An adaptive LTE listen-before-talk scheme towards a fair coexistence with Wi-Fi in unlicensed spectrum

Over the last years, the technological growth has led to a tremendous increase of wireless devices such as smartphones, tablets, laptops and wearable technologies. Additionally, the number of electronic devices that exchange information wirelessly is growing day by day, pushed by the evolution and consolidation of the Internet of Things (IoT). According to Qualcomm, the amount of wireless traffic is expected to further increase by a factor of 1000 by 2020 [1]. This massive amount of information is exchanged between devices using different types of technologies such as Long Term Evolution (LTE), IEEE 802.11, IEEE 802.15.4 and Bluetooth. Lately, sub-gigahertz bands are exploited by technologies like LORA and SIGFOX in order to achieve wide range communications. Additionally, high frequency bands such as mmWave are used for multi-gigabit speeds (IEEE 802.11ad). It becomes clear that the wireless network capacity will soon become a bottleneck for the increased wireless traffic.

Concurrently, the licensed spectrum used by the mobile operators becomes very scarce. The limitation of the licensed spectrum in combination with the high cost of a licensed frequency band have pushed the mobile operators to investigate other technological solutions that can support in meeting the 1000x challenge requirements. These solutions include among others, enhanced massive Multiple-Input Multiple-Output (MIMO), Carrier Aggregation, cloud computing services, as well as LTE operation in the unlicensed spectrum (LTE-U). Among various other solutions, the last one has attracted significant attention from the wireless community. Several mechanisms, such as Listen Before Talk (LBT) have been proposed towards the coexistence of LTE-U and other well-established technologies in the unlicensed spectrum, such as Wi-Fi [2].

In markets like the U.S., China and South Korea where a Clear Channel Assessment (CCA) mechanism (also known as LBT) is not required, LTE can operate in the unlicensed spectrum using techniques such as Carrier Sense Adaptive Transmission (CSAT) [3]. CSAT, proposed by Qualcomm, exploits duty cycle periods in order to give to Wi-Fi transmission opportunities (TXOP). It divides the time into LTE ON and LTE OFF slots. During an ON period, LTE transmits without sensing the medium, while during an OFF period it remains silent giving TXOP to other coexisting networks.

Recently, key players of the mobile world have proposed standards to the 3rd Generation Partnership Project (3GPP), which specify the LTE operation in unlicensed spectrum. 3GPP announced the operation of LTE Licensed-Assisted Access (LTE LAA) [4], as an enhancement towards 3GPP LTE Release 13. LTE LAA allows operators to transmit in the unlicensed spectrum using a secondary cell, alongside a primary cell operating in the licensed band that they own. Using LTE LAA, a mobile operator can offload the LTE network into the unlicensed spectrum, when this is required. The licensed spectrum can be used to ensure the transmission of the crucial LTE control signals without interference. Additionally, applications that require high Quality of Service (QoS) (e.g. video streaming) can exploit the advantages of the interference-free licensed spectrum. LTE LAA requires a CCA procedure before each transmission in the unlicensed spectrum. This way, the mechanism can be applicable worldwide, including markets like Europe and Japan, where CCA is mandatory.

In order to decouple LTE from the operators, leading wireless stakeholders proposed the LTE operation solely in the unlicensed spectrum as a standalone wireless solution. To this end, they formed the MulteFire Alliance [5]. Hence, LTE can be deployed by Internet Service Providers (ISPs), building owners, cable companies, etc. The underlying technique proposed by the alliance builds on elements of 3GPP LTE LAA.

Although the LTE LAA standard defines that a CCA procedure must be performed before a transmission burst, it also defines four different channel access priority classes. Each channel access priority class specifies among others the duration of the transmission burst that follows a successful CCA procedure. This duration ranges from 2 to 10 ms [2]. On the contrary, a Wi-Fi packet transmission when frame aggregation is not enabled or supported by the 802.11 standard typically lasts a few hundreds of \({\upmu }\)s [6]. Furthermore in [7], it has been assessed that for 802.11n with frame aggregation, 50% of the packets are transmitted within 30 \({\upmu }\)s, while 80% of the packets are transmitted within 1 ms. It is clear that the ratio between LTE and Wi-Fi channel occupancy is not balanced. This can result to unfair coexistence between co-located LTE LAA and Wi-Fi networks.

