Secure Inter-Frame Space Communications for Wireless LANs - MDPI

future internet Article

Secure Inter-Frame Space Communications for Wireless LANs Il-Gu Lee Department of Convergence Security Engineering, Sungshin University, 02844 Seoul, Korea; [email protected] Received: 19 April 2018; Accepted: 3 June 2018; Published: 4 June 2018

Abstract: The internet of things (IoTs) offers a wide range of consumer benefits, from personal portable devices to internet-connected infrastructure. The wireless local area network (WLAN) is one of the potential candidates for IoTs technology to connect billions of smart devices. Long-range WLAN is widely deployed in dense networks as an alternative to cellular networks or satellite internet access because of its low cost, high performance, and existing ecosystem. However, due to the nature of unregulated communications in industrial, scientific, and medical (ISM) bands, WLANs experience interferences from other radios such as radars and frequency hopping devices. Once interference is detected at a WLAN device, the channel is avoided and other channels become crowded. Thus, it degrades network throughput and channel utilization. In this paper, a secure inter-frame space communication system design is proposed for WLANs to share the ISM bands with other radio systems that generate periodic radio signals. The proposed secure inter-frame communication scheme achieves the goal by designing accurate radar detection and secure inter-frame space communication. The simulation results demonstrate that the proposed scheme significantly increases the receiver sensitivity and user datagram protocol throughput. Keywords: secure communications; WLAN; interference; radar

1. Introduction Wireless local area networks (WLANs) such as IEEE 802.11b/g were initially deployed in 2.4-GHz bands, in which industrial, scientific, and medical (ISM) devices are allowed to operate [1]. As WLANs were deployed widely and became an essential feature of everyday life, more spectrum was required because the 2.4-GHz band has only three channels. Thus, WLANs were extended to operate in the 5-GHz (IEEE 802.11a/n/ac/ax) and Sub-1-GHz (IEEE 802.11af/ah) ISM bands, which are also used by many other radio systems such as cordless phones, DECT (Digital Enhanced Cordless Telecommunications), and radars [2]. Because of their dependence on the same channel frequencies, the possibility of interference is higher. Several studies have investigated the interference effect, and several approaches to using new protection mechanisms in WLAN systems have been reported [3–6]. However, their scope was confined to demonstrating deteriorated performance due to interference for WLANs, and coexistence by channel avoidance as a passive way [3,4]. Some of the new protection mechanisms require modification of the current standard [5,6]. To protect the channel medium in the current WLAN standards, WLANs basically operate in the manner of carrier detection and avoidance. For the carrier detection and avoidance operation, spectrum-sharing mechanisms, such as dynamic frequency selection (DFS) and transmit power control (TPC), are required for WLANs [7,8]. Dynamic frequency selection and TPC allow WLANs to share channels with various radio systems, which are operating on the 5-GHz ISM channel frequencies. However, these conservative channel-access mechanisms are not efficient in terms of channel utilization and network throughput. Future Internet 2018, 10, 47; doi:10.3390/fi10060047

