FD MAC Protocol Design for Co-Existing WLANs in 5G Cellular Networks

In this paper, we design full-duplex (FD) medium access control (MAC) protocol for co-existing wireless local area networks (WLANs) in 5G cellular networks (WCFD-MAC). Our design considers some significant features of 5G networks, exceptionally, beamforming and FD capabilities at both, the base station and user equipment. FD communications may generate interferences, namely self-interference (SI) in bidirectional FD (BFD) and interuser interferences in three-node FD (TNFD). Several efforts have been performed to mitigate the SI in BFD communications. However, the inter-user interferences in TNFD are still considered as a major drawback in FD networks. These interferences must be mitigated by an efficient MAC protocol design. WCFD-MAC protocol allows two neighboring users to simultaneously participate in TNFD communication without interfering with each other by using directional transmission and a three-node angle condition (TAC). Directional transmission combined with TAC increases system throughput. WCFD-MAC protocol allows a new half-duplex (HD) communication scheme referred to as three-node HD (TNHD). This scheme may occur when bidirectional FD (BFD) and TNFD communications cannot occur. TNHD scheme includes device-todevice (D2D) communication and allows energy efficiency, which is one of the keys requirements of 5G wireless networks. Simulations results show that WCFD-MAC protocol achieves higher throughput than existing works in the literature.


INTRODUCTION
Multi-radio access technology (RAT) carrier aggregation is a technique that allows channels from different RATs to be aggregated and allocated to the end-user [1]. LTE-WLAN aggregation (LWA) emerged as a new alternative technology to LTE-U/LAA (LTE-Unlicensed/LTE Licensed Assisted Access). Release 13 includes LTE-WLAN aggregation where LTE (Long Terme Evolution) and WLAN (Wireless Local Area Network) are aggregated at the packet data convergence protocol (PDCP) layer, using a framework very similar to dual connectivity [2]. Furthermore, release 13 specifications allow aggregation of a primary cell, operating in licensed spectrum to deliver critical information and guaranteed quality of service (QoS), and a secondary cell, operating in an unlicensed II. RELATED WORKS AND MOTIVATION The work presented in [1] designed a scalable MAC protocol for D2D communication for future 5G networks. In this design, a three tier 5G heterogeneous architecture was proposed, where a macro-cell BS co-exists with multiple small cell WLAN BSs, which allows traffic offload from the cellular system to the WLAN system in a dense environment. To address the resource allocation in the WLAN small cell, the authors designed a small cell MAC protocol (SC-MAC) based on the PCF (Point Coordination Function) access mechanism. The feasibility and practical deployment possibility of integrated LTE-WLAN systems for the 5G mobile networks have been highlighted in [21]. They stated that the small cell BS and the WLAN BS are wired, or collocated within an integrated physical element, and can exchange through an interface such as the X2 interface. The work in [22] considered mmWave integrated heterogeneous networks for 5G cellular, where both cellular and WLAN systems utilize mmWave for their communications. To handle the performance degradation challenges caused by the increasing density of the WLANs in 5G unlicensed network scenarios, a channel observationbased scaled backoff (COSB) mechanism is proposed in [23]. The proposed COSB is based on CSMA/CA without request to send clear to send (RTS/CTS) mechanism that can avoid the hidden terminal problem. Furthermore, COSB can reduce the collision probability in a dense deployment scenario, however, it will introduce more delay due to the observation time slots introduced after the channel become idle for DIFS time. Moreover, 5G networks FD capability and beamforming technology were not considered in the designed mechanism. In [24], FD MAC protocol design for 5G mobile wireless networks is provided. In the proposed MAC protocol, different schemes such as BFD, TNFD, and HD communications have been considered. The proposed design is based on the CSMA/CA and RTS/CTS mechanisms. However, beamforming/directional transmission and the Brazilian Microwave and Optoelectronics Society-SBMO received 16  inter-user interference in TNFD communication were not considered in the proposed design. In [25], another FD MAC protocol design for 5G cellular WLAN, called FDDS-MAC (FD dynamic scheduling MAC) aims to enhance the spectrum usage on FD communications in 5G mobile wireless networks, by increasing BFD transmission probability. FDDS-MAC uses pulse/tone signals instead of RTS/CTS for channel reservation, since those signals can be decoded in 5µs. However, the decoding of the pulse/tone signal is challenging. Moreover, beamforming/directional transmission and TNFD scheme have not been considered in FDDS-MAC protocol design. To fully exploit FD benefit in heterogeneous wireless LANs, FD MAC (HFD-MAC) protocol based on CSMA/CA and RTS/CTS mechanisms have been proposed in [10], where the co-existence of the FD nodes and HD nodes is considered. HFD MAC protocol suppresses inter-user interference in TNFD communications, however, it does not allow two nodes to simultaneously communicate with the BS when they are in the transmission range of each other, in other when they are too closer.
Many existing FD MAC protocols have been addressed for ad-hoc mode networks deployment [26]- [30]. Although, some other works were focused on FD MAC protocol design for the infrastructure mode [31], [32].

