Page 207 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 193

procedure, the mobile device is also informed which cell of the Active Set will be the
serving E‐DCH cell. The serving cell is defined as being the cell over which the network
later controls the bandwidth allocations to the mobile device.
Once the E‐DCH has been successfully established, the mobile device has to request
a bandwidth allocation from the Node‐B. This is done by sending a message via the
E‐DCH, even though no bandwidth has so far been allocated. The bandwidth request
contains the following information for the Node‐B:
UE estimation of the available transmit power after subtracting the transmit power
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already necessary for the DPCCH and other currently active DCHs;
indication of the priority level of the highest‐priority logical channel currently established
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with the network for use via the E‐DCH;
buffer status for the highest‐priority logical channel;
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total buffer status (taking into account buffers for lower‐priority logical channels).
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Once the Node‐B receives the bandwidth request, it takes the mobile device’s
information into account together with its own information about the current noise level,
the bandwidth requirements of other mobile devices in the cell and the priority informa-
tion for the subscriber that it has received from the RNC when the E‐DCH was initially
established. The Node‐B then issues an absolute grant, also called a scheduling grant, via
the E‐AGCH, which contains information about the maximum power ratio the mobile
can use between the E‐DPDCH and the E‐DPCCH. As the mobile has to send the E‐
DPCCH with enough power for it to be correctly received at the Node‐B, the maximum
power ratio between the two channels implicitly limits the maximum power that can be
used for the E‐DPDCH. This in turn limits the number of choices the mobile device can
make from the TFC set that was initially assigned by the RNC. Therefore, as some TFCs
can no longer be selected, the overall speed in the uplink direction is implicitly limited.
Furthermore, an absolute grant can be addressed to a single mobile device only or to
several mobile devices simultaneously. If the network wants to address several mobile
devices at once, it has to issue the same Enhanced Radio Network Temporary ID (E‐
RNTI) to all group members when their E‐DCH is established. This approach mini-
mizes signaling when the network wants to schedule mobile devices in the code domain.
Another way to dynamically increase or decrease a grant given to a mobile device or a
group of mobile devices is the use of relative grants, which are issued via the optional
Relative Grant Channel (E‐RGCH). These grants are called relative grants because they
can increase or decrease the current power level of the mobile step by step with an inter-
val of one TTI or slower. Thus, the network is quickly able to control the power level and,
therefore, implicitly control the speed of the connection every 2 or 10 milliseconds.
Relative grants can also be used by all cells of the Active Set. This allows cells to influence
the noise level of E‐DCH connections currently controlled by another cell to protect
themselves from too much noise being generated in neighboring cells. This means that
the mobile device needs to be able to decode the E‐RGCH of all cells of the Active Set.
As shown in Figure 3.45, each cell of the Active Set can assume one of three roles:
One of the cells of the Active Set is the serving E‐DCH cell from which the mobile
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receives absolute grants via the E‐AGCH (cell 4 in Figure 3.45). The serving E‐DCH
cell can, in addition, instruct the mobile device to increase, hold or decrease its power
via commands on the E‐RGCH.