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In IEEE 802.11ac and IEEE 802.11ax standards, WLANs support 80-MHz and 160-MHz bandwidth operations [9,10]. Although IEEE 802.11ac/ax can adjust the bandwidth dynamically using pre-defined protocols between the transmitter and receiver, these wide-bandwidth modes may not work in the real world because they are operated in an “all-or-nothing” mode against non-802.11 interferences. In other words, WLANs may fail to communicate in the wide bandwidth if the channel is affected by other radios’ interferences. For example, in the 5-GHz radar bands, WLANs should not cause interference in the radar systems. The International Telecommunication Union (ITU) Recommendation on DFS provides test patterns and threshold criteria to test DFS implementations [8]. According to the standards in WLANs and ITU Recommendation, a DFS system should meet all the test criteria, and the system should avoid the radar channel automatically. However, it is not an efficient way of communication because WLANs and radar systems can share the frequency spectrum if WLANs detect radar more accurately and transmit data frames in idle time for the shared channel. As another example, frequency hopping radios are widely used in ISM bands because of their robustness, long range, and low cost. However, if a frequency hopping radio and a WLAN device exist in the same channel, the frequency hopping signal prevents WLANs from communicating in the channel. Eventually, the WLAN devices should change the channel. Since the radar systems and frequency hopping radios can be adopted for many purposes in the modern wireless systems, secure inter-frame space communication is required to improve channel utilization and network throughput. Radars were used for decades before the same frequency bands were included in 5-GHz ISM bands that were allocated for unlicensed use. Since frequency resources have become more valuable, frequency spectrum sharing has become indispensable. The ISM bands have been adopted by many local access specifications. Radar systems emit very strong pulse signal arrays that interfere with the network communication packet signals. This scenario is less threatening than the opposite case, since WLAN communication packets are retransmitted if they are not successfully received. A serious problem occurs in the opposite direction, that is, the WLAN’s packet signals interfere with radar operation. Although the WLAN’s signal is usually low-powered and limited within a small range, the radar detector is designed to detect a very small signal reflected by objects located far away from the radar transmitter. If a WLAN or a corresponding 5-GHz transmitter system works near a radar system, the operation of the radar system will be vulnerable and it can cause a serious information error. Radar systems are used for weather measurement, military information, etc. The false alarm of the radar system may cause much more serious problems than at WLAN communications. To avoid radar malfunction or false alarm, WLAN societies have developed a technique called dynamic frequency selection (DFS) to avoid using the radar-occupied frequency bands. Dynamic frequency selection is meaningful since radar systems usually use narrow frequency bands. Wireless local area network systems use wider frequency bands of 20 MHz or 40 MHz compared to radar systems. If a WLAN system detects a radar pulse signal, it leaves the frequency bands for another clear frequency band, just as it leaves a frequency band when the frequency band is too noisy. At the beginning of the WLAN operation, an access point (AP) can search the best channel or frequency band and can detect and avoid the frequency band that a radar system occupies. The term DFS has been used for avoiding the radar-occupied frequency bands. The DFS operation includes radar signal detection, decision, and frequency band change. The radar signal detection should be made in the physical layer (PHY), while decision and frequency change occur in the medium access control (MAC) layer. The radar signal detection tries to detect aspects of radar signals, such as the pulse width, frequency band, and pulse period, which represent radar signals well. In general, radar signals have many different types and parameters. It can be complicated for the signal detector to match all types of radar signals. The most popular method to detect radar signals is to measure the power of the received signal. Once the radar signal power is measured, the WLAN transceivers find and match the pulse width, pulse period, etc. Since the periods of the radar pulses are relatively long, this match process usually


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belongs to the MAC layer instead of the PHY. The normal period of radar pulse signals is wider than the WLAN packet lengths. The frequency change is also among the main functions of the MAC layer. This paper comprises the following: Section 2 explains the general background of the periodic interference detection technique. Section 3 explains the inter-frame space communications. Section 4 demonstrates the performance evaluation results, and the conclusion is given in Section 5. 2. Periodic Interference Detection Techniques Periodic interference detection depends on what characteristics of the interference are to be detected. Periodic interferences vary in carrier frequency, pulse period, staggered pulse repetition frequency (PRF), etc. One or a combination of these characteristics can be used to determine the interference. The difficult part is that the designed periodic-interference detection algorithm should detect as many periodic interferences as possible, throughout the world. This generality is difficult to achieve since not all the radios’ information is open to the public. 2.1. Detection of Carrier Frequency and Bandwidth If the carrier frequency is to be measured to determine a periodic interference, the assumed interference’s carrier frequencies must be known. If the received signal resides within a narrow frequency band centered around an assumed carrier frequency, the incoming signal may be determined as the assumed interference. This method needs to know all the interferences’ frequencies and their spectral band characteristics, or this information should be adaptively given according to the places near the known radio system. Many radio systems do not reveal which frequency they use and how it is used. This method needs a vast list of radio signal frequencies and bandwidth, and tries to analyze and match the frequency of the pulses [11]. It is also difficult to measure the tone frequency since the pulse width is short. With 20-MHz sampling, a pulse width of 1 µs provides only 20 samples. 2.2. Periodicity of Pulse or PRF Most of the radar pulses and frequency hopping signals have periodicity with either constant period or staggered period with two or more periods. If a radio system uses random periods between pulses, the interference detector cannot use this characteristic. A constant period or PRF requires the simplest detection algorithm that searches the periodic appearance of power. This approach is the easiest to develop, since radar signals have a simple periodicity scheme thus far [12]. 2.3. Signal Power and Pulse Width It is common to measure the power of the received signal and compare it with a threshold to determine if a signal exists. In addition to this, if pulse width is considered, most radar pulses are very short compared with WLAN frames. For example, many weather radar systems use 1-µs or even shorter pulses. If a power above a power threshold appears for less than a time threshold, it can be a radar pulse [13]. This approach needs fine tuning to decrease the false alarm probability. A large power signal can appear through other sources instead of the target periodic interference. Although an impulse noise may exceed a power threshold, it should not be determined as a periodic interference signal. One method of fine tuning is to match the received power with the known pulse power characteristics, such as constant power and abrupt appearance and disappearance, as well as short width. It should be noted that some radar signal pulses have a pulse width of over 100 µs. In this case, it is difficult to find an appropriate period threshold for the “short” pulse.

