Introduction

The number of mobile-connected devices is expected to increase drastically in the upcoming years, which requires the more efficient design of the cellular architecture. Global mobile traffic flow will grow seven-fold between 2017 and 2022. Per capita mobile devices will be 1.5 times, the network connection speed will increase up to 28.5 Mbps, 5G connection will generate approximately 2.6 times more traffic compared to 4G, and average 11 GB traffic will generate by smartphones up to 2022 [1]. D2D communication is the most innovative technology which contributes to the reduction of the heavy traffic problem.

D2D communication is categorized as in-band D2D, i.e., licensed spectrum, and out-band D2D i.e., unlicensed spectrum. In-band communication uses a cellular spectrum for both D2D and cellular links. In-band communication is classified as underlay and overlay. Underlay D2D communication shares the same radio resources for cellular and D2D communication while overlaying D2D communication uses a dedicated cellular spectrum. In-band D2D communication can expand spectral efficiency (SE) by reusing spectrum resources (i.e., underlay) or by allocating dedicated cellular resources to D2D users (i.e., overlay). Interference caused by, D2D users to cellular users or cellular users to D2D users, is the biggest problem for D2D communication.

Out-band D2D communication uses an unlicensed spectrum so interference between D2D link and cellular link affects the system least. Out-band communication is classified as ‘controlled’ and ‘autonomous’ D2D communication. Out-band D2D uses other wireless technologies such as Wi-Fi direct [2], ZigBee [3], or Bluetooth [4] for connection between user equipment (UEs). If control of the second interface/technology is given to the cellular network [5,6,7,8] then, this is controlled out-band D2D communication. On the other hand, keep the cellular communication control, and leave the D2D communications control to the users [9], then this is autonomous out-band D2D communication. Remember that the out-band D2D communication suffers from the uncontrolled nature of unlicensed spectrum.

The transmit rate of data can be increased up to 1Gbps for a distance of 1000 m [10] in D2D communication. In recent years, the 5G cellular networks have attracted the attention of researchers because of the milestones of 5G, which include almost zero delay, 100 billion links, and 1000 times improved throughput. 5G cellular network uses a D2D communication system to improve SE, system capacity, resource utilization, coverage extension, traffic offloading, energy efficiency (EE), and quality of services [11]. Direct communication between two or more devices without the involvement of the base station (BS) is known as D2D communication. Wi-Fi and Bluetooth are the technologies doing the same job but efficiency is low.

Cellular congestion reduces, by offloading users’ traffic with the help of the D2D network. D2D communication requires efficient physical algorithms, user and control plane protocol, and network architecture for cellular network offloading. D2D communication is popular for small-cell networks but network performance and offloading the traffic of the BS can be enhanced by D2D communication for underlying long-term evolution (LTE).

D2D communication was introduced by 3GPP (LTE release 12). LTE D2D communication uses proximity-based services for efficiency, flexibility, dynamicity, and security. One point must be noted that before the D2D communication, Wi-Fi and Bluetooth have supported short-range communication but they use unlicensed spectrum, manual pairing, low-security features, and are independent from the cellular network. Unlicensed spectrum generally doesn’t have a problem for low densities but has major limitations on proximity-based services along with security. Additionally, any form of D2D communication based on Wi-Fi and Bluetooth runs parallel with radio operations which may be the cause of significant drain on device batteries. Figure 1 explains, the basic concept of D2D communication along with the role of BS.

Fig. 1
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D2D communication

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D2D communication has some design challenges [11,12,13,14] like a choice between LTE uplink and downlink, a choice between OFDMA (orthogonal frequency division multiple access) and SC-FDMA (single carrier frequency division multiple access), a choice between static and dynamic allocation for radio resources, a choice between synchronous or asynchronous D2D discovery, different propagation characteristics, interference, the network controlling, reuse of protocols, adding of new features and protocol along with algorithms, frequency discovery, nature and quantity of information, privacy and security, and flexibility options based on a verity of approaches.

