Page 246 - From GMS to LTE

232 From GSM to LTE-Advanced Pro and 5G

other signal. In such a setup the amplitude and phase of the resulting signal can then be
controlled by changing only the two amplitudes of the two input signals.
The amplitude of one of the two input sine waves represents the I‐component of the
signal, the other one the Q‐component. In other words, the I‐ and Q‐amplitudes from
Figure 4.7 for one of the points are then used during each transmission step to set the
amplitude of each signal.
On the receiver side the same operation is performed in reverse, i.e. the amplitude
and phase encoded in a single input signal is given to two processing chains and each
recovers one of the amplitudes that was used at the transmitter side.
The I and Q signals are also referred to as the ‘baseband’ signals. This is because the
I/Q values do not change at the frequency of the carrier wave, for example, at 2600 MHz
or 2.6 million times a second, but only once for every transmission step. Also, the I/Q
values are independent from the carrier frequency, which means that the same values
are applied to modulate the final carrier wave independently of whether the transmis-
sion takes place at 800 MHz or 2600 MHz. As will be described in more detail below
each transmission step takes 66.667 microseconds and hence a new I/Q value pair for
each subcarrier has to be generated approximately 15,000 times a second. As the I/Q
value pairs are generated by the digital part of the modem chip, the GSM/UMTS/LTE
modem is also referred to as the ‘baseband processor’ and is separate from the ‘applica-
tion processor’ which runs a device’s user operating system, such as Android.
Data transmission in LTE is organized as follows: The smallest transmission unit on
each subcarrier is a single transmission step with a length of 66.667 microseconds. A
transmission step is also referred to as a symbol and several bits can be transmitted per
symbol depending on the modulation scheme. If radio conditions are excellent, 64‐
6
QAM is used to transfer 6 bits (2 = 64) per symbol. 3GPP Release 12 has added 256‐
8
QAM modulation that encodes 8 bits (2 = 256) per symbol, potentially for use in
small‐cell deployment scenarios where devices can be close to the transmitter and
hence have a very good signal‐to‐noise ratio. Under less ideal signal conditions, 16‐
QAM or QPSK (Quadrature Phase Shift Keying) modulation is used to transfer 4 or 2
bits per symbol. A symbol is also referred to as a Resource Element (RE).
As the overhead involved in assigning each individual symbol to a certain user or to a
certain purpose would be too great, the symbols are grouped together in a number of
different steps as shown in Figure 4.8. First, seven consecutive symbols on 12 subcarri-
ers are grouped into a Resource Block (RB). A RB occupies exactly one slot with a duration
of 0.5 milliseconds.
Two slots form a subframe with a duration of 1 millisecond. A subframe represents
the LTE scheduling time, which means that at each millisecond the eNode‐B decides
which users are to be scheduled and which RBs are assigned to which user. The number
of parallel RBs in each subframe depends on the system bandwidth. If a 10 MHz carrier
is used, 600 subcarriers are available (see Table 4.3). As an RB bundles 12 subcarriers, a
total of 50 RBs can be scheduled for one or more users per subframe.
The network has two options for transmitting a subframe. The first option is Localized
Virtual Resource Blocks (LVRBs), which are transmitted in a coherent group as shown
in Figure 4.8. In this transmit mode, the eNode‐B requires a narrowband channel feed-
back from the mobile device to schedule the RBs on subcarriers that do not suffer from
narrowband fading. The second option is to transfer data in Distributed Virtual
Resource Blocks (DVRBs), where the symbols that form a block are scattered over the