In this article, we discuss a way that fairness can be achieved between LTE-U and Wi-Fi. We define a new adaptive LTE-U transmission scheme according to which LTE can transmit in unlicensed spectrum using a variable TXOP time. This TXOP period is followed by a variable muting period in order to give channel access opportunities to other potentially co-located networks, such as Wi-Fi. Before a TXOP, LTE must perform a CCA procedure in order to determine the availability of the channel. This scheme can be used for LTE transmissions in the unlicensed spectrum next to the primary cell that an operator uses in the licensed spectrum similar to LTE LAA. The proposed scheme is evaluated through simulations. Finally, we discuss how the configurations of TXOP and muting period can be selected by a network in order to provide fair coexistence. The main contribution of this work is summarized as follows:The remainder of the article is organized as follows. In Sect. 2 we present an overview of the main characteristics of LTE and Wi-Fi that lead to coexistence issues when the two technologies operate next to each other in their traditional form. Section 3 discusses the current literature on LTE-U and especially LTE LAA. Section 4 defines the problem that arises when LTE LAA coexists with traditional Wi-Fi networks that do not use frame aggregation and describes the proposed solution. Next, in Sect. 5 we discuss about fair coexistence in unlicensed spectrum and the approach that we follow in this article. Section 6 presents the simulation platform that has been used. In Sect. 7, we discuss the simulation scenarios that are studied. Then, in Sect. 8, we present and discuss the obtained results for each investigated scenario. In Sect. 9, we discuss the way that a selection of the configuration parameters can be done for the proposed scheme towards fair coexistence. Finally, in Sect. 10, we conclude the paper and discuss plans for future work.

This section briefly discusses the main differences between LTE and Wi-Fi that result in unfair coexistence, when the two technologies operate in the proximity of each other in their traditional form.

LTE uses multi-user versions of the Orthogonal Frequency Division Multiplexing (OFDM) digital modulation scheme, called Orthogonal Frequency Division Multiple Access (OFDMA) for the DL and Single-Carrier Frequency Division Multiple Access (SC-FDMA) for the UL [8]. LTE divides the time domain in time slots of 0.5 ms duration. Each time slot corresponds to 7 OFDM symbols when Normal Cyclic Prefix (CP) is used and to 6 OFDM symbols for Extended CP. An LTE radio frame has a duration of 10 ms and consists of 10 subframes. Each subframe consists of 2 slots. The frequency domain is divided into subcarriers each one occupying 15 KHz of bandwidth. Combining one time slot and 12 subcarriers, LTE defines the Resource Block (RB), which is the unit of transmission.

LTE is a scheduled technology designed to operate in the licensed spectrum. Hence, it does not require to sense the medium before a transmission. The LTE base station named Evolved NodeB (eNB) schedules the different User Equipment (UE) transmissions on a subframe basis, meaning that every 1 ms the assignment of the subframes to the active UE can change. The time-frequency structure and the assignment of the resources to different UEs is depicted in Fig. 1.

On the other hand, Wi-Fi uses OFDM digital modulation scheme and the Distributed Coordination Function (DCF) as the fundamental mechanism to access the wireless medium, which is designed to be asynchronous and decentralized [6]. Wi-Fi uses a Carrier Sensing Multiple Access with Collision Avoidance (CSMA/CA) mechanism to compete for the channel access. Before a transmission, Wi-Fi has to sense the medium to determine if it is busy or idle This procedure is known as Clear Channel Assessment (CCA). Figure 2 illustrates the CSMA/CA procedure. Only if the medium is idle for DCF Inter-Frame Space (DIFS) the node can transmit. Otherwise and also prior to a new transmission immediately after a successful transmission, the node has to postpone its transmission for DIFS plus a random backoff time. The backoff time indicates how many idle time slots a node has to sense before a transmission. The number of the timeslots is specified by the backoff counter that is randomly selected within the range of the Contention Window (CW). If the transmission is not successful and an acknowledgement (ACK) is not received the node schedules a retransmission after a new exponential backoff period until the maximum number of retransmissions is reached.

CCA consists of two functions named Carrier Sense (CS) and Energy Detection (ED). The CS function refers to the ability of the receiver to detect and decode a received Wi-Fi preamble. The ED function is used when the received signal cannot be decoded. According to the standards the threshold of CS and ED for 20 MHz bandwidth is − 82 and −62 dBm respectively [6].

