8/12/2019 Node Synchronization

1/4

Node SynchronizationPrecision timing is vital in a UMTS Terrestrial Radio Access Network (UTRAN) and both RNC and RBSrequire a clock source with a precision of at least 0.05ppm9 for wide area base station deployment .

Data communication between RNC and UE through an RBS uses frame transport. A frame correspondsto a certain amount of time, usually 10ms. In the downlink scenario it is the RNC which decides when aframe should leave the radio interface in the RBS and thus be transmitted to the UE, i.e. the framereceiver needs a reference to determine when to forward the frame. This is addressed by the use offrame counters in RNC and RBS. The RNC Frame Number (RFN) is used by the RNC and the Node BFrame Number (BFN) is used by the RBS.

Node Synchronisation is used to handle the phase difference between the RFN and BFN frame counters.The Node Synchronisation mechanism determines the offset between the RFN and BFN counters as wellas measuring the latency of the interface between RNC and RBS (Iub)

Notice the values of for example T1 and T2 in figure 1.2. The RFN has incremented to 4094 just beforethe downlink Node Synchronisation is initiated and the value of T1 is 40941.250 ms. The nodesynchronization control frame is sent by the RNC and received by the Node B (RBS) just after the BFNchanged to 149 and likewise T2 has a value of 1492.500 ms, i.e. a frame corresponds to 10ms.

Transport Channel SynchronizationTransport Channel Synchronisation is the concept of synchronizing frame transport between RNC andRBS. This section describes the transport channel synchronization mechanism valid for all downlinktransport channels, such as the Forward Access Channel (FACH), Dedicated Channels (DCHs), etc.

8/12/2019 Node Synchronization

2/4

In addition to the RFN and BFN the transport channel synchronisation uses two additional frame counters,the Cell System Frame Number (SFN) and the Connection Frame Number (CFN). The SFN is equal toBFN adjusted with T cell. The T cell offset is used within the same Node B to skew differentcells10 in order to avoid colliding Synchronisation Channel bursts. The CFN is the frame counter used fortransport channel synchronisation between UE and UTRAN and relates to the SFN by means of theFrame Offset.

Figure 1.3 illustrates the scenario of a downlink data frame sent from RNC through RBS to UE withsubsequent response [3]. Sending data over a network interface always results in some sort of delaybefore the information is received at the other end. In order to handle latency variations on the Iub theRBS has a Receiving Window which allows for buffering of received frames scheduled for transmissionon the radio interface. The RBS will respond to the RNC with a timing adjustment control frame for eachdata frame received out of the configured Receiving Window.

8/12/2019 Node Synchronization

3/4

Figure 1.4 illustrates the receive window handling for a downlink data frame. A frame can be receivedwith the following status:

Too EarlyThe data frame is received too early to be buffered in the RBS. The boundary between Earlyand Too Early depends on the available buffer space in a particular Node B.

EarlyThe data frame is received before the desired receive window but can be buffered until thescheduled transmission time.

OKThe data frame is received within the Receiving Window.

Late The data frame is received after the desired receive window but still early enough to be processedand sent through the radio interface in time.

Too LateThe data frame is received too late to be transferred. This might be before the scheduledtransmission time but always after the Latest Time Of Arrival (LTOA).

8/12/2019 Node Synchronization

4/4