Performance of the Medium Access Control Protocol
for the High Speed Downlink Packet Access
Gregory Manuel
Nokia Research Center
P.O. Box 407, 00045 NOKIA GROUP, Finland
ABSTRACT
In this paper, the Medium Access Control (MAC) protocol for the High Speed Downlink Packet Access (HSDPA)
of the Wideband CDMA (WCDMA) radio transmission is
studied. In a packet radio network, the delay performance
is of special interest. HSDPA is a new concept, which enhances downlink performance of the WCDMA air interface to lower packet delays and higher throughput. The
HSDPA MAC protocol is shortly introduced and its impact
to the buffering requirements is studied. The retransmission probability and Transport Format selection probability are presented as distributions. Finally, the performance
results are analysed from the simulated measures of packet
delay distributions and window occupancy distributions.
KEY WORDS
WCDMA, High Speed Downlink Packet Access, MAC.
1 Introduction
The Wideband Code Division Multiple Access (WCDMA)
air interface [1,2,3,4,5] has defined common, dedicated and
shared transport channels [6]. Particular interest in this paper is the analysis of WCDMA cells with the HSDPA concept. The proposed techniques of this concept include short
Transmission Time Interval (TTI) with fast feedback signalling, packet scheduling at the base station and radio link
adaptation techniques [7,8]. The radio link adaptation techniques include multicode transmission, Fast Hybrid Automatic Repeat reQuest (FHARQ) and Adaptive Modulation
and Coding (AMC) [7,8,9,10]. The HSDPA concept is applicable on a new type of transport channel, which is intended to carry interactive and background packet traffic
and optionally streaming. The traffic on the active bearers
is available as Transport Blocks transmitted on the High
Speed Physical Downlink Shared Channel (HS-PDSCH)
[6,7,8,9]. The HSDPA operation requires control channels
as the High Speed Downlink Shared Control Channel (HSSCCH) in the downlink and High Speed Dedicated Physical Control Channel (HS-DPCCH) in the uplink [6,7,8,9].
Section 2 shortly gives the reference to the HSDPA concept, section 3 describes the simulator and the simulation
assumptions. In section 4, the window occupancy and
packet delay distributions are analysed, the results are concluded in section 5.
Mika Rinne
Nokia Research Center
P.O. Box 407, 00045 NOKIA GROUP, Finland
2 High Speed Downlink Packet Access
In the WCDMA downlink, the limiting resources are the
transmission power of the base station sector (i.e. the cell)
and the channelization codes of the Orthogonal Variable
Spreading Factor (OVSF) code-tree [5,6]. It is expected
that the throughput will increase and packet delays decrease, if the code-tree resource is shared effectively. As
many terminals in the cell may share the HSDPA codetree, it is favourable to adapt the Transport Format (TF)
[6,7,8] according to the measured channel conditions at the
receiver. During good channel conditions at a receiver terminal, a larger Transport Block (TB) [8,9] with more multicodes, higher modulation and less channel coding can be
created, whereas during worse channel conditions a smaller
Transport Block has to be created. The reverse feedback is
specified as the Channel Quality Indication (CQI) report
and FHARQ acknowledgements (A or N) [5,6,7,8,9]. As
the transmissions of the Transport Blocks are very temporal i.e. 2 ms (three slots), it is possible to efficiently use
the channel dynamics and operate the channel at very low
Es/No still achieving the target bit error probability by retransmissions of the Transport Blocks without large delays.
The physical channel structure and the transmission protocol are depicted in Figure 1. Here, N-channel processes
are active for one base station (NodeB) and for two terminals (User Equipment, UE). The performance of HSDPA
was analysed earlier in [11,12,13,14], HARQ transmission techniques in [15,16,17,18,19,20,21,22,23], incremental redundancy in [24, 25,26] and AMC e.g. in [27,28].
Figure 1. The physical channel structure and transmission
protocol for the High Speed Downlink Packet Access with
six FHARQ channels.
