Page 226 - From GMS to LTE

212 From GSM to LTE-Advanced Pro and 5G

easily adapt and the number of narrowband carriers is simply reduced. Several bandwidths
have been specified for LTE: from 1.25 MHz up to 20 MHz. In practice, channels with a
bandwidth of 10, 15 and 20 MHz are typically used. All LTE devices must support all
bandwidths and which one is used in practice depends on the frequency band and the
amount of spectrum available to a network operator. With a 20 MHz carrier, datarates
beyond 100 Mbit/s can be achieved under very good signal conditions.
Unlike in HSPA, the baseline for LTE devices has been set very high. In addition to the
flexible bandwidth support, all LTE devices have to support Multiple Input Multiple
Output (MIMO) transmissions, a method which allows the base station to transmit
several data streams over the same carrier simultaneously. Under very good signal con-
ditions, the datarates that can be achieved this way are beyond those that can be
achieved with a single‐stream transmission.
In practice the LTE air interface comes in two variants. Most networks around the
world use Frequency Division Duplex (FDD) to separate uplink and downlink transmissions.
In the US and China and to some degree in Europe and other parts of the world, spectrum
for Time Division Duplex (TDD) has also been assigned to network operators. Here, the
uplink and downlink transmissions use the same carrier and are separated in time.
While a TDD mode already exists for UMTS, it has come to market many years after the
FDD version and there are significant differences between the two air interface architectures.
Hence, it has not become very popular. With LTE, both FDD and TDD have been speci-
fied in a single standard and the differences between the two modes are mostly limited
to layers 1 and 2 on the air interface. All higher layers are not affected and a higher reuse
of both hardware and software on the network side is possible. On the mobile device
side, early LTE devices were either FDD or TDD capable. Today, many LTE devices are
both FDD and TDD capable and can thus address market demands of countries or
regions in which both air interface standards are used. The standard even includes
Carrier Aggregation of FDD and TDD channels so network operators that have deployed
both air interfaces can significantly benefit from their TDD spectrum.
The second major change in LTE as compared to previous systems has been the adop-
tion of an all‐Internet Protocol (IP) approach. While UMTS used a traditional circuit‐
switched packet core for voice services, for Short Messaging Service (SMS) and other
services inherited from GSM, LTE relies solely on an IP‐based core network. The single
exception is SMS, which is transported over signaling messages. An all‐IP network
architecture greatly simplifies the design and implementation of the LTE air interface,
the radio network and the core. With LTE, the wireless industry takes the same path as
fixed‐line networks with DSL, fiber and broadband IP over TV cable, where voice
telephony is also transitioned to the IP side. Quality of Service (QoS) mechanisms have
been standardized on all interfaces to ensure that the requirements of voice calls for a
constant delay and bandwidth can still be met when capacity limits are reached. While
from an architectural point of view, this is a significant advance, implementation, in
practice, has proven to be difficult. As a consequence, a significant number of current
LTE networks still use a mechanism referred to as Circuit‐Switched Fallback (CSFB) to
UMTS or GSM for voice calls. Since 2014, however, a growing number of networks and
devices have become Voice over LTE (VoLTE) capable and thus no longer require a
fallback to other radio networks except when a user leaves the LTE coverage area. More
details on CSFB can be found at the end of this chapter while Voice over LTE is dis-
cussed in more detail in Chapter 5.