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3. Secure Inter-Frame Space Communication 3. Secure Inter-Frame Space Communication 3. Secure Inter-Frame Space Communication

Periodic Interference Detection 3.1.3.1. Periodic Interference Detection

3.1.The Periodic Interference Detection The implemented periodic interference detection method is based on signal the signal power implemented periodic interference detection method is based on the power and and pulse pulse periodicity. Thus far, this periodic property of radar systems has been simple and clear for periodicity. Thus far, this periodic property of radar systems has been simple and clear for most cases. The implemented periodic interference detection method is based on the signal power and most cases. This means that periodic some simple periodic detectioncan algorithm can perform wellradar for most This means that some simple detection algorithm perform well for most pulse periodicity. Thus far, this periodic property of radar systems has been simple and clear cases. for radar cases. Figure 1 shows the block diagram of the DFS algorithm implemented. Figure showsThis the means block diagram of simple the DFSperiodic algorithm implemented. most1cases. that some detection algorithm can perform well for most radar cases. Figure 1 shows the block diagram of the DFS algorithm implemented.

Time Stamp/ Memory Decision DFS Time Width Stamp/ Memory Decision DFS Width Figure 1. Block diagram of the dynamic frequency selection (DFS) algorithm. Figure 1. Block diagram of the dynamic frequency selection (DFS) algorithm. Power Measure Power Measure

Figure 1. Block diagram of the dynamic frequency selection (DFS) algorithm.

To detect the periodicity of pulses, a receiver needs to measure the pulses. Since the tone To detectand the periodicity pulses, a receiver needs to measure Since the tone frequency frequency pattern areofnot known for all the radio devices, the its pulses. power is a good measure for To detect the periodicity of pulses, a receiver needs to measure the pulses. Since the tone anddetecting pattern the are pulses not known for all the radio devices, its power decreases is a good with measure formeasurement detecting the asynchronously. Of course, the sensitivity power frequency and pattern are not known for all the radio devices, its power is a good measure for pulses asynchronously. Of course, the sensitivity decreases with compared compared to synchronous tone signal measurement. Figure 2 power shows measurement the block diagram of theto detecting the pulses asynchronously. Of course, the sensitivity decreases with power measurement synchronous tone signal measurement. Figure 2 shows the block diagram of the implemented periodic implemented periodic interference detector. compared to synchronous tone signal measurement. Figure 2 shows the block diagram of the Measuring the signal power requires averaging over a window. If this window is short, the interference detector. implemented periodic interference detector. averaging is not enough,power so that its averaged power over will not give a secure measurement. Sudden Measuring the window. this window short, Measuring thesignal signal powerrequires requires averaging averaging over aa window. IfIfthis window is is short, thethe noise can crash the detection algorithm. Accordingly, it is important to have a long enough window averaging is not enough, so that its averaged power willwill not not givegive a secure measurement. Sudden noise averaging is not enough, so that its averaged power a secure measurement. Sudden size forthe averaging. The maximumAccordingly, window size is limited by thetoshortest pulse width, even as short cannoise crash detection algorithm. it is important have a long enough window size can crash the detection algorithm. Accordingly, it is important to have a long enough window as 0.5 µ s, which is defined window in Reference [14]. With by thisthe short pulse pulse width,width, only 10 samples areas forsize averaging. The maximum size is limited shortest even as short for averaging. The maximum window size is limited by the shortest pulse width, even as short available with 20-MHzinsampling. The designed detector uses eight samples averaging theavailable power, 0.5as µs,0.5 which is defined Reference [14]. With short onlyfor 10only samples are µ s, which is defined in Reference [14].this With thispulse shortwidth, pulse width, 10 samples are that is, a window size of 0.4 µ s. Thus, for every 0.4 µ s, the averaged power output is passed tois, aa with 20-MHz sampling. designed eightuses samples for averaging the power, that available with 20-MHz The sampling. The detector designeduses detector eight samples for averaging the power, comparator that compares it with a threshold and determines if there is a signal. window of 0.4 µs. the averaged output is passed comparator that is,size a window sizeThus, of 0.4for µ s.every Thus,0.4 forµs, every 0.4 µ s, thepower averaged power output to is apassed to a comparator compares it withand a threshold andifdetermines if there is a signal. that compares that it with a threshold determines there is a signal. Shift Register

