Digital Communications Semester Project
On
Comparing ARQ and HARQ Protocols for WSN and
MIMO SYSTEMS
Student:
Praveen Francis Rego
Matriculation Number: 30104837
Supervisor:
Marc Selig
September, 2011
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Declaration:
I hereby declare that this project work is my own work and has not been
submitted in any form for another degree or diploma at any university or
other institute of tertiary education. Information derived from the published
and unpublished work of others has been acknowledged in the text and a list
of references is given in the bibliography.
Praveen Francis Rego
Kassel, Germany
September, 2011
Signature…………
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Acknowledgement
I thank the Almighty God for providing me the knowledge and instilling in
me the confidence to perform all my undertakings successfully.
I am grateful to my supervisor Marc Selig, who has given me the opportunity
to work in this project and for all his help and guidance.
I am always grateful to my parents and relatives for their constant support
and encouragement at all times. Besides, I thank the entire comlab faculty for
their dedicated service in imparting high quality education and thereby
creating in me an unbound curiosity in specific fields of study. Furthermore, I
thank my dearest friends for their companionship, support, and
encouragement.
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Abstract:
A major concern in data communications is the effective and efficient control
of transmission errors caused by channel noise so that error-free data can be
delivered to the user. In view of this concern, to achieve error-free data,
various Data Link Layer error-control schemes were proposed. Automatic-
repeat-request (ARQ) schemes and Hybrid ARQ (HARQ) have been
considered.
The recent strides made in wireless communications and electronics have
encouraged the development of low-cost, low power, multifunctional sensor
nodes that are small in size and communicate reliably over short distances.
The tiny sensor nodes which consist of sensing, data processing, and
communicating components constitute the sensor network. Energy
conservation is one of the most important issues in wireless sensor networks
(WSN’s), where nodes are battery powered. The efficient transmission of data,
necessitate reliable and energy efficient error control schemes.
Although ARQ has been in place for many years and extensive studies exist
with respect to ARQ strategies primarily from the error correction point of
view, recent research investigate the integration of MIMO and ARQ so as to
exploit the best advantages of the combined schemes.
The aim of this project is to investigate and compare the performance of
existing ARQ and HARQ protocols applicable in wireless sensor networks
(WSN) and in multiple input multiple output (MIMO) systems.
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Contents Chapter 1 -Introduction .......................................................................................................... 7
1.1 Stop-and-wait (S-W) ARQ Scheme ................................................................................ 8
1.2 Go-back-N (GBN) ARQ scheme ..................................................................................... 9
1.3 Selective-repeat (SR) ARQ scheme ............................................................................... 9
1.4 Hybrid-ARQ (HARQ) scheme ....................................................................................... 10
Chapter 2- ARQ’S and HARQ’S in Wireless Sensor Networks ............................................... 13
2.1 Background and Motivation ....................................................................................... 13
2.2 Channel Aware Link Layer ARQ Protocol .................................................................... 13
2.3 Previous Related Work on ARQ protocols .................................................................. 14
2.4 Proposed channel aware ARQ protocol ...................................................................... 15
2.5 Cooperative and Non-Cooperative ARQ protocols for energy harvesting wireless
sensor nodes ..................................................................................................................... 18
2.6 Energy efficient adaptive error control (AEC-RSSI) Protocol ...................................... 21
Chapter 3- ARQ’s and HARQ’s Protocols in MIMO Systems ................................................. 25
3.1 Background and Motivation ....................................................................................... 25
3.2 MIMO single ARQ (MSARQ) and MIMO multiple ARQ (MMARQ) .............................. 25
3.3 HARQ-MIMO retransmission techniques ................................................................... 29
3.4 Cooperative Multicell ARQ in MIMO Cellular Systems ............................................... 32
Conclusion ......................................................................................................................... 35
Bibliography ...................................................................................................................... 36
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Chapter 1 -Introduction
The transmission of data over a channel introduces transmission errors. These
errors reduce the performance of the system. So, to achieve an error free
communication, error control has to be implemented to combat against the
transmission errors. Usually, error control is the task of the data link layer and
two error control schemes namely, automatic repeat request (ARQ) and forward-
error -correction (FEC) are mentioned [1].
The data before being transmitted over the channel is channel encoded.
Channel encoding is done to add redundant bits to the information. The
redundant bits provide robustness against the channel errors. This increases
the reliability of data transmission over the channel. In the ARQ
retransmission protocol, once the encoded data has been transmitted over the
channel the correctness of the code is checked by means of the parity check
bits appended to the data. If there is no error, the data transmitted is assumed
error free and a positive acknowledgment (ACK) is sent via the feedback
channel to the transmitter. The parity check bits are removed from the data
block and delivered to the user. However, in presence of an error, a
retransmission of the same data is requested again. This is notified to the
transmitter by means of a negative acknowledgement (NAK). Retransmission
continues via feedback channel until error-free data is received by the receiver.
Due to the simplicity and associated high reliability, ARQ schemes are widely
used in data communication systems [1].
