Introduction
High speed downlink packet access
(HSDPA) is the new technology which is introduced in 3GPP Release 5. As the
name itself suggests, this will enable the user to achieve high data rates in
the downlink while on the move.
Motivation behind HSDPA
- Sophisticated UE applications
need higher bit rates,
- Primary target of HSDPA is to
enhance system throughput with minimum changes in network architecture,
- HSDPA id Release 5 an
extension to WCDMA Release '("99")
Transition from Present UMTS
system
- Current WCDMA network can be
upgraded to support HSDPA
- To implement HSDPA it does
not required to change network architecture dramatically
- In a general scenario there
can be HSDPA supported cells and regular cells. This mean that a
particular time there can be some users in a particular cell who are
experiencing high data rate because of HSDPA as compared to other users
who are in a cell where HSDPA is not supported.
- If HSDPA is not accessible at
UE location it will use normal Release 99 (DCH) to communicate at
regular service speeds
The Release 99 or current UMTS
system provide data rates of 384Kbps to 2Mbps. HSDPA will increase peak data
rates up to 14Mbps.
How HSDPA differs from current
UMTS system
Various methods for packet data
transmission in WCDMA downlink already exist in Release'99. The three
different channels in Release'99/ Release 4 WCDMA specifications that can be
used for downlink packet data are:
- Dedicated Channel (DCH)
- Downlink-shared Channel
(DSCH)
- Forward Access Channel
(FACH).
The basic requirements for HSDPA
are to carry high data rate in the downlink. The HSDPA technology will:
- Increase the UTRAN network
capacity
- Reduce the round trip delay
- Increase the peak data rates
up to 14 Mbps
In order to achieve this few
architectural changes have been made in the R99 architecture.
The transport channel carrying the
user data with HSDPA operation is denoted as the High-speed Downlink-shared
Channel (HS-DSCH) known as downlink "fat pipe".
As discussed above the primary
motivation behind HSDPA was to achieve high data rates by not disturbing to
the current UMTS architecture too much. Thus it's clear that by implementing
the HSDPA the current UMTS architecture is maintained and some other features
or functionalities are added on top of the existing architecture.
So the question arises is that to
implement HSDPA (Release 5) which new features comes in, what goes out from
the existing UMTS (Release 99) and what is added onto it.
In HSDPA (Release 5) three new
transport channels are introduced. They are:
- HS-DSCH (High Speed Down
link Shared Channel)
TO support the HS-DSCH Operation Two Control Channels are added
- HS-SCCH (High Speed Shared
Control Channel)
DL channel
- HS- DPCCH (High Speed
Dedicated Physical Control Channel)
UL Channel
With HSDPA two fundamental
features of WCDMA are disabled which is:
- Variable SF
- Fast Power Control
These two features are replaced by
- Adaptive Modulation and
Coding (AMC)
- Fast retransmission strategy
(HARQ)
- Scheduling Algorithm
Thus the comparison for the DSCH
(UMTS) and HS_DSCH (HSDPA) can be made as:
Feature
|
DSCH
|
HS_DSCH
|
Variable spreading factor
|
Yes
|
No
|
Fast power control
|
Yes
|
No
|
Adaptive modulation and coding
(AMC)
|
No
|
Yes
|
Multi-code operation
|
Yes
|
Yes, Extended
|
Fast L1 HARQ
|
No
|
Yes
|
Note: HARQ: Hybrid Automatic Repeat request
HSDPA Features Explanation
Adaptive Modulation and Coding
(AMC)
One of the major techniques
introduced in WCDMA is power control. The idea is to increase the
transmission power when the quality of the received signal is poor and
decrease the transmission power when the quality of the received signal
exceeds a given threshold. This results in reliable communication between the
transmitter and the receiver. Also, since the power control technique reduces
the unnecessary intercell and intracell interference caused by excessive
transmit power, the overall system capacity is increased.
An alternative technique to the
power control in dealing with the time varying effects of the wireless
channel is to “ride” the fading profile of the channel. Instead of trying to
keep the signal quality at the receiver constant, once can change the
modulation and the coding scheme of the transmitted signal in such a way that
more information bearing bits are transmitted when the channel condition is
good, and the less information bearing bits are transmitted when the channel
condition deteriorates. This technique is known as adaptive modulation and
coding (AMC), or link adaptation. Compared to conventional power control
technique, AMC can lead to much higher system capacity for packet radio
systems.
