Wimax technology has reshaped the framework of broadband wireless internet service. It provides the internet service to unconnected or detached areas such as east South Africa, rural areas of America and Asia region. Full duplex helpers employed with one of the relay stations selection and indexing method that is Randomized Distributed Space Time are used to expand the coverage area of primary Wimax station. The basic problem was identified at cell edge due to weather conditions (rain, fog), insertion of destruction because of multiple paths in the same communication channel and due to interference created by other users in that communication. It is impractical task for the receiver station to decode the transmitted signal successfully at the cell edges, which increases the high packet loss and retransmissions. But Wimax is a outstanding technology which is used for improving the quality of internet service and also it offers various services like Voice over Internet Protocol, Video conferencing and Multimedia broadcast etc where a little delay in packet transmission can cause a big loss in the communication. Even setup and initialization of another Wimax station nearer to each other is not a good alternate, where any mobile station can easily handover to another base station if it gets a strong signal from other one. But in rural areas, for few numbers of customers, installation of base station nearer to each other is costlier task. In this review article, we present a scheme using R-DSTC technique to choose and select helpers (relay nodes) randomly to expand the coverage area and help to mobile station as a helper to provide secure communication with base station. In this work, we use full duplex helpers for better utilization of bandwidth.

1. Randomized – Distributed Space Time Selection
Scheme for Full Duplex Relay Helpers in
Development of Next Generation Wimax Network
Naveen Chauhan1
, A. L. N. Rao2
and Vineet Sharma3
1
Amity School of Engineering & Technology, Noida, India.
Email: rscholar_naveen@live.com
2
G.L.B.I.T.M., Greater Noida, India.
3
K.I.E.T. Ghaziabad, India.
2
Email: drrao.nit@gmail.com
3
Email: drvineetsharma.cse@gmail.com
Abstract— Wimax technology has reshaped the framework of broadband wireless internet
service. It provides the internet service to unconnected or detached areas such as east South
Africa, rural areas of America and Asia region. Full duplex helpers employed with one of
the relay stations selection and indexing method that is Randomized Distributed Space Time
are used to expand the coverage area of primary Wimax station. The basic problem was
identified at cell edge due to weather conditions (rain, fog), insertion of destruction because
of multiple paths in the same communication channel and due to interference created by
other users in that communication. It is impractical task for the receiver station to decode
the transmitted signal successfully at the cell edges, which increases the high packet loss and
retransmissions. But Wimax is a outstanding technology which is used for improving the
quality of internet service and also it offers various services like Voice over Internet
Protocol, Video conferencing and Multimedia broadcast etc where a little delay in packet
transmission can cause a big loss in the communication. Even setup and initialization of
another Wimax station nearer to each other is not a good alternate, where any mobile
station can easily handover to another base station if it gets a strong signal from other one.
But in rural areas, for few numbers of customers, installation of base station nearer to each
other is costlier task. In this review article, we present a scheme using R-DSTC technique to
choose and select helpers (relay nodes) randomly to expand the coverage area and help to
mobile station as a helper to provide secure communication with base station. In this work,
we use full duplex helpers for better utilization of bandwidth.
Index Terms— Randomized-Distributed Space Time Code, Co-channel Interference, Relay
Assisted Communication.
I. INTRODUCTION
Wimax i.e. Worldwide Interoperability for Microwave Access system is a standard technology of IEEE
family. The IEEE 802.16 standard for broadband wireless internet service is known as WIMAX. A working
group of broadband wireless access developed and deployed the standard (802.16) in 1999, used to provide
wireless mobile internet service (which broadend the bandwidth usage) across the cities and rural areas,
DOI: 02.ITC.2014.5.29
© Association of Computer Electronics and Electrical Engineers, 2014
Proc. of Int. Conf. on Recent Trends in Information, Telecommunication and Computing, ITC

2. 89
facilitate its services and features in telecommunication sector which supports for voice over internet
protocol, internet protocol TV, video conferencing, High definition video clarity and prime connection for
acquiring services such as cloud computing. IEEE 802.16m Wimax standard amended as an enhancement of
advanced techniques and services to support extended or better cell coverage area and higher system
capacity. In order to achieve higher performance gathered by subscribers at cell edges, the concept of [1]
mobile multi-hop relay (MMR) system has been defined. It specifies architecture for multi-hop transmission.
