Enhanced Radio Access Technologies for Next Generation Mobile Communication phần 8 pot

EVOLUTION OF THE WCDMA RADIO ACCESS TECHNOLOGY 203

3. HSUPA – HIGH SPEED UPLINK PACKET ACCESS

(“ENHANCED UPLINK”)

Enhanced Uplink, also sometimes referred to as HSUPA or High Speed Uplink

Packet Access, was introduced in 3GPP release 6 (finalized in 2005) to complement

HSDPA and further improve the WCDMA packet-data support, with focus on

the uplink, mobile-terminal-to-network, direction. Jointly, HSDPA and Enhanced

Uplink are often referred to as HSPA or High Speed Packet Access.

The aim of Enhanced Uplink is to further improve the WCDMA support for

packet-data services, targeting

– significantly improved uplink system capacity

– further reduced delay/latency with focus on the uplink

– possibility for significantly higher uplink data rates

To achieve these targets, Enhanced Uplink introduces improved base-station￾controlled uplink scheduling allowing for more efficient utilization of the uplink

radio resources. Base-station-controlled uplink scheduling also enables the possi￾bility to provide significantly higher instantaneous uplink data rates to a single user,

without the risk for system instability. With Enhanced Uplink, peak uplink data rates

beyond 5.7 Mbps can be provided in the uplink in case of good channel conditions.

In addition, Enhanced Uplink also introduces support for fast Hybrid ARQ

with soft combining also for the uplink. Similar to the downlink, fast Hybrid

ARQ with soft combining for the uplink provides both improved system efficiency

and possibility for significantly reduced delay. It should be noted that a reduced

uplink delay is beneficial also for downlink data transfer due to its positive impact

on the overall radio-interface round-trip time. Thus the introduction of Enhanced

Uplink also implies a further improvement in the WCDMA downlink packet-data

performance.

These techniques are introduced into the WCDMA standard as part of a new

transport-channel type, the Enhanced Dedicated Channel or E-DCH. In addition to

a 10 ms TTI, the E-DCH also supports a TTI of 2 ms, reducing the radio-interface

delays, allowing for fast adaptation of the transmission parameters, and enabling

fast retransmissions.

Unlike the downlink direction, the WCDMA uplink is inherently non-orthogonal

even within the cell. Fast power control is therefore needed for the uplink also in

the case of E-DCH transmission, in order to handle the so-called “near-far problem”

and to ensure coexistence on the same carrier with terminals and services not relying

on the E-DCH for uplink traffic. The E-DCH is transmitted with a power offset

relative to the WCDMA power-controlled uplink control channel, the DPCCH. By

adjusting the maximum allowed E-DCH/DPCCH power offset, the uplink scheduler

at the base station can control the E-DCH data rate, see further below.

Enhanced uplink also retains the uplink macro diversity (“soft handover”)

supported in earlier WCDMA releases. In practice, the support for uplink macro

diversity implies two things:

(1) Uplink data transmissions can be received by multiple cells, more specifically

the cells in the so-called Active Set of the mobile terminal

204 CHAPTER 6

(2) Mobile terminals can be jointly power controlled by multiple cells, more specif￾ically by all the cells in the Active Set

There are two reasons for supporting uplink macro diversity also for E-DCH:

– Receiving transmitted data at multiple cell sites provides a macro-diversity gain

which offers the possibility for improved coverage and cell-edge data rates also

for E-DCH

– Power control from multiple cells is beneficial in terms of limiting the amount

of interference generated in neighbor cells.

One cell within the Active Set of a mobile terminal is defined as the E-DCH

serving cell. The E-DCH service cell is the cell that has the main responsibility for

scheduling of the uplink transmissions from the mobile terminal.

As discussed in Section 1.2, HSDPA introduced the support for higher-order

modulation in case of downlink (HS-DSCH) transmission. As described, higher￾order modulation for the downlink is useful in situations where the data rates,

without the possibility for higher-order modulation, would be bandwidth limited

rather than power/SIR limited.

However, on the uplink the situation is somewhat different with regards to

higher-order modulation

– Due to the use of mutually non-orthogonal codes for different mobile terminals

in WCDMA, there is no need to share channelization codes between mobile

terminals on the uplink. Thus, there is less probability for the uplink to be

“bandwidth” limited, compared to the downlink.

– Due to power limitations, very high SNR occurs less frequently for the uplink

compared to the downlink. This further reduces the probability for the uplink to

be bandwidth limited rather than power limited.

For these reasons and in order to reduce the mobile-terminal complexity, higher￾order modulation was not introduced as part of the Enhanced Uplink. Once again,

note that even without the support for higher-order modulation, uplink data rates

beyond 5.7 Mbps can be supported with Enhanced Uplink.

3.1 Fast Base-station-controlled Scheduling

Similar to HS-DSCH, Enhanced Uplink introduces fast base-station-controlled

scheduling also for the uplink. However, due to fundamental differences between

the downlink and uplink transmission directions, the basic scheduling principles are

quite different between the downlink and the uplink.

– For the downlink, the cell transmit power and the set of channelization codes

are the shared radio resources. The task of the downlink scheduler at the base

station is to ensure as efficient utilization as possible of these resources, e.g. by

means of channel-dependent scheduling, while also taking e.g. quality-of-service

requirements into account.

– For the uplink, the shared resource is instead the amount of tolerable interference

at the cell site. The fundamental task of the uplink scheduler is to control the

uplink transmissions from the different mobile terminals so that the overall uplink