A METHOD OF SCHEDULING HYBRID AUTOMATIC REPEAT REQUESTS HARQ
PROCESSES
This invention relates to incremental redundancy or hybrid ARQ Type II or III
retransmission techniques in mobile communications systems and is particularly
applicable to cellular systems.
The most common technique for error detection of non-real time services is based on
Automatic Repeat request (ARQ) schemes which is combined with Forward Error
Correction (FEC), called hybrid ARQ. If an error is detected by Cyclic Redundancy Check
(CRC), the receiver requests the transmitter to send additional bits. From different existing
schemes the stop-and-wait (SAW) and selective-repeat (SR) continuous ARQ are most
often used in mobile communication.
A data unit (PDU) will be encoded before transmission. Depending on the bits that are
retransmitted three different types of ARQ are e.g. defined in S. Kallel, R. Link, S.
Bakhtiyari, IEEE Transactions on Vehicular Technology, Vol. 48 #3, May 1999
"Throughput Performance of Memory ARQ Schemes".
• Type I: The erroneous PDU is discarded and a new copy of that PDU is retransmitted
and decoded separately. There is no combining of earlier and later versions of that PDU.
• Type II: The erroneous PDU that needs to be retransmitted is not discarded, but is
combined with some incremental redundancy bits provided by the transmitter for
subsequent decoding. Retransmitted PDU's sometimes have higher coding rates and are
combined at the receiver with the stored values. That means that only little redundancy is
added in each retransmission.
• Type III: Is the same as Type II only that every retransmitted PDU is now self-decodable.
This implies that the PDU is decodable without the combination with previous PDU's. This
is useful if some PDU's are so heavily damaged that almost no information is reusable.
This invention is related to Type II and Type III schemes, where the received (re)
transmissions are combined. These schemes can be seen as a link adaptation
technique, since the redundancy can be adapted according to the channel
conditions as for example described in 3GPP TSG RAN, "Physical Layer Aspects of
High Speed Downlink Packet Access TR25.848 V5.0.0" and in Amitava Ghosh ,
Louay Jalloul, Mark Cudak, Brian Classon, "Performance of Coded Higher Order
Modulation and Hybrid ARQ for Next Generation Cellular CDMA Systems",
Proceedings of VTC 2000.
Another technique that falls under this category of link adaptation, is adaptive
modulation and coding (AMC). A description of AMC can be found in the above-
mentioned documents. The principle of AMC is to change the modulation and coding
format in accordance with variations in the channel conditions or system restrictions.
The channel conditions can be estimated e.g. based on feedback from the receiver.
In a system with AMC, users in favourable positions e.g. users close to the cell site
are typically assigned higher order modulation with higher code rates (e.g. 64 QAM
with R=3/4 Turbo Codes), while users in unfavourable positions e.g. users close to
the cell boundary, are assigned lower order modulation with lower code rates (e.g.
QPSK with R=1/2 Turbo Codes).
In the following, different combinations of coding and modulation will be referred to as
Modulation Coding Scheme (MCS) levels.
A transmission will be split into Transmission Time Intervals (TTI), whereas the MCS
level could change each TTI interval (for HSDPA the TTI is equal to 2 ms).
Thus, depending on the channel conditions, different MCS levels can be scheduled.
Packet size depends on MCS level and number of orthogonal codes allocated for a
particular transmission. We will refer to MCS level and number of codes as Transport
Format and Resource Combination (TFRC).
Apart from MCS used, bit combining method also influences the robustness of
packets to the transmission errors.
There are different combining schemes, Chase Combining (CC) and Incremental
Redundancy IR), that can be used for bit combining. In Chase Combining, always the
same information and parity bits are sent to be combined and every version of packet
is self decodable. The set of parity bits is always obtained by using the same
puncturing scheme. Incremental Redundancy may use different sets of parity bits
(obtained by different puncturing schemes) in consecutive packet transmissions. All
these groups of obtained from different transmissions have to be stored in the soft
buffer for combining. Hence, Incremental Redundancy provides more reliable
transmission at the expense of increased soft buffer memory requirements.
