Rohde & Schwarz CMU-K20, Руководство по эксплуатации

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Signalling: Receiver Quality Measurements
CMU-K20/K21/K22/K23/K24
1115.6088.12 4.122 E-12
Main Service Traffic Mode /
Coding Scheme CMU Signalling
State Signalling Channel /
Service Selection Receiver Quality
Meas. Mode /
Application Results
available
Packet Data
(Option CMU-
K42, GPRS) CS1 to CS4 TBF Established BLER BLER BLER
Packet Data
(Option CMU-
K43, EGPRS) MCS1 to MCS9 TBF Established BLER BLER
BLER
1
Principle of the
measurement
Like any other
Receiver Quality
measurement, the PDTCH BER test is based on the
comparison of the output signal generated by the CMU with the signal received and decoded
by the device under test (mobile station). To this end, the mobile station is set to return the
received signal to the CMU in loopback mode. In the case of packet data channels, the MS
loops back the packet data after demodulation and channel decoding (see BER/DBLER loop
in
Fig. 4-55
on p.
4.120
ff.
)
.
Frame structure
The CMU provides a GPRS or EGPRS signal with a 52-multiframe structure as shown in
Fig.
4-56 below
. Each 52-multiframe contains 12 blocks of 4 consecutive frames (B0 to B12), 2
idle frames (X) and 2 frames used for the Packet Timing Advance Control Channel (X). All
blocks in the signal are coded and modulated with the same coding and puncturing scheme.
The mobile station returns the data bits of the received blocks using the coding and
puncturing scheme signalled via RF connection (GPRS Test Mode B) or set via an external
test interface (reduced signalling). This means that the loopback is done on a block by block
basis. The modulation and coding schemes MCS7, MCS8 and MCS9 carry 2 RLC/MAC
frames which are coded separately.
52 TDMA Frames
B0 B1 B2 X B3 B4 B5 X B6 B7 B8 X B9 B10 B11 X
Fig. 4-56 52-multiframe for PDCH
Block structure
Each GPRS/EGPRS radio block is divided into the header information including the Uplink
State Flag (USF) and the data bits. The CMU uses downlink header with fixed (circuit
switched) or configurable (packet-data) USF and ignores the uplink header received from the
MS. This means that only the data bits of a radio block contribute to the BER and DBLER
calculation. However, in packet-data mode, where the USF can be explicitly set, the CMU is
also capable of evaluating the relative number of blocks with a USF correctly received by the
MS under test. This ratio is referred to as USF BLER.
On the other hand, the MS returns the received data even if the block check sequence
indicates that the block was not decoded correctly. In this case the MS calculates a new
block check sequence for the received data. The same timeslot is used for downlink and
uplink, however, the uplink signal is delayed by 3 timeslot periods.
Difference
between DBLER
and BLER
This Data Block Error Rate is not exactly the BLER that is defined in the GSM
recommendations, because the possibility of an error in the header is not taken into
account. But if the probability for an error in the data field, which depends on the
used coding scheme, is much higher than the probability for an error in the header,
then the calculated Data Block Error Rate is a good approximation to the BLER.
The difference between the BLER defined in GSM 11.10 and the DBLER measured
by the CMU varies from one coding scheme to another. For coding scheme CS4,
where no additional effects due to channel coding occur, the difference is
1 Depending on the hardware configuration, up to 2 or 4 DL timeslots can be measured simultaneously; see
BLER measurement
above.
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