The AMC is aimed at changing the
modulation and coding format in accordance with variations in the channel
conditions. The channel conditions can be estimated based on either on the
feedback from the receiver or from the transmission power of the other
downlink channels under power control.
Thus according to the above
principle, in system with AMC, users in the favourable positions, such as
close to the cell site or at the peak of a fading profile, are typically
assigned a higher-order modulation with higher code rates, such as
64QAM/16QAM and ¾ rate turbo codes. Whereas users in the unfavourable
position, such as ones close to the cell boundary or at the lower peak of a
fading profile, are assigned a lower-order modulation with lower coding
rates, such as QPSK with ½ rate turbo codes.
If there are large number of users
in cell and the channel condition of the different users vary with time,
which is normally the case, the base station can choose to serve users in
favourable condition and use high modulation scheme and the coding rates most
of the time, and the system capacity would be greatly improved. Another
advantage of AMC is that since the transmitted power is fixed (no fast power
control is used), the interference to the other users is significantly
reduced.
Thus the main benefits of the AMC
can be summarised as follows:
- Higher data rates can be
achieved for users in favourable conditions, which in turn increase the
average throughput of the cell.
- Interference is reduced due
to link adaptation based on the variation in the modulation and coding
scheme instead of variation in the transmit power.
Hybrid ARQ (HARQ)
In order to make sure that the
data reaches the terminal error free, two basic error-control strategies used
in any data communication are forward error correction and Automatic repeat
request:
- Forward error correction
(FEC): In this information bearing
bits are protected by some coding scheme employing redundancy bits, and
most errors caused by the channel impairment can be recovered at the
receiver by the decoder. The main advantage of FEC is that no feedback
channel is required and so the delay is only contained in the decoding
process. The disadvantage of the FEC is that the decoded data is always
delivered to the user regardless of whether it is correct or incorrect.
- Automatic repeat request
(ARQ): In this scheme a high-rate detection codes are normally used and
a transmission is requested if the received data is found to be
erroneous. The functionalities of the ARQ include the following:
- Error detection: The receiver detects error
in the so called PDU.
- Positive acknowledgement: The receiver returns a
positive ACK to the successfully received error free PDUs.
- Retransmission after timeout: The transmitter
retransmits the PDU that has not been acknowledged after a
predetermined amount of time.
- Negative acknowledgement and
retransmission:
The receiver return a negative acknowledgement (NACK) to the erroneous
PDUS and transmitter retransmit them.
There are three types of basic ARQ
schemes:
- stop-and-wait (SAW): In this system the
transmitter sends off a packet and then waits for an acknowledgment. A
positive ACK will encourage sender to send he next PDU in the queue
whereas the negative ACK will prompt the sender to retransmit the PDU.
Stop-and-wait is the simplest scheme but inherently inefficient because
of the idle time spent waiting for the acknowledgment of each
transmitted packet before the transmitter starting the next
transmission.
- Selective Repeat (SR): is the most theoretically
efficient ARQ scheme among the three basic ones and it has been employed
by many systems, including the Release 99 UTRAN radio link control (RLC)
layer. SR is theoretically insensitive to delay and has the favourable
property of repeating only those blocks that have been received in
error. To accomplish this task, however, the SR ARQ transmitter must
employ a sequence number to identify each block that it sends. SR may
fully utilize the available channel capacity by ensuring that the
maximum block sequence number (MBSN) exceeds the number of blocks
transmitted in one round-trip feedback delay. The greater the feedback
delay, the larger the maximum sequence number must be. Unfortunately,
there is a major problem in the implementation of selective repeat ARQ
in HSDPA - high memory requirement in the mobile terminal. This is due
to the fact that the mobile terminal must store soft samples for each
transmission of a block.
