It has already been explained how important reassembly and filter procedures are to ensure precise performance measurement. The special conditions of UTRAN FDD mode require another look at them to investigate some special cases of data stream combining that are necessary if identical data is transmitted via several radio and UTRAN interfaces.
All previous statements regarding RLC reassembly are valid as long as the UE is in CELL_DCH mode and only one radio link belongs to the active set as shown in the Figure 1.28.
This scenario is characterised by the fact that bidirectional transfer of RLC transport blocks on a single Uu/Iub interface connection is enabled. Radio connection is served by a single cell and each transport block sent via this single cell can be directly monitored without a special combining/filtering mechanism on Iub. If no softer or soft handover or transport channel type switching is performed this situation remains unchanged until the end of the call.
Figure 1.28 shows an example for the transport of an RRC measurement report message sent by UE to SRNC. The contents determine the length of the message. It depends on the
Figure 1.27 Protocol versions and releases by 3GPP
Figure 1.28 RLC reassembly – UE in CELL_DCH, one radio link in active set
total length of the RRC message how many RLC transport blocks are necessary to transport the whole RRC message. In the example it is assumed that the number of transport blocks necessary for transmission of this message is three. These three RLC transport blocks – each of which has its unique sequence number – are sent in sequence, first on the radio interface and then on the Iub physical transport bearer (AAL2 SVC). Only the RLC transport blocks actually exist on both interfaces and the original RRC measurement report message is not reassembled before transport blocks have been received by SRNC. However, the protocol analysers that capture these transport blocks on Iub – no matter if these analysers are equipped with performance measurement applications or not – need to reassemble the original RRC message in the same way as it is done by SRNC. By the way, this is true for both uplink and downlink RRC messages.
Now assume that the call goes into a soft handover situation, in which a second cell belonging to a different Node B provides a second radio link that is part of the active set for reasons explained in Figure 1.7.
Imagine that the same RRC measurement report message is sent by the UE again. Now – as shown in Figure 1.29 – the same three RLC transport blocks containing segments of this message are sent twice. Each transport block is sent simultaneously on two different radio links. Since two different Node Bs are involved in this scenario these identical transport blocks can also be monitored on two different Iub interfaces – except there is one – and only one! – RRC measurement report that is sent in active connection between UE and SRNC.
Two different radio links (ẳtwo different cells) have been used for the transport of identical RLC transport blocks. This means, in the case of different radio transmission conditions, it must be expected that on individual physical channels of each cell individual bit errors will occur. The uplink bit error rate is a radio interface KPI that is mostly estimated from the error rate of pilot bits sent on the uplink dedicated physical control channel. Node B then encodes this measured bit error rate as a quality estimate value, which is appended to
Figure 1.29 UE in CELL_DCH – two radio links in active set – soft handover
every transport block set sent by the FP in the uplink direction. If two identical RLC blocks are received by SRNC the radio network controller decides from the quality estimate which of the two identical blocks is assumed to have fewer bit errors and is hence used for reassembly. The other block is discarded. If a transport block has been protected by a cyclic redundancy check and a CRC error has been indicated for this block, SRNC usually discards this block anyway, no matter which bit error rate has been measured. Instead of the quality estimate other uplink radio quality parameters can be used to determine which of the two or more identical transport blocks is expected to be the best one in the uplink direction. The signal-to-interference ratio (SIR) reported by NBAP for each uplink dedicated physical channel is a good alternative decision maker, but there is no explicit definition by 3GPP of how the quality of uplink frames is acquired.
This procedure itself is officially called selection combining (3GPP 25.401), but it is also known as macro-diversity combining. It applies to all uplink data transmissions in the case of soft handover situations. In downlink there is also a combination of data streams on the UE side, but this maximum ratio combining follows different rules and is explained in Figure 1.30. Macro-diversity is a feature that is nowadays only available in SRNC as far as the author knows (3GPP offers the possibility to implement selection combining in DRNC also) and it only applies to uplink data streams, no matter if they carry RRC signalling or payload (voice/data).
How radio link combining is realised on the downlink is seen when analysing a special case of soft handover that is called softer handover. In this case cells that provide radio links belonging to the same active set are controlled by the same Node B as shown in Figure 1.30.
In softer handover the uplink and downlink radio signals are combined behind the rake receiver antenna of UE or Node B using maximum ratio combining. The rake receiver is a special antenna design that allows the device to send/receive the same data simultaneously on several radio channels. It is now possible to add received signal levels coming from different radio links, when it is known that the received data is identical. The higher the
Figure 1.30 UE in CELL_DCH – two radio links in active set – softer handover
resulting signal level the better signal-to-noise ratio can be achieved. Based on this effect, it might be possible that two different radio links, which would be not good enough for signal transmission in stand-alone mode, become good enough to be used in softer handover due to this so-called micro-diversity combining (a synonym for maximum ratio combining of uplink data streams in Node B).
However, since signals are already merged right behind receiving antennas there is just one uplink/data stream monitored on Iub while there are two independent data streams on the radio interface. The quality estimate attached to uplink transport blocks also represents a combined value, an average of bit error rate measured on two (or more) radio links, and as a result it is not possible to find out on which radio link of the active set more bit errors occurred. This means that in this scenario it is no longer possible to measure on Iub which of the radio links of active connection has been better or worse. This limitation is quite important, because it disables the correlation of some performance-related data monitored on Iub to a single cell (example: uplink bit error rate).
In the future it is planned to introduce another feature that has an impact on RLC reassembly processes using downlink data transmission. This feature is shown in Figure 1.31 and is called site selection diversity transmission (SSDT) and it reduces the overhead of downlink data transfer due to softer and soft handover scenarios. Basically it is defined that with SSDT only the best of all radio links in the active set will be used for transmission of downlink data.
While in soft handover there are still identical uplink RLC frames (containing the same RRC message) sent on the different Iub interfaces in uplink direction there is onlyoneIub interface/oneradio link used for downlink data transmission. The single downlink radio link is the one with the best quality. The quality indicator in this case is the feedback information (FBI) transmitted on the uplink dedicated physical control channel (DPCCH).
An indicator in the NBAP Radio Link Setup Request message shows if SSDT can be enabled for a defined radio link or not. While the feature is currently not used yet
Figure 1.31 UE in soft handover, SSDT enabled
(May 2006) it is expected to become important for Iub transport resource optimisation in later deployment stages of UMTS networks.
This section ends by having a final look at the CELL_FACH state illustrated in Figure 1.32. Here neither macro-diversity nor micro-diversity combining applies, but uplink and downlink transport blocks are sent on different Iub physical transport bearers (different VPI/VCI/CID).
This is not a problem for protocol analysers except for the fact that the used transport channel can be changed if the UE is ordered to go back to CELL_DCH state right in the middle of transmitting RLC transport blocks belonging to the same higher layer message.
Therefore it is possible that the first RLC transport block of an RRC message transmitted in the uplink direction is sent on RACH while the second transport block that carries a fragment of the same RRC message is sent on DCH (and hence, on different Iub VPI/VCI/
CID again). Switching of the transport channel is indicated if the RRC Cell Update Complete message shown in the lower part of Figure 1.32 contains the RRC state indicatorẳ‘CELL_DCH’.