2.1
Air interface protocols for the HSDPA
concept
The Radio Link Control (RLC) protocol [29] operates
equally for the Dedicated Channel (DCH) and for the
High Speed Downlink Shared Channel (HS-DSCH)
transport [6]. The Medium Access Control protocol
[8,30] in the Radio Network Controller (RNC) will decide
on which transport channel the Protocol Data Units
(MAC-d PDU) will be transmitted. For the HS-DSCH
transport, the MAC is further split to the base station
MAC called the High Speed MAC (MAC-hs), which
enables fast radio resource allocation [8,30]. The MAC-hs
takes care of the Transport Block scheduling, FHARQ
channel allocation and Transport Format selection. The
forward signalling on the HS-SCCH serves the MAC-hs
at the receiver and contains beside terminal identification parameters for Transport Format and Resource
Indication (number and indexes of the channelization
codes and modulation), FHARQ process identifier, TB
size, redundancy version and new data indicator [9].
The reverse signalling on the HS-DPCCH contains
the CQI-report and the FHARQ acknowledgements [9].
In order to perform effectively, the MAC-hs has a
buffer and a transmission window for TBs. Each TB
includes a Transmission Sequence Number (TSN) in its
header to allow in-sequence reception [30]. At the transmitter, every TB in the window has to wait for a positive
acknowledgement from the receiver and the window starts
from the first non-acknowledged TB. At the receiver, the
MAC-hs window exists as well, because the TBs received
from the several FHARQ processes need to be reordered
for (nearly) in-sequence delivery to the RLC protocol [29].
The mechanisms to read the reordering window are timer
based (Timer T1) and limited-window based [30]. Timer
(T1) is always running for a correctly received TB with
TSN higher than the next expected TSN and the window
is moved only if the next expected TSN was received and
correctly decoded, if T1 triggers for time-out or if the
window limit was reached [30]. Thus, when T1 triggers
for a given next expected TSN, all time-out TSNs will be
dropped from the window, or if the window gets full, the
receiver will follow the upper edge of the window and
the TSN at the bottom of the window is always dropped,
when a new TB with a higher TSN is received. At the
transmitter, the MAC-hs buffer is loaded by new data from
the RLC buffers over the Iub-interface.
3 A protocol simulator for the WCDMA
radio interface
This paper presents simulation studies of the HSDPA protocol in a WCDMA cell. The simulator contains selected
features of the Radio Resource Control (RRC) the Radio Link Control (RLC) and the Medium Access Control
(MAC) protocols [29,30,31]. The simulator operates with
WCDMA parameters [5,6], and the physical performance
is available as instantaneous BLock Error Rate (BLER)
probability as a function of signal to interference ratio
from the lookup tables [4,5,6,32] and from Jakes model
[33]. Traffic is generated here by a (non-real time) packet
traffic model [32]. This model generates packets by the
Poisson process, where the size of each packet is randomly selected from the Pareto distribution having a specified cut-off value [32]. The traffic source parameters are
shown in Table 1. The statistics are collected over all
packet calls in all sessions during a long simulation.
TABLE I. Summary of the traffic source parameters.
Traffic source parameters
Settings
Session arrival process
Poisson
Session inter-arrival rate
High
Mean number of packet
500
calls in a session
Packet source rate
1,500 kbps
Reading time
2 s
Mean number of packets in
25
a packet call
Packet inter-arrival time
1.536 ms
Datagram size distribution
Pareto
Mean packet size
288 byte
Maximum packet size
1,500 byte
a of Pareto distribution
1.1
k of Pareto distribution
81.5
TABLE II. Summary of the RLC/MAC parameters.