5G systems, offer network-controlled D2D communication that has the capability of local traffic offloading. Traffic offloading is defined as, the separation of local traffic with global traffic by the use of short-distance communication link management. It is also helpful in the removal of the load burden of the network and reduction in the effort for managing traffic at central network nodes. D2D communication provides low latency communication which is generated by local communication links between users in proximity. In this way, D2D communication plays a very vital role for 5G. Real-time implementation of D2D in a 5G network faces some challenges, i.e., interference management, network discovery, proximity services, context-aware services, network coding, and security. Future direction and emerging aspects for D2D in 5G are V2V (vehicle to vehicle) communication, mm-Wave technology, social D2D networks, energy harvesting, simultaneous wireless and information power transfer (SWIT), pricing, incentive.

IoT transfers data, over the network, without requiring human-to-human or human-to-computer interaction. With the help of unique identifiers, IoT interrelates computing devices, mechanical devices, and digital devices. Due to the utility of IoT, 5G placed IoT in a vital position in its ecosystem. So, the compatibility of IoT with D2D communication is a challenging task for future 5G.

Spatial diversity, underlying in-band D2D communication can increase spectral efficiency, and this is possible by interference management, mode selection, resource allocate on, and network coding. Wireless communication ought to use the spectrum efficiently. SE may be defined as [15]:

$$\eta = \frac{R/B}{K} \;{\text{b/s/Hz/Site}}$$
(1)

Here and now, R is the bit rate, B is Bandwidth and K is the size of a cluster. R/B is the link efficiency. Generally, SE is described as the net data rate in bits per second (bps) divided via way of means of bandwidth in Hz. To higher apprehend, let’s have a gadget that has a channel bandwidth of three MHz and raw data rate of 17 Mbps and overhead is 3 Mbps (net data rate = 17 − 3 = 14 Mbps), so the SE will be:

$${\text{SE}} = \frac{{14*10^{6} }}{{3*10^{6} }} \approx 4.67 \;{\text{b/s/hz}}$$
(2)

D2D communication uses higher order modulation technique. Suppose we have an LTE network that uses D2D communication. Here raw data rate is a combination of payload and overhead. Let’s, channel BW is 2 MHz, the raw data rate is 15 Mbps, overhead is 2 Mbps. So, the net data rate will be 13 Mbps (15 − 2). Now, (SE) will be 6.5 bits/sec/Hz (13 × 106/2 × 106) (Table 1).

Table 1 LTE occupied bandwidth and number of subcarriers for specified channel bandwidth
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Here and now;

$${\text{Symbol}}\;{\text{ Rate}} = \left( {{\text{No}}{. }\;{\text{of}} \;{\text{subcarriers}}} \right)*\left( {{\text{Data}} \;{\text{Rate}}\;{\text{ of}}\;{\text{individual}}\;{\text{ carrier}}} \right)$$

For channel BW 20 MHz, each subcarrier is able to carry data at max. rate of 14 Kbps. Note that sub-carrier spacing is 15 kHz. So:

$$\begin{aligned} {\text{Symbol}}\;{\text{ rate }} = & \, 1200*14 \times 10^{3} \\ = & \, 16.8 \times 10^{6} \;{\text{symbol/s}} \\ \end{aligned}$$

For 16 QAM; number of bit transmitted is 4 (24). So, the raw data rate will be 67.2 Mbps (1.68 × 106 * 4). For down link; assume 4 × 4 MIMO used, now, the data rate will be 268 Mbps (67 * 4). Here 25% is overhead. So, net data rate will be 201 (268 − 67) Mbps. Similarly, for uplink, there is MIMO not allowed and 25% overhead, so, net data rate will be 51 Mbps (= 67 − 16.75). So, downlink SE for 16QAM = 201 × 106/20 × 106 = 10 bits/s/Hz and uplink SE for 16QAM = 51 × 106/20 × 106 = 2.55 bits/s/Hz. Similarly, downlink SE for 64QAM = 300 × 106/20 × 106 = 15 bits/s/Hz and Uplink SE for 64QAM = 75 × 106/20 × 106 = 3.7 bits/s/Hz again downlink SE for 256QAM = 403.2 × 106/20 × 106 = 20 bits/s/Hz and uplink SE for 256QAM = 101 × 106/20 × 106 = 5 bits/s/Hz similarly downlink SE for 1024QAM = 504 × 106/20 × 106 = 25.2 bits/s/Hz and uplink SE for 1024QAM = 126 × 106/ 20 × 106 = 6.3 bits/s/Hz (Fig. 2).