Basically in conventional single relay transmission network only fixed relays are used, those are dedicated
and static device. The relays stations work on decode and forward (DF) strategy. In this strategy, source
station sends its signal [2], [3] to air, relay station first receive it and apply the DF strategy to decode the
signal successfully and then regenerate the same received source signal to transmit through multi hops to the
destination.
Wimax cannot deliver 70 Mbps data rate over a distance of 31 Miles; it can operate at higher bit rate or over
a long coverage but both the factors are not possible simultaneously. Due to this type of operating
disadvantage i.e. at above distance of 31 Miles Wimax results in delivering very low data rate or maximum
packet loss. On the other hand by reducing the distance between base station and destination station below 1
Km allows a subscriber to operate at very high bit rate. This problem is faced at cell edges of Wimax
coverage area. For the solution of this problem either we have to compromise at cell coverage area or go with
another alternate i.e. cooperative relaying. A vivid range of prior literature on cooperative relaying exists in
multi-hoping communication network. However this review article differs from prior work by selecting the
relay stations randomly, there is no prior requirement of helpers before initiation of signal transmission.
Cooperative communications have been proposed to achieve high throughput at cell edges similar to that of
MIMO (Maximum input maximum output) system. It replaces the conventional direct communication
architecture or single hop communication system into multi-hop transmission model [4]. In below presented
figure 1, we presented direct communication architecture between base station and subscribers; mostly
subscribers are taken as mobile nodes. Wimax i.e. Worldwide Interoperability for Microwave Access system
is a standard technology of IEEE family. The IEEE 802.16 standard for broadband wireless internet service is
known as WIMAX. A working group of broadband wireless access developed and deployed the standard
(802.16) in 1999, used to provide wireless mobile internet service (which broadend the bandwidth usage)
across the cities and rural areas, facilitate its services and features in telecommunication sector which
supports for voice over internet protocol, internet protocol TV, video conferencing, High definition video
clarity and prime connection for acquiring services such as cloud computing. IEEE 802.16m Wimax standard
amended as an enhancement of advanced techniques and services to support extended or better cell coverage
area and higher system capacity. In order to achieve higher performance gathered by subscribers at cell
edges, the concept of [1] mobile multi-hop relay (MMR) system has been defined. It specifies architecture for
multi-hop transmission. Basically in conventional single relay transmission network only fixed relays are
used, those are dedicated and static device. The relays stations work on decode and forward (DF) strategy. In
this strategy, source station sends its signal [2], [3] to air, relay station first receive it and apply the DF
strategy to decode the signal successfully and then regenerate the same received source signal to transmit
through multi hops to the destination.
Wimax cannot deliver 70 Mbps data rate over a distance of 31 Miles; it can operate at higher bit rate or over
a long coverage but both the factors are not possible simultaneously. Due to this type of operating
disadvantage i.e. at above distance of 31 Miles Wimax results in delivering very low data rate or maximum
packet loss. On the other hand by reducing the distance between base station and destination station below 1
Km allows a subscriber to operate at very high bit rate. This problem is faced at cell edges of Wimax
coverage area. For the solution of this problem either we have to compromise at cell coverage area or go with
another alternate i.e. cooperative relaying.
A vivid range of prior literature on cooperative relaying exists in multi-hoping communication network.
