Hybrid type-II ARQ/AMS supported by
Channel Predictive Scheduling in a Multi-User Scenario
Nilo Casimiro Ericsson, UU
Sorour Falahati, CTH
Anders Ahlén, UU
Arne Svensson, CTH.
IEEE Vehicular Technology Conference - VTC'00-Fall
Boston, MA, USA, September 24-28, 2000, pp. 1804-1811.
© 2000 IEEE
In future packet based wireless communication systems,
the downlink will put a
high demand on the rate of data transmission
to mobile terminals. An obstacle
in this context is the time-variability of the channel. To achieve a high
throughput also over fading channels,
adaptive methods for adjustment of, for
example, the modulation alphabet and the coding complexity, together with
time-slot scheduling can be used. In our previous publications we
describe two substantially different approaches to packet data transmission
over fading channels, using adaptive methods for modulation and coding.
One approach is the Hybrid type-II ARQ scheme combined with an
Adaptive Modulation System (AMS). The idea behind
the proposed HARQ-II/AMS is to reduce the signalling constellation
size and/or the coding rate at each (re)transmission attempt.
Therefore, more resistance against channel impairments can be
provided in the latter transmission attempts while still having the
prospect of a high transmission rate in the first attempt.
The drawback of this system is the delay imposed by
the method: The adaptation
of coding rate/modulation is only controlled by the ACK/NAK feedback
signal which increases the system delay in poor channel conditions.
The performance of this system was previously studied and evaluated
only for the single user case, where no scheduling algorithm was
The other approach is based on the knowledge and
prediction of the channel quality,
allowing for scheduling of the transmission for one or multiple users.
Predictions of the different users' channel conditions provide a basis for a
detailed scheduling of the transmission, combining time-slot allocation and
adaptive modulation. This approach can also take into account the desired
error-probability and priority of the different users, and, the traffic
situation at hand. Moreover, it utilizes the frequency band efficiently, since
the different users are allocated time-slots when
their transmission conditions
are predicted to be favorable, allowing them to use a high modulation level.
The resulting constant and low (user-specified)
error-rate provides the error
correcting codes with manageable data, avoiding bandwidth consuming
re-transmissions. The main drawbacks are the
computational complexity, and the
sensitivity to channel prediction errors.
In this paper, the two methods are combined and the performance of the
combined approach is evaluated and compared
to the two individual methods.
The combined method successfully reduces the
drawbacks of the individual
methods described above: Knowledge of the channel conditions
gives a hint on the initial coding rate/modulation to use, reducing the delay
due to numerous NAKs, and, the ARQ-scheme
provides robustness against
channel prediction errors.
Based on the required bandwidth for each user, and their priorities,
the scheduler allocates the time-slots to them, giving priority to
re-transmissions due to NAK-signals. In case of a NAK-signal, the
coding rate/modulation is chosen to be less than the one used in the
previously unsuccessfully transmitted packet, according to the Hybrid
type-II ARQ/AMS scheme which is demonstrated in Table 1 where the
modulation scheme and coding rates are ordered with respect to the
retransmission attempt. This scheme is based on the rate compatible
convolutional codes at the parent rate 1/3 with constraint length
7 and puncturing period 2. The codes at higher or lower rates than
the parent code rate are obtained by utilizing optimum puncturing or
repetition, respectively. The transmitter uses 16-QAM,
8-PSK, QPSK and BPSK where a constant symbol
rate and constant average
symbol energy are assumed. Generalizing to the N-user system and
employing M time slots, for each new packet, the rate and the
modulation level are picked from the M-by-N matrix of predicted values
for the different users, described above.
This approach results in a reduction of the system delay due to the
prior information of the channel which is available at the
transmitter. Additionally, applying channel coding improves the error
correction capability of the system and provides robustness against
the channel prediction errors, especially in situations where there
are considerable changes in the channel conditions during the
prediction time interval. Moreover, the error correction is done in an
adaptive way, suitable for the time varying channel. The full-length
paper contains a full description of the proposed system, the
simulation results and proper references to the related work.
by Nilo Casimiro Ericsson, Oct. 2004.
Licenciate Thesis by Nilo C. Ericsson.
IEEE VTC-fall 99 conference paper
on scheduling over fading links.
PCC Workshop'98 conference paper
Globecom'99 conference paper
Postscript, 326K ;
The research project;
The Wireless IP Project within PCC;
entry in list of publications
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