In our previous work [9], we studied the impact of traditional LTE operating in unlicensed spectrum on Wi-Fi using Off-The-Shelf (OTS) hardware equipment using the LTE testbed of IMEC [10]. In this study, three different levels of LTE signal have been examined, representing different possible levels of LTE impact on Wi-Fi. The results show that the Wi-Fi performance, in terms of throughput and latency, can be significantly affected by LTE. In [11], the authors performed an experimental evaluation to study the impact of LTE LAA on Wi-Fi performance in indoor office environment. The study includes analysis of LTE LAA interference for five different scenarios. Based on this analysis the authors provide LTE LAA Medium Access Control (MAC) designs to deal with coexistence issues with Wi-Fi. Several other studies [12‐14] evaluate the Wi-Fi performance degradation based on mathematical models and simulations. All studies come to the same conclusion, namely that coexistence mechanisms are required to enable coexistence between co-located LTE and Wi-Fi networks.

The authors in [15] evaluate through simulations the performance impact of LTE and Wi-Fi when both networks operate in the same frequency. They propose a coexistence mechanism similar to CSAT that exploits periodically blank LTE subframes during an LTE frame in order to give opportunity to Wi-Fi to transmit. They conclude that the network topology, as well as the number and order of the blank subframes lead to different coexistence results.

Towards a global coexistence technique that respects the regional regulations, 3GPP announced the LTE LAA standards in Release 13, including the description of a CCA procedure [2]. Initially, LTE LAA is scheduled to operate for DL only and within the 5 GHz channel. Towards Release 14, it is expected to be extended to 2.4 GHz unlicensed band and for both DL and UL traffic. The transmission in the unlicensed spectrum can be done via a secondary cell operating alongside the primary cell owned by the operator. This feature can be enabled using the Carrier Aggregation mechanism that has been introduced in 3GPP LTE Release 10 [16].

In [17], the authors provide a description of the LTE LAA mechanisms including motivation and use cases to which it can be applied. They present a coexistence evaluation methodology and results, which have been contributed by 3GPP.

The authors of [18] present a detailed overview of LTE LAA in Release 13. They show how the introduction of CCA and the discontinuous transmission impose changes in different LTE components such as the DL physical channels, the hybrid automatic repeat request (HARQ) feedback procedures, etc. Simulation results are presented to show that coexistence with Wi-Fi can be enabled in a range of scenarios. Moreover, an overview of LTE LAA enhancements beyond Release 13 is given.

In [19] two non-coordinated and two coordinated network management approaches to enable coexistence are proposed. Regarding the non-coordinated techniques, the first one proposes eNB to perform CCA on different channels and to switch to a different channel after a transmission, while the second proposes LTE to offer transmission opportunities of variable duration to Wi-Fi after a transmission based on the occupancy of the medium. Concerning the coordinated methodologies, the first one proposes a Network Function Virtualization (NFV) interconnection to combine the Wi-Fi network and the LTE-U service provider. Channel selection and seamless transfer of resources between the two technologies can be enabled, using the in-the-cloud control of distributed Access Points (APs). The second method proposes the management of coexistence using the X2 interface among the eNBs. The eNBs can exchange information and schedule Almost Blank Subframes (ABS) in different subframes giving this way more opportunities to any Wi-Fi network that is located potentially within their proximity. In the aforementioned schemes, the different Radio Access Technologies (RATs) are under the control of the same mobile operator.

The authors in [20] propose an LBT protocol for LTE LAA that enhances the coexistence with Wi-Fi and increases the overall system performance. This LBT scheme consists of two different mechanisms named on-off adaptation for channel occupancy time and short-long adaptation for idle time. The first mechanism is responsible to adapt the channel occupancy time of LTE based on the load of the network, while the second one adapts the idle period based on the CW duration of Wi-Fi.

In [21], the authors propose an LBT mechanism for LTE LAA that aims to share the medium in a fair way towards the increase of the overall system performance. The mathematical analysis of the proposed LBT scheme is validated via simulations. The results show that a proper selection of LAA channel occupancy and backoff counter can increase the performance of Wi-Fi.

In [22], the coexistence between LTE LAA and Wi-Fi is studied using LBT category 4 channel access scheme. The behaviour of LAA eNB is modelled as a Markov Chain and the obtained throughput is adopted as performance metric. The proposed LBT scheme uses an adaptive CW size for LTE LAA. According to the results, the proposed scheme outperforms the fixed CW size.