However this review article differs from prior work by selecting the relay stations randomly, there is no prior
requirement of helpers before initiation of signal transmission. Cooperative communications have been
proposed to achieve high throughput at cell edges similar to that of MIMO (Maximum input maximum
output) system. It replaces the conventional direct communication architecture or single hop communication
system into multi-hop transmission model [4]. In below presented figure 1, we presented direct
communication architecture between base station and subscribers; mostly subscribers are taken as mobile
nodes. Some of the mobile nodes are located at cell edges, due to strong attenuation, natural sources trees,
and forests etc, interference created by other users in the ongoing communication and relatively low transmit
power; the direct communication between BS and MS is put down.

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Fig. 1 Direct Communication between BS and SSs in Wimax system in presence of natural sources
Most of the existing literature work on cooperative relaying network has adopted the half duplex protocol, in
which relay station transmit and receive the source signal in different time-slots. This orthogonality results in
inefficient use of channel bandwidth. Due to inefficient use of channel bandwidth the dispersion variation of
packet delay becomes unmanageable. On the other hand, full-duplex relaying [5], [6] overcomes the
drawbacks specified in the above half duplex scenario by allowing the relay stations to transmit and receive
in the same time-slot and [7] frequency band. This simultaneous process results in efficiently use of system
resources and channel bandwidth and outperforms the half duplex protocol. In this work, we study the effect
of selection and indexing scheme randomized- Distributed Space Time for mobility environment with respect
of half duplex and full duplex protocols. The full duplex (FD) protocol requires only one channel [8] for end-
to-end transmission, due to this; FD introduces loop back interference (to be described in section III in detail)
due to signal leakage between the relay station output and input. We study the feasibility of the full duplex
protocol in the presence of residual loop back interference, we consider a threshold point where FD mode can
outperforms the HD mode.
The rest of this article is organized as follows. Randomized Distributed Space Time scheme is introduced in
Section II with their Physical Layer and MAC layer specifications. In Section III, these models are applied to
check feasibility with FD protocol and analysis the problem loop back interference introduced due to FD
mode.
II. SPECIFICATION OF R-DSTC SCHEME
Wimax, A Metropolitan access network technology is much demanded in compacted and hardly populated
areas such as East Africa and North America. It has various advantages like supporting high data rates and
offering ultra-wideband. The enhanced standard IEEE 802.16j introduced mobile multi-hop relaying. A
Wimax conventional multi-hoping relay [1] network mainly consists of three network components as shown
in Fig. 2: the base station, subscriber stations and the relay stations, See Fig 2. Due to low transmit power and
other responsible factors, at cell edges any mobile node can communicate with the help of relative relay
station.
On the other hand, those subscriber’s nodes have strong transmit power capability, can communicate directly.
As presented in Fig 2. home subscriber 1 has enough power signals to communicate with base station
directly. Relay-assisted multi-hop communications have been shown to acquire significant [9] efficiency
boosters at cell edge’s problems over the legacy of direct communication between base station and
subscribers. In multi-hop relay networks, the advantage of multiple relays enhances the reliability and
throughput performance. Benefits of multi-hoping includes to overcome dead spots, reduction of transmit
power and increases the capacity gain. The term diversity technique is single most imperative [10] patron for
reliable wireless communication, since the channel acquiring for communication may suffer from effects of
fading, high attenuation due to addition of destructive multi-paths in the same communication media and due

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Fig. 2. Network elements specified in IEEE 802.16j standard
to interference produced by other users in the same communication channel. It is impractical to diagnose the
transmitted signal at destination unless some less affected copy of the transmitted signal is lended to the
receiver in case of strict attenuation. For reliable communication, it is mandatory to transmit multiple copies
of the signal but with short delay. The overall communication overhead for the above technique is not
desirable in the situation where number of subscribers is large. Another reliable technique has been
introduced i.e. Space Time Coding. It is a method which has been utilized to [4] elevate the reliability of data
transmission in wireless communication by using multiple transmits antennas. Source transmits multiple
redundant copies of a data stream in the air in a hope that at least some of them (redundant copies) may
survive the physical path. Recently, several amendments have been introduced for cooperation among relays
to provide spatial gains without utilizing multiple transmit antennas.