In the forward-error-correction (FEC) scheme of error control, the encoded
data (codeword) has error-detection as well as error-correction capability. But,
when the error cannot be located and corrected, the erroneous codeword is
transmitted to the user. No retransmission is possible in this scheme and when
uncorrected errors are still present due to decoding failure, it is hard to
achieve high system reliability. ARQ schemes are often preferred over FEC
schemes for error control in data communication systems such as packet-
switched data networks and computer communication networks [1].
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In the subsequent sub-section a brief account of the various forms of the ARQ
schemes are mentioned. Depending on the way retransmission is carried out,
there are three basic types of ARQ schemes. Stop-and-wait (S-W), go-back-N
(GBN) ARQ and selective-repeat (SR) ARQ [1].
1.1 Stop-and-wait (S-W) ARQ Scheme
In [1], it is said that the stop-and-wait protocol is the simplest of the ARQ
schemes. The transmitter transmits the codeword, stops and waits (idling) for
an ACK/NAK from the receiver before it continues further transmissions.
This is shown in Fig. 1.1 below. In case a perfect transmission where there are
no errors, the receiver sends ACK via the feedback channel. On receiving the
ACK, the transmitter transmits the next codeword in the queue. But on
receiving NAK, the transmitter once again retransmits the same codeword.
Retransmissions continue until the transmitter receives an ACK. The
scheme although simple, has the drawback of being inefficient mainly because
of the time wasted in waiting for an ACK/NAK from the receiver. During this
time no data is transmitted and this results in low data throughput of the
system. Increasing the block length can be considered as a solution to reduce
the waiting time (idle time) for the ACK/NAK. But it also introduces more
errors due its increase in size. Moreover, due to constraints of data formats in
practical system, long code lengths are impractical to be considered.
Fig. 1.1. Stop-and-wait ARQ scheme [1]
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1.2 Go-back-N (GBN) ARQ scheme
In this scheme, the transmitter continuosly transmits the codewords without
waiting for the acknowledgement from the receiver. This is shown in Fig. 1.2
below. An ACK/NAK is received after one round-trip delay. Round-trip
delay is defined as the time elapsed between the transmission of a codeword
and the reception of ACK/NAK for that codeword. Thus, in one round-trip
delay a certain number of codewords N can be transmitted. When the receiver
encounters an error for a code word i, it deletes it and each of the succeeding
N-1 codewords. When the transmitter receives an NAK for a particular
codeword i, the control goes back to the codeword i and retransmits it until
the transmitter receives an ACK for it. This is then followed by the
retransmission of the succeeding N-1 codewords irrespective of whether they
were error-free or not. The major drawback of this protocol is that unnecessary
retransmissions of error free codewords following a codeword detected in
error results in reduced throughput and extra energy consumption [1].
Fig.1.2. Go-Back-N ARQ with N=7 [1]
1.3 Selective-repeat (SR) ARQ scheme
In the selective-repeat ARQ scheme, introduced in [1], the transmitter
continuosly transmits the codewords without waiting for the
acknowledgement from the receiver. This is shown in Fig. 1.3 below. The
receiver on detecting a transmission error, sends a NAK. On receiving the
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NAK for a particular codeword, the transmitter only retransmits the NAK’ed
codeword and continues with the transmission of other codewords in the
transmission buffer. The data received from all codewords at the receiver
must be ordered sequentially matching to the transmission sequence before
being sent to the user. A receiver buffer is needed to store the error-free
codewords in sequence following the codeword in error. On correctly
receiving the codeword in error on retransmission, the receiver buffer releases
any error-free codewords so as to sequence the transmitted codewords in
correct order. To avoid codewords being lost due to buffer overflow resulting
due to small buffer size, large receiver buffer storage is needed for the
selective repeat ARQ system [1].
Fig. 1.3. Selective-repeat ARQ [1]
1.4 Hybrid-ARQ (HARQ) scheme
It is seen from [1] that although the basic ARQ protocols provide error control
capabilities, they have several drawbacks. In order to overcome the
drawbacks, a hybrid scheme consisting of the combination of ARQ and FEC
were used and is referred to as hybrid ARQ. The FEC subsystem increases the
system throughput by correcting most of the frequently occurring error
patterns in the transmitted codewords. When a detectable but not correctable
error pattern is being realized, an ARQ retransmission is requested thus
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increasing the system reliability. As a result, the combination of FEC and ARQ
scheme provides higher reliability than a system operating only with FEC, and
higher throughputs than a system operating only with ARQ.
Hybrid ARQ schemes can be classified into two categories namely, type-1 and
type-2 schemes [1]. The type-1 schemes uses a code designed for error-
detection and error-correction. On receiving a codeword in error, the receiver
tries to correct the error depending on the error-correcting capability of the
code. If the number of errors is within the error-correcting capability of the
code then the errors are corrected and transmitted to the receiver or stored in
the receiver buffer. This constitutes the FEC action. If the error cannot be
corrected, the codeword is rejected and then the receiver asks for a
retransmission (ARQ action) of the same codeword until it is able to be
decoded correctly.