A.
Network model In this paper, we consider a covered small cell with WLAN BS. We assume that the BS and all the UEs use the same frequency band to exchange control frames and data packets. Each UE and BS have FD features, meanings that they can transmit and receive simultaneously using the same frequency band and can mitigate the self-interference. Also, both the BS and UEs can transmit directionally and omnidirectionally. To avoid deafness problems, in our FD MAC protocol all the control frames except for the ACKs frames are transmitted omnidirectionally, while the data packets are transmitted and received directionally by performing a directional beam in the direction of the receiver and transmitter, respectively. When the BS transmits a control frame omnidirectionally, all the UEs in the covered small cell can hear it and update their NAVs (Network Allocation Vectors) accordingly. Directional transmission will be possible after hearing a transmitted control frame and estimating the transmitter direction to then beam form in the direction of the incoming control frame B.
Antenna system model The BS and all the UEs are assumed to be equipped with massive MIMO to perform beamforming technology using phased array systems (adaptive beamforming system) which can determine the angle of arrival (AoA) of the incoming signal and then switch the most appropriate beam. Also, this system can direct the beam in the exact direction needed and can move it in real time [33]. After hearing an RTD control frame signal from the transmitter, the indented receiver will beam form toward the transmitter direction, since it can estimate the exact location direction of the transmitter. Also, after hearing the corresponding CTSD, the transmitter can beam form toward the receiver direction. We assume that all the UEs and BS have the same beam width, 2 b , which is symmetric concerning the desired transmission direction. We assume an adaptive beamforming system which can perform: Omnidirectional transmission: omnidirectional transmission is used to transmit all the control frames apart from the ACK frame which is transmitted directionally. Thus, inter-user interference due to hidden/exposed can be avoided.
Directional transmission: only the data packets and ACK frames are transmitted directionally after the control frame exchanges. This will make possible TNFD communications in the covered small cell at the same time. Thereby, spatial reuse and spectral efficiency increase. Directional antennas transmission can allow a neighboring UE of the PT to simultaneously receive a data packet from the BS (PR and ST). Furthermore, it can allow a neighboring UE of the PR to simultaneously transmit a data packet to the BS (PT and SR) without interfering with the PR reception. Fig. 2  In TNFD communication, interference can occur between the UEs, from ST to PR and from PT to SR. Therefore, the following section will analyze how these interferences may occur and how we can mitigate them.

IV. TNFD INTER-USER INTERFERENCES AND MITIGATION
In this section, we will first introduce how inter-user interferences can occur in TNFD communications when directional transmission is employed. Therefore, we will present our approach to suppress them by using an angle condition based on the beam of the employed antennas.

A. Interference from ST to PR
Let us suppose that in Fig. 2 To mitigate these inter-user interferences in TNFD communications, we will establish some conditions referred to as TAC (Three-node Angle Condition) based on the transceiver directional antenna beams width and the angle between them at the ST side or the PT side.

C. TNFD inter-user interferences mitigation
In this subsection, we propose an efficient method to mitigate the inter-user interferences that occur when two adjacent UEs participate simultaneously in TNFD communication. Our approach is based on a condition called in this paper TAC as mentioned above. In other words, two UEs can participate simultaneously in TNFD communication when the separation angle between the locations of these UEs is sufficiently high than a threshold value at the PT side or the ST side depending on if the ongoing TNFD communication is source-based TNFD or destination-based TNFD. Before formulating TAC, we are interested to derive the angle (between the elements) that must be greater than a given threshold value. Fig. 3 a)  B and 2 1 B is greater than a given threshold value. The value of q depends not only on the width of the beam of the employed antennas [34] but also on the value of  as shown in Fig. 3 a), and it is expressed as where 2 b is the width of the beam as mentioned above, and  is the angular separation of the To avoid inter-user interference from PT to SR, the BS must always select an SR whose location satisfies the TAC. When there are many UEs whose locations satisfy the TAC, the BS can randomly select one of them before sending the SR-NCTS control frame with the address of the selected UE as the SR for the ongoing TNFD communication. To avoid interference from ST to PR, the BS must check first the TAC between the ST and the PR, and will only allocate the channel to an ST whose location satisfies the TAC. When there is more than one UEs whose positions satisfy the TAC, the BS will select one whose location form the highest angle q for the PR. In the following subsection, we give the way to derive the angle q at the BS side. D.