Figure 1 shows a high level diagram of the UMTS architecture,
Figure 2 illustrates the current architecture of UTRAN,
Figure 3 shows a user plan radio interface architecture of HSDPA,
Figure 4 shows exemplarily the timing relations of an HARQ process,
Figure 5 shows the high level architecture of an HSDPA base station,
Figure 6 illustrates a high level architecture of an HSDPA mobile station,
Figure 7 illustrates the dynamic HARQ process configuration according to the
present invention, and
Figure 8 illustrates a possible format of MAC-hs control information, and
Figure 9 illustrates an example of an HARQ process configuration.
The high level architecture of Universal Mobile Telecommunication System (UMTS)
is shown in Figure 1. The network elements are functionally grouped into Core
Network (CN), UMTS Terrestrial Radio Access Network (UTRAN) and User
Equipment (UE). UTRAN is responsible for handling all radio-related functionality,
while CN is responsible for routing calls and data connections to external networks.
The interconnections of these network elements are defined by open interfaces as
can be seen in the Figure. It should be noted that UMTS system is modular and it is
therefore possible to have several network elements of the same type.
Figure 2 illustrates the current architecture of UTRAN. A number of RNCs (Radio
Network Controllers) are connected via wired or wireless links (lub) to the CN. Each
RNC controls one or several base stations (Node Bs) which in turn communicate via
wireless links (not shown) with the UEs.
High Speed Downlink Packet Access (HSDPA) is a new technique that is
standardised (see for example, 3GPP TSG RAN "Physical Layer Aspects of High
Speed Downlink Packet Access TR25.848" V5.0.0 or 3GPP TSG RAN TR 25.308:
"High Speed Downlink Packet Access (HSDPA): Overall Description Stage 2",
V5.2.0). It shall provide higher data rates in the downlink by introducing
enhancements at the Uu interface such as adaptive modulation and coding. HSDPA
relies on hybrid Automatic Repeat Request protocol (HARQ) Type ll/lll, rapid
selection of users which are active on the shared channel and adaptation of
transmission format parameters according to the time varying channel conditions.
The invention is particularly applicable to HSDPA but is not restricted to this system.
Therefore the data transmission does not necessarily have to be in the downlink nor
does it depend on a particular radio access scheme.
The User Plane Radio Interface Protocol Architecture of HSDPA is shown in Figure
3. The HARQ protocol and scheduling function belong to the Medium Access Control
High Speed (MAC-hs) sublayer which is distributed across Node B and UE. It should
be noted that an SR ARQ protocol based on sliding window mechanisms could be
also established between RNC and UE on the level of the Radio Link Control (RLC)
sublayer in an acknowledged mode. Parameters of the protocols are configured by
signalling in the control plane. This signaling is governed by a Radio Resource
Control (RRC) protocol. The service that is offered from RLC sublayer for point-to-
point connection between CN and UE is referred to as Radio Access Bearer (RAB).
Each RAB is subsequently mapped to a service offered from MAC layer. This
service is referred to as Logical Channel (LC).
The performance of high speed packet transmission may depend on technical
characteristics of the mobile UE capabilities. These could be signaled from the UE
entity to the RNC entity during connection establishment using the RRC protocol.
Over a feedback channel information is sent from the receiver to the transmitter that
notifies the transmitter whether a data packet has been acknowledged (ACK) or not
(NAK). Usually there is some delay until ACK/NAKs can be sent, due to processing
time the transmitter spends on demodulation and decoding. HARQ Type ll/lll
schemes put severe requirements on the receiver's memory size to store the soft
decision values for subsequent combining. This buffer is in the following called soft
buffer.
One method to overcome this constraint is to introduce a very fast feedback channel
without an involvement of the Radio Link Control (RLC) protocol in RNC and UE. A
scheduler is located in Node B so that retransmissions can be rapidly requested thus
allowing small delays and high data rates.
The functional behaviour of one HARQ process is illustrated in Figure 4. A physical
channel is used to transmit data to a receiver. In this case it is a so-called HS-DSCH
(High Speed - Downlink Shared Channel), where different users are time multiplexed.