The type-2 schemes operate adaptively depending on the varying channel
conditions. In the first transmission, the message to be sent is encoded with
parity check bits having only error-detection capability. On detecting an error,
the receiver stores a copy of the error message in the receiver buffer and at the
same time asks for a retransmission. The codeword retransmitted is not the
original codeword but rather a block of parity check bits. The parity check bits
have error correcting capability chosen depending on the previous
erroneously transmitted codeword. When the parity check bits are received it
tries to correct the errors of the codeword stored in the receiver buffer. There
is no guarantee that the decoding will be successful and in the case of a
decoding failure, the receiver requests for second retransmission of the
NAK’ed codeword [1].
HARQ with soft combining is classified into (a) chase code combing and (b)
incremental redundancy combining.
(a) Chase code combining is a technique used to combine repeated data
packets at the receiver. The successive retransmitted data packets are identical
copies of original transmission having same code rate but have different
weights associated to each of them. Chase combining can be considered as
additional repetition coding and therefore has no coding gain. The weights are
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measure of the reliability of the packet. The decoder is a code combiner based
on maximum likelihood estimation of transmitted information. The soft
decisions statistics of previously received data packets are combined so as to
obtain the best estimate of the transmitted information packet and thus
improve the SNR of the received signal [11, 17].
(b) In the incremental redundancy scheme of soft decision combining, each of
the retransmitted data packets has different coded bits and different code rates
for the same set of information bits. They possess a coding gain since several
different codes are combined to form a lower rate code with stronger error
correction capabilities [11].
The type-1 scheme is different from soft chase combining with respect to the
storing/deleting of the erroneously received data packet. In chase combining,
the erroneous codewords when detected at the receiver are not deleted but
rather stored in the receiver buffer to combine with subsequent retransmissions
of the original data packet.
The type-2 scheme, though similar to the incremental redundancy soft code
combining technique, differs in the retransmission process. In type-2 scheme
the retransmitted codeword consists of only the parity check bits and not the
data. But in the incremental redundancy of code combining, the subsequent
retransmissions comprise of data along with different encoding bits each time.
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Chapter 2- ARQ’S and HARQ’S in Wireless Sensor Networks
2.1 Background and Motivation
The growth of wireless communications and the rapid progress in electronics
has encouraged the development of senor nodes that consume less power but
are able to guarantee efficient sensing and processing of data so as to bring
about reliable communications. With this as the objective, recent research is
based on designing efficient sensor networks [3].
Throughput and energy efficiency are important yardsticks to judge the
performance of a protocol [1, 4, 7, 9]. Most of sensor nodes are battery
powered and have low energy capabilities. Suitable protocols are designed so
as to achieve maximum throughput with high reliability of the transmitted
sensed information. The reliability of the transmitted link is prone to errors
due to random nature of the channel. So in order to ensure error-free
transmission of the link, data link error control schemes are used.
In the subsequent subsections the different error control protocols employed
in WSN’s shall be discussed highlighting their main features, operation and
bringing out the differences between them in terms of their achievable
performances.
2.2 Channel Aware Link Layer ARQ Protocol
In [4], the authors propose a channel aware link layer ARQ protocol. The
motivation behind the proposal is that the conventional ARQ protocols as
discussed in chapter 1 have no adaptive capabilities to track changes in
channel conditions. So, due to lack of channel state information, the
conventional ARQ methods have the tendency to waste energy on
unnecessary transmissions/retransmissions due to ill-conditioned channels.
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Thus the proposed channel-aware ARQ protocol aims to minimize the
unnecessary wastage of energy by inducing channel aware properties into
the ARQ transmission/retransmission process. In the paper, stop-and wait (S-
W) ARQ protocol has been used for the adaptive process. With the main
objective being to achieve energy efficient link layer communications, four
channel aware based protocols are mentioned. The protocols make use of the
channel state information incorporated in the feedback process to choose the
appropriate time instant for transmission/retransmission. In order to have
clarity of comparison between the conventional S-W, the related work already
done on channel probing, and the proposed protocols in [4], each of them will
be discussed briefly below.
2.3 Previous Related Work on ARQ protocols
The basic stop-and-wait (S-W) ARQ protocol or schemes possess no channel
checking capability, and transmission of a data frame is considered successful
only if the channel is good for the transmission period plus the ACK period. If
during the ACK period, the received signal strength is below a certain
predefined threshold power, the received data frame is assumed corrupted
and thereby rejected at the receiver.
The probing based S-W ARQ protocol is said to operate similar to the basic
S-W ARQ under good channel conditions. Probing implies the channel
tracking mechanism under bad conditions. When the channel is corrupted, on
receiving a corrupted data packet the protocol has the ability to send a NAK to
the transmitter. On receiving the NAK, the transmitter stops the transmission
of a new data packet and enters into the probing mode to check the quality of
the channel. Once an ACK for the channel condition is received, the
transmitter reverts back to transmitting mode to retransmit the data packet
again. Though not explicitly stated, it is assumed that the channel remains
good for the data transmission period following an ACK. The important
point to be considered is that the channel probing period (the duration for
which the probing is carried out) is not chosen optimally due to lack of
channel fading properties [4].