Angle q estimation
In this sub-section, we present our approach to how BS can estimateq. Our approach is firstly based on the cellular user position estimation method based on the AOA (Angle of Arrival) presented in [35]. Then, we will apply mathematical cosine and sine rules to this method to derive the angle q. The method in [35] is based on the fact that when a BS receives a signal from a UE it can know its AOA and the corresponding coordinates. Because of noisy measurements in the UE position estimation, the authors in [35] proposed to collect information data from multiple base stations in order to resolve the ambiguities resulted from multiple crossings of the lines of position and to improve the positioning accuracy. Note that the information collection can be easily performed in the dense deployment scenario by allowing cooperation between different BSs.  In order to effectively avoid the interference in TNFD communications, the angle q must be estimated with more exactitude, therefore we adopted the proposed method in [35]. The 1 UE can be simultaneously associated with many base stations. Fig. 4 However, since the angles a , d , and g , subject to measurements errors, the three lines do not intersect at the same point. A reasonable method to estimate the UE position is to find the point of intersection of each two lines mutually and average the result.   Fig. 5 to derive the angle q.
The angles 1 f and 2 f can be calculated as  [36] and HFD-MAC protocol presented in [10]. Therefore, some control frames are the same as those presented in [10] and IEEE 802.11 standard. Furthermore, the CSMA/CA backoff algorithm is the same as in [10], [36]. The control frames used in this paper are RTSD, CTSD, RTSC, SR-NCTS, ST-NCTS, and ACK. Their formats are shown in Fig. 6. The ACK frame is the same as the standard of IEEE 802.11, then it is not shown in Fig. 6.
The main differences with the one presented in [10] are: -Our design uses omnidirectional and directional antenna transmission, instead of omnidirectional only.
-our design allows two adjacent UEs to simultaneously transmit and receive to and from the BS in TNFD communication, which is impossible in [10].  NDI control frame in source-based TNFD scenario as occur in [10], in our design the BS will immediately select one of these UEs and then transmit ST-NCTS frame with its address.
-A new HD scheme is proposed (TNHD) in our MAC design, which can enhance the throughput of UEs located at the small cell border. The meaning of DI bits is the same as those in [10] and are presented in Table I. According to the location and traffic condition at FD UEs involved in this communication scenario, four possible cases arise as depicted in Fig. 7: BFD, TNFD (destination-based TNFD and sourcebased TNFD), HD, and TNHD.

A. Bidirectional FD communication (BFD)
After receiving an RTSD frame from BS, if a UE also has a data packet for BS, it replies with a CTSD frame with a DI value of 11. Then, the communication becomes bidirectional, and both BS and UE transmit simultaneously their data to each other after exchanging RTSD/CTSD control frames.
After a SIFS time, the ACK frames are transmitted to each other simultaneously as the data packets.
In this case, the BS becomes at PT/SR and the corresponding UE becomes PR/ST simultaneously.
Note that communication can also be initiated by a UE. if the PR does not have a data packet to the PT, however, it has a data packet for another UE, thus the PR becomes ST and the receiver of its data packet becomes SR. This scenario is referred to as destination-based three nodes FD. The PR/ST replies with the SR-NCTS frame where it will indicate the address of an SR whose position satisfies the TAC with respect PT in the SR address field as shown in Fig. 6. The SR-NCTS frame will inform both PT and SR about their data exchange information as proposed in [10]. After receiving the SR-NCTS, the SR replies with another CTS control frame to the PR/ST, and the data packet exchanges will occur between the three network elements after a SIFS time. Note that, the SR must be a UE whose location satisfies the TAC which can reply to PR/ST and TNFD communication will occur even if they are in the transmission range of each other thanks to directional transmission unlike the MAC protocol proposed in [10], [37]. After the data transmission, ACK frames are transmitted directionally and simultaneously from the PR to the PT and from the SR to the ST.
However, if the PR (BS) does not receive the SR-NCTS from SR, it will receive the data packet from PT after SIFS plus CTS times. Other nodes update their NAV times according to the time defined by the control frames. This case is shown in Fig. 8.

