UMTS Idiots Guide

The GSM Evolution

The basic evolution of GSM towards third-generation capabilities.

The GSM Evolution

Functionality

Drivers for 3G

IMT2000 Family Concept
Worldwide Terrestrial System Roaming
Standardised Air-Interface(s) and networks(s)
Multi-vendor Sourcing
Pan-European Spectrum Regulation Volume Market Potential
IP Multimedia Extension
Broadband Capability
Wideband Delivery
Information Delivery

Terminals

Small and Light
Browser Capability
Bluetooth
Not just Telephones

3G Concept

The IMT-2000 (UMTS) has been designed to carry multimedia services, which means that multi-channels can be created, each with independent QoS and data rates. These data rates range up to 2Mbps for Time Division Duplex (TDD) in building Only, and 384kbs for Frequency Division Duplex for outdoor applications.

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HSCSD (High Speed Circuit Switched Data)

Mobil Rx
Data Rate - 14.4kb/s (One time-slot)
One restriction that HSCSD places upon multiple timeslot links is that the timeslots allocated must be consecutive. As can be seen from the diagram below the use of two timeslots is relatively simple to implement. The mobile is still able to run through its standard routine of receive, transmit, monitor a neighbour within an 8 timeslot frame. With 3 or more time slots being used there is an overlap between the receive and transmit times and implementation involves substantial hardware changes in the mobile station. I.e. the use of an RF duplexor. It would seem at first sight as if there is no overlap when using 3 timeslots but there is due to the timing advance applied to the up-link.

Mobile Rx
HSCSD - Data Rate - 28.8kbs (2 Timeslots) Mobile Tx

Data Rate - 28.8kbs (2 Timeslots)

HSCSD Throughput

The maximum theoretical throughput in HSCSD is utilising all timeslots (8 x 14.4kbs = 115.2kbs). Will require a duplexor in the Rx and a second Rx to monitor a neighbouring cell. The aim of HSCSD is to provide a mixture of services with different air interface user rates by a single physical layer structure. The available capacity of a HSCSD configuration is several times the capacity of a TCH/F, leading to a significant enhancement in the interface data transfer rate.

Terminal Vendours are launching 4+1 HSCSD terminals

4 down-link TS
1 up-link TS

GPRS Coding Schemes

GPRS uses a packet-mode technique to transfer high-speed data signalling in an efficient manner. GPRS optimises the use of network and radio resources. Strict separation between the radio sub-system and network sub-system is maintained, allowing the network sub-system to be reused with other radio access technologies. GPRS does not mandate changes to an installed MSC base.

New GPRS radio channels are defined, and the allocation of these channels is flexible; form 1 to 8 radio interface timeslots can be allocated per TDMA frame, timeslots are shared by the active users, and up and down-link are allocated separately. The radio interface resources can be shared dynamically between speech and data services as a function of service load and operator preference. Various radio channel coding schemes are specified to allow bit rates from 9 to more than 150kbps per user.

Applications based on the standard data protocols are supported, and interworking is defined with IP networks and X.25 networks. Specific point-to-point and point-to-multipoint services are supported for applications such as traffic telematics. GPRS allows SMS transfer over GPRS radio channels.

Multiple Mobiles can use a single timeslot
Multiple Mobiles can share multiple timeslots
A single Mobile could in principle use multiple timeslots (Max 8)
Mobiles could in principle use one of the four channel coding schemes

Table 2

Terminal Vendors are launching 4+1 GPRS terminals

4 down-link TS
1 up-link TS

So, in the down-link the throughput could reach 13.4 x 4 = 56.6kbs

In the up link, the peak throughput could reach 14.4kbs

….however a carrier supports 8 timeslots with existing voice users.

GSM network operators will have to double the Tx base to support GPRS???

Best Effort???

If we compare cell size for the various coding scheme rates (1 to 4), the increasing data rate reduces the cell size for a given QoS.

Table3
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EDGE Modulation Concept

EDGE will use a 8-PSK modulation system. This will allow 3 bits to be sent when 1 is sent in GSM. Thus the data rate will increase from 21.4kbs to 3 x 21.4 = 64.2kbs. With 8 timeslots 513.6kbs will be possible.

GSM and EDGE Compared

EDGE allows GSM operators to provide the same data rate as FDD-CDMA in UMTS. However, they will need to re-design their cell plan and provide additional coverage/cells. Moving to EDGE reduces the cell to 9% which will require a large number of BTS's to be added to ensure the same QoS/GoS and user density.