Recently, two new cooperation protocols [3], [4]; Decode-and-Forward and Amplify-and-forward have been
introduced to achieve the expected spatial gains without using multiple antennas, which was mandatory
checkpoint in space-time-coding method. The problem identified in multi-mobile relaying network is
selection and indexing of the relay nodes. For indexing and perfect selection of relay nodes, various solutions
are being proposed, one such effective solution specified in [1], named Distributed Space Time coding
(DSTC). It is a physical layer technique used to authorize the capabilities of relay station, so that they act as a
helper in a multi relaying wireless network. DSTC turn to use spatial diversity gain and achieves more than
expected range of end-to-end transmission, better throughput performance and less bit error rate. If we see
highlights, a Wimax system is typically designed and deployed in highly mobile environment, more than
80% of subscribers are mobile subscribers. This may lead to a substantial overhead in a mobile network to
identify the location of mobile subscribers where the channel statistics vary frequently. Due to this, DSTC
has various flaws over mobility environment such as before each transmission of signal all the relays which a
central node controller wants to [8] recruit are selected and indexed, so that they know who wants to
participate in cooperation and which signal stream will be transmitted by different antennas of relay station.
In distributed environment with mobility, this leads to extra signalling overhead for monitoring the location
of mobile subscriber to select and index relaying for them.
For mobility environment, the drawbacks identified in DSTC scheme can be directed by employing an
effective variation of DSTC called, Randomized-Distributed Space Time Coding, which expel the
requirement of space time codeword simulation and reduces the coordination between the source and the
relays. Main flaws specified in DSTC scheme respect to mobility environment has been addressed by R-
DSTC unique [1], [8], [11] operation and service. This emerged physical layer technique allows every
intermediate node, not only indexed helpers simulated and selected in previous (DSTC) scheme for each
subscriber individually, to potentially work as a relay station as long as it successfully decodes the source
signal and eager to participate in cooperation. This result leads to perform better utilization of system
resources (bandwidth usage etc) and maintain the factors to control the loss of packets. As much as mobile
subscribers change their location, there is no prior requirement for indexing relay helpers for their signal
transmission. In this emerged technique, any source mobile node immediately sends its signal to air; if the

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signal capacity is too poor for direct communication with base station then any relay node with relaying
capability (to be described in Section IV) can decode the signal reliably and then relay it to air after re-
encoding. Therefore, the intermediate helpers with relaying capability can be conveniently selected on
demand.
III. FEASIBILITY OF FULL DUPLEX RELAY HELPERS
Most of the existing research has been worked on cooperative communication by assuming half duplex
relaying, where relay nodes works [3] to support the source transmission by transmitting and receiving the
source data in orthogonal1
channels. Since delay variations increases time by time, this leads to inefficiently
utilization of the channel bandwidth and ineffective impact on throughput performance. On the other hand
full duplex relaying consider simultaneously transmission and reception of signal which uses same frequency
and time channel at the radio relay, which leads to result more efficiently utilization of the spectral
efficiency. The full duplex mode requires only one channel for the end-to-end transmission. The channel can
be divided either in frequency division mode or in time division mode. In frequency division mode, the
uplink [12] and downlink channels are located on separate frequencies.
Fig. 3. Frequency Division Duplexing model
Fig. 4. Effect of loop back interference at relay station
In the above drawn Fig 3. We presented a frequency division model. Base station sends a frame after each
periodic [13] time in 2.5 to 20ms. The whole frequency channel divided into different frequency slots, in
which uplink subframe and downlink subframe are sent through separate frequency sub channels. A fixed-
1
Orthogonality is a technique used to utilize one common channel completely either for transmission purpose or for
reception of signal. Thus there is no interference possible at destination. Because following node is busy in listening the
status of channel.

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duration frame is used for both uplink and downlink transmissions, which is transmitted after each 2.5 to
20ms.
