On Adaptive Energy-Efficient Transmission in WSNs

One of the major challenges in design of wireless sensor networks (WSNs) is to reduce energy consumption of sensor nodes to prolong lifetime of finite capacity batteries. In this paper, we propose energy-efficient adaptive scheme for transmission (EAST) in WSNs. EAST is an IEEE 802.15.4 standard compliant. In this scheme, open-looping feedback process is used for temperature-aware link quality estimation and compensation, wherea closed-loop feedback process helps to divide network into three logical regions to minimize overhead of control packets. Threshold on transmitter power loss ( RSSI loss ) and current number of nodes ( n c (t)) in each region help to adapt transmit power level ( P level ) according to link quality changes due to temperature variation. Evaluation of the proposed scheme is done by considering mobile sensor nodes and reference node both static and mobile. Simulation results show that the proposed scheme effectively adapts transmission P level to changing link quality with less control packets overhead and energy consumption as compared to classical approach with single region in which maximum transmitter P level assigned to compensate temperature variation.


I. INTRODUCTION
WSNs are currently being considered for many applications; including industrial, security surveillance, medical, environmental and weather monitoring.Due to limited battery lifetime at each sensor node; minimizing transmitter P level to increase energy efficiency and network lifetime is useful.Sensor nodes consist of three parts; sensing unit, processing unit and transceiver [1].May 11, 2014 DRAFT Limited battery requires low power sensing, processing and communication system.Energy efficiency is of paramount interest and optimal WSN should consume minimum amount of power.
In WSNs, sensor nodes are widely deployed in different environments to collect data.As sensor nodes usually operate on limited battery, so each sensor node communicate using a low power wireless link and link quality varies significantly due to environmental dynamics like temperature, humidity etc.Therefore, while maintaining good link quality between sensor nodes we need to reduce energy consumption for data transmission to extend network lifetime [2], [3], [4].IEEE802.15.4 is a standard used for low energy, low data rate applications like WSN.This standard operate at frequency 2.45 GHz with channels up to 16 and data rate 250 kbps.
To efficiently compensate link quality changes due to temperature variations, we propose a new scheme for P level control EAST, that improves network lifetime while achieving required reliability between sensor nodes.This scheme is based on combination of open-loop and closedloop feedback processes in which we divide network into three regions on basis of threshold on RSSI loss for each region.In open-loop process, each node estimates link quality using its temperature sensor.Estimated link quality degradation is then effectively compensated using closed-loop feedback process by applying propose scheme.In closed-loop feedback process, appropriate transmission P level control is obtained which assign substantially less power than those required in existing transmission power control schemes.
Rest of the paper is organized as follows: section II briefs the related existing work and motivation for this work.In section III, we provide the readers with our proposed scheme.In section IV, we model our proposed scheme.Experimental results have been given in section V.

II. RELATED WORK AND MOTIVATION
To transmit data efficiently over wireless channels in WSNs, existing schemes set some minimum transmission P level for maintaining reliability.These schemes either decrease interference among sensor nodes or increase unnecessary energy consumption.In order to adjust transmission P level , reference node periodically broadcasts a beacon message.When nodes hear a beacon message from a reference node, nodes transmit an ACK message.Through this interaction, reference node estimate connectivity between nodes.
In Local Mean Algorithm (LMA), a reference node broadcasts LifeMsg message.Nodes trans- Radio irregularity results in radio signal strength variation in different directions, but the signal strength at any point within the radio transmission range has a detectable correlation with transmission power in a short time period.There are three main reasons for the fluctuation in the RSSI loss .First, fading causes signal strength variation at any specific distance.Second, the background noise impairs the channel quality seriously when the radio signal is not significantly stronger than the noise signal.Third, the radio hardware doesnt provide strictly stable functionality [10].
Since the variation is small, this relation can be approximated by a linear curve.The correlation between RSSI loss and transmission P level is approximately linear.Correlation between transmission P level and RSSI loss is largely influenced by environments, and this correlation changes over time.Both the shape and the degree of variation depend on the environment.This correlation also dynamically fluctuates when the surrounding environmental conditions change.
The fluctuation is continuous, and the changing speed depends on many factors, among which the degree of environmental variation is one of the main factors [11].
Propose energy efficient transmission scheme EAST helps efficiently compensate link quality changes due to temperature variation.To reduce packet overhead for adaptive power control temperature measured by sensors is utilized to adjust transmission P level for all three regions based on RSSI loss .Compared to single region in which large control packets overhead occur even due to small change in link quality.Closed-loop feedback process is executed to minimize control packets overhead and required transmitter P level .