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2.4 Proposed channel aware ARQ protocol
The proposed fading channel aware ARQ scheme in [4] considers the channel
fading property and the current channel state to decide the correct time
instant for the transmission/retransmission of the data packet. The four
channel aware approaches are :
a) Channel-aware probing is based on average fading delay which is a
function of the receiver threshold power, AFD ( ): AFD is the average fading
duration for which the received signal remains below a certain threshold
power. The receiver on receiving a data frame notifies the transmitter whether
the received signal is above or below the preset receiver threshold power
level. This is accomplished via the ACK/NAK acknowledgement. On
receiving the NAK, the transmitter enters the probing mode until the received
signal power is greater than the receiver threshold power level. The receiver
threshold power is dependent on the signal fading margin and this affects
the fading dependent threshold power.
b) The second protocol ( is similar to but in addition to receiver
threshold power it makes use of the current signal level at the receiver and
transmits it to the transmitter via NAK. This protocol has the advantage of
being aware of the depth of fading channel and reporting it to the transmitter.
This additional information can be utilized in recovering faster from a deep
fade condition thereby improving the throughput and the energy efficiency.
c) The third proposed protocol ( ) utilizes the current signal level at
receiver in addition to its slope. The knowledge of slope characteristics
provides valuable information regarding the dynamics of the fading channel
to know whether it is improving or degrading further. The current channel
state and its past history obtained from the slope are sent via the NAK.
d) The fourth protocol is similar to the third protocol with the only difference
that the current signal level and its slope at the receiver are sent for both ACK
as well as NAK. So, on receiving an ACK, the transmitter does not
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immediately transmit a new data frame, rather it estimates for how long the
signal would remain good, and based on that decides on transmitting new
data or stays away from immediate transmission foreseeing bad channel
conditions. Also, based on the slope of the current signal level, a new data
frame transmission or probing will be initiated. The incorporated
modifications in the transmission timing decisions are expected to increase the
throughput, energy and delay efficiencies for the transmission process.
The simulation results in [4] clearly show the superiority in the performances
of the proposed fading channel-aware schemes over the basic S-W and the
probing schemes. The probing period for the probing scheme is fixed for two
time slots indicated by t = 2 in the figures below.
Fig.2.1. Throughput versus fading margin (transmit power) of [4]
The plot in Fig. 2.1 shows that has achieved significantly higher
throughput and the performance gain is more significant at moderate fading
margin. The plot of energy efficiency of compared in Fig. 2.2 indicate
that in general, a probing ARQ achieves a significant reduction in
unsuccessful attempts with respect to basic ARQ thereby increasing the
energy efficiency.
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Fig. 2.2. Energy efficiency performance of [4]
In [6], the authors propose another channel-aware adaptive ARQ. The paper
addresses the issue of inefficient utilization of the channel due to lack of
channel state information when the Berkley MAC (B-MAC) protocol is used.
B-MAC is a carrier sensing medium Access (CSMA) based protocol designed
for low power wireless sensor networks. B-MAC is a link layer protocol. It
assesses the channel by making use of the clear channel assessment (CCA)
functionality. CCA is used for tasks such as channel arbitration i.e. to check if
the channel is free or not, and for the acknowledgement of the link layer
frames to ensure reliability. The acknowledgement of link layer frames is
optional and the protocol can operate with or without ACK. The B-MAC
protocol has a bi-directional interface that allows the network services to
adaptively change the duty cycle of the B-MAC protocol depending on the
service needs. Although the B-MAC protocol is energy efficient with better
reliability than other MAC protocols, in scenarios where bursts of data have to
be transmitted reliably from different sensors to a destination (base station)
there occurs an inefficient utilization of the channel due to collision and loss of
ACK [6].
In [6], the authors propose an adaptive ARQ scheme for better channel
utilization and higher data rates. The adaptive error control is based on a fuzzy
logic control. The fuzzy logic control function selects the appropriate
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retransmission time instant for sending a particular data frame based on
signal strength of the nodes and distance between the sender and receiver
nodes. The performance of the proposed adaptive ARQ is compared with the
B-MAC protocol via simulation. Simulation results show the superiority of the
proposed adaptive ARQ protocol. The superiority in performance is expressed
in terms of higher delivery rate output.
Fig. 2.4. Data delivery rate (y) vs. Number of sensor motes (x) [6]
2.5 Cooperative and Non-Cooperative ARQ protocols for energy harvesting
wireless sensor nodes
The wireless sensor nodes in a sensor network need to be battery powered so
as to perform the tasks of sensing, processing, and communication. The
batteries may be either periodically replaced over time or the batteries may be
self–rechargeable i.e. recharge themselves by harvesting energy from the
surrounding environment. The energy harvesting technique provided is a
very attractive maintenance-free solution for low power sensor nodes [7].
