Page 179 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 165

the timing advance is not possible. This is because while a mobile device is in soft handover
state, all Node‐Bs of the Active Set receive the same data stream from the mobile device.
The distance between the mobile device and each Node‐B is different though, and thus each
Node‐B receives the data stream at a slightly different time. For the mobile device, it is not
possible to control this by starting to send data earlier, as it only sends one data stream in the
uplink direction for all Node‐Bs. Fortunately, it is not necessary to control the timing
advance in UMTS as all active subscribers transmit simultaneously. As no timeslots are
used, no collisions can occur between the different subscribers. To ensure the orthogonal
nature of the channelization codes of the different subscribers it would be necessary, how-
ever, to receive the data streams of all mobile devices synchronously. As this is not possible,
an additional scrambling code is used for each subscriber, which is multiplied by the data
that has already been treated with the channelization code. This decouples the different
subscribers and thus a time difference in the arrival of the different signals can be tolerated.
The time difference of the multiple copies of a user’s signal is very small compared to
the length of a frame. While the transmission time of a frame is 10, 20, 40 or 80 milliseconds,
the delay experienced on the air interface of several Node‐Bs is less than 0.1
milliseconds even if the distances vary by 30 km. Thus, the timing difference of the
frames on the Iub interface is negligible.
If a subscriber continues to move away from the cell in which the radio bearer was
initially established, there will be a point at which not a single Node‐B of the S‐RNC is
part of the transmission chain anymore. Figure 3.29 shows such a scenario. As this state
is a waste of radio network resources, the S‐RNC can request a routing change from the
MSC and the SGSN on the Iu(cs)/Iu(ps) interface. This procedure is called a Serving

All user data is sent via two RNCs After the SRNS relocation
and the Iur interface only a single RNC is used

SGSN SGSN

Iu(ps)
Iu(ps)

RNC 1 RNC 2 RNC 1 RNC 2
Iur Iur

Iub Iub

Node-B 1 Node-B 2 Node-B 3 Node-B 1 Node-B 2 Node-B 3

Figure 3.29 SRNS relocation procedure.