The authors in [23] describe and evaluate a channel switch function that is used to determine the LTE LAA channel dynamically. This way, LTE LAA can exploit the spectrum in a more flexible way. They propose an enhanced LBT scheme with channel switch that uses a frozen period to select the appropriate channel. The channel switch is done based on a proportional fair based dynamic channel switch method that is analytically presented. The results show that the proposed scheme can increase the overall system performance.

In [24], a MAC layer for LTE-U is proposed that uses an LBT algorithm and channel reservation packets. Both synchronous and asynchronous LBT are examined. Additionally, improvements to the LTE link adaptation algorithm are proposed in order to cope with potential collisions. Simulation results indicate that the performance of Wi-Fi can be improved by the proposed MAC design. Furthermore, the channel reservation mechanisms increase the LTE-U cell edge performance.

In our previous work [25], we extensively studied the concept of LTE-U. Initially, we provide a detailed analysis of the current state-of-the-art regarding LTE-U and Wi-Fi. Furthermore, the article presents a classification of techniques that can be applied between co-located LTE and Wi-Fi networks. This classification in combination with the study of the literature revealed the lack of cooperation schemes among co-located networks that can lead to more optimal use of the available spectrum. In order to fill this gap, several concepts of cooperation techniques that can enhance the spectral efficiency between coexisting LTE and Wi-Fi networks are proposed. Additionally, the proposed cooperation schemes are compared between each other in terms of complexity and performance.

Finally, the authors in [26] provide a detailed survey of the coexistence of LTE-U and Wi-Fi on 5 GHz with the corresponding deployment scenarios. They provide a detailed description of the coexistence-related features of LTE-U and Wi-Fi, the coexistence challenges, the differences in performance between the two different technologies and co-channel interference. They extensively discuss the proposed coexistence mechanisms between LTE-U and Wi-Fi in the current literature. Furthermore, the survey discusses the concept of the scenario-oriented coexistence, in which coexistence-related problems are solved according to different deployment scenarios.

Although the 3GPP standards specify that the channel must be sensed by a CCA procedure before a transmission, the ratio between LTE LAA and Wi-Fi transmission opportunities is not balanced, especially in the case that Wi-Fi does not support or use frame aggregation. According to the best of our knowledge, the current literature lacks of a mechanism that can adapt the LTE-U channel access after a CCA in order to provide equal channel opportunities to other co-located networks such as Wi-Fi. The following aspects render our proposal novel and valuable. Firstly, the proposed scheme is flexible as it adapts the LTE-U channel access in order to provide fair coexistence with networks in unlicensed spectrum based on various parameters such as the number of the co-located networks and the type of traffic that has to be served (e.g. delay-sensitive traffic). Secondly, the CCA procedure ensures that the mechanism can be applicable worldwide. Thirdly, this scheme can provide fair coexistence not only to Wi-Fi but also to other well-established technologies in unlicensed spectrum, such as 802.15.4 and Bluetooth. Finally, the proposed variable TXOP followed by a variable muting period does not have an impact on time-sensitive LTE traffic, as it can still be transmitted via the licensed band of the operator.

Recently, 3GPP announced the LTE LAA standards as part of LTE Release 13. LTE LAA defines that a CCA procedure [2] must be performed before an LTE transmission in the unlicensed spectrum. This way, the standard can be applicable worldwide, as it respects the regional regulations in markets like Europe and Japan where a CCA procedure is mandatory.

Initially, LTE LAA (as defined in Release 13) is scheduled to operate within the 5 GHz unlicensed spectrum and for Downlink (DL) traffic only, while the Uplink (UL) traffic will be maintained in the licensed spectrum. In a later phase towards Release 14, it is expected to be extended to 2.4 GHz unlicensed band including both DL and UL traffic. According to LTE LAA Release 13, an eNB will be able to activate and deactivate a secondary cell operating in the unlicensed spectrum. Via this cell only DL data traffic can be sent through the Physical DL Shared Channel (PDSCH). The LTE control signals and the UL traffic will be maintained in the licensed anchor via the Physical UL Shared Channel (PUSCH). Especially for the LTE control signals whose transmission is time-critical, the licensed anchor can guarantee a safe and interference-free transmission.