In [7], the GAP4S system architecture as show in Fig. 2.5 was considered
where the location of sensor nodes are confined to a fixed distance
surrounding a power rich base station. The base station serves as an access
point and provides an extension to larger communication network. Each of the
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sensor nodes transmits data directly to the base station via the uplink wireless
channel. The energy harvesting or recharging is done via the downlink
microwave channel. The base station being rich in power has the
responsibility of scheduling the sensor nodes transmission time instants and
the relays to be chosen for each sensor node.
Fig. 2.5. GAP4S system architecture [7]
The main objective of the authors in [7] was to achieve maximum saturation
throughput from the sensor nodes to the base station while maintaining
reliable and fair transmission of the sensed information. Maximum saturation
throughput is defined in the paper as the maximum load that can be tolerated
by the sensor node without exceeding its energy harvesting rate. To fulfill the
second requirement of reliable transmission of the sensed information, ARQ
protocols were used. Two classes of ARQ protocols were considered, namely,
the conventional non-cooperative ARQ (ARQ-NC) protocol and the
cooperative ARQ protocol (ARQ-C).
The conventional ARQ protocol transmits the data directly to the base station
and waits for an acknowledgement. On receiving an ACK within the time
allotted for the ACK to be received, a new frame is transmitted. Otherwise the
same frame is retransmitted until the data frame is received correctly at the
base station.
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The Cooperative ARQ protocol (ARQ-C) makes use of the relaying
phenomenon where the sensor node chosen as the relay overhears the
information intended to the base station. It saves a copy of it in its buffer. On
receiving NAK for a particular date frame, it is the relay that has to retransmit
the data frame and not the source node. In doing so, unnecessary wastage of
energy due to retransmission from the source node is avoided. The relay offers
its energy in retransmitting the received NAK’ed data frame. To state it in
another way, the source sensor node borrows energy from the relay so that
its energy consumption rate does not exceed its recharging rate.
Fig. 2.6. Three node network scenario [7]
As mentioned earlier, it is the responsibility of the base station for choosing
the relay/relays for each sensor node for a data frame transmission. There are
many possibilities of choosing the relay for a sensor node. Either one sensor
node may act as relay for many source nodes or multiple sensor nodes can act
as relay for one source node. But in the latter case it is assumed that for every
data frame transmitted from a source node, a different relay would act each
time. It is said in [7] that using several relays for a single source node
improves the load and energy consumption balancing.
It was observed through simulation that for lower values of transmitted
energy per bit, the Cooperative ARQ protocol achieves a higher saturation
throughput than non-cooperative protocol. This is accomplished by the energy
borrowing scheme where energy deficient nodes ask neighboring energy rich
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nodes to retransmit the data frames. Thus the Cooperative ARQ protocol
achieves a saturation throughput which is twice as much as that of the non-
cooperative conventional ARQ when the energy is the limiting factor in the
system.
The authors in [7] have extended their work and proposed ARQ- protocol
in [8]. The protocol is a recursive version of the ARQ-C protocol where several
relays would be iteratively selected by the BS to overhear and store a copy of
the data frame intended to the receiver. The reason behind this proposal is
based on the assumption that the relays have a higher probability to
retransmit the data correctly compared to the previous relays or the source
node. Also, the relay being much closer to the receiver (BS) requires lesser
energy to retransmit the data. The simulation results in paper [8] show that
the saturation throughput of the ARQ- protocol is much better than both,
the ARQ-C and the non-cooperative ARQ protocols. However, the complexity
of the base station is slightly higher than the ARQ-C protocol.
2.6 Energy efficient adaptive error control (AEC-RSSI) Protocol
In [9], an energy efficient adaptive error control scheme is proposed. The AEC-
RSSI is a hybrid scheme comprising of the two data link layer error control
mechanisms namely ARQ and HARQ. Depending on the communication
distance between the sensor nodes, either of the two error control schemes is
adaptively chosen. When the transmitter sensor node receives NAK
information, it makes use of the information to calculate the location of the
receiver sensor node.
Energy efficiency is often considered as a metric for evaluating the
performance. To quantify and compare the performance of the AEC-RSSI
scheme with other schemes, first the energy efficiencies of the ARQ, FEC and
HARQ schemes are considered. Then energy efficiency performances are
compared by means of a mathematical approach. The comparison is done so
as to add clarity to distinguish the schemes mentioned later in the section.
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The transmission energy efficiency of WSN’s takes into consideration the
energy consumption and reliability of the transmission link. It is given by
(1- )….(2.1)
Where, is the energy consumption throughput, represents the link
layer frame error rate. The communication reliability can be expressed by
r = (1 - ) which represents the probability that a link layer frame is
successfully transmitted. r is further defined as ratio of energy consumed by
the link layer data frame ( to the total energy consumption ( . The
total energy comprises the energy needed to start/receive a data frame in
addition to the energy needed for coding/decoding depending on the error
control scheme. For the FEC scheme the decoding energy is included in the
total energy but for the ARQ scheme it is neglected.
