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Measurements

Test setups

Abis versus loopback

In normal operation, the GSM basestation receives signals from the mobile over

the air interface, demodulates and decodes the signal, then sends the data to

the basestation controller (BSC) over the network interface (Abis). Controlling

the basestation using Abis is a critical part of R&D test and type approval. Abis

testing emulates actual operation of the basestation, and verifies functions such

as signal strength (RX-LEV), signal quality (RX-QUAL), and frame erasure

indication (FEI).

It is common in manufacturing to test the basestation or its transceiver units

(TRX’s) using the loopback method. During loopback the received signal is

demodulated, decoded, then encoded, modulated and re-transmitted. Recently

many manufacturers have begun using the loopback method in final test

because the Abis interface can be device independent.

Static versus multipath propagation

Static testing refers to the ideal propagation condition. The test signal is

connected directly to the TX or RX port. Static testing is the method most

used in manufacturing and service. Multipath propagation testing refers to

the simulation of various conditions that create interference in the propagation

environment. The GSM specification describes three different propagation

conditions: Typical urban terrain (TU), rural area terrain (RA), and hilly terrain

(HT). Each terrain will include a speed parameter. For example, TU50 specifies

an urban fading profile, which is passed through at a speed of 50 km/hr. RF

channel fading simulators are used in R&D and during full type approval.

Test

system

Combiner

distribution

modules

C/D

Transceiver

modules

TRX

Abis

loopback

(internal)

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Agilent Technologies Switch/Measure Manual do Utilizador Página 28