A hybrid ARQ (HARQ) system
consists of an FEC subsystem contained in an ARQ system. The function of the
FEC system is to reduce the frequency of retransmission by correcting the
error patterns that occur most frequently, thus ensuring a high system
throughput. When a less frequent error pattern occurs and is detected, the
receiver requests a retransmission instead of passing the erroneous data to
the user. This increases the system reliability. There are three type of HARQ
- HARQ Type I: In this the same protocol
data unit (PDU) is retransmitted until the receiver accepts it as
error-free or until the maximum number of allowed retransmission
attempts is reached. HARQ Type I scheme is best suited for
communications systems in which a fairly constant level of noise and
interference is anticipated in the channel. In this case, an adequate
amount of redundancy can be built into the system to correct the vast
majority of errors so the number of retransmissions can be kept to a
minimum. For non stationary channels, such as the ones encountered in
mobile communications, the HARQ Type I can be very inefficient.
- HARQ Type II: This is particularly suited
for time varying channels. In this scheme, the concept of incremental
redundancy is employed and the received PDUs are concatenated to form
corrupted code words from increasingly longer and lower rate codes. In
the first transmission, the PDU may be coded with a high-rate code (low
redundancy) for error detection and correction. If the receiver detects
the presence of errors in the PDU, it saves the erroneous PDU in a
buffer and at the same time requests a retransmission. Unlike HARQ Type
I, what is retransmitted in HARQ Type II is not the original PDU but a
block of new data. The new data are formed based on the original PDU and
the error correcting code used. When the new PDU is received, it is used
to correct the errors in the erroneous PDU stored previously in the
buffer. If the second attempt fails again, the receiver will request a
further retransmission and this process continues until satisfactory
results are achieved.
- HARQ Type III: also belongs to the class
of incremental redundancy ARQ schemes. With HARQ Type III, however, each
retransmission is self-decodable. Chase combining (also called HARQ Type
III with one redundancy version) involves the retransmission by the
transmitter of the same coded data packet. The decoder at the receiver
combines these multiple copies of the transmitted packet weighted by the
signal-to-noise ratio (SNR) of the received signal for each attempt.
The performance of different type
of HARQs depends on the condition of the radio channel. Considering all
factors, including spectral efficiency, implementation complexity, and
robustness, it appears that Chase Combining offers a very attractive
compromise. In UTRAN, it is the scheduler that determines the redundancy
version parameters for the HARQ functional entities in node Bs.
Combining the two complementary
techniques, AMC and HARQ, leads to an integrated robust and high-performance
solution, in which AMC provides the coarse data rate selection, whereas the
HARQ provides for fine data rate adjustment based on channel conditions.
Scheduler
The scheduler for HSDPA is
referred to as being fast due to the fact that, compared with Release 99
specifications; the scheduler is moved from RNC to node Bs to reduce delays
so faster scheduling decisions can be made. In addition to other
functionalities, such as the choice of redundancy version and modulation and
coding scheme, a fundamental task of the scheduler for HSDPA is to schedule
the transmission for users. The data to be transmitted to users are placed in
different queues in a buffer and the scheduler needs to determine the
sequential order in which the data streams are sent. The scheduling
algorithms are:
- round-robin method: This algorithm selects the
user packets in a round robin fashion. In this method, the number of
time slots allocated to each user can be chosen to be inversely
proportional to the users’ data rates, so the same number of bits is
transmitted for every user in a cycle. Obviously, this method is the
“fairest” in the sense that the average delay and throughput would be
the same for all users. However, there are two disadvantages associated
with the round-robin method. The first is that it disregards the
conditions of the radio channel for each user, so users in poor radio
conditions may experience low data rates, whereas users in good channel
conditions may not even receive any data until the channel conditions
turn poor again. This is obviously against the spirit of the HSDPA and
it would lead to the lowest system throughput. The second disadvantage
of the round-robin scheduler is that there is no differentiation in the
quality of services for different classes of users.
- Maximum C/I
(carrier-to-interface) ratio method: In this method, the scheduler attempts to take
advantage of the variations in the radio channel conditions for
different users to the maximum, and always chooses to serve the user
experiencing the best channel condition, that is, the one with maximum
carrier-to-interference ratio. Apparently, the max C/I scheduler leads
to the maximum system throughput but is the most unfair, as users in
poor radio conditions may never get served or suffer from unacceptable
delays.