RLC/MAC parameters
Settings
RLC mode
Acknowledged mode
RLC transmissions
Limited {10}
RLC PDU size
440 bit
RLC window size
256 PDU
Polling scheme
Periodic timer
(Every k frames)
Poll timer
10 frames
Poll prohibit timer
10 frames
MAC-hs window size
{4,16,32} TB
Timer T1
{38,50,76,90} ms
The RRC protocol model includes simple procedures to
setup the radio bearers and simple algorithms to schedule packets. The RLC protocol takes care of processing
the PDUs, i.e. polling schemes, retransmissions and selective acknowledgements. The MAC-d schedules the logical channels and selects the transport channel. The MAChs schedules the HSDPA transmissions, selects the Transport Format and creates the Transport Block for each TTI.
Fast L1 signalling is modelled on the HS-SCCH channel
in the downlink and on the HS-DPCCH channel in the
uplink with randomly distributed signalling errors2 . The
RLC/MAC parameters are summarised in Table 2. The
HSDPA transport is modelled with multicode transmission,
FHARQ processes and AMC. The data is available in the
2 This is justified as these channels have fast power control, whereas
HS-PDSCH has not.
TABLE III. Summary of the transport channel parameters.
TrCH parameters
Settings
RNC scheduler
Round Robin
Iub delay
Distributed
[0,30] ms1
DCH bitrate
Variable
Associated DCH
30 kbps
HS-DSCH parameters
TTI
2 ms (3 slot)
NodeB scheduler
Round Robin
Spreading factor
16 (fixed)
Number of codes, Nc
Max {Nc{5,10,15}}
L1 transmissions
Limited {2,4,6,8}
FHARQ channels
6
FHARQ processes
{1...6} per UE
Transport Format
Channelization codes
{1...max{Nc}}
Channel code rate
{1/2,3/4}
Modulation
{QPSK,16QAM}
Channel parameters
HS-PDSCH BLER
50% for first
transmission
HS-SCCH BLER
1%
HS-DPCCH p{N|A}
1%
HS-DPCCH p{A|N}
1e-4
UE velocity
3.6 kmph, Jakes
Simulation time
1000 s
base station buffers as MAC-d PDUs, which are assembled
to a TB after the TF is selected for each TTI. Thus, exactly one Transport Block per TTI is triggered for transmission. The HS-SCCH and HS-DPCCH control information
are formed respectively, and each TB delivery is attached
appropriately with the TSN. In the receiver, Chase combining is implemented and the retransmitted symbols are
soft-combined to the already received symbol energy to increase the probability of correct decoding for the TB [24].
The transport channel parameters are given in Table 3.
4 Analysis of the packet delay and MAC-hs
window occupancy
In this section, the impact of MAC-hs operation and
parameters to the observed packet delay distributions are
analysed. The MAC-hs window occupancy (memory size)
is also compared for each case. The packet delay here
is the peer-to-peer delay for a network SDU (e.g. a TCP
segment) from the time, when the SDU was created at the
traffic source in the network till it was received, correctly
decoded and reassembled as an identical SDU at the
terminal. Any core network delays are not included in the
presented delay statistics. Figure 2 shows the distributions
of the physical layer (L1) and RLC transmissions. It
can be seen that four L1 transmissions was typically
enough and the probability of RLC retransmissions was
2 Iub
delay was set to 10 ms for the reverse link RLC STATUS PDU.
about 5%, which is much lower than e.g. on the DCH
operating at 20% BLER. Increasing the number of L1
transmissions further, may make the probability of RLC
retransmissions even lower. However, the distribution of
RLC transmissions seem to have a long tail, meaning that
when the channel dynamics is slow and the transmissions
occur during worse channel conditions, it takes a long time
and many retransmissions before high enough Es/No is
available for the correct decoding of the Transport Block.
If even more L1 transmissions are allowed, the FHARQchannel is unnecessarily occupied for a longer time, still
not decreasing the RLC retransmission probability notably.
These properties of channel dynamics would also argument
gains for other than Round Robin types of schedulers.
Figure 2. The RLC transmission probability distribution
for the HSDPA with channel operation point at about
BLER 50% with number of L1 transmissions limited to (a)
two and (c) four. The RLC transmission probability shown
in (b) and (d) respectively.