III. PROPOSED ENERGY EFFICIENT TRANSMISSION SCHEME
In this section, we present energy efficient transmission scheme that maintains link quality due to temperature variation is as follows [12]: To compensate RSSI loss estimated from Eq.( 1) we have to control output P level of radio transmitter accordingly.Relationship between required transmitter P level and RSSI loss is formulated by Eq.( 2) using least square approximation [12]:  Nodes are placed at different locations in a square area of 100*100m and distance (d i ) between them is from 1 to 100m.For given environment temperature (T i ) can have values in range -10C 0 RSSI loss due to the temperature variation can be formulated using the relation between RSSI loss and the temperature experimented in Bannister et al [12].Equation for the RSSI loss for the temperature variation is as follows: Relation betweenP level and RSSI loss is formulated by using a least square approximation [12]: Maximum, minimum and average value of RSSI loss for all nodes in network can be formulated as: RSSI loss (min) = min(RSSI loss (i)) RSSI loss (max) = max(RSSI loss (i)) RSSI loss (avg) = (min(RSSI After finding maximum and minimum values of RSSI loss we will define upper and lower limit of RSSI loss to divide network into three regions and also set counter to count number of nodes in each region.Let suppose we have set counter zero initially and then define upper and lower bound and check condition, nodes that follow this condition are considered to be in region A ∀ i ǫ N. RSSI loss (Amax) = max(RSSI loss (i)) RSSI loss (Amin) = RSSI loss (avg) + 2 (10) count=0; Given that ∀ i ǫ N; RSSI loss (i) ≤ RSSI loss (Amax) and RSSI loss (i) > RSSI loss (Amin) Similarly we define upper and lower limits for region B and C and also check nodes that follow given conditions are said to be in region B and C respectively.
RSSI loss (Bmax) = RSSI loss (avg) + 2 (11) Given that ∀ i ǫ N; RSSI loss (i) ≤ RSSI loss (Bmax) and RSSI loss (i) > RSSI loss (Bmin) To apply our proposed scheme EAST we need to define threshold on RSSI loss for each region for energy efficient communication between sensor nodes.Threshold on RSSI loss for each region depends upon RSSI loss of all nodes in a particular region and number of nodes in that region.
Threshold on RSSI loss for each region is defined as: regions is defined as given below: Here P RR A , P RR B and P RR C are packet reception ratio for regions A, B, C respectively.
RSSI loss for each region on basis of propose scheme for given conditions like threshold RSSI loss and n c (t) is formulated as: Given that ∀ i ǫ N: Given that ∀ i ǫ N: Estimation of P level for new RSSI loss is formulated as ∀ i ǫ N:

Pt (dBm)
Pt (dBm) Fig. 6.Transmitter Power ature is high RSSI loss has high value means low quality link and vise versa.After estimating RSSI loss for each node in WSN we compute corresponding transmitter P level to compensate RSSI loss .Fig5 shows range of P level on y-axis for given RSSI loss that is between (20-47) and also variation of required P level for sensor node with changing temperature that is at low temperature required P level is low and for high temperature required P level is high.
As we have earlier estimated RSSI loss for each sensor node on the basis of given meteorological temperature that helps to estimate required P level to compensate RSSI loss .That power level only helps to compensate RSSI loss due to temperature variations.To compensate path loss due to distance between each sensor node in WSNs, free space model helps to estimate actual required transmitter power.After addition of required P level due to temperature variation and distance, we estimate actual required P t between each sensor node.Fig6 shows required P t including both RSSI loss due to temperature variation and free space path loss for different May 11, 2014 DRAFT In Fig7, we have shown P level using classical approach for three regions and in Fig8, P level for the proposed technique; EAST.We can clearly see the difference between P level assigned.
To show P level for each region, we take the difference between the assigned P level s using EAST and classical technique, as can be seen in the figures 9, 10, 11.As we know that in classical approach, there is no concept of sub regions, so, for the sake of comparison with the proposed technique; EAST, we have shown P level for different regions using classical approach.
After estimating RSSI loss for nodes of each region, we have estimated required P level for nodes of each region that we clearly see in Fig7, in region A, P level lies between (40-45), for region B (30-35) and for region C (20-25).It means that for region A required P level high than both other region that also shows that for that region temperature and RSSI loss is large.For region B required P level is between both region A and C and for C region required P level is less than both other two regions.We have earlier seen in Fig7 P level for each region assigned using classical approach.After applying proposed technique we see what P level required for each region.We can clearly see difference between P level as shown in Fig8, that required P level decreases for each region and for region A it decreases maximum.Fig9,10,11 respectively shows required P save for region A,B and C after implanting proposed technique.P save up to 2.3 for region A, 1.7 for B and 1.5 for C. requirement that cause maximum P save .We can clearly see maximum P save 12dBm to 20dBm for center location.When reference node move from center to one of the corner (0, 0) of square region P save remains constant approximately around 1dB, fact is that number of nodes near reference node region having same RSSI loss mean constant temperature and they need approximately same P level near threshold.P save for reference node movement from (0, 0) to (0, 100) fluctuate between -5dBm -6dBm and at two moments we observe maximum P save because number of nodes near reference node have to increase their P level to meet threshold is minimum.
Movement of reference node from (0, 100) to (100, 100) causes P save between -4dBm -12dBm and only one time peak P save .Similarly when reference node move from (100, 100) to (100, 0) P save remains in limits between -4dBm-7dBm and only one time maximum P save .From this figure it is also clear that for region A reference node location at center gives maximum P save that enhances network lifetime.We can also see variation of P save with respect to time that basically depends upon nodes near reference node have what RSSI loss if nodes have less RSSI loss then threshold then we have to increase P level that decrease P save and if nodes have large RSSI loss then threshold then we need to decrease P level that enhances P save .It is also clear from result that peak maximum and minimum P save comes at same time.
Similarly we can see P save for similar pattern of reference node mobility considering regions B and C. For region B in Fig13 when reference node at center location (50, 50) P save remains between 14dBm-20dBm, from center to (0, 0) P save remains between 0 -1dBm.When reference node moves from (0, 0) to one of the corner of square region (0, 100) P save fluctuate between 0 -4dBm.Reference node movement from (0, 100) to (100, 100) cause P save -1dBm-5dBm.Reference node movement from (100, 100) to (100, 0) P save -4dBm-5dBm.This figure also indicates that P save for region B is maximum when reference node at center location.For reference node mobility from center to (0, 0) P save remains constant due to constant RSSI loss near reference node region.For other reference node movements P save remains approximately constant due to less variations in RSSI loss .Compared to region A where P save goes to peak maximum and minimum value in region B P save remains on average approximately constant and less variation occurs, fact is that nodes in region B have approximately same RSSI loss near threshold.In future, firstly, we are interested to work on Internet Protocol (IP) based solutions in WSNs [14].Secondly, as sensors are usually deployed in potentially adverse environments [15], so, we will address the security challenges using the intrusion detection systems because they provide a necessary layer for the protection.