( )2 ( )2 ( )2

+ + ++

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+

+ +

+

2

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Power Threshold Power Threshold

+ +

Shift Register Shift Register

+ + Shift Register Count Threshold Count Threshold

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rad_det rad_det

Valid Packet Valid Packet Figure 2. Block diagram of periodic an interference detector.

Figure an interference interferencedetector. detector. Figure2.2.Block Blockdiagram diagram of of periodic periodic an


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With this many averaging samples, the power measurement has a problem in the start time. If the start time is, inopportunely, in the middle of the 0.5-µs pulse, the two consecutive averaged powers will divide the total power of the pulse and fail to exceed a threshold. This problem can be eliminated with the moving average window. Since it is a moving average, a pulse will provide a plateau shape and the maximum power is always available. These moving averaged powers are first compared with a threshold to save the memory space instead of saving the power information. Here, the power threshold is set by WLAN standards as −62 dBm. Wireless local area network devices can communicate through an idle channel in the unit of the 20-MHz channel, which is not used by other radios. For example, if the WLAN’s frequency bands are used by a complete 40-MHz band or upper or lower 20-MHz band, the periodic interference detection is performed for upper and lower 20-MHz bands. If WLAN uses the lower 20-MHz band and a periodic interference is detected in the same 20-MHz band, the WLAN must coordinate 20-MHz and 40-MHz transmissions so that they do not interfere with the periodic interferer. If the periodic interference is detected in the upper 20-MHz band only, the WLAN can continue to transmit 40-MHz bandwidth packets during idle time and transmit 20-MHz bandwidth packets through the lower 20-MHz band during the interference time. In the case of conventional WLANs, the WLAN must leave the lower 20-MHz band for the same conditions. Although the periodic interference detector uses moving average power and the decision is made every clock, the decision should be time-stamped and reported to the upper layer, MAC. If this does not have a period of 0.4 µs, the time stamp value needs more accurate resolution to represent the time. In terms of a typical pulse period of 1 ms, 0.4 µs is within an error margin and finer resolution is not required. At the report time interval 0.4 µs, the periodic interference detector counts how many times the averaged power exceeds the threshold. This count value is compared with a count threshold to determine the periodic interference signal. If this count threshold is small, say 1, one peak can cause the periodic interference decision, which can increase the false alarm probability. If all eight values are to exceed the threshold, it may miss a short real interference pulse. This way of implementation increases the flexibility of the design, so that a user can change the sensibility of the periodic interference detector after the hardware is fixed. The final decision needs to be delayed to block the valid WLAN packets. The valid WLAN packet is determined after the SIGNAL field is checked. Since the SIGNAL field comes after a short and two long preambles, and some receiver processing delay is required, 64 radar signal decision samples (0.4 µs × 64 = 25.6 µs) are delayed. If a WLAN packet valid signal occurs during this period, the periodic interference decision is nullified until the receiver enable signal is activated again. 3.2. Implementation of Secure Inter-Frame Space Communications The periodic interference detection bit is determined and updated every 0.4 µs at the PHY. This one-bit data does not really determine the existence of the periodic interference signal, but rather the existence of a powered signal. For the determination of the periodic interference signal, this one-bit data stream needs to be saved and the periodicity of pulses should be searched to decide the existence of a periodic interference signal. Since the periodic pulses occur for durations as large as 100 ms, it is difficult for PHY to handle this long period, which is well beyond one PHY packet period. Note that the PHY enable signal is on only during PHY transmit and receive packet periods. The MAC layer usually deals with a longer period enabled continually. The data stream of the single periodic interference detection bit is passed to the MAC layer. This interface bit is added to the existing MAC-PHY interface. The interface change is not easy once the MAC-PHY data interface is fixed. However, the periodic interference detection bit has nothing to do with WLAN packet communication. If they share the same interface, some information can be lost. For example, while WLAN packet data is being transferred to MAC, the periodic interference detection bit may lose the time to smear on the same interface path. The opposite scenario is also possible. The existing data interface is based on the packet structure. When a packet is received, PHY starts to transfer the header information with packet length information and then the received data.