- Proportional fairness or
R[n]/Rav Method:
This method takes into account both the short-term variation of the
radio channel conditions and the long-term throughput of each user. In
this method, the user with the largest R[n]/Rav is served first, where
R[n] is the data rate in the current time slot n and Rav is the average
data rate for the user in the past average window. The size of the
average window determines the maximum duration that a user can be
starved from data, and as such it reflects the compromise between the
maximum tolerable delay and the cell throughput. According to this
scheduling scheme, if a user is enjoying a very high average throughput,
its R[n]/Rav will probably not be the highest. Then it may give way to
other users with poor average throughput and therefore high R[n]/Rav in
the next time slots, so the average throughput of the latter can be
improved. On the other hand, if the average throughput of a user is low,
the R[n]/Rav could be high and it might be granted the right of
transmission even if its current channel condition is not the best.
The figure below illustrates the
performance of different scheduling algorithm
Fast scheduling and AMC, in
conjunction with HARQ, is a way of maximizing the instantaneous use of the
fading radio channel in order to realize maximum throughput. The HSDPA technology
enables higher-rate data transmission through a higher-modulation and coding
rate and limited retransmissions, while keeping the power allocated to
HS-DSCH channel in a cell constant. Notwithstanding, the slow power control
is still needed to adjust the power sharing among terminals and between
different channel types.
HSDPA Impact on Radio Access
Network And UE Architecture
All Release’99 transport channels
presented earlier in this document are terminated at the RNC. Hence, the
retransmission procedure for the packet data is located in the serving RNC,
which also handles the connection for the particular user to the core
network. With the introduction of HS-DSCH, additional intelligence in the
form of an HSDPA Medium Access Control (MAC) layer is installed in the Node
B. This way, retransmissions can be controlled directly by the Node B,
leading to faster retransmission and thus shorter delay with packet data
operation when retransmissions are needed. With HSDPA, the Iub interface
between Node B and RNC requires a flow control mechanism to ensure that Node
B buffers are used properly and that there is no data loss due to Node B
buffer overflow.
Although there is a new MAC
functionality added in the Node B, the RNC still retains the Release’99/Release
4 functionalities of the Radio Link Control (RLC), such as taking care of the
retransmission in case the HS-DSCH transmission from the Node N would fail
after, for instance, exceeding the maximum number of physical layer
retransmissions.
The key functionality of the new
Node B MAC functionality (MAC-hs) is to handle the Automatic Repeat Request
(ARQ) functionality and scheduling as well as priority handling. Ciphering is
done in any case in the RLC layer to ensure that the ciphering mask stays
identical for each retransmission to enable physical layer combining of
retransmissions.
Similar to Node B a new MAC
entity, MAC-hs is added in the UE architecture. The functionality of the
MAC-hs is same as on the Node B side.
Node B protocol stack in R99
Node B Protocol stack in R-5
Transport and Control Channel in
HSDPA
High Speed Downlink Shared Channel
(HS-DSCH)
The HS-DSCH is allocated to users
mainly on the basis of the transmission time interval (TTI), in which users
are allocated within different TTIs.
HS-DSCH has the following
features:
- TTI = 2ms (3 time slots):
This is to achieve short round trip delay for the operation between the
terminal and the Node B for retransmissions. TTI in R99 is 10ms
- Adding higher order
modulation scheme, 16 QAM, as well as lower encoding redundancy has
increased the instantaneous peak data rate. In the code domain
perspective, the SF is fixed; it is always 16, and multi-code
transmission as well as code multiplexing of different users can take place.
The maximum number of codes that
can be allocated is 15, but depending on the terminal (UE) capability,
individual terminals may receive a maximum of 5, 10 or 15 codes.
The physical channel carrying
HS-DSCH transport traffic is termed as HSPDSCH, and each HS-PDSCH is
identified by its specific channelization code. Therefore, there can be up to
15 HS-PDSCHs in a cell. This means that for HS-PDSCH, both QPSK and 16QAM
modulation schemes can be used and these modulation schemes lead to different
slot formats. With 16QAM, a single HS-PDSCH channel can achieve a data rate
of 960 kbps. Using 15 HS-PDSCH channels (codes), HSDPA can produce the
headline data rate of 14.4 Mbps.