Fig2 shows complete flowFig. 2 .
Fig2 shows complete flow chart for reference node.Node senses temperature by using locally installed sensor and checks if temperature change detected.If there is any temperature change, compensation process is executed on the basis of Eqs(1,2).Nodes send an ACK message including temperature change information with a newly calculated P level .Apply ing this temperature-aware compensation scheme we can reduce overhead caused by conventional scheme in changing temperature environments.

Fig. 7 .Fig. 8 .
Fig. 7. Power level using Classical Approach for regions A, B, and C

Fig. 9 .Fig. 10 .
Fig. 9. Difference of Power level save between Classical Technique and EAST for region A

Fig12Fig. 11 .
Fig12 describes the effect of reference node mobility on P save for region A. Reference node move around boundaries of square region and nodes in a region considered to be static.When reference node is at center location (50, 50) of network maximum nodes around reference node have large RSSI loss than threshold so we need to reduce P level to meet threshold P level

Fig. 12 .
Fig. 12. Transmitter power save in region A for different Reference Node Locations

Fig. 13 .
Fig. 13.Transmitter Power save in region B for different Reference Node Locations

level to use for each of
May 11, 2014 DRAFT mit LifeAckMsg after they receive LifeMsg.Reference nodes count number of LifeAckMsgs and transmission P level to maintain appropriate connectivity.For example, if number of LifeAckMsgs is less than NodeMinThresh; transmission P level is increased.In contrast, if number of LifeAckMsgs is more than NodeMaxThresh transmission; P level is decreased.As a result, they provide improvement of network lifetime in a sufficiently connected network.However, LMA only guarantees connectivity between nodes and cannot estimate link quality [5].Local Information No Topology/Local Information Link-state Topology (LINT/LILT) and Dynamic Transmission Power Control (DTPC) use RSSI loss to estimate transmitter P level .Nodes exceeding threshold RSSI loss are regarded as neighbor nodes with reliable links.Transmission [6]evel also controlled by Packet Reception Ratio (PRR) metric.As for the neighbor selection method, three different methods have been used in the literature: connectivity based, PRR based and RSSI loss based.In LINT/LILT, a node maintains a list of neighbors whose RSSI loss values are higher than the threshold RSSI loss , and it adjusts the radio transmission P level if number of neighbors is outside the predetermined bound.In LMA/LMN, a node determines its range by counting how many other nodes acknowledged to the beacon message it has sent[6].Adaptive Transmission Power Control (ATPC) adjusts transmission P level dynamically according to spatial and temporal effects.This scheme tries to adapt link quality that changes over time by using closed-loop feedback.However, in large-scale WSNs, it is difficult to support scalability due to serious overhead required to adjust transmission P level of each link.The result of applying ATPC is that every node knows the proper transmission P during temperature variation in wireless environment.It utilizes open-loop process based on sensed temperature information according to temperature variation.Closed-loop feedback process based on control packets is further used to accurately adjust transmission P level .By adopting both open-loop and closed-loop feedback processes we divide network into three regions A, B, C for high, medium and low RSSI loss respectively.levelby utilizing both number of current nodes and temperature sensed at each node.Since power controller is operated not merely by comparing number of current nodes with desired nodes but by using temperature-compensated P level , so that it can reach to desired P level rapidly.If temperature is changing then temperature compensation is executed on basis of relationship between temperature and RSSI loss .Network connectivity maintained with low overhead by reducing feedback process between nodes which is achieved due to logical division of network.Transmission power loss