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Putting the periodic interference detection bit data into this structure may delay the transfer of the Internet 2018, 10, x FOR PEER REVIEW 6 of 10 data.Future It should be noted that the periodic interference detection bit does not have the pulse length information. Although the pulse length is notinterference used for deciding signal, data. It should be noted that the periodic detectionthe bit periodic does not interference have the pulse lengthit may give additional information about the periodic interference signal. If the pulse length is excessively information. Although the pulse length is not used for deciding the periodic interference signal, it large,may the pulse can be eliminated from thethe periodic interference category. Oncelength the periodic give additional information about periodic interferencesignal signal. If the pulse is excessively large, the can is bepassed eliminated from the periodic Once interference detection bitpulse stream to the MAC layer, itinterference should be signal storedcategory. so that the upper periodic bitbit stream is passed to the layer, it should be stored2500 so that layerthe fetches the interference stored data.detection This raw stream has one bit MAC per 0.4 µs, which provides bits per the upper layer fetches storedThat data.is, This raw bit stream hasduring one bit 100 per ms, 0.4 µ250,000 s, whichbits provides millisecond, a typical pulsethe period. if there is one pulse should be 2500 bits per millisecond, a typical pulse period. That is, if there is one pulse during 100 ms, stored in the worst case. Of course, there can be some false alarms, such as noise between250,000 pulses. bits should be stored in the worst case. Of course, there can be some false alarms, such as noise The important design criterion in this memory design is to save memory. One-bit memory width between pulses. is not efficient either. The periodic interference detection information needs the start time and pulse The important design criterion in this memory design is to save memory. One-bit memory length in the calculation of periodicity. MAC, thedetection periodic interference detection bit stream is width is not efficient either. The periodicIn interference information needs the start time and transformed to a 32-bit time stamp format. That is, every time a periodic interference detection bit is pulse length in the calculation of periodicity. In MAC, the periodic interference detection bit stream on, the corresponding time stamp is saved. The first 22 bits represent theinterference pulse startdetection epoch, and is transformed to a 32-bit time stamp format. That is, every time a periodic bit the on, the corresponding time is saved. Thesignal first 22detection. bits represent the 22 pulse and the epochis has a resolution of 0.4 µs,stamp as used in radar Then bitsstart canepoch, represent a linear µ s, ass.used radar signal Then 22 bits can represent a linear time epoch framehas of 2a22resolution × 0.4 µsof=0.4 1.6777 Thein remaining 10detection. bits represent the pulse length, that is, up to 22 × 0.4 µ s = 1.6777 s. The remaining 10 bits represent the pulse length, that is, up to 10 time frame of 2 2 × 0.4 µs = 409.6 µs, which can cover over 100-µs-long radar pulses. It should be noted that the 10 × 0.4 µ s = 409.6 µ s, which can cover over 100-µ s-long radar pulses. It should be noted that the pulse2length information is available at the end of the pulse when the periodic interference detection pulse length information is available at the end of the pulse when the periodic interference detection bit goes from on to off. The write timing is at the end of a pulse, although the start time is already bit goes from on to off. The write timing is at the end of a pulse, although the start time is already savedsaved in the front of the pulse. in the front of the pulse. The memory depth should alsoalso be decided in terms of the write and read speeds. The memory depth should be decided in terms of the write and read speeds.The Thewrite writespeed should be slower than the read speed, the memory will be full subsequent data would speed should be slower than the readotherwise speed, otherwise the memory will be and full and subsequent data be lost. Thebesize memory is set as isit set stores time stamp data.data. ThisThis memory depth would lost.ofThe size of memory as it 64-bit stores 64-bit time stamp memory depthcan can store over approximately 1 ms appear if signals appearin10 in 100 Thenlayer the upper layer in over store approximately 1 ms if signals 10 times 100times µs. Then theµ s. upper implemented implemented in a CPU Unit) (Central Processing can have enough time to read a CPU (Central Processing processer canUnit) haveprocesser enough time to read the memory at itsthespeed. memory size at itsisspeed. memory sizeThis is 64memory × 32 bits size = 2 Kb. memory size be adjusted The memory 64 × The 32 bits = 2 Kb. canThis be adjusted to acan smaller size if to thea used smaller size if the used CPU has faster read speed. The memory is set as a circular first-in-first-out CPU has faster read speed. The memory is set as a circular first-in-first-out (FIFO) or FIFO with dual (FIFO) or FIFO with dual ports. Since the write and read timings are not synchronized, FIFO is the ports. Since the write and read timings are not synchronized, FIFO is the best choice to transfer data. best choice to transfer data. The time stamps are written any time a signal is detected. The upper The time stamps are written any time a signal is detected. The upper layer reads the FIFO any time, layer reads the FIFO any time, independently of the write timing. Figure 3 shows the diagram of the independently of FIFO. the write timing. Figure 3 shows the diagram of the implemented FIFO. implemented FIFO

