Page 253 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 239

later the signal arrives. As in GSM, the network then informs the mobile device to adjust
its transmission time. The farther the mobile device is from the base station, the earlier
it has to start its transmissions for the signal to reach the base station at the right time.
This is also referred to as the timing advance and the system can compensate transmis-
sion distances of up to 100 km.
When a mobile device has been granted resources, that is, it has been assigned RBs on
the PUSCH, the shared channel is used for transmitting user data and also for transmit-
ting lower‐layer signaling data, which are required to keep the uplink connection in
place and to optimize the data transmission over it. The following lower‐layer informa-
tion is sent alongside user data packets:
The Channel Quality Indicator (CQI) that the eNode‐B uses to adapt the modulation
●
and coding scheme for the downlink direction.
MIMO‐related parameters (see Section 4.3.9).
●
HARQ acknowledgments so that the network can quickly retransmit faulty packets
●
(see Section 4.3.10).
In some cases, lower‐layer signaling information has to be transferred in the uplink
direction, while no resources are assigned to a mobile device on the uplink shared chan-
nel. In this case, the mobile can send its signaling data such as HARQ acknowledgments
and scheduling requests on the Physical Uplink Control Channel (PUCCH), which uses
the first and the last RBs of a carrier. In other words, the PUCCH is located at the edges
of the transmission channel.
As in the downlink direction, some information is required for the receiver, in this
case the eNode‐B, to estimate the uplink channel characteristics per mobile device.
This is done in two ways. During uplink transmissions, Demodulation Reference
Signals (DRS) are embedded in all RBs for which a mobile has received uplink sched-
uling grants. The symbols use the fourth symbol row of the RB for the purpose. As
the eNode‐B knows the content of the DRS symbols, it can estimate the way the data
transmission on each subcarrier is altered by the transmission path. As the same
frequency is used by all mobile devices, independent of the eNode‐B with which they
communicate, different phase shifts are used for the DRS symbols depending on the
eNode‐B for which the transmission is intended. This way, the eNode‐B can filter out
transmissions intended for other eNode‐Bs, which are, from its point of view, noise.
A second optional reference is the Sounding Reference Signal (SRS), which allows the
network to estimate the uplink channel quality in different parts of the overall channel
for each mobile device. As the uplink quality is not necessarily homogeneous for a
mobile device over the complete channel, this can help the scheduler to select the best
RBs for each mobile device. When activated by the network, mobile devices transmit
the SRS in every last symbol of a configured subframe. The bandwidth and number of
SRS stripes for a UE can be chosen by the network. Further, the SRS interval can be
configured between 2 and 160 milliseconds.

4.3.9 MIMO Transmission

In addition to higher‐order modulation schemes such as 64‐QAM and 256‐QAM,
which encode 6 bits or 8 bits respectively in a single transmission step, 3GPP Release 8
specifies and requires the use of multi‐antenna techniques, also referred to as MIMO,
in the downlink direction. While this functionality has also been specified for HSPA in