due to temperature variation formulated using relationship between RSSI loss and temperature experimented in Bannister et al.. Mathematical expression for RSSI loss May 11, 2014 DiagramIn order to assign minimum and reachable transmission P level to each link EAST is designed.EAST has two phases that is initial and run-time.In initial phase reference node build a model for nodes in network.In run-time phase based on previous model EAST adapt the link quality to dynamically maintain each link with respect to time.In a relatively stable network, control overhead occurs only in measuring link quality in initial phase.But in a relatively unstable network because link quality is continuously changing initial phase is repeated and serious overhead occur.Before we present block diagram for proposed scheme some variables are defined as follows (1)Current nodes in a region n c (t) (2) Desired nodes in a region n d (t) (3) Error: e(t) = n d (t) − n c (t),(4) P level .Fig1 shows system block diagram of proposed scheme.PRR, ACK, and RSSI loss used to determine connectivity.ACK estimates connectivity but it cannot determine link quality.PRR estimates connectivity accurately but it causes significant overhead[8].In our scheme, we use RSSI loss for connectivity estimation, which measures connectivity with relatively low overhead.May 11, 2014DRAFTPower controller adjusts transmission P P t [dBm] = [η * (E b /N 0 ) * mkT B * (4πd/λ) 2 + RNF ] + RSSI loss(3)Parameters for propose scheme are,(1) Threshold RSSI loss for each region.(2) Desired nodes in each region n d (t) = n c (t) − 5, (3) Transmission power level P level for each region.RSSI loss is estimated for logical division of network, number of nodes with high RSSI loss considered in region A, medium RSSI loss considered in region B, and with low RSSI loss in region C.If (RSSI loss ≥ RSSI loss Threshold) and (n c (t) ≥ n d (t)) then Threshold transmitter P level assigned if for similar case (n c (t) < n d (t)) then similar transmitter P level assigned and if (RSSI loss < RSSI loss Threshold) then by default keep same transmitter P level .
(1,2)2)Based on Eqs(1,2), we obtain appropriate P level to compensate RSSI loss due to temperature variation. Tompensate path loss due to distance between each sensor node in WSN, free space model helps to estimate actual required transmitter power.After addition of RSSI loss due to temperature variation in Eq.(3), we estimate actual required transmitter power between each sensor node.For free space path loss model we need number of nodes in a network (N), distance between each node (d), (E b /N o ) depends upon (SNR), spectral efficiency (η), frequency (f ) and receiver noise figure ((RNF )):ThresholdRSSI loss is minimum value required to maintain link reliability.Reference node broadcasts beacon message periodically to nodes and wait for ACKs.If ACKs are received May 11, 2014 DRAFT from nodes then 2: N ← Number of nodes in Network 3: d ← Distance between each node and ref erence node 4: T ← T emperature f or each node 5: RSSI loss ← T ransmission power loss f or each node 6: P level ← P ower level f or each node 7: P t ← T ransmitter power f or each node 8: Region A ← HighRSSI loss 9: Region B ← MediumRSSI loss 10: Region C ← LowRSSI loss 11: n c (t) ← Current number of nodes 12: n d (t) ← Desired number of nodes 13: if RSSI loss (A, B, C) ≥ RSSI loss (T hreshold) then 14: if n c (t)(A, B, C) ≥ n d (t)(A, B, C) then 15: RSSI loss (new)(A, B, C) = RSSI loss (T hreshold) loss (new)(A, B, C) = RSSI loss (A, B, C) 20: if RSSI loss (A, B, C) < RSSI loss (T hreshold) then 21: RSSI loss (new)(A, B, C) = RSSI loss (A, B, C) 22: end if 23: P levsl (Save)(A, B, C) = P level − P level (new)(A, B, C) May 11, 2014 DRAFT 17)P RR is also an important metric to measure link reliability.Here count A are n d (t) and count Ā number of nodes not present in region due to mobility and (count A -count Ā) are n c (t).It is defined as number of nodes present in a region at particular time n c (t) to number of desired nodes n d (t) in a region.Similarly we can define P RR for regions B and C. P RR for all three