As apparent from the figure, a transmitter Base Station (Node B) transmits to a
receiver called User Equipment (UE1). Node B transmits (Tx) a data packet A to the
UE1. Before the data is received (Rx) by the UE1 there is a propagation delay. UE1
will demodulate and decode the packet A. After a UE1 processing time of t rx process
an ACK or NAK will be sent (depending on if the packet A has been received
correctly or not). In this case UE1 sends a NAK assuming that Packet A has not been
received correctly. If the NAK has been received and decoded correctly by the
transmitter (tpr0pa introduced once again by the radio channel), the transmitter can
decide to resend the data packet after a processing time t tx process- Thus the number
of data packets that have to be stored depends on the number of simultaneously
active HARQ processes.
A high level architecture of HSDPA Base Station is depicted in Figure 5. It is
assumed there are #1...# X different data flows (logical channels) with data packets
to be transmitted from the Node B to the User Equipment (UE). The set of HARQ
transmitting and receiving entities, located in Node B and UE respectively, will be
referred to as HARQ processes. The maximum number of HARQ processes per UE
is usually predefined. These data flows can have different Quality of Services (QoS),
e.g. delay and error requirements and may require a different configuration of HARQ
instances.
The scheduler will consider these parameters in allocating resources to different
UEs. The scheduling function controls the allocation of the channel (HS-DSCH) to
different users or to data flows of the same user, the current MCS level in one TTI
and manages existing HARQ instances for each user.
A data flow or even a particular packet of a data flow may have a different priority.
Therefore the data packets can be queued in different priority queues. Different data
flows with similar QoS requirements may also be multiplexed together (e.g. data flow
#3 and #4). Besides the HS-DSCH that carries the data packets there is control data
which is mapped onto a High Speed - Shared Control Channel (HS-SCCH). This
could carry data such as the HARQ process ID, the modulation scheme, code
allocation, transport format etc. that is needed by the receiver to correctly receive,
demodulate, combine and decode the packets.
As said before, the scheduler decides which of the N HARQ processes shall be used
for transmission. Each HARQ process can have different window sizes. In HSDPA
there is only a single HARQ process scheduled each TTI and each process works as
a SAW protocol which corresponds to selective repeat ARQ with window size 1. In
the example illustrated in Figure 4, a retransmission can be scheduled after 5
transmission time intervals (TTI). It is not possible to schedule the same HARQ
process earlier if packet combining shall be used because the processing is still
ongoing. The HARQ process number as well as the sequence number has to be
signalled separately to allow a proper combining even if the packet is not received
correctly. In HSDPA the 1 bit sequence number is called New Data Indicator (NDI).
Each time a new packet has been sent, the NDI is incremented. In HSDPA the
HARQ process ID and the NDI are signalled on the HS-SCCH.
Furthermore in HSDPA each packet has a Transmission Sequence Number (TSN)
for reordering of correctly received packets. This information is signalled inband in an
header that is part of the packet. The TSN is increased for new each packet that is
send by the transmitter. The receiver will check the TSN after successful decoding of
an packet and deliver the packet only to higher layer if there is no previous packet of
that data flow missing. In case of missing packets the received packet will be stored
in the reordering buffer to wait for outstanding packets and to ensure in sequence
delivery to higher layer. If the reordering buffer is full because the receiver is waiting
for an outstanding packet for a long time the transmission must be stopped to avoid
dropping or overwriting of packets. This situation is called stalling and can reduce the
data throughput significantly. The stalling can be mitigated by different means such
as time out timer, window forwarding etc. The receiver recognizes that it will not
receive certain packets anymore and continues operation.
Usually a retransmission has a higher priority compared to new transmissions to
reduce overall delay. Thus a packet will be scheduled every 6 TTI for successive
erroneous decoding. A basic method is to adapt the number N of HARQ processes
or the window size of a ARQ process to the round trip time. A practical
implementation in this case would be an N channel Stop-and-Wait ARQ process.
Continuous transmission while considering the round trip delay can be assured by
switching between the HARQ processes every TTI. To support different priorities, a
new transmission can be initiated on a HARQ process at any time even though there
is a retransmission pending for that process. This will cause the UE soft buffer of the
process to be flushed.
In a system using N-channel SAW ARQ processes the number of HARQ processes
is chosen according to the round trip delay to provide continuous transmission while
minimizing the number of processes. In the same manner will the window size be
selected according to the RTT for window based ARQ mechanisms. Since the RTT
can vary during transmission so the initial configuration may not be optimum
anymore.