E.
Computational complexity In this paper, our design considers directional transmission, which is performed by the beamforming technique and AoA estimation. The AoA estimation algorithm must have high resolution and low computational complexity. Our design can use the algorithm proposed in [38] since it is computationally more efficient and it doesn't require an exhaustive search through all possible steering vectors for AOA estimation. The computational complexity of this algorithm was derived in [39]. Additionally, our design follows the user scheduling algorithm based on CSMA/CA and RTS/CTS mechanisms of the IEEE 802.11 standard, thus having the same computational complexity as this standard.

VI. PERFORMANCE EVALUATION
In this section, the performance of our MAC protocol design is evaluated through simulations based on Bianchi's model [40] and the transmission probability derivation method presented in [10]. To this end, we will present in the first subsection, transmission probability derivation, then we will present throughput analytical expression derivation after the impact of SI cancellation consideration, and finally, we will present simulation results.

A. Transmission probability
To derive the probability equations in this paper, our approach follows the method developed by where µ is the average data rate of the BS.
In the following subsections, we will present the transmission probability expressions for BFD, TNFD, HD.

1.
Bidirectional FD transmission probability Independently from which network element initiates the transmission (BS or UE), BFD transmission can occur. Therefore, we can identify two scenarios for BFD communication to obtain the BFD transmission probability ( BFD P ).

Scenario 1 (BS initiates the communication): when the BS begins the communication by sending an
RTSD control frame to a UE, if this UE has a data packet to send to the BS at the same time, thus, BFD transmission will occur between them. To obtain BFD P corresponding to scenario 1, we make the following observations related to the conditional probability: i) the conditional probability that the packet arriving at the BS is for a UE, which is  Table II.

Scenario 2 (UE initiates the communication): when a UE begins the communication by sending an
RTSD control frame to the BS, only if the BS has a data packet to send to the corresponding UE at the same time, BFD transmission will occur between them. To obtain BFD P corresponding to scenario 2, we make the following observations related to the probability theory: i) the conditional probability that a packet arriving at BS is sent by a UE is UB total nl l ; ii) the probability that BS also has at least one packet to send to the corresponding UE within the time 1 . Now, we can write BFD transmission probability BFD P corresponding to two scenarios presented above as

Three-node FD transmission probability
TNFD communication can occur whether the BS initiates the communication. In this case, we can also identify two scenarios for TNFD communication in order to obtain the TNFD transmission probability ( TNFD P ).  condition that must be satisfied is ''the corresponding UE has no data packet in its buffer to send to the BS''. The second condition is ''another UE has a data packet to send to the BS at the same time''.
To obtain TNFD P corresponding to the scenario 2, we make the following observations related to the probability theory: i) the probability that the BS at least has a data packet to send to a UE is  Table II. Now, we can write TNFD transmission probability TNFD P corresponding to the two scenarios presented above is expressed as

B.
Impact of SI cancellation coefficient The SI cancellation performance depends on several factors, such as system bandwidth, antenna displacement error, and transmit signal amplitude difference [24], and channel estimation error at the receiver side and active/passive cancellation robustness.
Let H E , B E and T E be the effective packet payloads of HD, BFD and TFD transmission, respectively.
Their expressions are given by where 1 E and 2 E are the packet payloads transmitted in both directions between the BS and one UE in BFD transmission, or packet payload exchanged during a TNFD transmission between the BS and two other UEs. k ( 0 1 k £ £ ) denotes the SI cancellation coefficient. When k approaches 0, this means that the SI causes large interference on FD communication. When k approaches 1, it implies that the SI causes little interference on FD communication. Note that the differentiation between the effective packet payloads corresponding respectively to BFD and TNFD transmissions is due to the fact that BFD links are more affected by the SI than TNFD links [41], since in BFD scheme each network element involved in the communication has to mitigate SI, while in TNFD scenario, only one network element (BS) has to mitigate SI.

C.
Throughput expression To derive the throughput analytical expression for our design, our approach is based on Bianchi's model [40] which was developed for HD communication scenarios. From Bianchi's throughput model derived in [40], the system throughput can be expressed as ( ) HD H BFD B TNFD T HD BFD TNFD S C C idl slot P E P E P E S P P P T P T P T where 1 idl tr P P = -, tr P denotes the probability that there is at least one transmission in the considered slot time and it depends on the parameters of exponential backoff algorithm (minimum and maximum backoff window size) and of the number of nodes in the covered small cell and can be obtained as in [40]. The meaning of all the parameters in (18) are reported in Table II T T  T  T  T  T  T  T The collisions can occur only on the RTSD frames in the systems where each packet is transmitted through the RTSD/CTSD access mechanism [40].