>
The HSDPA specification does
permit simultaneous transmissions. For instance, two to four users can be
supported within the same TTI by using different subset of the channelization
codes allocated to HS-DSCH.
High Speed Shared Control Channel
(HS-SCCH)
Besides user data, the node B must
also transmit associated control signalling to user terminals, so terminals
scheduled for the upcoming HS-DSCH TTI can be notified. Similarly, additional
lower-layer control information such as the transport format, including the
modulation and coding schemes to be used, and hybrid ARQ related information
must be transmitted. This control information applies only to the user
equipment that is receiving data on the HS-DSCH and is transmitted on a
shared control channel, HS-SCCH.
The UTRAN needs to allocate a
number of HS-SCCHs that correspond to the maximum number of users that will
be code-multiplexed. If there is no data on the HS-DSCH, then there is no
need to transmit the HS-SCCH either. From the network point of view, there
may be a high number of HS-SCCHs allocated, but each terminal will only need
to consider a maximum of four HS-SCCHs at a given time. The HS-SCCHs that are
to be considered are signalled to the terminal by the network. In reality,
the need for more than four HS-SCCHs is very unlikely. However, more than one
HS-SCCH may be needed to better match the available codes to the terminals
with limited HSDPA capability.
Each HS-SCCH block has a
three-slot duration that is divided into two functional parts.
- The first slot (first part)
carries the time-critical information that is needed to start the
demodulation process in due time to avoid chip level buffering.
- The next two slots (second
part) contain less time-critical parameters including Cyclic Redundancy
Check (CRC) to check the validity of the HS-SCCH information and HARQ
process information.
For protection, both HS-SCCH parts
employ terminal-specific masking to allow the terminal to decide whether the
detected control channel is actually intended for the particular terminal.
The HS-SCCH is a fixed rate
(60kbps) DL channel and uses SF=128 that can accommodate 40 bits per slot
(after channel encoding) because there are no pilot or Transmit Power Control
TPC bits on HS-SCCH.
The HS-SCCH used half-rate
convolution coding with both parts encoded separately from each other because
the time-critical information is required to be available immediately after
the first slot and thus cannot be interleaved together with Part 2.
The HS-SCCH Part 1 parameters
indicate the following:
- Codes to despread. This also
relates to the terminal capability in which each terminal category
indicates whether the current terminal can despread a maximum of 5, 10
or 15 codes.
- Modulation to indicate if
QPSK or 16 QAM is used.
The HS-SCCH Part 2 parameters
indicate the following:
- Redundancy version
information to allow proper decoding and combining with the possible
earlier transmissions.
- ARQ process number to show
which ARQ process the data belongs to.
- First transmission or
retransmission indicator to indicate whether the transmission is to be
combined with the existing data in the buffer (if not successfully
decoded earlier) or whether the buffer should be flushed and filled with
new data.
Parameters such as actual channel
coding rate are not signalled but can be derived from the transport block
size and other transport format parameters.
The terminal has single slot
duration to determine which codes to despread from the HS-DSCH. The use of
terminal-specific masking allows the terminal to check whether data was
intended for it. The total number of HS-SCCHs that a single terminal monitors
(the Part 1 of each channel) is at a maximum of 4, but in case there is data
for the terminal in consecutive TTIs, then the HS-SCCH shall be the same for
that terminal between TTIs to increase signalling reliability. This kind of
approach is also necessary not only to avoid the terminal having to buffer
data not necessarily intended for it but also as there could be more codes in
use than supported by the terminal capability. The downlink DCH timing is not
tied to the HS-SCCH (or consequently HS-DSCH) timing.
Uplink High Speed Dedicated
Physical Control Channel (HS-DPCCH)
The uplink direction has to carry
both ACK/NACK information for the physical layer retransmissions as well as
the quality feedback information to be used in the Node B scheduler to
determine to which terminal to transmit and at which data rate. It was
required to ensure operation in soft handover in the case that not all Node
Bs have been upgraded to support HSDPA. Thus, it was concluded to leave
existing uplink channel structure unchanged and add the needed new
information elements on a parallel code channel that is named the Uplink High
Speed Dedicated Physical Control Channel (HS-DPCCH). There is one HS-DPCCH
for each active terminal using HSDPA services.