Different data flows can have different QoS and will therefore also have different
process configurations (e.g. maximum number of retransmissions). A scheduler may
preempt a certain transmission according to QoS attributes (priority, delay
requirement, guaranteed bit rate and other parameters) known by the scheduler. The
formulation preemption of higher priority data over lower priority data will be used in
the following although the reason for preemption may be a QoS attribute other than
priority (e.g. delay requirement).
After the UE data was scheduled to the appropriate HARQ process the transport
format (e.g. transport block size) and resource combination (e.g. number and index
of codes) for the data need to be selected. Depending on the channel conditions
different MCS levels and thus packet sizes can be scheduled.
The UE HSDPA architecture is shown in Figure 6. It can be noted that each HARQ
process is assigned a certain amount of soft buffer memory for combining the bits of
the packets from outstanding retransmissions. Once a packet is received
successfully, it is forwarded to the reordering buffer providing the in-sequence
delivery to RLC sublayer. According to the conventional architecture, the reordering
queue is tied to a specific priority.
It should be noted that the available soft buffer size depends on the UE radio access
capability parameters. Processing time of UE for a certain MCS level and minimum
inter-TTI interval (minimum time between two successive scheduling instants) can
also be considered as capability parameters. These are signaled from the UE to the
RNC by RRC protocol and further from RNC to Node B .
One constraint for current communication systems is that different priorities as part
of QoS requirements of data need to be supported efficiently. Future packet
switched applications will have low rate signalling (e.g. session initiation protocol )
which is more delay critical than the data. Thus signalling in parallel to the data
stream itself will have higher priority. In particular in mobile communication systems
there is high priority radio resource signalling such as to prepare for or to carry out
handover when changing the serving cell. Other radio resource management
information may also be scheduled in-between of data transmission. This signalling is
usually of low rate, but has to be very fast to avoid packet or even call drops.
Further, downlink messages are generally significantly larger than uplink messages
since they typically include more parameters as described in more detail in 3GPP
TSG RAN TS 25.331 "RRC Protocol Specification", V 5.0.0. At the same time, the
signalling between RNC and UE using radio bearers mapped on dedicated channels
is slow due to delays in the transport network between RNC and Node B and due to
a larger TTI of dedicated channels. For example, as mentioned in 3GPP TSG RAN
TS 34.108 "UE Conformance Testing", V 4.1.0, signalling radio bearers configured
for downlink interactive traffic with a peak rate of 2048 kbps is configured with a
payload in RLC packets of 136 bits and a TTI of 40 ms, that is with data rate of 3.4
kbps. For a typical RRC message size of 150 octets, signalling delay is 390 ms,
assuming transport network delay of approximately 30 ms. For the payload
corresponding to the lowest MCS in HSDPA (240 b), HSDPA TTI is equal to 2 ms
and minimum inter-TTI interval is equal to 2 ms, signaling delay is 20 ms assuming 2
retransmissions per packet. Thus, it may be beneficial to route some signalling traffic
over HSDPA connection.
Due to deep and long fades, which are likely when a mobile is located near the cell
edge, it may happen that all HARQ processes are simultaneously in the state of
combining packets. In such cases it could be required to handover to a different cell.
Some signalling is required for this purpose. However, scheduling any new data to
the occupied processes will result in flushing the contents of the UE soft buffer for
these particular processes. This causes an inefficient use of radio resources because
already transmitted packets (although not received correctly and currently in the
process of combining) are discarded. It should be noted that the packet size of the
data that is discarded could be quite large compared to the one of higher priority
signalling.
Another problem that occurs in case of insequence delivery to higher layer is stalling.
The flushing of packets could cause gaps in the reordering entity. Already
successfully received packet can not be delivered to higher layer because previous
packets are missing. If the flushed data can be resend, the problem is less severe,
but still more retransmissions will be required because combined bits of the
unsuccessfully received packets were discarded.
The object of the present invention is consequently to avoid flushing of lower priority
packets in the soft buffer of the UE when they are superseded by higher priority data.
This object is solved by a method of HARQ process configuration according to claims
1 and 2. According to the invention, some HARQ processes are either reserved or
additional HARQ processes are pre-configured for high priority data. This allows the
efficient support for data flows of different priorities and in particular for delay critical
signalling.
