|
Introduction:
Compressed mode, also known as the
Slotted Mode, is needed when making measurements on another frequency
(inter-frequency) or on a different radio technology (inter-RAT). In the
Compressed Mode the transmission and reception are stopped for a short time
and the measurements are performed on other frequency or RAT in that time.
After the time is over the transmission and reception resumes. To make sure
that the data is not lost, the data is compressed in the frame making empty
space where measurements can be performed.
Compressed mode is not necessary.
If the UE has a second receiver it can make measurements on that receiver
while continuing with the transmission/reception on the first receiver. This
does not happen in practise as the cost would go up. The UE capabilities
define whether a UE requires compressed mode in order to monitor cells on
other FDD frequencies and on other modes and radio access technologies. UE
capabilities indicate the need for compressed mode separately for the uplink
and downlink and for each mode, radio access technology and frequency band.
A UE shall support compressed mode
for all cases for which the UE indicates that compressed mode is required. A
UE does not need to support compressed mode for cases for which the UE
indicates that compressed mode is not required. For these cases, the UE shall
support an alternative means of making the measurements. The UE shall support
one single measurement purpose for one transmission gap pattern sequence. The
measurement purpose of the transmission gap pattern sequence is signaled by
higher layers.
The figure above gives an idea of
how the frame is compressed for performing measurements. In compressed
frames, TGL slots from N first to N last are not used for transmission of
data. As illustrated in figure, the instantaneous transmit power is increased
in the compressed frame in order to keep the quality (BER, FER, etc.)
unaffected by the reduced processing gain. The amount of power increase
depends on the transmission time reduction method. What frames are
compressed, are decided by the network. When in compressed mode, compressed
frames can occur periodically, as illustrated in figure, or requested on
demand. The rate and type of compressed frames is variable and depends on the
environment and the measurement requirements.
Parameterisation of the compressed
mode [3]
In response to a request from
higher layers, the UTRAN shall signal to the UE the compressed mode
parameters.
A transmission gap pattern
sequence consists of consecutive occurrences of transmission gap pattern 1,
where transmission gap pattern 1 consists of one or two transmission gaps.
See figure below.
The following parameters
characterise a transmission gap pattern:
- TGSN (Transmission Gap
Starting Slot Number): A transmission gap pattern begins in a radio
frame, henceforward called first radio frame of the transmission gap
pattern, containing at least one transmission gap slot. TGSN is the slot
number of the first transmission gap slot within the first radio frame
of the transmission gap pattern;
- TGL1 (Transmission Gap Length
1): This is the duration of the first transmission gap within the
transmission gap pattern, expressed in number of slots;
- TGL2 (Transmission Gap Length
2): This is the duration of the second transmission gap within the
transmission gap pattern, expressed in number of slots. If this
parameter is not explicitly set by higher layers, then TGL2 = TGL1;
- TGD (Transmission Gap start
Distance): This is the duration between the starting slots of two
consecutive transmission gaps within a transmission gap pattern,
expressed in number of slots. The resulting position of the second
transmission gap within its radio frame(s) shall comply with the
limitations of [1]. If this parameter is not set by higher layers, then
there is only one transmission gap in the transmission gap pattern;
- TGPL1 (Transmission Gap
Pattern Length): This is the duration of transmission gap pattern 1,
expressed in number of frames;
The following parameters control
the transmission gap pattern sequence start and repetition:
- TGPRC (Transmission Gap
Pattern Repetition Count): This is the number of transmission gap
patterns within the transmission gap pattern sequence;
- TGCFN (Transmission Gap
Connection Frame Number): This is the CFN of the first radio frame of
the first pattern 1 within the transmission gap pattern sequence.
In addition to the parameters
defining the positions of transmission gaps, each transmission gap pattern
sequence is characterised by:
- UL/DL compressed mode
selection: This parameter specifies whether compressed mode is used in
UL only, DL only or both UL and DL;
- UL compressed mode method:
The methods for generating the uplink compressed mode gap are spreading
factor division by two or higher layer scheduling and are described in
[1];
- DL compressed mode method:
The methods for generating the downlink compressed mode gap are
spreading factor division by two or higher layer scheduling and are
described in [1];
- downlink frame type: This
parameter defines if frame structure type 'A' or 'B' shall be used in
downlink compressed mode. The frame structures are defined in [1];
- scrambling code change: This
parameter indicates whether the alternative scrambling code is used for
compressed mode method 'SF/2'. Alternative scrambling codes are
described in [4];
- RPP: Recovery Period Power
control mode specifies the uplink power control algorithm applied during
recovery period after each transmission gap in compressed mode. RPP can
take 2 values (0 or 1). The different power control modes are described
in [5];
- ITP: Initial Transmit Power
mode selects the uplink power control method to calculate the initial
transmit power after the gap. ITP can take two values (0 or 1) and is
described in [5].
The UE shall support simultaneous
compressed mode pattern sequences which can be used for different
measurements. The following measurement purposes can be signalled from higher
layers:
- FDD
- TDD
- GSM carrier RSSI measurement
- Initial BSIC identification
- BSIC re-confirmation.