The HS-DPCCH is divided into two
parts as shown in Figure below and carries the following information:
ACK/NACK transmission, to reflect the results of the CRC
check after the packet decoding and combining. Downlink Channel Quality
Indicator (CQI) to indicate which estimated transport block size, modulation
type and number of parallel codes could be received correctly (with
reasonable BLER) in the downlink direction.
CQI
- Information is Based on
(Cpich) Common Pilot Channel
- Is used to estimate the
following:
- Transport Block Size
- Modulation Type
- No. of Channelisation Codes
(that can be supported at a given reliability level)
- The feedback of CQI can be
set as Network parameter in a Predefined steps of 2ms
- When longer feedback cycles
are used, the (PDCH) Power Control Command can be used to update the
channel quality estimation
- CQI (0-30) => UEs
estimates highest value with probability of TB error < 10% for the
next transmission slot, and with various power assumptions
DL-DPCH & UL-DPCH
- The DL-DPCH carries HSDPA
Indicator (HI)
- HI indicates which HS-SCCH
subset the terminal should monitor in order to obtain the signalling
information,
- The UL_DPCH carries power
control signals for the DL-DPCH,
- The UL-DPCH power control
signal information and feedback information from HS-DPCCH can be used by
the network to estimate the channel quality.
HSDPA Architecture
The figure below illustrates the
HSDPA architecture for both UE and the network
MAC- hs
Functions, such as adaptive
modulation and coding and fast scheduling, are placed in node B. In contrast,
in the Release 99 UTRAN architecture, the scheduling and transport-format
selections are performed in the radio network controller (RNC). For HSDPA, it
is advantageous to move parts of the functionality from RNC to node B, thus
forming a new Node B entity, MAC-hs. The MAC-hs is responsible for handling
scheduling, HARQ, and transmit format (TF) selection. Apparently, some
upgrading is needed in the node B to enable the MAC-hs functionalities. The
consensus among the 3G network vendors is to implement MAC-hs in the channel
coding card.
There is one MAC-hs entity in the
UTRAN for each cell supporting HS-DSCH. The MAC-hs is responsible for
handling the data transmitted on the HS-DSCH. Furthermore, it is responsible
for managing the physical resources allocated to HSDPA. MAC-hs receive
configuration parameters from the higher layers.
The functional entities included
in MAC-hs are shown in the figure below
- Flow control: This is the companion flow
control function to the flow control function for existing dedicated,
common, and shared channels in RNC. This function is employed to limit
layer 2 signalling latency and reduce discarded and retransmitted data
as a result of HS-DSCH congestion.
- Scheduling/Priority handling: This function manages
HS-DSCH resources between HARQ entities and data flows according to
their priority. There is one priority queue for each MAC-d protocol data
unit (PDU) in the MAC-hs. Based on status reports from associated uplink
signalling in HS-DCCH, either new transmission or retransmission is
determined. A new transmission can be initiated instead of a pending
retransmission at any time to support the priority handling.
- HARQ: One HARQ entity handles the
hybrid ARQ functionality for one user. One HARQ entity is capable of
supporting up to eight HARQ processes of stop-and-wait HARQ protocols.
There is one HARQ process per HS-DSCH per TTI.
- TFRC selection: This is to select an
appropriate transport format and resource combination (TFRC) for the
data to be transmitted on HSDSCH.
To summarize, the MAC-hs needs to
perform the following tasks when dealing with HSDPA traffic:
- Decode the higher-layer
information regarding UE capability and required QoS for initial
connection.
- Decode the ACK/NAK and CQI
transmitted in the uplink and check the power level of the downlink
dedicated physical channel.
- Make a scheduling decision on
which terminal is due to receive data among the terminals having data in
the transmission buffer.
Set the transport format
combination indicator (TFCI) and HARQ parameters in the downlink shared
control channel HS-SCCH
In the following section the
function of two major functional entities in MAC-hs, the scheduler and the
HARQ unit are explained further.