comparator

adiff

count

re

EMPTY

Memory

count

we

FULL

CLK

Figure 3. Block diagram of first-in-first-out (FIFO) to store time stamps of radar detect signals.

Figure 3. Block diagram of first-in-first-out (FIFO) to store time stamps of radar detect signals.

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A FIFO needs a FULL and an EMPTY signal to protect over-writing and over-reading. A A FIFOFIFO needs a FULL and an EMPTY protectand over-writing andbe over-reading. A dual-port dual-port has a write address and asignal read to address, they should properly managed. The FIFO writeaddresses address and a read address, and should be properly managed. The write and writehas anda read must not be twisted. Thethey write address must be ahead of the read address read addresses mustthe notwrite be twisted. The writearound addressand must be ahead ofread the read address continually. continually. When address wraps points to the address again, the FIFO When write address wraps and points toand the reaches read address again, the FIFO stop shouldthe stop writing. When the around read address follows the write address, the should FIFO should writing. When the read address follows and reaches the write address, the FIFO should stop reading, stop reading, which is the EMPTY signal. One more logic can be found in the case when write and which is the EMPTY signal. simultaneously One more logic can found in the case when write read enables are read enables are activated withbeFIFO FULL signal. This case is and a special FULL case, activated simultaneously FIFO FULL signal. This is aFULL special FULL case, when write and read when write and read canwith work simultaneously, evencase in the situation. There is no such case in can simultaneously, evenwriting in the FULL situation.one There is no the EMPTY situation the work EMPTY situation because data requires cycle. To such makecase the in FULL and EMPTY logic because one cycle. To make the FULL and EMPTY simple, the FIFO uses simple, writing the FIFOdata usesrequires the same clock source: 40 MHz for both write and logic read ports. the same clock source: 40 MHz for both write and read ports. 4. Performance Evaluation 4. Performance Evaluation As shown in Section 3, a radar signal consists of a train of pulses that typically have a short As shown in narrow Section frequency 3, a radar band. signalThat consists a train of pulses that typically have a short pulse width and is, a of transmitted radar signal is a simple sinusoidal pulse width and narrow frequency band. That is, a transmitted radar signal is a simple sinusoidal wave modulated by On-Off keying or OOK. In this case, the important design aspect is to control the wave modulated On-Off keying or several OOK. Indifferent this case,types the important designpulses, aspectitisistoimportant control the timings of pulsesby and bursts. To test of radar signal to timings of pulses and bursts. To test several different types of radar signal pulses, it is important to set the signal parameters, such as PRF, pulse width, and burst period, as input parameters so that the set the signal such as PRF, pulse width, and burst period, as input parameters so that the simulator canparameters, change them easily. simulator can change them easily. Figure 4 shows the block diagram of the radar transmitter. With the input signal parameters set shows thepower, block diagram of the radar transmitter. With the parameters set by by aFigure user, 4the signal frequency, and phase are computed andinput thensignal fed into the sinusoidal agenerator. user, the signal power, frequency, and phase are computed and then into the with sinusoidal generator. On the other hand, the timings of the pulse train are fed computed counters, burst On the other hand, the timings of the pulse train are computed with counters, burst counter, and pulse counter, and pulse counter. The pulse ON signal enables the sinusoidal wave generator to output the counter. The pulse ON signal enables the sinusoidal wave generator to output the radar signal pulses. radar signal pulses.