If the Node B receives low rate and delay sensitive data such as higher priority
signaling, it will switch to the reserved or additional HARQ processes (if required),
instead of using engaged HARQ processes which would cause the UE soft buffer to
be flushed. Furthermore the Node B or RNC will signal to use a separate reordering
queue for such data to avoid delays caused by reordering for in sequence delivery.
According to the conventional architecture, any HARQ process can be used for any
priority queue. According to one embodiment of the invention, it is proposed to
restrict the use of HARQ processes. Some HARQ processes may be limited for
specific data flows of high priority, while other HARQ processes may maintain full
flexibility. Such HARQ processes of limited use will be called reserved HARQ
processes. By this it is ensured that high priority data can be sent without waiting for
completion of outstanding retransmissions or flushing the UE soft buffer of an HARQ
process.
It should be noted that the restricted use of some HARQ processes limits the
scheduling in particular for continuous transmission. It also reduces data throughput
since the number of HARQ processes with full flexibility is insufficient for continuous
transmission during round trip delay.
In another embodiment of the invention the number of HARQ processes is increased
with respect to the minimum required by RTT to accommodate higher priority data.
These additional HARQ processes which may also have limited functionality are in
the following called additional HARQ processes. The limited functionality will most
likely be caused by reserving smaller soft buffer sizes for additional HARQ
processes. Thus, only some (lower) MCS can be scheduled with this processes.
Soft buffer memory that is required for one HARQ process depends on the following:
• type of bit combining (Incremental Redundancy, Chase Combining)
and
• highest possible TFRC, i.e. the highest possible MCS and maximum
number of orthogonal codes to be used with a particular process.
Once an HARQ process is added, it may be possible to restrict its usage just for
certain type of bit combining, MCS levels and number of orthogonal codes.
In the following, the embodiments of the invention will be described in further detail.
An example of process configuration is shown in Fig. 9. Fig. 9 plots soft buffer
distribution among HARQ processes. We assume that 5 processes are enough to
support continuous transmission during one round trip time (case a). Some memory
in the soft buffer is still available. Thus, 5 HARQ processes may be configured to so
to support higher TFRC or to support more reliable transmission (Incremental
Redundancy instead of Chase Combining) - case b. Alternatively, if needed, some
processes may be added. In the figure, case c, one process with unrestricted
functionality (A1) and the other with restricted functionality (A2) in terms of maximum
allowed MCS and/or transmission reliability are configured.
The following decision making rules may be envisaged for the embodiment of the
invention assuming initial configuration of HARQ processes. If the soft buffer size of a
UE is such that no HARQ process can be added, one HARQ process may be
reserved. If the soft buffer size allows, the process can be added. In this case, the
process may be configured only for certain (lower) TFRCs or may support less
reliable transmission (Chase Combining instead of Incremental Redundancy), thus
enabling more efficient use of the soft buffer for high priority and low rate data.
Additional HARQ processes also cause an increased signalling range for identifying
the HARQ process to the UE. The signalling of the HARQ process ID via a shared
control channel is usually done by a fixed number of bits. Additional bits for the
signalling may not be necessary because the number of HARQ processes that can
be signalled is in the range of the power of two (e.g. 8 HARQ processes).
The method of configuring an additional HARQ process consequently requires that
additional soft buffer memory is reserved for combining. In order to address such
constraints, once an HARQ process is added, it may be possible to restrict its usage
just for certain MCS levels, e.g. restrict its usage for certain packet sizes. Thus, the
soft buffer size for such limited HARQ processes is minimized. This is illustrated in
query 400 with subsequent steps 420 and 440 in figure 7.
An additional advantage of adding an HARQ process resides in the fact that all
ongoing regular HARQ processes are not affected and that consequently the data
throughput is not reduced.
In case query 300 results in that available soft buffer size is not sufficient for adding a
further HARQ process, the transmitter pre-configures at least one reserved HARQ
process for instantaneous transmission of delay sensitive data (step 340).
13
Hence, no additional soft buffer memory is required at the receiver, but a reduced
data throughput is the result, since the number of HARQ processes with full
functionality is reduced.
Variable RTT shall be monitored by the Node B in order to dynamically configure
reserved processes. Furthermore, the state of the HARQ processes should be
considered when scheduling high priority data in order to prevent soft buffer flushing.