Before a transmission, an eNB must perform the CCA procedure in order to sense the channel in the unlicensed spectrum. When the channel is sensed as busy, the eNB must defer its transmission and perform an exponential backoff. If the medium is sensed as idle, the eNB starts a transmission burst with a duration varying form 2 up to 10 ms, depending on selected channel access priority class. Table 1 shows the definitions of the different channel access priority classes. The smaller the number of the class, the higher the priority.

From the table, it can be seen that each priority class uses different \(\hbox {T}_{\mathrm{m cot,p}}\) that refers to the maximum channel occupancy time for the specific class p. According to the standard, for the priority classes 3 and 4, the \(\hbox {T}_{\mathrm{m cot,p}}\) equals to 10 ms if the absence of any other co-located unlicensed technology sharing the same spectrum band can be guaranteed on a long term basis. In a different case, it is limited to 8 ms. An eNB cannot continuously transmit in unlicensed spectrum for a period longer than \(\hbox {T}_{\mathrm{m cot,p}}\). After the end of the \(\hbox {T}_{\mathrm{m cot,p}}\), it must perform a CCA procedure to estimate again the occupancy of the channel.

On the other hand, in traditional Wi-Fi network without frame aggregation, an AP or a Station (STA) transmits only one packet after it successfully estimates the medium as idle. Such a Wi-Fi packet transmission typically lasts a few hundreds of \({\upmu }\)s. After the transmission of the packet, it has to compete again to access the medium against other co-located networks by performing a CCA procedure. In several still widely used Wi-Fi standards such as 802.11a/g frame aggregation is not supported. Even if frame aggregation is available (e.g. 802.11n/ac [27]), often it is not used depending on the traffic type (e.g. low latency constraints) [28].

It is clear that the transmission durations of LTE LAA and Wi-Fi are not balanced as the TXOP duration of LTE LAA is significantly longer compared to a single packet transmission of Wi-Fi. Moreover, as both networks perform an exponential backoff after they sense the channel as busy, it is possible for an LTE LAA network to gain consecutive times access to the channel forcing Wi-Fi to postpone its transmission for even longer period of time. This can lead to unfair coexistence between co-located LTE and Wi-Fi networks. Especially in the case of multiple LTE LAA networks, a co-located Wi-Fi network will be impacted drastically as it has to compete against more networks that are able to gain access to the channel for considerably longer duration.

In order to deal with this serious concern, we propose a new adaptive channel access scheme for LTE-U. According to this scheme, LTE has to perform a CCA before a transmission. If the CCA estimates the channel as idle, then the LTE LAA eNB transmits for a variable duration called TXOP in a range of 2 up to 20 ms. This TXOP is followed by a variable muting period in a range of 0 up to 20 ms. During the muting period, the LTE-U network that has finished a transmission of a TXOP duration has to remain silent in order to give channel access opportunities to other co-located networks (e.g. Wi-Fi or another LTE-U). After the end of the muting period (or at the end of the TXOP in case of zero muting period), the eNB has to perform again a CCA procedure before a new TXOP. In this solution, the introduction of the muting period can cause problems for delay sensitive traffic. In this case, similar to LTE LAA, a primary cell operating in licensed spectrum can still be used for time sensitive transmissions. In the rest of the article, we will refer to the proposed scheme as muting LTE-U (mLTE-U). Figure 3 illustrates the proposed scheme.

This scheme can offer high coexistence flexibility as the mLTE-U behaviour can be adapted based on various parameters, such as the number and the type of the co-located networks, the QoS requirements that a network has to serve (e.g. best effort traffic, video traffic, etc.) and the load of the different networks. For instance, when an mLTE-U network coexists with multiple Wi-Fi networks, then the proposed scheme has to be adapted so that mLTE-U transmits using a short TXOP followed by a relatively long muting period. The Wi-Fi networks can exploit this period to further gain channel access. According to another example-scenario, an mLTE-U that serves a video streaming coexists with a Wi-Fi network that serves best-effort traffic. In this case, the mLTE-U transmission scheme has to be modified in order to use a higher TXOP followed by a shorter muting period for Wi-Fi transmissions.

The purpose of the proposed scheme is to enhance the coexistence and increase the fairness among the co-located LTE-U and Wi-Fi networks. A fair coexistence scheme should offer all the available networks equal opportunities to the medium. It is important to point out the difference between fairness among different available technologies and fairness among the different coexisting networks, as it is depicted in Fig. 4.