Figure 3 shows the probability of selecting a given
Multicode, Modulation and Coding Scheme (MCS) for a
Transport Format during a TTI [9]. Here, the MCS is an
ordered set of multicode, modulation and channel coding
choice arranged in the order of increasing Es/No requirement for increasing TB payload. The MCS index points to
a unique selection in this set. The terminal is proposing the
highest MCS, which meets the BLER target in reference to
the most recently measured pilot channel. The statistics of
the proposed MCS (on the left) shows that in these channel
conditions, the terminal was proposing more often lower
MCS. With higher power allocation and better average
channel conditions, the probability of proposing higher
MCS could of course increase. The statistics of the selected MCS (on the right) was even more weighting lower
MCS, either because there was not enough data available
in the buffer at the moment of transmission or because
for the window-limited MAC-hs in Figure 7 respectively.
The delay distributions show that when T1 time-out value
was increased, the MAC-hs window memory requirement
gets higher and the packet delays are allowed to increase.
For a given T1 value, the MAC-hs window size does not
largely depend on the given MCS set. However, buffering
larger TBs will take more memory both in the transmitter
and in the receiver. The packet delay distributions do not
show big differences for different T1 settings. For a given
MCS with 5 multicodes, the probability of getting higher
packet delay is higher than for the MCS with 10 or 15
multicodes, which have no large mutual difference.
Figure 3. Probability of a given MCS proposed by the terminal (left) and the MCS selected by the base station (right)
with different MCS set available up to (a and b) 5, (c and
d) 10 and (e and f) 15 multicodes respectively.
Figure 4. The cumulative distribution of the MAC-hs window size occupied by the number of TBs for different
Timer T1 setting and MCS set.
a retransmission occurred3 . With a more strictly limited
MCS, the probability of selecting a high MCS in the set
is larger in comparable channel conditions, as shown in
Figure 3 (a) and (b) for up to 5 multicodes. The MCS
distributions in Figure 3 (e) and (f) present the ultimate
case of MCS selection up to 15 multicodes, which makes
the probability of selecting a high MCS in the set lower.
Figure 4 shows the result of the MAC-hs window occupancy in number of TBs as a function of T1 setting.
Figure 5 shows the result of the MAC-hs window occupancy in number of TBs as a function of limited window
size. The MCS set was limited to maximum of 5, 10 and
15 multicodes respectively. In both cases, the TBs in the
MAC-hs window may be of different size, because of the
MCS selection algorithm. The packet delay distributions
are shown for the Timer T1 limited MAC-hs in Figure 6 and
3 The MCS was not allowed to change during the retransmissions in
these simulations.
Figure 5. The cumulative distribution of the MAC-hs window size occupied by the number of TBs for different
MAC-hs window size limit and MCS set.
Next, the MAC-hs window size was limited to 4, 16
and 32 respectively. With a small window, the memory
requirements are low and the forced packet delays are
small. Here, a high MCS set with 15 multicodes makes a
difference in packet delay, as larger TBs can be formed
larger and not that strictly limiting L1 retransmissions, the
packet delay distributions are not drastically different for
any of the MCS sets.
5 Conclusions
Figure 6. The cumulative distribution of packet delay for
different Timer T1 setting and MCS set.
Performance of the High Speed Downlink Packet Access
protocol was analysed in the WCDMA air interface with
the multicode transmission, N-channel FHARQ and AMC.
The MAC-hs parameters have an impact to the performance by allowing a high physical channel BLER operation point e.g. of order 30-50% and still making the RLC
protocol BLER very low e.g. of order 5%. The results are
shown for selected channel conditions, packet traffic model
and Iub delay distributions as typical conditions and the results were analysed as a function of the HSDPA MAC protocol parameters. The packet delay was observed to depend
on the Transport Format set available and on the MAC-hs
parameter settings as window timers and window limits.
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Figure 7. The cumulative distribution of packet delay for
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MCS yields lower packet delays. With the window limit
set to 32, the window was not observed to be full and it
would already show the maximum requirement for the
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