To avoid idle periods due to lack of available HARQ processes, it is necessary that
number of HARQ processes is matched to the round trip time.
Round trip time (RTT) dominantly depends on UE and Node B processing time, UE
minimum inter-TTI interval and timing of the shared channels. While the HS-SCCH
and the HS-DSCH in the downlink are shared channels, the ACK/NAK in the uplink
are sent on a dedicated channel. The timing of the shared channels needs to be
aligned with other UE's. Therefore there can be different offsets between the
channels having an effect on the round trip delay. Finally, during the time between
traffic bursts, continuous transmission supported by all HARQ processes is not
necessary. In summary the number of HARQ processes to support data transmission
depends on capabilities, configuration and traffic statistic and can also vary
dynamically.
If there are any additional HARQ processes existing, Node B will monitor RTT, traffic
burstiness and QoS (required throughput) in order to estimate the number of
processes that are necessary for transmission. Should the number be smaller than
the one estimated at the time of process addition, these processes will be deleted
and some of the remaining processes reserved for high priority data.
The following decision making rules may be envisaged for the embodiments of the
invention. If the soft buffer size of a UE is such that no HARQ process can be added,
one HARQ process may be reserved. If the soft buffer size allows, the process can
be added. In this case, the process may be configured only for certain (lower) MCS
levels, thus enabling more efficient use of the soft buffer for high priority and low rate
data. Decision process is depicted in figure 7 as described above.
Node B may use a separate reordering queue for some of reserved or additional
HARQ processes. In the conventional standard, reordering queues are tied
exclusively to priorities of certain data flows. Priority is not the only cause for packet
preemption. For example, even though some data flows have same priorities, some
of the packets may be more delay critical than others. In this case it would be
beneficial to route these packets to the HARQ process with a separate reordering
queue. This avoids additional delay due to reordering. Therefore, having reordering
queue per priority and having it per HARQ process are two possible options.
Some disadvantages of the conventional RRC signalling have been outlined above in
connection with figure 3. It is particularly important to minimize the delay of the
signaling used for HARQ process configuration and reconfiguration during RTT
monitoring. When a control message related to MAC-hs needs to be carried to the
UEs, the information is first sent from the MAC-hs in the Node B to the RRC in the
RNC and only then RRC entity in RNC can forward the signalling message to
corresponding entity in the UE. Thus, when high priority data shows up for the UE in
MAC-hs buffers, delays will be introduced due to RRC signalling before the first
packet can be sent to that UE. However, if the signalling is to be implemented
between RNC and UE, than the formats of control packets between Node B and
RNC (NBAP protocol) and of control packets between RNC and UE have to be
specified. A possible control information format is illustrated in figure 8.
Since decisions on HARQ process addition/reservation are supposed to be carried
out in the scheduler of Node B, it would be beneficial to send the signaling message
directly from Node B to the UE. It should be noted that this solution does not
precludes RRC signaling, but only complements it. To send this signaling
information, MAC-hs packets with Number of MAC-d PDUs set to 0 can be used, and
the control information can be put into payload. The semantics of the fields is as
follows.
E/l bit denotes the options of explicit and implicit signaling. If explicit signaling of
buffer allocation is used, soft buffer allocation (field Memory Partitioning) is sent as a
vector whose length corresponds to maximum number of HARQ processes. If
explicit signaling of reordering buffer allocation is used, a Reordering Buffer
Configuration field is a vector of the same length denoting whether the reordering
buffer is allocated per process or per priority. Implicit signaling is a default option in
both cases and denotes uniform soft buffer partitioning and allocation of reordering
buffer per priority respectively.
To support continuous transmission during one RTT, variation of RTT shall be
monitored by the Node B in order to dynamically configure reserved processes. If
RTT decreases, smaller number of HARQ processes is needed. Thus, more
functionality (higher maximum supported MCS, Incremental Redundancy instead of
Chase Combining) can be allocated to additional processes. Should RTT increase,
higher number of HARQ processes is needed. This, for example, may require further
reduction in functionality of additional processes.
The invention discloses an intelligent method for a flexible configuration of multiple
parallel hybrid ARQ processes. The method minimizes the required buffer in the
mobile station to store the soft values for combining while reducing the latency to
transmit packets of different priority. The latency is reduced without having to flush
the bits corresponding to outstanding retransmissions in the soft buffer of the UE.