The comparison of the energy efficiencies of the different schemes (FEC, ARQ,
and HARQ only) was considered for fixed length data frames and for variable
communication distances. The maximum number of retransmissions for ARQ
and HARQ was set to 1. In the figure below it is seen that the energy
efficiencies of all the schemes increased with the increase in payload size of
the data frame and for communication distances up to 40 meter. Within the 40
meter range, the ARQ scheme showed the best performance compared to FEC
and HARQ. This was because the additional decoding energy consumed by
the FEC and HARQ was far more than the energy consumed by ARQ due to
retransmissions. However, for distances greater than 40 meter, HARQ showed
better performance.
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a) Frame length=750 bytes. b) Frame length=1500 bytes.
. Fig. 2.7. Energy efficiency of FEC, ARQ, and HARQ [9]
The AEC-RSSI protocol in [9] comprises of the ARQ and HARQ error control
mechanism. The protocol makes use of the received signal strength indicator
(RSSI) for power control. Based on the RSSI the distance between the sensor
nodes is calculated. Depending on the distance, either ARQ or HARQ scheme
is adaptively chosen for error control depending on their respective
efficiencies. When the distance of communication is less than 40 meters, ARQ
is used due to its higher efficiency in this range. With distance greater and
equal to 40 meters HARQ is used for the error control of the lost link layer
frames.
In [9], it is shown the superiority in the performance of the proposed AEC-
RSSI scheme compared to the other schemes when the communication
distance is greater than 40 meters. Furthermore, the comparison in
performance is shown in Fig. 2.8 below. The benefits of the protocol was due
to the fact that AEC-RSSI protocol design has the inherent multiple
redundancy protection, which incorporates channel encoding technology and
diversity combining (soft combining) technique of retransmitting the link
layer frames.
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a) Length of link layer frame is 750 bytes . b) Length of link layer frame is 1500bytes.
Fig. 2.8. Comparison of AEC-RSSI, FEC, ARQ, and HARQ [9]
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Chapter 3- ARQ’s and HARQ’s Protocols in MIMO Systems
3.1 Background and Motivation
Research work carried out over the last decade has confirmed that by utilizing
multiple transmit and receive antennas multi-input multi-output (MIMO)
transceivers provide substantial spectral efficiency and diversity gain [2, 10,
16].
Although the diversity exploited through MIMO technique provides
substantial robustness to the errors introduced by the physical layer, practical
systems often encounter harsh, distortive or fading channels that can lead to
packet failure despite the robustness provided by MIMO diversity. So there
arises a necessity to provide additional protection to the transmitted signal
against channel errors. Error control schemes can be used to provide the
additional robustness thereby increasing the overall system throughput [16].
With the aim of improving the diversity provided by the ARQ scheme, several
antenna precoding schemes have been designed so as to increase the mutual
information throughput of the system. Single and multiple ARQ MIMO
protocols have been proposed and their performance is evaluated based on
linear and non-linear detection schemes [11, 13, 16].
In the subsequent sub-sections of this chapter, the various ARQ and HARQ
protocols of the data link layer in conjunction with the MIMO technique of the
physical layer will be explained.
3.2 MIMO single ARQ (MSARQ) and MIMO multiple ARQ (MMARQ)
The design of some MIMO systems is based on the BLAST (BELL Labs
Layered Space- Time) architecture, where substreams from the individual
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antennas are transmitted over different layers independently. BLAST is a
communication technique utilized in MIMO for achieving very high data rates
in rich scattering environments [2].
The encoding of the data can be done either by joint encoding or separate per-
antenna encoding. When the entire data packet is encoded and interleaved
before being demultiplexed into parallel data streams, the encoding is called
joint encoding. This is shown below in Fig. 3.1 (a).The joint encoding has the
limitation of increased complexity which grows with increase in number of
transmitter antennas. In per-antenna encoding the input data packet is
demultiplexed into parallel data streams and then each of the streams is
separately encoded and radiated from the corresponding transmit antenna as
shown in Fig. 3.1 (b) [11].
Fig. 3.1 (a) MIMO with joint coding. Fig.3.1 (b) MIMO with per-antenna coding [11].
In [11], a multi-user downlink packet data transmission is considered. The
base station is equipped with multiple antennas. In the joint encoding scheme,
the entire data packet has a single cyclic redundancy check (CRC) appended
to the data. The substreams being transmitted via different antennas
encounter different channel conditions and hence different error statistics.
When one or two substreams may be corrupted the entire data packet has to
be retransmitted owing to the single CRC for the entire packet. This leads to
27
unnecessary retransmissions of the entire data packet reducing the
throughput efficiency of the system. Since the entire multiple transmit
antennas share a common single ARQ, the scheme of retransmission is called
MIMO single ARQ (MSARQ).This is shown in Fig. 3.2 (a) below.
To overcome the drawback of the MSARQ scheme, per-antenna encoding with
multiple ARQ processes has been proposed. A single CRC is attached to each
substream and detection of each substream is done independently. Thus only
those substreams which are corrupted will be retransmitted from that
particular transmitter antenna. The scheme of retransmission is called MIMO
multiple ARQ (MMARQ) and is shown in Fig. 3.2 (b) below.