According to the first approach (Fig. 4a), the wireless resources are divided among the co-located networks according to the different wireless technologies that are used. Hence, in the case of two coexisting wireless technologies such as LTE and Wi-Fi, half of the time the medium is used by LTE and half of the time is used by Wi-Fi. In our opinion, such an approach is not always fair as it does not take into consideration the number of the LTE and Wi-Fi networks respectively. For instance, if there are multiple co-located Wi-Fi networks and one LTE-U network, it would not be fair to Wi-Fi to split the time that the different technologies access the channel to the half.

Regarding the second approach (Fig. 4b), the medium is shared according to the number of the co-located networks in a technology-agnostic manner. Consequently, a coexistence mechanism that belongs in this category does not discriminate the coexisting networks based on the type of the wireless technology that they use. Instead, the distribution of the resources is done based on the number of the co-located networks and ideally based on several characteristics, such as the type and the amount of traffic that must be served.

In an ideal scenario in which all the different networks are aware of the requirements of each other and can exchange information, or a central coordinator is in charge of communicating with each network, collecting their traffic requirements and coordinating their transmissions, the distribution of the wireless resources could be done in a really fair manner.

However, in the wireless world, several diverse networks that have been designed, each having completely different principles in order to serve different requirements, are forced to coexist and compete for the wireless resources. Furthermore, the channel access mechanisms used by different technologies vary significantly among each other. Even between nodes of the same wireless technology equally time sharing of the wireless resources is not guaranteed. Wi-Fi is an indicative example of such a scenario. One of the basic principles of traditional Wi-Fi (without frame aggregation) is the equal division of the channel between the users. Hence, only one packet is transmitted by each node after the medium is sensed as idle. Nevertheless, very often there is a case in which a node faces better channel quality than another. Thus, the node with better channel conditions can perform a faster transmission, using a high Modulation and Coding Scheme (MCS) profile compared to the other. This way the node with the lower MCS profile occupies the channel for longer duration to transmit exactly the same number of bytes.

In the case of LTE and Wi-Fi coexistence, the two technologies that compete for the wireless resources are diverse having major design differences. The obtained throughput together with the channel occupancy are good indicators for the fairness that a coexistence technique can offer. Hence, in the rest of the article, the obtained throughput and the channel occupancy are adopted as key performance indicators for the evaluation of the proposed scheme. Towards a fair coexistence in line with the second approach that discussed above, the parameters of mLTE-U are selected in such a way that each participating network can achieve an equal ratio of throughput, compared to the maximum throughput that it can be achieved during the standalone operation.

In order to evaluate the proposed scheme, experiments have been performed using the NS3 network simulator, which is an event-based and flexible simulation platform. The simulator allows the design of scenarios in which multiple LTE networks can coexist together with multiple Wi-Fi networks in the unlicensed spectrum. The LTE and Wi-Fi networks are able to operate using the same channel and can interfere with each other.

During the experiments, the LTE has been set to operate in the 5 GHz unlicensed band. As it is mentioned in Sect. 4, mLTE-U can transmit using a variable TXOP period, which ranges from 2 up to 20 ms. In addition, a muting period has been introduced to the LTE channel access scheme. This muting period ranges from 0 up to 20 ms and starts after the completion of a TXOP period. The maximum duration of both TXOP and muting period can be set to even higher values. However, we believe that this range is long enough to showcase the effect that the proposed scheme can have on the coexistence between mLTE-U and Wi-Fi networks in unlicensed spectrum.

Before an mLTE-U node starts a transmission, it has to complete a CCA procedure. The CCA parameters have been configured in order to be similar to the Wi-Fi LBT Category 4 procedure. Table 2 summarizes the specific mLTE-U parameters that have been used.

Regarding the Wi-Fi network, 802.11n mode has been selected in order to allow operation in 5 GHz unlicensed band. Additionally, frame aggregation is disabled so that we can investigate the traditional 802.11 transmission, according to which a single packet is transmitted after the channel is estimated as idle. Additionally, the network is configured to operate in SISO mode, so that the Wi-Fi operation can be comparable to other popular 802.11 standards that does not support MIMO mode such as 802.11a/g. Table 3 lists all the related parameters that have been used for the configuration of the Wi-Fi network.

The common simulator parameters are presented inTable 4.

