Page 160 - From GMS to LTE

146 From GSM to LTE-Advanced Pro and 5G

however, is that the interference caused by this can be heard, for example, in radio
receivers which are close by. This can be observed with GSM mobile devices, for exam-
ple, which use only some timeslots on the air interface and thus have to frequently
activate and deactivate the transmitter. In UMTS, only the transmission on the I‐path is
stopped while the DPCCH on the Q‐path continues to be transmitted. This is neces-
sary, as power control and signal quality information need to be sent even if no user data
is transferred in order to maintain the channel. The transmission power is thus only
reduced and not completely switched off. The typical interference of GSM mobile
devices in radio receivers which are close to the device can thus not be observed
anymore with a UMTS mobile device.

3.5 The UMTS Terrestrial Radio Access Network (UTRAN)

3.5.1 Node‐B, Iub Interface, NBAP and FP
The base station, called Node‐B in the 3GPP standards, is responsible for all functions
required for sending and receiving data over the air interface. This includes, as shown
in Section 3.3, channel coding, spreading and despreading of outgoing and incoming
frames, as well as modulation. Furthermore, the Node‐B is also responsible for the
power control of all connections. The Node‐B just receives a transmission quality target
from the RNC for each connection and then decides on its own if it is necessary to
increase or decrease the transmission power in both uplink and downlink directions to
meet the target.
Size and capacity of a Node‐B are variable. Typically, the Node‐B is used in a sector-
ized configuration. This means that the 360‐degree coverage area of a Node‐B is divided
into several independent cells, each covering a certain area. Each cell has its own cell ID,
scrambling code and OVSF tree. Each cell also uses its own directional antennas, which
cover either 180 degrees (2‐sector configuration) or 120 degrees (3‐sector configura-
tion). The capacity of the Iub interface, which connects the Node‐B to an RNC, depends
mainly on the number of sectors of the Node‐B.
While GSM uses only some of the 64 kbit/s timeslots on an E‐1 link to the base station,
UMTS base stations require a much higher bandwidth. To deliver high datarates, Node‐
Bs were initially connected to the RNC with at least one E‐1 connection (2 Mbit/s). If a
Node‐B served several sectors, multiple E‐1 links were required. Owing to the rising
datarates enabled by HSPA, even the aggregation of several E‐1 lines has become insufficient.
E‐1 lines are also quite expensive, which is another limiting factor. In the meantime,
however, high‐speed fiber and microwave Ethernet lines have become available.
Therefore, in most networks, E‐1 connections to Node‐Bs have been replaced by links
based on these technologies, with the IP protocol replacing the ATM transport protocol
that was used over E‐1 lines.
For very dense traffic areas, like streets in a downtown area, a Node‐B microcell can be
an alternative to a sectorized configuration. A microcell is usually equipped with only a
single transceiver that covers only a very small area, for example, several dozens of meters
of a street. As the necessary transmission power for such a small coverage area is very
low, most network vendors have specialized micro Node‐Bs with very compact dimensions.
Usually, these micro Node‐Bs are not much bigger than a PC workstation.