Page 209 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 195

is called a nonscheduled grant. If the RNC decides that a certain constant bandwidth
and delay time is required for an uplink connection, it can instruct the Node‐B to
reserve a sufficiently large power margin for the required bandwidth. The mobile device
is then free to send data at this speed to the Node‐B without prior bandwidth requests.
If such E‐DCH connections are used, which is again implementation dependent, the
Node‐B has to ensure that even peaks of scheduled E‐DCH connections do not endan-
ger the correct reception of the nonscheduled transmissions.

3.11.4 E‐DCH Mobility
Very high E‐DCH datarates can only be achieved for stationary or low‐mobility sce-
narios owing to the use of low spreading factors and few redundancy bits. Nevertheless,
the E‐DCH concept uses a number of features to also enable high datarates in high‐
speed mobility scenarios. To this end, macro diversity (soft handover) can be used as
shown in Figure 3.45. This means that the uplink data is received by several cells, which
forward the received frames to the RNC. Each cell can then indicate to the mobile
device if the frame has been received correctly and thus, only the frame has to be
repeated if none of the cells was able to decode the frame correctly. This is especially
beneficial for mobility scenarios in which reception levels change quickly because of
obstacles suddenly appearing in between the mobile device and one of the cells of the
Active Set, as shown earlier. Furthermore, the use of soft handover ensures that no
interruptions in the uplink occur while the user is moving through the network with the
mobile device.
For capacity reasons, network operators usually use several 5 MHz carriers in a cell
today. If a device moves to a different location that is served by a cell with only a single
carrier, a soft handover procedure is used if the previous carrier and the new carrier are
on the same frequency. If the device is served on a carrier frequency that is not present
in the new cell, an interfrequency hard handover is required as the carrier of the new
cell cannot be decoded at the same time as the carriers in the current Active Set that
transmit on a different frequency.

3.11.5 E‐DCH‐Capable Devices
E‐DCH‐capable devices once again require increased processing power and memory
capabilities compared to HSDPA devices to sustain the high datarates offered by the
system in both downlink (HSDPA) and uplink (HSUPA) directions. To benefit from the
evolution of mobile device hardware, the standard defines a number of mobile device
categories that limit the maximum number of spreading codes that can be used for an
E‐DCH and their maximum length. This limits the maximum speed that can be achieved
with the mobile device in the uplink direction. Table 3.7 shows a number of typical E‐
DCH mobile device categories and their maximum transmission speeds under ideal
transmission conditions. The highest number of simultaneous spreading codes an E‐
DCH mobile device can use is four, with two codes having a spreading factor of two and
two codes having a spreading factor of four. The maximum user datarates are slightly
lower than the listed transmission speeds as the transport block also includes the frame
headers of different protocol layers. Under less ideal conditions, the mobile device
might not have enough power to transmit using the maximum number of codes allowed
and might also use a more robust channel coding method that uses smaller transport