Fig. 3.2. Transmitter structures for MSARQ AND MMARQ scheme [11]
In [11], the receiver side functionality consists of the HARQ combining and
the interference cancellation process. MMARQ and the MSARQ process
operate differently. In MMARQ, the HARQ combining technique and
interference cancellation step are blended advantageously. The content of the
detected substreams are validated by means of their cyclic redundancy check.
If the cyclic redundancy check detects uncorrectable error, a copy of the
corrupted stream will be stored and a retransmission will be asked for that
particular stream. A soft decision combining of all the received packets is done
so as to increase the probability of correct symbols being received. This is the
HARQ combining technique. The data obtained from HARQ combining is
28
then used for the interference cancellation. This ensures that the interference
contribution of the data stream is removed.
However, in MSARQ scheme the combination of packet combining and
interference cancellation cannot be done easily. Firstly each data stream is
decoded and interference cancellation is done. The data streams are then
demulitplexed into the single transmitted stream and correctness of the data
stream is then validated by means of the single CRC .The single CRC check is
ineffective in the sense that there may be several data streams which are
erroneous but have already undergone interference cancellation prior to CRC.
This result in a decoding error of each substream to propagate to substreams
decoded later. Based on the CRC, a retransmission of corrupted data frame is
requested and HARQ soft combining is performed.
MMARQ and MSARQ substream error performance carried out through
simulation in [11] show the superiority of the MMARQ scheme. The
performance of the MMARQ scheme achieves 26-40% throughput
improvement compared to MSARQ in case of perfect channel state and perfect
uplink feedback. However, comparatively a better performance of the
MMARQ scheme is achieved in case of non-ideal environment and non
perfect channel state information. MMARQ achieves a throughput
improvement of 30-45%. It is seen that MMARQ is relatively less sensitive to
estimation noise and feedback error.
In [13], a spatially multiplexed MIMO system is considered with channel state
information only at the receiver .Two types of HARQ schemes are considered,
namely single HARQ and multiple HARQ. In single HARQ the transmission
and retransmissions from all the layers is controlled by a single HARQ while
in multiple HARQ, the packets are preassigned to each antenna and
transmission/retransmissions take place only from the preassigned antenna.
Time division multiplexing is used for data transmission. To prevent
unnecessary retransmissions of entire data packet in single HARQ each time
slot is divided into a number of independent subtime slots.
29
Joint and separate detection was used for each of the HARQ schemes. In joint
detection the symbol received in current time slot and the retransmitted
symbols in subsequent time slots are jointly detected.
In [12], a joint detection combing scheme for single HARQ was proposed. The
joint detection was based on combing the soft decisions at receiver output
either prior to interference cancellation (pre-combining) or after the
interference cancellation (post-combining). When the detection was carried
out for linear zero-forcing (LZF) and the minimum mean-square error
(LMMSE) receivers, the precombining based HARQ achieved better
throughput gain than the post-combing HARQ scheme.
Similarly in [13], by virtue of simulation, it was seen that for a certain SNR in
joint and separate detection process, single HARQ achieves higher throughput
gain than multiple HARQ for a linear receiver structure. The superiority in
single HARQ is due to the random interleaving across the layers prior to
transmissions, thereby exploiting the diversity to develop robustness against
channel errors and this facilitates the detection process. But when the non-
linear interference cancellation (detection) is used by virtue of V-BLAST
architecture, multiple HARQ always outperforms the single-HARQ. Here, the
gain due to interference cancellation is far more than the gain achieved due to
interleaving across layers in case of single HARQ.
3.3 HARQ-MIMO retransmission techniques
In [15], the authors propose a cross layer design MIMO-HARQ protocol which
jointly exploits the spatial diversity of the MIMO transmission at the physical
layer and the link reliability of the data link layer. The proposed protocol
makes use of the MIMO V-BLAST architecture so as to benefit from the error
control performance in the link layer. The HARQ error control scheme is
adopted with selected repeat (SR) ARQ as the retransmission protocol.
30
The transmission of data packets is based on multiplexing the encoded
codeword of certain block length into sub-blocks via each of the several
transmitting antennas. This is shown in Fig. 3.3 below. Depending on the
retransmission technique adopted the cyclic redundancy check may or may
not be appended to each sub-block to validate the content. If the error pattern
can be corrected by the error-correcting code, then no retransmission is
required. Thus the transmission scheme by exploiting the spatial diversity
develops robustness against transmission error and at the same time
minimizes the number of retransmissions, in turn making better utilization of
resources.
Fig. 3.3. Dividing HARQ codewords over the transmit antennas [15]
Failure Block transmission rate (FBR) is a parameter which determines the rate
of failed transmission and it is considered as an important quality of service
criteria. By prioritizing the retransmission process the block error rate (BLER)
can be minimized. In [15], to ensure high reliability to the retransmitted data
various techniques are considered and are briefly discussed below.
In the basic retransmission technique, no CRC is used for each sub-block. Even
due to single error the whole block must be retransmitted. In any case the
entire block must be retransmitted, so the need for prioritizing the
retransmissions to new data transmission is no longer valid.
