Mumbai University > Computer Engineering > Sem6 > Mobile Communication and Computing

Marks: 10M

Year: May15, Dec15

• UMTS, short for Universal Mobile Telecommunications System, is a 3G networking standard used throughout much of the world as an upgrade to existing GSM module.

• UMTS makes use of WCDMA, a technology that shares much with CDMA networks used throughout the world, though it is not compatible with them.

• Base level UMTS networks are generally capable of downlink speeds as 384 kbps.

• The UMTS architecture takes advantage of the existing GSM and GPRS networks which serve as a core network in UMTS infrastructure.

• The UMTS is made up of 3 main components:

a) User Equipment:

It is assigned to a single user and contains all the functions needed to access UMTS services. It contains:

i. Mobile Equipment (ME) :

It is a radio terminal which is used to connect the UMTS subscriber with the fixed part of UMTS system via the radio interface Uu.

ii. UMTS Subscriber Identity Module (USIM):

A smartcard which contains the subscriber identity, authentication algorithms, encryption keys etc.

b) UMTS Terrestrial Radio Access Network(UTRAN):

It handles cell-level mobility. It is a system of base station and controller handling function related to mobility. It contains:

i. Nodes B (Base Stations):

• It converts the data between Uu radio interface and the Iub interface connecting a Node B with the RNC.

• It performs physical level processing such as channel coding, data interleaving, rate matching, modulation etc.

ii. Radio Network Controllers (RNC):

• RNC’s controls and manages radio resources to Node B.
• RNC performs the data-link layer processing and participates in handover operations.
• RNC is considered a single access point of UTRAN for the core network.
• It’s connected to a single MSC/VLR to route circuit-switched traffic and to a single SGSN to route packet switched traffic.

c) Core Network(CN):

The core network is shared with GSM and GPRS. The CN contains functions for intersystem handover, gateways to other networks and performs location management. It contains:

i. Home Location Register (HLR)

ii. Mobile Station Controller / Visitor Location Register (MSC/VLR).

iii. Gateway MSC: Connect UMTS to external circuit switch n/w (e.g PSTN)

iv. Serving GPRS Support Node (SGSN): It serves the Packet-switched traffic.

v. Gateway GPRS Support Node (GGSN): Connects UMTS to external packet switched. (e.g. Internet)

• For simultaneous access to the medium by base station and receiver station in a network, we used channel duplexing.

• FDD: One method to achieve this duplexing is to transfer the signals in both directions by sending them at different frequencies. Therefore, the two directions, i.e. user-station to base station and vice versa (uplink and downlink) are separated using different frequencies.

• Such a scheme is called frequency division duplex (FDD).

• TDD: In similar environment, when we assign different time-slots for uplink and downlink using same frequency ( not using two frequency as in FDD but only one frequency used ) we call it as Time Division Duplex (TDD).

• Duplexing is an important part of a wireless communication system. Here we have to type UTRA-FDD and UTRA-TDD.

UTRA-FDD:

• The FDD mode for UTRA used wideband CDMA (WCDMA) with direct sequence spreading (DSS).

• As per FDD’s definition, the uplink and downlink uses different frequencies

• The UTRA-FDD uses the following frequency band for transmission

  (MS->BS) Uplink → 1920 to 1980 MHz

  (BS->MS) Downlink → 2110 to 2170 MHz

• Each radio channel is divided into 10 millisec frames and each frame is further divided into 15 time slots. The time slots over here are not used for user separation (as in GSM) but for periodic functions.

UTRA-TDD:

• As TDD is used here, this UTRA mode i.e. UTRA-TDD separates uplink and downlink in time using a radio frame with 15 slots as in FDD.

• The duration of each frame with 15 slots is 10millisec with each slot having 2560 chips per slot.

• To satisfy the requirements of different users in terms of data rates, the TDD frame can be symmetrical or asymmetrical i.e. the frame can contain the same number of uplink and downlink slots or any other combination (e.g. 5slots for uplink & 10 for downlink out of total 15 in a single frame).

• However at least one slot must be compulsorily be allocated for uplink & downlink respectively (i.e. there cant be all 15 downlinks only…atleast 1 uplink should also be there)