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Receiver measurements in GSM basestations

Introduction

While transmitter power is limited by the GSM standards, there is no limit on

the ability of a receiver to acquire a low-level signal under adverse conditions.

As a result, receiver characteristics are often the differentiating factor in

competing systems. The ETSI GSM 11.21 standard is extensive, but can be

summarized with the following tests: sensitivity, signal quality, and selectivity.

These tests determine if the basestation can correctly receive signals within its

channel and reject interfering signals.

Metrics

Before performing receiver tests, it is important to understand the metrics used

to characterize a receiver’s performance. In GSM systems the primary measure

of receiver performance is sensitivity. Bit error rate (BER), residual bit error

rate (RBER), and frame erasure rate (FER) are the metrics used to evaluate

this performance.

BER

The most common performance metric for digital signal quality is BER.

BER is the ratio of the number of erroneous bits received to the total number

of bits received. Conceptually, the received bit stream is compared with the

transmitted bit stream to detect errors; however, most bit error rate testers

use pseudo-random bit sequences to eliminate the need for synchronization

between the received and transmitted bits.

Pseudo-random sequences have bit lengths of 2

–1 where n is commonly 9 or

15. For example, a PN9 sequence has 2

–1 or 511 bits. PN sequences have two

properties that make them ideal for BER testing. As the name implies, they

appear statistically random when the entire sequence is transmitted. Also,

the entire bit stream can be predicted from any given sequence. Because of

this second property, it is not necessary to compare the received bit stream

with the transmitted bit stream. Instead, the entire bit stream is quickly

constructed from the first correct sequence bits. All received bits are then

compared with this synthesized sequence. This technique eliminates the

difficult synchronization between the transmitted and received bits required

in the conventional approach. BER is monitored as the receiver is subjected to

low-signal levels, interference and fading.

RBER

The residual bit error rate (RBER) is performed on demodulated speech frames

that are not marked corrupt. A frame is labeled corrupt if the test on the parity

bits or cyclic redundancy check (CRC) is not successful. There are two types

of RBER: RBER Ib and RBER II. Each one evaluates different portions of the

demodulated, decoded speech frame. To better explain the different portions of

the speech frame, this note first examines how speech is coded in GSM.

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