The invention enables efficient HARQ process configuration depending on UE
capabilities and enables efficient soft buffer management in case of HARQ process
addition. Further, the invention decreases the possibility of stalling in the reordering
buffer by preventing the superceding of higher priority data by lower priority data and
by configuring reordering buffer per HARQ process. By adapting the number of
HARQ processes to the round trip time the use of the UE soft buffer is optimised.
It is clear to those skilled in the art, that the above described embodiments can be
combined, in particular it is possible to form a configuration, wherein an additional
HARQ process and at the same time have reserved HARQ process are used to
transmit delay sensitive high priority data.
We Claim :
1. A method of scheduling hybrid automatic repeat requests HARQ processes
involving packet combining in a mobile communication system, wherein a
plurality of HARQ processes are established in a transmitter and a receiver,
said method comprising the steps of:
configuring a plurality of HARQ processes for data flows having
predetermined quality of service requirements
characterized by
configuring an additional HARQ process for data flows of high priority, and
determining whether the scheduling of said additional HARQ process
requires to perform modifications in the Transport Format and Resource
Combination TFRC for the scheduled HARQ processes wherein, in the
case of requiring to perform modifications, the additional HARQ process
supports a lower modulation coding scheme MCS level or a lower
Transport Format and Resource Combination TFRC compared against the
modulation coding scheme MCS level or the Transport Format and
Resource Combination TFRC supported by other HARQ processes.
2. The method as claimed in claim 1, comprising the step of configuring a
minimum number of HARQ processes according to a system parameter.
3. The method as claimed in claim 1 or 2, comprising the steps of:
scheduling a plurality of data flows from at least one priority queue and
emptying the priority queue to one or a plurality of configured HARQ
processes for transmission.
4. The method as claimed in one of claim 1 or 2, wherein the additional HARQ
process has a limited functionality compared with a plurality of HARQ
processes.
5. The method as claimed in one of claim 1 or 2, wherein the additional HARQ
process supports Chase Combining or Incremental Redundancy according
to available memory size in the receiver's soft buffer.
6. The method as claimed in one of claim 1 or 2, wherein for the additional
HARQ process, a smaller soft buffer size is reserved at the receiver
compared with that reserved for one of a plurality of HARQ processes.
7. The method as claimed in one of claim 1 or 2, wherein the transmitter
signals to the receiver to use a separate re-ordering buffer for the additional
HARQ process.
8. The method as claimed in one of claim 1 or 2, wherein an HARQ process
identification is signalled to the receiver.
9. The method as claimed in one of claim 1 or 2, wherein the number of
HARQ processes and/or functionality of additional HARQ processes are
matched to the round trip delay caused by transmission time and
processing time at the receiver and the transmitter.
10. The method as claimed in claim 1, wherein the number of configured HARQ
processes varies dynamically in accordance with a system parameter.
11. The method as claimed in claim 2 or 10, wherein the system parameter is
at least one of round trip time, processing time, traffic burstiness, quality of
service, modulation coding scheme, timing of shared channels and
minimum transmission time interval.
12. The method as claimed in claim 1, wherein an HARQ process configuration
is signalled from the transmitter to the receiver by an HARQ protocol control
packet.
13. The method as claimed in claim 12, wherein an HARQ protocol control
packet is identified by inband signalling.
14. The method as claimed in one of claims 12 or 13, wherein control
information may be signalled explicitly or implicitly.

A method of scheduling hybrid automatic repeat requests HARQ
processes (320) involving packet combining in a mobile communication system,
wherein a plurality of HARQ processes are established in a transmitter and a
receiver, said method comprising the steps of: configuring a plurality of HARQ
processes for data flows having predetermined quality of service requirements
characterized by configuring an additional HARQ process for data flows of high
priority, and determining whether the scheduling of said additional HARQ
process requires to perform modifications in the Transport Format and Resource
Combination TFRC for the scheduled HARQ processes wherein, in the case of
requiring to perform modifications, the additional HARQ process supports a
lower modulation coding scheme MCS level or a lower Transport Format and
Resource Combination TFRC compared against the modulation coding scheme
MCS level or the Transport Format and Resource Combination TFRC supported
by other HARQ processes.