UMTS UTRA / UTRAN: terrestrial radio access network

- the air interface for UMTS is known as the UMTS terrestrial radio access, or UTRA, the UMTS terrestrial radio access network is known as the UTRAN.

Physical layer within UMTS / WCDMA is totally different to that employed by GSM. It employs a spread spectrum transmission in the form of CDMA rather than the TDMA transmissions used for GSM. Additionally it currently uses different frequencies to those allocated for GSM.

The UTRA, UMTS radio access is the technology that is the radio interface, and the network, or UMTS Radio Access Network is known as the UTRAN. Sometimes the UTRAN may also be known as the Radio Network Subsystem, or RNS.

UMTS radio access network, UTRAN

The UMTS Radio Access Network, UTRAN, or Radio Network Subsystem, RNS comprises two main components:

  • Radio Network Controller, RNC:   This element of the UTRAN / radio network subsystem controls the Node Bs that are connected to it, i.e. the radio resources in its domain.. The RNC undertakes the radio resource management and some of the mobility management functions, although not all. It is also the point at which the data encryption / decryption is performed to protect the user data from eavesdropping.
  • Node B:   Node B is the term used within UMTS to denote the base station transceiver. This part of the UTRAN contains the transmitter and receiver to communicate with the UEs within the cell. It participates with the RNC in the resource management. NodeB is the 3GPP term for base station, and often the terms are used interchangeably.

In order to facilitate effective handover between Node Bs under the control of different RNCs, the RNC not only communicates with the Core Network, but also with neighbouring RNCs.

Diagram of the 3G UMTS radio access network, UTRAN / radio network subsystem architecture comprising the RNC, Node Bs and the MSs
3G UMTS UTRAN Architecture

UTRAN interfaces

The UMTS standards are structured in a way that the internal functionality of the different network elements is not defined. Instead, the interfaces between the network elements is defined and in this way, so too is the element functionality.

There are several interfaces that are defined for the UTRAN elements:

  • Iub :   The Iub connects the NodeB and the RNC within the UTRAN. Although when it was launched, a standardisation of the interface between the controller and base station in the UTRAN was revolutionary, the aim was to stimulate competition between suppliers, allowing opportunities like some manufacturers who might concentrate just on base stations rather than the controller and other network entities.
  • Iur :   The Iur interface allows communication between different RNCs within the UTRAN. The open Iur interface enables capabilities like soft handover to occur as well as helping to stimulate competition between equipment manufacturers.
  • Iu :   The Iu interface connects the UTRAN to the core network.

Having standardised interfaces within various areas of the network including the UTRAN allows network operators to select different network entities from different suppliers.

UTRA uplink & downlink

When looking at the radio air interface and its associated properties, it is necessary to define the directions in which the transmissions are occurring. Being a full duplex system, i.e. transmitting simultaneously in both directions, it is necessary to be able to define which direction is which.

  • Uplink;   This may also sometimes be known as the reverse link, and it is the link from the User Equipment (UE) to the Node B or base station.
  • Downlink;   This may also sometimes be known as the forward link, and it is the link from the Node B or base station to the User Equipment (UE).

The terms Uplink and Downlink are the terms that are used with UMTS, and especially within Europe. The terms forward link and reverse link are more commonly used with the CDMA2000 technologies and also within North America.

Uplink and downlink transmission directions
Uplink and downlink transmission directions

UTRA FDD & TDD

In view of the fact that transmissions have to be made in both directions, i.e. in both uplink and downlink. It is necessary to organise the way these transmissions are made. Two techniques are used to ensure concurrent or near concurrent transmissions in both directions: frequency division duplex and time division duplex.

  • UTRA-FDD:   The frequency division duplex version of UTRA uses a scheme whereby transmissions in the uplink and downlink occur on different frequencies. Although this requires double the bandwidth to accommodate the two transmissions, and filters to prevent the transmitted signal from interfering with the receiver. Even though there is a defined split between uplink and downlink, effective filters are required.
  • UTRA-TDD:   The time division version of the UTRA uses uplink and downlink transmissions that use the same frequency but are timed to occur at different intervals.

Both UTRA-FDD and UTRA-TDD have their own advantages and disadvantages and therefore tend to be used in different areas.

While the UTRA-FDD and UTRA-TDD both belong to 3G UMTS and are contained within the 3GPP standards, they may have some slightly different parameters for their transmissions.


Key Specifications for UTRAN Operation for FDD & TDD
Parameter UTRA FDD UTRA TDD
Multiple access method CDMA TDMA, CDMA
Channel spacing 5 MHz 5 MHz (and 1.6MHz for TD-SCDMA)
Carrier chip rate 3.84 Mcps 3.84 Mcps
Spreading factors 4 .. 512 1 .. 16
Time slot structure 15 slots / frame 15 / 14 slots / frame
Frame length (ms) 10 10
Multirate concept Multicode, and OVSF[1] Multicode, multislot and OVSF[1]
Burst types N/A (1) traffic bursts
(2) random access burst
(3) synchronisation burst
Detection Coherent based on pilot symbols Coherent based on mid-amble
Dedicated channel power control Fast closed loop 1500 Hz rate Uplink: open loop 100 Hz or 200 Hz rate
Downlink: closed loop max 800 Hz rate

Notes
[1] OVSF = Orthogonal Variable Spreading Factor

By Ian Poole


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