D.
Simulation results In this subsection, we present simulation results of WCFD-MAC protocol design. In the simulation stage, some assumptions have been performed. We assumed that TAC is satisfied with the sourcebased TNFD scenario when the angle 1 q between the straight lines ( )( )

BS ST
   is greater than a given threshold value. Similarly, we assumed that TAC is satisfied with the destination-based TNFD scenario when the angle 2 q between the straight lines ( )( )

BS PT
  is greater than a given threshold value. We evaluated our method by performing simulations under non-saturation conditions, because in saturation conditions all the communications in the covered small cell may be BFD communications. Note that, saturation conditions assume that, always all the UEs and BS have a data with the previous works related to FD MAC protocol, particularly, with those obtained in [10], [24], [36], [42]. We selected [10] for comparison since it is the only work that addressed partially inter-user interference in TNFD communication for FD MAC protocol according to our acknowledgment. Thus, for a fair comparison, we used the same simulation parameters as in [10]. Additionally, we compare our simulation results with those obtained in [24], because it is the most cited work on FD MAC protocol design for 5G cellular networks in literature. These simulation parameters are shown in Table III. To evaluate the performance of our design, the following aspects have been evaluated:

1)
System throughput and comparison with previous works: Fig. 11 shows the performance of the system throughput as a function of the number of UEs of our proposed MAC protocol. Fig. 11 indicates that both WCFD-MAC analytical and WCFD-MAC simulation achieve higher throughput.
For our analytical throughput derivation, we used (22) where the proportion of occurrence of the FD links w was fixed to 0.90, while for the simulation throughput derivation, the proportion of occurrence of the FD links w varies according to the given threshold value of angle for TAC, which was not considered in the analytical throughput calculation. The threshold angle value for TAC was fixed to 6°(6 degrees). In other words, when the angle q is lower than 6°, the corresponding ST or SR cannot be selected for the ongoing TNFD communication. Therefore, the values of the two throughputs are different, but they have similar behavior and approximatively similar values, which can justify our simulation throughput derivation approach. Also, other parameters such as g and k were fixed to 0.3 (for a fair comparison with HFD-MAC) and 1 (perfect SI cancellation), respectively. where inter-user interference is suppressed by avoiding TNFD transmission when the two corresponding UEs are closer. This mechanism decreases the spatial reuse, thus decreasing the overall system throughput since TNFD will only occur when the ST and PR or when the PT and SR are out of the transmission range of each other, which will impose a high probability of HD communication than TNFD. WCFD-MAC protocol achieves higher throughput than HFD-MAC because our design simply requires a TAC to be satisfied, thus it allows TNFD communications with high probability with respect to HFD-MAC [10], thus, enhancing spectrum efficiency and special reuse. Another major problem with HFD-MAC protocol is that in a source-based TNFD communication scenario the UEs must perform a self-timer algorithm and the user whose self-timer reaches zero will become the ST.
This algorithm also may decrease the system throughput since it introduces more delay, which may not satisfy 5G cellular networks delay requirement. In WCFD-MAC protocol, the self-timer algorithm is suppressed, in order to minimize communication delay and enhance the system throughput. Hence, our proposed design enhances the spectrum efficiency by allowing better special reuse than all the existing FD MAC protocols, and it can at least double the system throughput of the traditional HD system. 2) Impact of the threshold value of angle on the throughput: Fig. 12 shows the variation of the system throughput with the number of the UEs in the covered cell with respect to different values of the threshold angle values assuming a perfect SI cancellation. When the value of the given threshold angle increases, the system throughput decreases, as shown in Fig. 12 with 6°, 15°and 30°of the threshold value of angle. Additionally, Fig. 12 indicates that when the threshold angle for q is 30°the decrease of system throughput becomes more important. This is due to the fact that the number of the UEs that cannot be selected for TNFD communication (because they cannot satisfy the established TAC) becomes high, thus imposing more HD communications, which may decrease the throughput.
When the value of the threshold angle for q is very small, for example, when this value is fixed to 6°, the achieved throughput at least doubles that of the traditional HD system of the IEEE 802.11 standard. Since beamforming/ directional transmission in 5G networks is assumed to be performed using massive MIMO and mmWave technologies, the beamwidths of the UEs and BS may be very  Fig. 13. Impact of SI cancellation coefficient (SIC) on the system throughput with q = 6°(6 degrees).
3) Impact of the SI cancellation coefficient on the system throughput: Another factor that affects the system throughput is the value of the SI cancellation coefficient k . Fig. 13 shows how the system throughput decreases when the SI cancellation algorithm is not efficient at the FD devices, including the BS and UEs. Different values of the SI cancelation coefficient k will have a different impact on system throughput. In Fig. 13, it is shown that when the SI coefficient k is 1 or approaches 1 ( 0.9 k = ), the achieved throughput at least doubles the throughput of the traditional HD system of the IEEE 802.11 standard. However, when k is 0.5 or less than this value 0.5, the achievable throughput is approximatively equal to or less than the throughput of the HD communication system. Better performance of SI cancellation obtained by combining passive suppression, analog and digital domain cancellations can further enhance the system throughput with a value of k that reaches 1. Fig. 14