Compute the amplitude of the Radar Signal with SNR

Compute the frequency of the Radar Signal compared with RLAN carrier center frequency

Sinusoidal Wave Generator

I, Q Radar Pulses

Enable

Count with 80MHz (Channel) Clock

Pulse ON

Pulse Period Counter

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Burst Period Counter

Figure4.4.Block Blockdiagram diagramof ofthe theradar radartransmitter. transmitter. Figure

The channel is assumed static throughout a burst and generated independently for each burst. The channel is assumed static throughout a burst and generated independently for each burst. The impulse response of the channel is composed of complex samples with randomly distributed The impulse response of the channel is composed of complex samples with randomly distributed phase phase and Rayleigh distributed magnitude with the average power decaying exponentially. In the and Rayleigh distributed magnitude with the average power decaying exponentially. In the simulation simulation model, the WLAN data sender and receiver, which have the proposed radar detector and model, the WLAN data sender and receiver, which have the proposed radar detector and secure secure inter-frame space communications system at 5 GHz, are modeled in the C programming inter-frame space communications system at 5 GHz, are modeled in the C programming language. language.

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A periodic interference detection algorithm has been designed and implemented. The designed algorithm utilizes the periodic property of the interferences. To do this, the periodic interference A periodic interference detectionsignal algorithm has been designed and implemented. Thedecision designed detector first measures the received power averaged over a moving window. The is algorithm utilizes the periodic property of the interferences. To do this, the periodic interference detector saved in a shift register to cover a period of interference detection. The count of the decisions is first measures the received signal power averaged over a moving window. The decision is saved in a compared with a threshold again. The final decision is delayed to see if the received signal is a valid shift register to cover a period of interference detection. The count of the decisions is compared with a packet. threshold again. The final decision is delayed to see if the received signal is a valid packet. The detection of the periodic interference is with PHY, while the decision bit stream is passed to The of MAC the periodic interference with PHY, while thea decision bit stream is passed the MACdetection layer. The layer transforms theis one-bit stream into 32-bit time stamp driven by to the MAC layer. The MAC layer transforms the one-bit stream into a 32-bit time stamp driven by signal detection activation. The transformed time stamps are stored and the upper layer fetches this signal detection activation. The transformed timepatterns. stamps are storedWLAN and thefactors upperare layer fetches this information to search the periodic interference Finally, considered to information to search the periodic interference patterns. Finally, WLAN factors are considered to decide the presence of the periodic interferences. Periodic interference detection is not only a power decide theproblem, presence but of the periodic interferences. Periodic interference is not a power detection also a network problem in the WLAN stations.detection The nodes andonly access point detection problem, but also a network problem in the WLAN stations. The nodes and access point should cooperate together to detect the periodic interferences. should cooperate together tothe detect the periodic interferences. Figure 5 demonstrates improved detection performance of the proposed detection scheme, Figure 5 demonstrates the improved detection performance the proposed scheme, and Figure 6 shows the improved maximum user datagram of protocol (UDP) detection throughput. The and Figure 6 shows the improved maximum user datagram protocol (UDP) throughput. The simulation simulation model was built on the bit- and cycle-accurate C simulator which is the same model used model built on the bit- and cycle-accurate C simulator is the same model for the for thewas implemented commercial chipset and was certifiedwhich through standard Wi-Fi used certification implemented commercial and was certified through certification process. process. In Figures 5 and chipset 6, “PRO” and “CON” refer to the standard proposedWi-Fi scheme and conventional In Figures 5 and 6, “PRO” and “CON” refer to the proposed scheme and conventional scheme, scheme, respectively. Ten thousand radar samples were used for this test. “W” and “P” refer to pulse respectively. Ten thousand radar samples were usedAs forathis test.shorter “W” and “P” refer and to pulse width width and periodicity of radar signals, respectively. result, pulse width periodicity and periodicity of radar degradation signals, respectively. result, shorter pulseofwidth periodicity resultthe in result in performance due to As thea limited resolution the and detector. However, performance degradation due to the limited resolution of the detector. However, the proposed scheme proposed scheme outperformed for test scenarios. In Figure 6, W50-P2000 was much better than outperformed for test scenarios. In Figure 6, W50-P2000 wasduration. much better than W50-P500 because W50-P500 because short periodicity increased the interfered However, all the cases of the short periodicity increased the interfered duration. However, all the cases of the proposed scheme proposed scheme outperformed when compared with the conventional scheme. outperformed when compared with the conventional In conventional WLAN systems, once a periodic scheme. interference signal is detected within the basic In conventional WLAN systems, a periodic interference signal within the basic service set either by an access point oronce station, they must follow the ruleistodetected escape the channel and service set either by an access point or station, they must follow the rule to escape the channel and search another clearer channel. The channel search can also be done by all stations as well as access search clearerresults channel. channelatsearch can also be done by all point stations as well asa access points.another The searched areThe gathered the access point. The access broadcasts move points. The searched results are gathered at the access point. The access point broadcasts a move command to every station and finishes DFS within a specified time period. These steps are command station andwith finishes DFS within a specified timeOn period. These stepsinare performedtoinevery the MAC layer the support of the upper layer. the other hand, theperformed proposed in the MAC layer with the support ofthe the wireless upper layer. On the other hand, significant in the proposed secure secure inter-frame communication, device could achieve throughput inter-frame communication, the wireless device could achieve significant throughput improvement by improvement by adopting periodic interference detection and data transmission in the secure adopting periodic interference detection and data transmission in the secure inter-frame space. inter-frame space.