31
In the Alamouti retransmission (AR) technique, the CRC is not appended to
each sub-block and on detecting an error, the entire data block is
retransmitted. Priority is give to the retransmission process where each
symbol of the retransmitted block is coded by the Alamouti coding scheme
([15], referenced to [10] within the paper).
Another retransmission technique is based on resending the entire block on
that antenna having the best channel conditions. This is called the transmit
antenna selection retransmission technique (TASR). It is the responsibility of the
receiver which has perfect channel state information (CSI) to feedback the CSI
to the transmitting antenna.
The third technique (AR+CRC) facilitates either an entire block or a sub-block
retransmission. The sub-block retransmission is made possible by the presence
of the CRC appended to the end of each sub-block data. Due to the individual
CRC’s there is added complexity to the buffering and processing.
The fourth technique (TASR+CRC) is also a combination of the best antenna
selection with CRC appended to each sub-blocks. Retransmission of a single
sub-block as well as entire block is possible.
Simulation results carried out in [15] validate the fact that the proposed
HARQ-MIMO retransmission techniques achieve better throughput and
minimize the block error rates. The Alamouti coding scheme achieves the
better performance compared to the transmit antenna selection scheme but at
the expense of added complexity. Also individual CRC’s appended to each
sub-block results in added complexity but in very little additional
performance gain.
32
3.4 Cooperative Multicell ARQ in MIMO Cellular Systems
In [14], multiple base stations are considered and each of the base stations is
equipped with multiple antennas. The cooperation among the base stations
helps in achieving better coverage and throughput in the MIMO cellular
system. The enhanced performance due to cooperative multipoint processing
(CoMP) is virtue of exploiting the spatial diversity obtained by means of the
broadcast nature of the wireless channel. With the objective of increasing the
reliability of the transmission links, by exploiting the broadcast nature of the
wireless transmission, ARQ protocol at the data link layer is combined with
the cooperative diversity at the physical layer.
In [14], a three-cellular uplink transmission is considered with one serving
base station and two relaying base stations. This is shown in Fig. 3.4 below.
The serving base stations are connected to its neighbors via wired back haul
links. The back haul links serve the two purposes. Firstly, the cooperation
among the base stations to facilitate better transmission performance is done
via back haul links. Secondly, it serves as pathway to increase the reliability of
the transmission link by facilitating the retransmission of the data packets
when the data packet received at the serving base station is corrupted.
Fig.3.4. CoMP for 3 cells [14]
33
To enhance the reliability of the transmission links, conventional ARQ
protocols and cooperative multicell ARQ protocols are considered. The
conventional ARQ protocol does not make use of the relaying neighboring
base stations and fails to exploit the diversity achievable through relaying.
The cooperative multicellular ARQ protocols are of three types namely,
opportunistic decode-and-forward (ODF), amplify and forward (AF) and compress
and forward (CF).
In ODF the neighboring BS’s overhear the information intended to the serving
BS and try decoding it. On successfully decoding it re-encodes the data again
and forwards it to the serving BS. Maybe, due to the failure of the link
between the relays and serving BS (destination), a NAK is sent to the source
terminal as well as to the BS’s involved in relaying. The communication for
retransmission is carried out via the back haul links between the cooperating
BS’s and the serving BS’s.
In AF protocol, all the neighboring multiantenna base stations take part in
retransmitting their information from each of their antennas. The AF protocol
does not decode the information rather only amplifies and forwards the
information. The absence of decoding makes the AF scheme less complex in
comparison to the ODF scheme of retransmission.
In the CF scheme the information received at the base station is compressed
and forwarded to the base station. The performance of the scheme improves
with the increase in the reliability of the link between the relay and the serving
base station.
In [14], the comparison between the conventional ARQ and the CoMP ARQ
protocols were based on the average packet error rate (PER) for each of the
protocols. The packet error rate depends on the instantaneous SNRs of the BS
being served and also on the effective instantaneous received SNR at serving
34
BS via the neighboring BS. Simulation results carried out show that all the
cooperative multicellular ARQ protocols provide much lower average PER.
Also the performance gap between each cooperative mutlicell ARQ protocol
reduces with increase in the SNR of the back haul links. Additionally, the AF
protocol performs the best compared to the other two protocols when the
reliability of the backhaul link is considered to be at a certain minimum. Also,
the performance of the proposed protocols improves with increase in number
of BS’s and with number of antennas at each BS.
35
Conclusion
In this project a literature survey has been conducted on the application of
error correction schemes in wireless sensor networks and in MIMO systems.
Throughout the survey, the significant benefits of exploiting the cross layer
analysis of the physical layer and the data link layer has been observed. The
data link layer error control schemes in conjunction with the physical layer
architecture of wireless sensor networks and MIMO have considerably
improved the overall system performance that can be achieved. The gain in
system performances are quantified based on certain common performance
yardsticks such as throughput and energy efficiencies. In particular, special
emphasis was laid on surveying the various ARQ’s and HARQ’s in wireless
sensor networks and in MIMO systems. Different protocols were compared
and contrasted, highlighting their performance characteristics. The remarkable
benefits of the cross layer analysis have fostered a continued research interest
in this specific field of study.
36
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