Scheduler
The scheduler is one of the most
important functional entities in determining the QoS and data rate of HSDPA
services, as it controls when and how to transmit data streams dedicated at
each terminal. For each terminal, the information available to the scheduler
includes the estimate of channel quality (CQI) received on the HS-DPCCH, the
knowledge of priority queues, and the HARQ processes and terminal capability.
Based on the information, the scheduler performs the following functions:
- Schedules all HSDPA users
within a cell.
- Services priority queues.
- Schedules MAC-hs PDUs based
on information from the HS-DSCH frame protocol. One terminal may be
associated with one or more MAC-d flows. Each MAC-d flow contains
HS-DSCH MAC-d PDUs for one or more priority queues.
- Determines the HARQ entity
and the queue to be serviced.
- Indicates the queue ID and
the transmit sequence number (TSN) to the HARQ entity for each MAC-hs
PDU to be transmitted.
- Schedules new transmissions
and retransmissions.
- Based on the status reports
from HARQ processes, the scheduler determines if either a new
transmission or a retransmission should be made. A new transmission can,
however, be initiated on a HARQ process at any time.
- Determines a suitable
redundancy version and modulation scheme for each transmitted and
retransmitted MAC-hs PDU and indicates the redundancy version to lower
layers.
HARQ
The HARQ unit is responsible for
handling the HARQ functionalities of all mobile terminals. There is one HARQ
functional entity per mobile terminal in UTRAN. Each functional entity can
manage up to eight parallel stop-and-wait HARQ processes. As the input, the
HARQ entity receives the acknowledgment (ACK/NAK) from the mobile terminal.
The HARQ entity sets the queue ID in transmitted MAC-hs PDUs based on the
identity of the queue being serviced. The HARQ entity sets the transmission
sequence number (TSN) in transmitted MAC-hs PDUs. The TSN is set to value 0
for the first MAC-hs PDU transmitted for one HS-DSCH and queue ID and it is
increased by one for each subsequent transmitted MAC-hs PDU. The HARQ entity
determines a suitable HARQ process to service the MAC-hs PDU and sets the
HARQ process identifier accordingly.
The HARQ process sets the new data
indicator in the transmitted MAC-hs PDUs. It sets the new data indicator to
value “0” for the first MAC-hs PDU transmitted by a HARQ process and then
increases the new data indicator with one for each transmitted MAC-hs PDU
containing new data. The HARQ processes received status messages. UTRAN
delivers received status messages to the scheduler.
Mobility Procedures
Once a terminal is in the
so-called CELL_DCH state when dedicated channels have been set up, it can be
allocated with one or more HS-PDSCH(s), thus allowing it to receive data on
the HS-DSCH. For dedicated channels, it is advantageous to employ the
so-called soft handover technique, which is to transmit the same data from a
number of Node Bs simultaneously to the terminal, as this provides diversity
gain. Owing to the nature of packet transmission, however, synchronized
transmission of the same packets from different cells is very difficult to
achieve, so only hard handover is employed for HS-PDSCH.
This is referred to HS-DSCH cell
change, and the terminal can have only one serving HS-DSCH cell at a time. A
serving HS-DSCH cell change message facilitates the transfer of the role of
serving HS-DSCH radio link from one belonging to the source HS-DSCH cell to
another belonging to the target HS-DSCH cell. In theory, the serving HS-DSCH
cell change can be decided either by the mobile terminal or by the network.
In UTRAN Release 5, however, only network-controlled serving HS-DSCH cell
changes are supported and the decision can be based on UE measurement reports
and other information available to the RNC. A network-controlled HS-DSCH cell
change is performed based on the existing handover procedures in CELL_DCH
state.
Since the HSDPA radio channel is
associated with dedicated physical channels in both the downlink and uplink,
there are two possible scenarios in changing a serving HS-DSCH cell: (1) only
changing the serving HS-DSCH cell and keeping the dedicated physical channel
configuration and the active set for handover intact; or (2) changing the
serving HS-DSCH cell in connection with an establishment, release, and/or
reconfiguration of dedicated physical channels and the active set.
