Rohde & Schwarz CMU-K20, Operating Manual

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Signalling: Receiver Quality Measurements
CMU-K20/K21/K22/K23/K24
1115.6088.12 4.118 E-12
Receiver Quality Measurements
The menu group
Receiver Quality
tests the transmission performance on the complete signal path from
the CMU to the device under test (mobile station) and back. To this end the mobile is set to test loop
operation where it returns the received data back to the tester. The measurement is especially suitable
to assess the sensitivity of the mobile station receiver at low RF power levels.
The popup menu
Receiver Quality Configuration
is used for configuration of the measurements; the
measurement results are directly indicated in the
Receiver Quality
menu.
The measurements in the menu group
Receiver Quality
assume that a call has been set up (signalling
state
Call Established)
. This is why they are not available in the mode
GSM400/850/900/1800/1900-MS
Non Signalling
.
Principle of the
measurement
The
Receiver Quality
measurement is based on the comparison of the output signal sent by
the CMU with the signal received (and possibly decoded) by the device under test.
To this end, the mobile station must either detect bit errors by itself and return the result or
return the received signal to the CMU in loopback mode. Error detection by the mobile phone
is used in the measurement of the Block Error Ratio (BLER); the loopback mode is used for
all other receiver quality tests.
Due to the higher signal level, the transmission errors produced on the way back (from the
mobile station to the CMU) can usually be neglected. However, frames destroyed on the way
back are detected in a cyclic redundancy check (CRC) and counted. They are not taken into
account in the calculation of transmission errors.
In most cases, a test SIM card is required for receiver quality measurements. The quality of
the mobile station transmitter can be tested separately in the
Modulation
measurement.
Bit classes
In the GSM system, the speech coder combines the speech information into data blocks with
a length of 260 bits (full rate version 1), the so-called frames. Within one speech frame, the
bits are divided into bit classes:
•
The 78
class II bits
have no error protection which is why they quickly produce
transmission errors.
•
The 132
class Ib bits
are partly protected against errors during channel coding (by added
guard bits).
The 50 most important
class Ia bits
are well protected. The mobile phone recognizes
erroneous
class Ia bits
and clears the complete frame if no correction is possible
Definition of
measured
quantities
The Bit Error Rate
(BER)
is the ratio of erroneous bits to the total number of
transferred bits in percent (also referred to as samples in the operating mode
BER
).
The CMU outputs the bit error rate according to bit classes:
BER = Erroneous bits / total number of bits * 100%
BER II = Erroneous class II bits / total number of class II bits * 100%
BER Ib = Erroneous class Ib bits / total number of class Ib bits * 100%
The Frame Erasure Rate
(FER)
is the ratio of frames recognized to be erroneous
and erased by the mobile to the total number of transferred frames in percent:
FER = Erroneous frames / total number of frames * 100%
The Residual Bit Error Rate
(RBER)
characterizes the quality of transmission of the
valid frames (not erased, therefore residual). It corresponds to the ratio of the
erroneous bits to the total number of transferred bits in percent, the numerator and
denominator referring only to the
valid frame (residual frames, RF)
:
RBER II = Erroneous class II bits (RF) / total number of class II bits (RF) * 100%
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