Figure 5. 5. Detection Detection error error probability probability vs. vs. radar radar input input power. power. Figure

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Figure Figure 6. 6. Maximum Maximumuser userdatagram datagramprotocol protocol (UDP) (UDP) throughput throughput vs. vs. data data size. size.

5. 5. Conclusions Conclusions In In this this paper, paper, aa secure secure inter-frame inter-frame space space communication communication system system was was designed designedand andits itssimulation simulation model was implemented. To design the secure inter-frame space communication algorithm, the model was implemented. To design the secure inter-frame space communication algorithm, characteristics of periodic interference signals the characteristics of periodic interference signalswere were investigated. investigated. Although Although most most periodic periodic interference signals have similar properties such as pulse width, pulse period, and interference signals have similar properties such as pulse width, pulse period, and burst burst period, period, some some military militaryradios, radios,such suchas asradars, radars,do donot notreveal revealtheir theirparameters. parameters.To Toimplement implementthe thespecial-purpose special-purpose periodic periodic interference interference signals, signals, all all the the parameters parameters should should be be set set as as adjustable adjustable parameters. parameters. The The periodic periodic interference signal generators have been implemented in the WLAN C models and a pulse interference signal generators have been implemented in the WLAN C models and a pulse train train is is available to test the proposed algorithm. Since a periodic interference signal is generated independently available to test the proposed algorithm. Since a periodic interference signal is generated independently of inter-frame space communication algorithms could be of WLAN WLANcommunication communicationpackets, packets,the thesecure secure inter-frame space communication algorithms could tested either alone or together with WLAN packets. One of major security requirements of wireless be tested either alone or together with WLAN packets. One of major security requirements of networks is availability issue. The availability refers to ensuring that the authorized users are indeed wireless networks is availability issue. The availability refers to ensuring that the authorized users capable of capable accessing a wirelessa wireless networknetwork when needed. The violation of availability, caused by are indeed of accessing when needed. The violation of availability, caused interferences, will results in the authorized users to become unable to access the wireless network. by interferences, will results in the authorized users to become unable to access the wireless network. The The proposed proposed secure secure inter-frame inter-frame space space communications communications scheme scheme improves improves the the availability availability under under the the interfered environments. As future work, the secure inter-frame space communication system will interfered environments. As future work, the secure inter-frame space communication system will be implemented and demonstrated in a real testbed, and optimization issues for more than one radar be implemented and demonstrated in a real testbed, and optimization issues for more than one will be investigated in the real testbed and the performance analyzed theoretically including radar will be investigated in the real testbed and the performance analyzed theoretically including mathematical models. mathematical models. Funding: Funding: This Thiswork workwas wassupported supportedby by the the Sungshin Sungshin University University Research Research Grant Grant of of 2017-1-11-055/1. 2017-1-11-055/1. Conflicts Conflicts of Interest: The The author author declares declares no conflict of interest.

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