Although an unsynchronized serving
HS-DSCH cell change is permissible, a synchronized one is obviously
preferable for ease of traffic management. In that case, the start and stop
of the HS-DSCH transmission and reception are performed at a given time. This
is convenient especially when an intranode B serving HS-DSCH cell change is
performed, in which case both the source and target HS-DSCH cells are
controlled by the same node B and the change happens between either
frequencies or sectors.
If an internode B serving HS-DSCH
cell change is needed, the serving HS-DSCH Node B relocation procedure needs
to be performed in the UTRAN. During the serving HS-DSCH node B relocation
process, the HARQ entities located in the source HS-DSCH node B belonging to
the specific mobile terminal are deleted and new HARQ entities in the target
HS-DSCH node B are established. In this scenario, different controlling RNCs
may control the source and target HS-DSCH node Bs, respectively.
Intranode B Serving HS-DSCH Cell
Change
Figure below illustrates an
intranode B serving HS-DSCH cell change while keeping the dedicated physical
channel configuration and the active set, using the physical channel reconfiguration
procedures. The transition from source to target HS-DSCH cells is performed
in a synchronized fashion, that is, at a given activation time. For clarity,
only the layers directly involved in the process are shown and the sequence
of the events starts from the top and finishes at the bottom.
In this scenario, the terminal
transmits a measurement report message containing intrafrequency measurement
triggered by the event change of best cell. When the decision to perform
handover is made at the serving RNC (SRNC), the node B is prepared for the
serving HS-DSCH cell change at an activation time indicated by
CPHY-RL-Commit-REQ primitive. The serving RNC then sends a physical channel
reconfiguration message, which indicates the target HS-DSCH cell and the
activation time to the UE. Since the same node B controls both the source and
target HS-DSCH cells, it is not necessary to reset the MAC-hs entities. Once
the terminal has completed the serving HS-DSCH cell change, it transmits a
physical channel reconfiguration complete message to the network.
It should be pointed out that, in
this particular case, it is assumed that HS-DSCH transport channel and radio
bearer parameters do not change. If transport channel or radio bearer
parameters are changed, the serving HS-DSCH cell change would need to be
executed by a transport channel reconfiguration procedure or a radio bearer
reconfiguration procedure, respectively.
Internode B Serving HS-DSCH Cell
Change
For terminals on the move, what
happens more often than the intra-node B serving HS-DSCH cell change is the
so-called internode B serving HS-DSCH cell change. For synchronized case, the
reconfiguration is performed in two steps within UTRAN.
To begin with, the terminal
transmits a measurement report message containing measurement triggered by
the event change of best cell. The serving RNC determines the need for hard
handover based on received measurement report and/or load control algorithms.
As the first step, the serving RNC establishes a new radio link in the target
node B. After this, the target Node B starts transmission and reception on
dedicated channels. In the second step, this newly created radio link is
prepared for a synchronized reconfiguration to be executed at a given
activation time indicated in the CPHY-RL-Commit-REQ primitive, at which the
transmission of HS-DSCH will be started in the target HSDSCH node B and
stopped in the source HS-DSCH node B.
The serving RNC then sends a
transport channel reconfiguration message on the old configuration. This
message indicates the configuration after handover, both for DCH and HS-DSCH.
The transport channel reconfiguration message includes a flag indicating that
the MAC-hs entity in the terminal should be reset. The message also includes
an update of transport channel-related parameters for the HS-DSCH in the
target HS-DSCH cell.
After physical synchronization is
established, the terminal sends a transport channel reconfiguration complete
message. The serving RNC then terminates reception and transmission on the
old radio link for dedicated channels and releases all resources allocated to
the UE. The process of internode B handover for HS-DSCH is shown in Figure
below.
It should be noted that in the
case of internode B handover, the radio link control (RLC) for
transmission/reception on HS-DSCH may be stopped at both the UTRAN and the
terminal sides prior to reconfiguration and continued when the
reconfiguration is completed, which could result in data loss during the
handover period. Furthermore, the transport channel reconfiguration message
indicates to the terminal that the MAC-hs entity should be reset and a status
report for each RLC entity associated with the HS-DSCH should be generated.
However, a reset of the MAC-hs entity in the terminal does not require
flushing the reordering buffers but delivering the content to higher
layers.
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