Page 227 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 213

Also, all interfaces between network nodes in LTE are now based on IP, including the
backhaul connection from the radio base stations. Again, this is a great simplification
compared to earlier technologies that were initially based on E‐1, ATM and frame relay
links, with most of them being narrowband and expensive. The standard leaves the
choice of protocols to be used below the IP layer open, which means that the physical
infrastructure becomes completely transparent and interchangeable. To further sim-
plify the network architecture and to reduce user data delay, fewer logical and physical
network components have been defined in LTE. In practice, this has resulted in round‐
trip delay times of less than 25–30 milliseconds. Optimized signaling for connection
establishment and other air interface and mobility management procedures have fur-
ther improved the user experience. The time required to connect to the network is in
the range of only a few hundred milliseconds and power‐saving states can now be
entered and exited very quickly.
To be universal, LTE‐capable devices must also support GSM, GPRS, EDGE and
UMTS. On the network side, interfaces and protocols have been put in place so that
data sessions can be moved seamlessly between GSM, UMTS and LTE when the user
roams in and out of areas covered by different air interface technologies. While in the
early years of deployment, LTE core network and access network nodes were often
deployed independent of the already existing GSM and UMTS network infrastructure,
integrated GSM, UMTS and LTE nodes are now used in practice.
LTE is the successor technology not only of UMTS but also of CDMA2000, mostly
used in the Americas. To enable seamless roaming between Code Division Multiple
Access (CDMA) and LTE, interfaces between the two core networks have been
specified. In practice, the user can thus also roam between these two types of access
networks while maintaining their IP address and hence all established communica-
tion sessions.
LTE, as specified in 3GPP Release 8, was a new beginning and also a foundation for
further enhancements. With subsequent 3GPP releases, new ideas to further push the
limits were specified as part of LTE‐Advanced and LTE‐Advanced Pro, to comply with
the International Telecommunication Union’s (ITU) IMT‐Advanced requirements for
4G wireless networks [1]. One major enhancement is Carrier Aggregation (CA), to bun-
dle up to five carriers of up to 20 MHz each in the same or different frequency bands to
reach datarates of several hundred megabits per second. At the time of publication,
aggregation of two and three downlink carriers has become the norm in many networks
and 3GPP has extended the standards to allow carrier aggregation beyond five carriers
in the future.
On the opposite side of the throughput scale, an emerging field of interest is small,
very power‐efficient Internet of Things (IoT) devices. Such devices are often only
equipped with small batteries that cannot easily be replaced or recharged. Also such
devices only communicate sporadically. Hence, their network requirements in terms of
efficiency and power consumption are significantly different from smartphones and
other devices requiring high throughput speeds. As a consequence, 3GPP has added a
number of enhancements to the standard, most importantly the Narrow‐Band IoT (NB‐
IoT) radio network that is based on LTE.
This chapter is structured as follows. First, the general network architecture and
interfaces of LTE are described. Next, the air interface is described for both FDD and
TDD systems. This is followed by a description of how user data is scheduled on the air