Before the beginning of a transmission burst mLTE-U must perform a CCA procedure. This means that the medium can be sensed as idle at any time. On the other hand, LTE is a scheduled technology and the scheduling is performed by the eNB on a sub-frame level, meaning that each 1 ms the assignment of the wireless resources to the active UE can change. Hence, as every data transfer starts at the subframe boundaries, an LTE reservation signal is used after the channel is sensed as idle and until the beginning of the next subframe in order to preserve the channel and force other nodes to backoff. In the best-case but very rare scenario in which the channel is estimated as idle in the beginning of a subframe, the transmission of a reservation signal is not necessary and thus it is omitted. Contrariwise, when the channel is sensed idle immediately after the beginning of a subframe, then the reservation signal lasts for the rest of the subframe and the data transmission starts at the beginning of the next subframe. The duration of the reservation signal is deducted from the TXOP duration of the mLTE-U. For this reason, the minimum examined TXOP is 2 ms. Figure 5 illustrates the usage of the reservation signal as described above.

In order to verify the coexistence issue that occurs when LTE LAA operates next to Wi-Fi, a related simulation scenario has been designed. According to this scenario, an LTE LAA network consisting of one eNB and one UE operates in the proximity of a Wi-Fi network that consist of one AP and one STA.

Towards the performance evaluation of the proposed scheme, various simulation scenarios have been designed. For each scenario, all the different combinations of TXOP and muting values have been tested. For both mLTE-U and Wi-Fi networks, we assume that one end-device is connected to one base station. In each network, high load UDP traffic is transmitted in the DL, meaning from the eNB to the UE for LTE and from the AP to the STA for Wi-Fi.

For the evaluation of the proposed scheme during the first four scenarios, the mobility of the end-nodes is not taken into consideration. In these scenarios, we study the performance of the mLTE-U scheme in cases of different mLTE-U and Wi-Fi network densities. The first examined scenario consists of one mLTE-U network and one Wi-Fi network. The distance between the LTE eNB and the Wi-Fi AP is 10 m, while the LTE UE and the Wi-Fi STA are located at a distance of 10 m from the eNB and the AP respectively. In the remainder of the article we refer to this scenario as reference scenario. For the other investigated static scenarios, the number of the mLTE-U and Wi-Fi networks ranges from one up to four networks for each type of technology. This way, various situations of high interest can be studied, such as:In every scenario with multiple mLTE-U and/or multiple Wi-Fi networks, all the available nodes (eNBs, UEs, APs and STAs) are deployed randomly in the proximity of each other (within 20 m). This way, the ED threshold is surpassed and the backoff mechanism of mLTE-U and Wi-Fi is triggered during every transmission. Figure 6 presents the investigated static coexistence scenarios.

Furthermore, the effect of mobility in the coexistence of mLTE-U and Wi-Fi is studied in an indicative mobile scenario. During the mobile scenario and similar to the reference scenario, one mLTE-U network, consisting of one eNB and one UE, coexists with one Wi-Fi network consisting of one AP and one STA. The UE is placed at a distance of 25 meters from the eNB and the STA is placed at a distance of 100 meters from the AP. During the execution of the scenario, the UE moves away from the eNB, while the STA moves towards the AP. The above described scenario is depicted in Fig. 7.

In the previous sections, the proposed scheme has been evaluated for different scenarios of high interest. Each of the scenarios investigates different density for mLTE-U and Wi-Fi networks and identifies the combinations of TXOP and muting period that can provide fair coexistence between LTE and Wi-Fi. The fair coexistence is defined in terms of equal throughput ratio achievement for each one of the co-located networks. In the investigated scenarios, all the networks consist of one end-device connected to one base station and they have equal traffic requirements. As it has been revealed from the simulation results, for each scenario multiple configurations can provide the desired fair coexistence. The biggest challenge is to identify and select the optimal parameter values that can guarantee fair coexistence.

This section discusses how these configurations can be automatically identified. This identification can be done taking several parameters into consideration, such as the amount of the co-located networks and the type of traffic that must be served. The traffic that must be served refers to the load of each network and the QoS requirements. The degree that a network can exploit the aforementioned parameters is related to the network architecture. Regarding the network architecture, the co-located networks can be either under the control of a central coordinator or can operate independently.