Fig. 2
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Spectral efficiency of M-ARY QAM for LTE downlink and uplink

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Here, we can understand that the SE depends on the order of the modulation, data rate, bandwidth, MIMO, number of subcarriers, size of overhead, subcarrier spacing, the data rate of the individual carrier, UL/DL, size of the cluster, and link efficiency.

The literature [16,17,18,19,20] suggested interference management algorithms. Additionally, article [6] uses resource blocks that are used by a cellular user in the proximity (system throughput increased by 41%), In [16], there is a dedicated control channel for D2D users (increase the average system throughput up to 374%), article [17] avoids the allocation of the same frequency-time slot to the cellular which is used by D2D user at that time (30% higher capacity gain), manuscript [18] defines an interference-limited area in which no cellular users can occupy the same resources as the D2D pair (gain of 129%), article [19] formed interference-limited areas according to the amount of tolerable interference and minimum SINR requirements for successful transmission, and the reference [20] uses Han-Kobayashi rate splitting techniques [22] (increase the throughput up to 650% higher).

The first order protocol with the necessary functionality and signaling can achieve throughput up to 65% [23]. Estimation of the achievable transmission rate in each mode through utilizing the channel measurements can get a 50% gain [24]. A joint D2D communication and network coding scheme [25], uses an interference-aware algorithm [18] and gets the 30% increment in the number of D2D users. Power control of BS and cellular devices can improve the area of the capacity region of the D2D link and BS-device link by 60% [26].

For overlaying in-band D2D communication, a base station (BS) assisted scheduling and D2D power control used in [27], and authors of [28, 29] focus on relaying to overcome the effect of interference. Article [29] consumes 90% fewer spectrum resources in comparison to the scenario with only one transmitter.

Out-band D2D communication has no interference dispute between D2D and cellular communications. Outbound controlled D2D communication uses the cellar network advance management features. The authors of [30] propose the ISM band for communication in LTE as the groups of D2D users who do not sense the same channel at that time and get a 25% increment in the throughput. By creating clusters among cellular users with the help of Wi-Fi, D2D communication can increase the throughput by up to 30% [7, 8, 31].

This manuscript will investigate the path to improve SE by various concepts, models, algorithms, and scenarios. Here we will provide a review for SE improvement for D2D communication based on clustering in the cellular network, frequency reusing, spectrum sharing, resource block allocation, and implementation of the new generation waveform.

Interference Control, Management, and Reduction Methods or Algorithms

An interference is that which modifies a signal in a disruptive manner because it communicates alongside a conversation channel among its supply and receiver. The period is regularly used to consult the addition of undesirable alerts to a beneficial signal.

There are four types of interference in wireless communication systems [32] (Fig. 3):

Fig. 3
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Type of interference and their causes

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The internal interference can be reduced with the help of multichannel [33] and scheduling of communication channel with the help of TDMA (Time-division multiplexing) [34] while external interference may be reduced by channel hopping [35], temporary diversity [36], error control mechanisms [37], and network coding [38]. Channel hopping, temporary diversity, error control mechanisms, and network coding also overcome the effect of self-interference additionally, the antenna diversity technique contributes to the same [39]. Interference due to signal attenuation can be controlled by power control [40], multipath routing, error control mechanism, and network coding.

Interference is at once associated with the bandwidth and data rate of the system. As in line with Eqs. (1) and (2), we can see that SE is likewise associated with the bandwidth and data rate. Of course, we can grow the SE with the aid of using enhancing the data rate in addition to lowering the bandwidth, however in each case, there may be an increment withinside the interference which reduces the effectiveness of the system. So, interference management and control together play a completely crucial position in D2D communication. Interference withinside the cellular network may be decreased with the assistance of power control, powerful frequency mission, the use of sensible techniques, and intermodulation.