The simultaneous transmission and reception of signal at relay nodes, introduces a problem called loop back
interference. In wireless communication, interference produced by other user’s communication, makes wrong
impact on system performance. The below mentioned problem is specified in [3], [5], [6], [14] in which
signal leakage occurs at relay station between the relay output and input. Basically, relay nodes receive
source data plus transmitting data which is transmitting by them (same relay nodes) and some additive white
Gaussian noise2
. See Fig 4. We have taken some [3], [15] abbreviations like ƴSR, ƴLI and ƴRD which
represents the channel gain factors with respective to frequency-flat-source-relay, residual loop interference
and relay-destination channels. As shown in the Fig 4. SS1 sends a signal to relay station using decode-and-
forward protocols, at the same time that same relay node sends a signal which is received from other SS2 and
simultaneously receives the signal from source station SS1. At relay station, both transmitting signal and
currently receiving signal are collapsed and relay station cannot determine the receiving signal. On the other
hand, the half-duplex mode eliminates the loop back interference, but this dwindles the end-to-end rate. In
full duplex mode, if we maintain the loop back interference value up to a controlled level by increasing the
distance and by placing the obstacles between the antennas, our model can outperforms the half duplex mode
easily. Now here a question arises, how much loop back interference level is acceptable? In order to analysis
the feasibility of full-duplex employed with a relay station in the presence of threshold point (controlled
level) of loop back interference. We focus on following issues- should loop back interference be permitted in
full duplex mode or full duplex mode can achieve the double performance that of achieved by half duplex
mode.
IV. RELAY RECRUITMENT AND MAC LAYER SPECIFICATION
The recruitment policy of relay nodes for R-DSTC scheme is different from DSTC scheme. Basically R-
DSTC scheme is used for distributed mobile environment, where channel statistics vary frequently. In DSTC
scheme, a predefined set of helpers are recruited before the initiation of signal [1] , [11] transmission, is
synchronized in a pattern that is being indexed for a specific antenna of a space time coding where each relay
node simulates a specific antenna of an underlying STC code.
Fig. 5A. Cooperative communication in mobility environment
Since a specific signal stream is transmitted by unique helper corresponding to its designated antenna, this
helper is recruited prior and indexed for transmission of this single signal stream until channel statistics does
not being changed. In conventional cooperative communication networks have only fixed relays for relaying
purpose. Let us assume we use DSTC scheme for cooperative relaying and selects and indexes some
2
Additive white Gaussian noise is random radio noise characterized by a wide frequency band. The additive property of
noise stands that received signal equals to the transmitted signal plus some noise, where additional noise has no relation
with signal.

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Fig. 5B. Cooperative communication in mobility environment
predefined helpers for a specific mobile subscriber. In the above presented Fig 5A, we consider cooperative
communication system in mobility mode. In Fig 5A. At time t1 a mobile subscriber is located at some
location inside the Wimax system coverage area. We assume at time t1, the distance between MS1 is
approximately 30 Miles and associated relay node is located in between MS1 and base station. Initially
distance between RS and MS 1 and between BS and RS are about 12 Miles and 20 Miles respectively. We
assume that using DSTC scheme before data transmission relay node 1 is associated with MS1 for
cooperative communication. But after time t2, MS1 is being moved to some other location, presented in Fig
5B. We can see that still relay node is associated with MS1 as a cooperative relaying. But now scenario has
been changed in terms of distance and power transmission. The distance between RS1 and MS1 (still located
at cell edge) is about 35 miles, which is impossible for relay station to reliably decode the source signal.
Faults which are addressed by above problems can be easily rectified by R-DSTC scheme. A large scale
Wimax system requires a huge number of relay stations for better performance of the model. In order to
provide high end-to-end transmission, we are taking three types of relaying nodes [1], [9] : RSs- standard
conventional fixed relay station, femto/picocell BSs- the fixed femto/picocell BSs with wireless relay
functionality, and mobile relays- any subscriber station with relaying capability, wishes to work as helper.