According to the coordinated approach, the identification of the participating networks and the collection of traffic information can be easy as the coordinator can directly communicate with each network. On the other hand, the existence of a coordinator increases the complexity of the network. Additionally, there always might be other networks in the neighbourhood that do not belong to the coordination scheme. Modifications to the wireless protocols are required in order to render each technology capable of communicating such type of information to the coordinator. The coordinator needs a careful design in order to be able to communicate with different technologies, collect and manage the required information. In such an ideal scenario, the coordinator will be responsible to tune the mLTE-U parameters in order to ensure that each network is able to achieve the required throughput. On the other hand, a non-coordinated approach is more realistic, as every network can be deployed arbitrarily. Such an approach requires lower complexity regarding the overall network architecture as each network operates independently. On the contrary, each network must be responsible to collect the information that is required in order to decide the appropriate configuration that enables fair coexistence with the co-located networks. Wireless technology recognition techniques [7, 30] are required to identify the amount and type of the wireless technologies that are in the proximity of each other. Based on this information, each mLTE-U network can decide the combinations of TXOP and the muting period that can offer the proportional fair throughput.

The discovery of the TXOP and muting period configurations that offer fair coexistence requires careful design. As a first approach, a heuristic technique can be used. According to such a technique, the eNB can try different configurations attempting to find the ones that offer a performance (e.g. throughput) that approaches its target. When a combination of TXOP and muting period is found, the eNB can evaluate other configurations by using neighbouring values for both TXOP and muting period. As it has been observed by the simulation result, neighbouring configuration values are more possible to offer fair coexistence. Hence, for instance if a TXOP duration of 4 ms followed by a muting period of 8 ms is a possible configuration, then a next possible combination could be a TXOP of 5 ms followed by a muting period of 8 ms. As in every learning technique, this method requires a convergence time to identify the desired configurations. In the beginning, the system can operate in acceptable bounds but as the time passes the considered heuristic algorithm approaches to the optimal configuration values in reasonable time. The complexity is in line with our previous work [31], in which two heuristic algorithms for joint power assignment and resource allocation in femtocells are evaluated and optimized in order to achieve the optimal solution in short time. The design of an algorithm that determines the configurations that can offer fair coexistence in an optimal way will be further examined in our future work.

Based on the identified combinations of TXOP and muting period that offer fair coexistence for a specific topology and according to the traffic requirements that must be satisfied, the network can select the optimal configuration that serves them better. For instance, in case of voice traffic (AC_VO), the network must choose a configuration that offers a short muting period and a long TXOP. On the contrast, a network that must serve best-effort traffic (AC_BE) can choose a configuration with longer muting period and shorter TXOP.

Algorithm 1 presents the complete procedure as it is described above and is required by an independent mLTE-U network to select an optimal configuration that enables fair coexistence with the co-located LTE or Wi-Fi networks. The algorithm takes as input the traffic requirements that the mLTE-U network has to serve. Then, periodically it performs a technology recognition in order to identify potential co-located LTE or Wi-Fi networks. Based on the discovered networks, the algorithm determines the possible values (TXOP and muting period) that can provide fair coexistence (e.g. using a heuristic technique). Finally, based on the traffic requirements, it selects the optimal parameters that enable fair spectrum sharing. Further study and optimization of the techniques that can identify an optimal mLTE-U configuration based on different topologies and traffic requirements for both LTE and Wi-Fi will be investigated in our future work.

This article proposes a new coexistence scheme that can enable a fair coexistence of LTE-U and Wi-Fi. As it is discussed, a fair coexistence can give to the participating networks opportunities to achieve equal performance in a technology-agnostic manner. The proposed coexistence scheme named mLTE-U, requires a CCA procedure before each mLTE-U transmission. When the CCA mechanism indicates the channel as idle, then the mLTE-U performs a transmission burst of variable duration followed by a muting period of variable duration. The muting period can give further transmission opportunities to coexisting Wi-Fi networks. The proposed mLTE-U scheme and the provided coexistence with Wi-Fi and other mLTE-U networks is evaluated in different scenarios of high interest. These scenarios include different mLTE-U and Wi-Fi network densities, as well as static and moving end-devices. Furthermore, we discuss the procedure according to which an mLTE-U network can select the parameters that can offer the required fair coexistence in a technology-agnostic manner, based on the number of participating networks and the traffic requirements that must be satisfied. The simulation results show that the proper configuration of mLTE-U according to the number of co-located networks can enable fair and harmonious coexistence in unlicensed spectrum.

In the near future, we will further investigate and analyse techniques towards the optimal selection of the mLTE-U parameters that can enable fair coexistence with co-located wireless technologies.