Page 261 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 247

UE–MME
signaling messages

NAS Application
• Encapsulation of higher layer
RRC IP User data protocols
UE–eNodeB • Ciphering
signaling PDCP • Header compression
• Lossless handover support
RLC
• Segmentation
MAC • Reassembly
• ARQ
• Multiplexing of radio bearers Physical layer
• QoS management
• HARQ
• Fast signaling
(e.g., bandwidth requests / grants)

Figure 4.15 Air interface protocol stack and main functions.

The first unifying protocol layer to transport IP, RRC and NAS signaling messages is
the PDCP layer. As discussed in the previous section, it is responsible for encapsulating
IP packets and signaling messages, for ciphering, header compression and lossless
handover support. One layer below is the RLC. It is responsible for segmentation and
reassembly of higher‐layer packets to adapt them to a packet size that can be sent over
the air interface. Further, it is responsible for detecting and retransmitting lost packets
(ARQ). Just above the physical layer is the MAC. It multiplexes data from different radio
bearers and ensures QoS by instructing the RLC layer about the number and the size of
packets to be provided. In addition, the MAC layer is responsible for the HARQ packet
retransmission functionality. And finally, the MAC header provides fields for address-
ing individual mobile devices and for functionalities such as bandwidth requests and
grants, power management and timing advance control.

4.4 TD‐LTE Air Interface

Most initial LTE deployments around the world used FDD (frequency division duplex),
which means that data is transmitted and received simultaneously on two separate
channels. In addition, a TDD (time division duplex) variant of the LTE air interface has
been specified, which is referred to as TD‐LTE or TDD‐LTE. While the majority of LTE
deployments are in FDD spectrum today, spectrum assigned to TDD operation is now
also widely in use in China, the US and to some degree in Europe. Apart from using a
single channel for both uplink and downlink transmissions, most other parameters of
the air interface are identical to the FDD implementation described earlier. Some differ-
ences exist, however, which are described in this section.
As TD‐LTE only uses a single channel, a transmission gap is required when switching
from reception (downlink) to transmission (uplink) mode. This is also referred to as the
guard period and its length is determined by the time it takes to switch the operation