Page 229 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 215

4.2.1 LTE Mobile Devices and the LTE Uu Interface
In the LTE specifications, as in UMTS, the mobile device is referred to as the User
Equipment (UE). In 3GPP Release 8, UE classes 1 to 5 have been defined in TS 36.306
[2]. In later 3GPP releases additional UE classes have been defined which are required
for the higher speeds offered by carrier aggregation. Table 4.1 shows a selection of typi-
cal UE device classes in use at the time of publication. Unlike in HSPA where devices
support a wide range of different modulation and coding schemes because the standard
evolved over time, all LTE UEs support the very fast 64‐QAM (Quadrature Amplitude
Modulation) in the downlink direction and antenna diversity.
In the uplink direction, only the slower but more reliable 16‐QAM support is required
for terminal classes 1–4, which results in a maximum theoretical speed of 50 Mbit/s in
the uplink direction in a 20 MHz carrier. Some of the higher device classes also require
the support of 64‐QAM, which enables a theoretical top speed of 75 Mbit/s in a 20 MHz
carrier.
Except for UE category 1, which was never introduced in practice for smartphones
and other end‐user devices, all mobile devices have to support MIMO transmission in
the downlink direction. With this advanced transmission scheme, several data streams
are transmitted on the same carrier frequency from multiple antennas from the base
station to multiple antennas in the mobile device. If the signals reach the receiver via
different paths, for example, because of reflections at different angles from objects
caused by the spatial separation of the transmitter and receiver antennas, the receiver
can distinguish between the different transmissions and recreate the original data
streams. The number of transmit and receive antennas dictates the number of data
streams that can be sent in parallel. Most LTE networks and devices today use 2 × 2
MIMO, that is, two transmit and two receive antennas. Up to this day it remains a chal-
lenge to fit four independent antennas in a small mobile device as the built‐in antennas
have to support many frequency bands and additional network technologies such as
GSM and UMTS in the same or in different frequency bands. Despite this challenge the
first networks and devices have appeared that support 4 × 4 MIMO in practice.
In practice, peak datarates observed are between 100 and 150 Mbit/s under ideal con-
ditions per 20 MHz carrier with 2 × 2 MIMO. As high‐end devices and networks sup-
port aggregation of several carriers, individual device throughput under ideal conditions
can be much higher. In practice, average speeds per carrier are usually lower because of

Table 4.1 LTE UE categories.

Category 3 4 6 9 12

Maximum downlink datarate with carrier aggregation 100 150 300 450 600
Typical number of aggregated carriers in downlink 1 1 2 3 4
Maximum uplink datarate 50 50 50 50 100
Typical number of aggregated carriers in uplink 1 1 1 1 2
Number of receive antennas 2 2 2 2 2
Number of MIMO downlink streams 2 2 2 2 2
Support for 64‐QAM in the uplink direction No No No No No