The UE shall support one
compressed mode pattern sequence for each measurement purpose while operating
in FDD mode, assuming the UE needs compressed mode to perform the respective measurement.
In case the UE supports several of the measurement purposes, it shall support
in parallel one compressed mode pattern sequence for each supported
measurement purpose where the UE needs compressed mode to perform the
measurement. The capability of the UE to operate in compressed mode in uplink
and downlink is given from the UE capabilities.
The GSM measurements Initial BSIC
identification and BSIC re-confirmation are defined in [6].
Higher layers will ensure that the
compressed mode gaps do not overlap and are not scheduled to overlap the same
frame. The behaviour when an overlap occurs is described in [7]. UE is not
required to support two compressed mode gaps in a frame.
In all cases, higher layers have
control of individual UE parameters. Any pattern sequence can be stopped on
higher layers' command.
The parameters TGSN, TGL1, TGL2,
TGD, TGPL1, TGPRC and TGCFN shall all be integers.
Different Methods of Frame
Compression
There are three different methods
through which the frame compression can be achieved
- Puncturing: The symbol rate can be
reduced by puncturing the data bits. This method is practical only for
short TGLs. This is because there is an upper limit to the amount of
data that can be punctured and then receovered at the other end. The
advantage is that it is a simple method and there is no need to change
the speading code as in the other method. This method has been removed
from the latest version of the specifications.
- Changing the Spreading Factor
(SF): In this method the frame in
which compression is to be carried out, SF is reduced by 2 so the same
amount of data can be transmitted in half the time and the remaining
time measurements can be done. This is the most popular method used in
practice as its very straightforward.
- Higher Layer
scheduling: The
higher layers are aware of the compressed mode schedule, so they could
lower the data rate in the frame measurements need to be done thus
avoiding the need for new spreading factor and new channelisation codes.
Higher layers sets restrictions so that only a subset of the allowed
TFCs are used in a compressed frame. The maximum number of bits that
will be delivered to the physical layer during the compressed radio
frame is then known and a transmission gap can be generated. Note that
in the downlink, the TFCI field is expanded on the expense of the data
fields and this shall be taken into account by higher layers when
setting the restrictions on the TFCs. Compressed mode by higher layer
scheduling shall not be used with fixed starting positions of the TrCHs
in the radio frame.
Frame structure in the uplink [1]
The frame structure for uplink
compressed frames is illustrated in figure below:
Frame structure types in the
downlink [1]
There are two different types of
frame structures defined for downlink compressed frames. Type A maximises the
transmission gap length and type B is optimised for power control. The frame
structure type A or B is set by higher layers independent from the downlink
slot format type A or B.
- With frame structure of type
A, the pilot field of the last slot in the transmission gap is
transmitted. Transmission is turned off during the rest of the
transmission gap (figure a). In case the length of the pilot field is 2
bits and STTD is used on the radio link, the pilot bits in the last slot
of the transmission gap shall be STTD encoded assuming DTX indicators as
the two last bits in the Data2 field.
- With frame structure of type
B, the TPC field of the first slot in the transmission gap and the pilot
field of the last slot in the transmission gap is transmitted.
Transmission is turned off during the rest of the transmission gap
(figure b). In case the length of the pilot field is 2 bits and STTD is
used on the radio link, the pilot bits in the last slot of the
transmission gap shall be STTD encoded assuming DTX indicators as the
two last bits of the Data2 field. Similarly, the TPC bits in the first
slot of the transmission gap shall be STTD encoded assuming DTX indicators
as the two last bits in the Data1 field.
(a) Frame structure type A
(b) Frame structure type B
Transmission gap position [1]
(a) Transmission gap position
(b) Transmission gap positions with different Nfirst
Transmission gaps can be placed at
different positions as shown in figures a and b (above) for each purpose such
as interfrequency power measurement, acquisition of control channel of other
system/carrier, and actual handover operation.
When using single frame method,
the transmission gap is located within the compressed frame depending on the
transmission gap length (TGL) as shown in figure a(1). When using double
frame method, the transmission gap is located on the center of two connected
frames as shown in figure a(2).
Parameters of the transmission gap
positions are calculated as follows.
TGL is the number of consecutive
idle slots during the compressed mode transmission gap:
TGL = 3, 4, 5, 7, 10,14
Nfirst specifies
the starting slot of the consecutive idle slots,
Nfirst =
0,1,2,3,…,14.
Nlast shows the
number of the final idle slot and is calculated as follows;
If Nfirst +
TGL £ 15, then Nlast = Nfirst +
TGL –1 ( in the same frame ),
If Nfirst + TGL
> 15, then Nlast = (Nfirst + TGL – 1) mod
15 ( in the next frame ).
When the transmission gap spans
two consecutive radio frames, Nfirst and TGL must be chosen so that at least
8 slots in each radio frame are transmitted.
References:
|
