probably focus on the most commonly deployed style of UMTS/UTRAN worldwide: FDD radio interface and GSM-MAP core network.

Please do not just copy-paste large sections of text from the specifications themselves into the wiki. First, it's a copyright violation. Second, it defeats the purpose of the wiki. So if you can't/won't rewrite something to make it more understandable, just make a reference to the specification document and section/paragraph, like this: "3GPP 25.331 10.5.1". Small quotations are good, but please put them in italics like this and note the document and paragraph from which the text was quoted.

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If you have a specific question about a specific part of the specification, please put the question into the wiki using italicized text like you see here.

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Other Web Resources for Understanding UMTS

General Comments on UMTS

Like GSM/GPRS, UMTS defines logical and physical channels, but there is a greater variety of physical channel types and there's is a new intermediate sublayer, the transport layer, between the logical and physical channel. The existence of transport channels permits the grouping of logical channels with similar bearer performance (e.g., error rate) requirements.

UMTS Channels

Logical Channels

  • BCCH (Broadcast Control Channel) - Downlink only. Broadcast. Functionally similar to GSM BCCH. Carries a cycle of repeating System Information Blocks (SIBs).
  • PCCH (Paging Control Channel) - Downlink only. Unicast. Functionally similar to GSM Paging Channel (PCH). Carries paging requests.
  • CCCH (Common Control Channel) - Downlink/uplink. Unicast. Functionally similar to GSM Access Grant Channel (AGCH) and Random Access Channel (RACH). Primarily carries messages related to RRC connection setup.
  • DCCH (Dedicated Control Channel) - Downlink/uplink. Functionally similar to GSM SDCCH, SACCH, or FACCH, depending on the context. Carries user-specific control plane traffic.
  • DTCH (Dedicated Traffic Channel) - Downlink/uplink. Functionally similar to the GSM PDTCH or TCH. Carries user plane traffic.

Transport Channels

  • BCH (Broadcast Channel) - Downlink only. Carries the BCCH.
  • PCH (Paging Channel) - Downlink only. Carries the PCCH.
  • FACH (Forward Access Channel) - Downlink only. Carries the downlink CCCH. May carry one or more downlink DCCH and/or DTCH depending on the context. May carry the BCCH.
  • DCH (Dedicated Channel) - Downlink/uplink. Carries the DCCH and/or the DTCH.
  • RACH (Random Access Channel) - Uplink only. Carries the uplink CCCH. May carry the uplink DCCH and/or DTCH depending on the context.

Physical Channels

The physical channels use different slot formats, documented in 3GPP 25.211 Section 5. The mapping of transport channels onto physical channels is defined in 3GPP 25.211 Section 6.

  • P-CCPCH (Primary Common Control Physical Channel) - Carries the BCH.
  • S-CCPCH (Secondary Control Control Physical Channel) - Carries one or more FACHs and the PCH.
  • PICH (Paging Indication Channel) - Carries a bit mask of reduced paging information. Its existence allows the handset to sleep more of the time thereby conserving battery. Does not carry any higher-layer data.
  • P-SCH (Primary Synchronization Channel) - Supports the first stage of cell synchronization: word sync.
  • S-SCH (Secondary Synchronization Channel) - Supports the second stage of cell synchronization: frame sync.
  • PRACH (Physical Random Access Channel) - Carries the RACH.
  • AICH (Access Indication Channel) - Indicates the base station's reception of a PRACH preamble transmission, which is the go-ahead signal for the handset to transmit the PRACH burst. Does not carry any higher-layer data. The UMTS access procedure is more complex to insure proper initialization of uplink power control.
  • P-CPICH (Primary  Common Pilot Channel) - Channel state reference and physical cell identifier. Does not carry any higher-layer data. Similar in purpose to the GSM FCCH/SCH logical channels.
  • DPDCH (Dedicated Physical Data Channel) - Carries one or more DCH.
  • DPCCH (Dedicated Physical Control Channel) - Carries physical link control information such as power control bits and the transport format combination indicator. Does not carry any higher-layer data.

UMTS Layers

Layer 1 - PHY

The UMTS PHY is based on  WCDMA. Key features of CDMA:

  • A single company has a strangle-hold on the IP.
  • In the real world, with Doppler and multipath and varying propagation delays, nothing is really orthogonal, so you usually get well less than half the theoretical capacity.
  • It requires you to control uplink power to within about 3 dB to prevent the near-far effect.
  • It requires you to process large swaths of bandwidth continuously even for low-rate signals.
  • The main noise source that degrades receiver performance is other users, not thermal noise, so per-user performance varies wildly as the traffic mix changes.
  • Since the UMTS CDMA signal has a high crest factor (similar to Gaussian noise), the amplifiers require a lot of extra headroom, making them much less efficient than the constant modulus amplifiers used in GSM/GPRS.

Most UMTS deployments utilize frequency-division duplexing (FDD). Unlike GSM, in which transmit and receive bursts are separated in time, UMTS-FDD requires simultaneous transmission and reception. Thus, instead of a simple switch, the RF front-end in a UMTS handset utilizes a duplexer. Small duplexers are typically quite lossy, requiring the (already inefficient) PAs in handsets to produce even more power.

UMTS Frequency Bands

The primary UMTS operating frequency band is the IMT-2000 band at 2100 MHz. This band is used for UMTS throughout most of the world including Asia, Europe, Australasia, Africa, and parts of South America. In the United States, the IMT-2000 band was not (entirely) available at the time UMTS was being rolled out. Therefore, the 1900 MHz "PCS" band and the 850 MHz "cellular" bands are used for UMTS in the U.S. and countries that typically follow U.S. frequency band planning in North America and parts of South America.

The 3GPP technical specifications permit UMTS to be operated in numerous, often region-specific frequency bands - a total of 14 as of R8 of the specs. As traffic has grown, UMTS has been deployed in in more and more of these operating bands - a process colloquially referred to as spectrum "re-farming" - including 900 MHz (GSM), 1800 MHz (DCS), and 1700/2100 MHz (U.S. AWS).

The 3GPP technical specifications support the use of virtually any downlink/uplink frequency pairing in FDD mode, though it is extremely rare for networks to utilize non-standard operating bands and duplex spacings.

FDD Radiomodem

This is everything from the formatted radio frame data bits to the antenna, including:

  • spreading
  • scrambling
  • pulse shaping
  • radio frequency tuning and power control

Forward Error Correction

From 3GPP 25.212 4.2. This is the encoding/transmission path between a transport block and a radio frame:

  • transport block concatenation
  • parity word append
  • code block segmentation
  • convolutional or turbo encoding
  • rate-matching (puncturing or repeating channel bits in coded blocks to match input and output rates)
  • interleaving
  • segmentation of coded blocks into radio frames
  • more interleaving

Special Cases

Layer 2

Layer 1-2 Interface

Data structures exchanged between L1 and L2 are called transport blocks. UMTS defines concatenation and rate-matching mechanisms in L1 that allows great flexibility in transport block size on most L1-L2 interfaces. However, there is a maximum code block size, defined in 3GPP 25.212 4.2.2.2 that determines the maximum available data rates for given channel types. See DecodingUMTS/FEC for more details.

Medium Access Control (MAC)

Radio Link Control (RLC)?

Layer 3

Layer 2-3 Interface

Radio Resource Control (RRC)

Layer 4

Layer 3-4 (AS-NAS) Interface

Call Control (CC)?

Service Management (SM)?

Mobility Management (MM)?

GPRS Service Management (GSM)?

GPRS Mobility Management (GMM)?

Packet-Switched (PS) Services

Circuit-Switched (CS) Services

User Equipment

SIM vs USIM