A. Relay Node Discovery
In IEEE 802.16j standard, some basic functionalities and services offered by conventional relay stations are
predestined and known by the base station. Other additional relay stations such as any subscriber and sub-
base stations are required to identify the functionalities and services by base station. Any additional relay
stations can likely work as a helper and required to endorse their services. When any station enters into the
network either it is fixed relay station or any additional subscriber, it instantly delivers a basic capability
request (SBC-REQ) message to the base station announcing their essential services it can support. In the
pattern of SBC-REQ message specified in [1] includes a bit to classify the cooperative capacity of the
additional subscribers are empowered or not. That message also contains various information such as number
of antennas available for relaying purpose, mobility model and exact location of the relay node. In reply of
SBC-REQ message base station acknowledged by sending subscriber basic capability response (SBC-RSP)
message. Based on this transmission, the base station explores the existence of all available helpers.
B. Selection of Relay Nodes
In order for selection and indexing the helpers/relay nodes for each subscriber station, the base station
performs recruitment strategy, if it requires. At cell edges, most of the subscribers communicate through
cooperative relaying. It is consider that the direct communication from the source to destination is stagnant
due to strong fading of signal and comparatively low transmit power. As specified by author in [1], [4], [12],
[15] base station periodically broadcasts the downlink channel descriptor (DCD) and uplink channel
descriptor (UCD) message in a Wimax system to all nodes. The DCD is type of medium access control
management message and it encompasses information regarding frame duration codes, downlink burst

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profiles and parameters required for transmission such as modulation and coding techniques. Basically
Wimax system has divided its MAC layer into three sub layers parts- convergence sub layer, common part
sub layer and security sub layer. The common part layer is mostly responsible for connection establishment,
resource allocation required for connection establishment and scheduling between base station and subscriber
stations. The SS establishes a connection with BS using the contention slots [12] in the uplink sub-frame. For
entering and registering a new SS into the network, subscriber must follow the following procedures-
I. Base Station periodically sends the Uplink/Downlink map messages and Uplink channel descriptor
& Downlink channel descriptor into the air.
II. When any new subscriber enters into the network, it receives the downlink map message. It tries to
establish physical synchronization with the base station.
III. In the second step, subscriber waits for the uplink channel descriptor message to obtain the required
parameters for uplink channel.
IV. After successfully obtaining the uplink channel parameters, subscriber again receives the
uplink/downlink map to extract the time slot information for uplink communication.
V. Performing above steps successfully, in the next periodic time slot 2.5- 20ms; subscriber wait for
transmission opportunity to perform ranging operation.
Initially, when any new SS enters into the network, it waits for Downlink/uplink –map message; which is
periodically sent by base station after a certain interval. As well as base station sends downlink channel
descriptor messages and uplink channel descriptor messages. After receiving the DCD successfully, SS
establishes the physical synchronization by retrieving the frequency offset information. Now SS knows
various information such as frame duration, downlink center, frequency and base station ID etc and waits for
uplink channel information. In the next step, SS retrieves parameters from UCD message that contain
information about uplink center frequency, bandwidth request opportunity size and most important is
cooperation control information element for our model. This information is required to index the relaying
helpers, which is embedded inside UCD message. After performing the ranging operation, a sixteen-bit
unique identification number is allotted within the network for each one BS to one SS that is referred to as
connection identifier (CID). Afterwards, allotted CID is used to establish the communication via relaying
helpers or all available nodes concert their transceivers to the signal transmitted through these CID’s and
relay it to the destination successfully.
V. CONCLUSIONS
In this article, we presented an effective cooperative model for Wimax system. Through literature review,
we identify that proposed model would outperform the previous model and check the feasibility of the model
in presence of loop back interference. We can use some physical obstacles between the transmission and
receiving antenna of the relay station to overcome the effects of loop back interference. We investigated that
our model would lead over distributed space time code model in a distributed environment with mobility.
There is a lot of scope for analysis and justify our assumption to calculate the performance measures in terms
of BER (bit error rate), throughput calculation that will help and positively support our assumption in favor.
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