Due to the non-orthogonal uplink transmission in W-CDMA the principles applied for the newly defined
transport channel Enhanced Dedicated Channel (E-DCH) are fundamentally different from HSDPA.
The shared resource in the system is the received interference at the Node B and a transmission at a single
UE can impact the raise over thermal noise as received by different Node B. Continuous uplink power
control is still an essential means of link adaptation due to the well known near-far problem. Consequently
it was decided to support soft handover for E-DCH to minimize intercell interference. Unlike HSDPA
the scheduler is not aware of the transmission buffer status, channel state and the UE transmission capabilities.
Partly this information will be signalled to the Node B via control signalling.
For the support of the new functionality several new physical channels were introduced.
• E-DPDCH: E-DCH Dedicated Physical Data Channel for dedicated uplink data transmission.
During data transmission so-called Scheduling Information such as buffer status, data priority
and power headroom can be piggybacked.
• E-DPCCH: E-DCH Dedicated Physical Control Channel with the associated control data
for E-DPDCH detection and decoding. For the support of the scheduler there is a Happy Bit that
informs if the UE has sufficient resources for transmission.
• E-HICH: E-DCH HARQ Acknowledgement Indicator Channel to transmit HARQ feedback
• E-RGCH: E-DCH Relative Grant Channel to grant dedicated resources (up, down, hold) to a
• E-AGCH: E-DCH Absolute Grant Channel is a shared channel that allocates an absolute resource
for one or several UE.
In Figure 3 the E-DCH data and signalling flow is illustrated. Based on the rate request (Scheduling Information
or Happy Bit) the Node B may respond with a resource allocation via the absolute or a relative
grant. The UE will use the grant for data transmission and the Node B will acknowledge the received
The HARQ protocol defined for HSDPA and for E-DCH is based on an n-channel stop-and-wait protocol.
Since out of sequence delivery is a regular event for this protocol, there is a reordering function in
place to provide in-sequence delivery to higher layer protocols. Unlike in HSDPA this function is contained
in a separate sub-layer called MAC-es. MAC-es is located in the RNC since E-DCH supports soft
handover and the packets can be received by different Node Bs. It must also be noted that the
ACK/NACK reception is not reliable and there may be unwanted repetitions or even packet losses
caused by ACK/NACK misinterpretations at the sender. In that case RLC can recover the packets if configured
in acknowledged mode (AM).
BENEFITS FOR END-TO-END PERFORMANCE
Besides an increase in radio and transport network efficiency for packet based services, HSPA improves
user perception by significantly increased peak data rates and a reduced overall latency. Peak data rates
depend on the supported reference classes. Typically the operator will upgrade the network successively.
The first terminals will be a data cards enabling 1.8 Mbit/s peak data rate.
At the final state of HSPA Release 6 deployment a maximum of 14.4 Mbps will be supported in the
downlink and 5.76 Mbps in the uplink. However it should be emphasized that the peak data rates are
temporary rates at the physical layer and neglect protocol overhead at the different layer. Furthermore an
optimistically high channel code rate at the physical layer is assumed. HSPA networks are not expected to
be deployed before 2007.
In terms of end-to-end delay significant enhancements can be expected due to fast Node-B HARQ retransmission
as well as reduced transmission time interval. Fast HARQ by the Node B will save at least
two times Iub transmission delay compared to RLC ARQ retransmission. Note that the Iub is susceptible
to congestion due to missing statistical multiplexing on the low capacity last mile. HARQ uses on synchronous
ACK/NACK feedback and does not rely on infrequent event based RLC status reports. Furthermore
the interleaving delay decreases proportionally to the TTI reduction. On the other hand the
HARQ generally operates at higher block error rate and will thus have a higher number of retransmissions.
In general there is no easy calculation of the system throughput and latency reduction due to various functions
performed at the different layer. All protocol functions must be modelled realistically to take into
account the impact of encapsulation, segmentation, retransmission, reordering etc. Results will be highly
dynamic and depend on the selected scenario and parameters. Furthermore the gain for a single link may
also not necessarily turn into improvement of overall system performance. Simulations on system level
considering multiple cells and multiple users are well established as means to evaluate system performance
in today’s complex mobile communication systems. Due to the high complexity those simulations are very
time consuming and generally run offline.
Nevertheless, in our research effort Nomor Research has implemented a standard compliant UMTS system
with the enhanced features of HSPDA and E-DCH in our RealNeS platform. The Real-time Network
Simulation (RealNeS) tool with our HSPA implementation as described above allows applications to be
tested live and even provide means to perform measurements and parameter reconfigurations in real-time.