4) Energy efficiency (EE):
where, Col E denotes the energy consumed in collided control frame transmission. Tx E denotes the total energy consumed in BFD, TNFD, HD or TNHD successful transmission, respectively, and it was derived using the expressions of energy consumption for FD transmission as in [43]. For further details on the calculation of these parameters, the interested reader is directed to [43] and the references within. For all the communication scenarios depicted in Fig. 14

6)
Collision probability: Fig. 15 shows a comparison of the collision probabilities between WCFD-MAC protocol, T2F Single Round protocol presented in [42] and IEEE 802.11 standard basic scheme [36]. For all these MAC protocols, the collision probability grows proportionally with the number of UEs in the covered small cell, because when increases the number of the contending UEs, more UEs will try to access the channel simultaneously, which will result in a collision increase. As shown in Fig. 14, WCFD-MAC achieves the lowest collision probability even when increases the number of UEs in the covered cell. This high performance is due to the RTSD/CTSD mechanism which is omnidirectionally transmitted for the channel reservation in order to prevent collisions.
Hence, our proposed WCFD-MAC protocol achieves high performance in terms of collision probability.

VII. CONCLUSION AND FUTURE WORKS
In this paper, we designed WCFD-MAC protocol for co-existing WLANs in 5G cellular networks considering all possible communications schemes in an FD covered small cell including BFD, TNFD and HD. To mitigate the inter-user interferences in TNFD communications, we proposed an optimal TAC based on the width of the beams of the antennas, AOA, and mathematical cosine and sine rules.
The proposed TAC allows two neighbor UEs to simultaneously participate in TNFD communication without interfering with each other. This increases the overall system throughput of the covered cell.
Our MAC protocol includes a new HD (TNHD) communication scheme that aims to enhance the throughput of the small cell boundary UE and enhance energy efficiency by reducing the energy consumption at both, the BS and UE. The achieved throughput of WCFD-MAC protocol was compared to the previous works, namely HFD-MAC, FD-MAC, and traditional HD IEEE 802.11 standard. It has been shown that WCFD-MAC protocol performs higher throughput than these works and it can double the throughput of the HD IEEE 802.11 standard when the given threshold value for TAC is small or equal to 15°assuming perfect SI cancellation. However, when the SI cancellation coefficient reaches 0.5 WCFD-MAC achieve approximatively the same throughput of the IEEE 802.11 standard with the threshold angle value for TAC fixed to 6°. Due to significant performance improvements and the fact that it considers some important features of 5G networks for application in cellular scenarios the proposed protocol can be a solution for the co-existing (integrated and nonintegrated) WLANs in future 5G mobile networks.
When estimating the angle q at the BS side in order to verify the TAC, we assumed cooperation between the BS of the covered small cell and other BSs in its vicinity, since the 5G network densification strategy requires coordination/cooperation between different BSs. In current cellular networks, there is coordination/cooperation among the BSs, however, there is a need to develop a coordination/cooperation mechanism between the cellular BSs and WLAN BSs for the 5G heterogeneous networks. When the cooperation mechanism is performed among BSs (including the cellular BSs and WLAN BSs), the estimation of the angle q has lower complexity. In our future work, we will focus on implementing a more efficient power saving algorithm by developing a user scheduling approach that takes into account power allocation and coordination/cooperation among different BSs in a dense deployment scenario. Moreover, we have only considered symmetric traffic between the uplink and downlink. An interesting research topic will be how to improve WCFD-MAC protocol by taking into account asymmetric traffic conditions.