Research paper [41] evaluates the SE strategies in D2D communication for 5G, right here this article provides the demanding situations withinside the control of interference. The authors of [42] investigated and analyzed the essential idea at the back of the D2D communication gadget to increase an efficient D2D resource allocation scheme with decreased interference. The work in [43] suggests the power control strategies which facilitate to lessen the impact of interference wherein interference management algorithm maximizes the overall performance of D2D communication in LTE-A networks for uplink and downlink transmissions. Moreover, the most appropriate routing selection strategies reduces the entire route distance which facilitates to limit the interference and enhance the general network capacity. The manuscript [44] represents a survey of the interference management for D2D applications for 5G. The article [45] affords the special famous and AI-based interference mitigation and RA strategies advanced in D2D communications which might be contemplated withinside the article [46]. Research article [46] surveyed the artificial intelligence (AI) conventional RRM (Radio Resource Management) and interference mitigation techniques where the authors discussed the mitigation of interference through mode selection, power control, and RRM together with traditional strategies. The MAAC (Multi-Agent Actor-Critic) is an algorithm that is proposed by the researchers of [47], that is used to keep away from interference (Table 2).

Table 2 Summary of interference control management and reduction methods and algorithm for D2D communication
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Clustering in Cellular Network

Short reuse distances and low transmitting powers by the D2D network, lead to weighty acquisitive interference in the network. Such a densely deployed D2D network requires BS with large capacity, but it also introduces large delays in communication. This problem can be solved by dividing the network into several clusters, where each cluster has its BS for D2D communication. D2D multicast in the cluster, then the user transmits the same file separately to every receiver, which saves time and network resources. Multicast can be used for cellular communication which provides continuity for data sharing in the cellular network [57]. Figure 4 shows the basic clustering of D2D communicating devices in the cellular network, where D2D enabled devices to make small clusters and they are connected to BS.

Fig. 4
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Basic D2D clustering

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A set of UEs which communicate with each other for a common objective may be defined as a cluster. Any UE can be added/removed from the cluster at any time by opening/ending a channel configuration. Every cluster must have a master node and at least two worker nodes, and all these nodes perform operations by a shared network. Clusters do not reuse the frequency. While frequency used in one cell can be reused by another cell within the same cluster. So, clusters can overcome the effect of co-channel interference and adjacent channel interference. The size of clusters depends upon the traffic load of the network. But one point must be remembered that the small cluster size generates co-channel interference.

D2D communication performs clustering operations by assigning devices to direct communication mode by utilizing cellular network resources. Direct communication mode removes/ reduces core network involvement while enhancing BS resource utilization. Few services, i.e., mobile multiplayer gaming, file sharing, mobile advertising, streaming, and cooperative downloading could be benefited by clustering operation. D2D cluster mode decides to use radio resources, modulation and coding scheme, transmit/receive slot allocation, and link adaptation [58]. The further cluster head is assigned to each device group. These cluster heads are responsible for the allocated resources division between cluster members. The network controls the cluster member by the control link to the cluster head.

Clustering operation increases resource availability (one severs fails another server in the cluster can pick up the workload), strategic resource usage (allow to use resources flexibility), increased performance (multiple machines provide greater processing power), greater scalability (as complexity increases, resources can grow), and simplified management (clustering simplifies the management of large or rapidly growing system).

Many researchers provide their efforts to improve the D2D communication performance, i.e., increment the pair of D2D, SE, Energy efficiency (EE), sum-rate, throughput, minimization in delay, and complexity. They use uplink and downlink or uplink/downlink transmission for underlay and overlay D2D communication. A few of them are summarized in Table 3.

Table 3 Summary of clustering scheme and algorithm for D2D communication
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Frequency Reusing and Spectrum Sharing

A cellular area can reuse the same radio frequency if they are separated by a considerable distance. This is a frequency reusing concept. For cellular communication, band frequencies are allocated to each cell covered by BS, while an adjacent cell uses a different band of frequencies to avoid interference. This concept can improve the SE but co-channel interference is a major concern. Upcoming technology of communication works for short distances and indoor [73] so the concept of frequency reuse is very useful. Figure 5 shows the basic concept of frequency reusing in cellular communication where there are three clusters and seven cells considered for the shake of the moment. Frequency is named as 1, 2, 3, 4, 5, 6, and 7 allotted for each cell in the cluster. Each cluster cell reuses the same frequency.

Fig. 5
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Frequency reusing

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An important concern for D2D communication while using the frequency reuse concept is the interference between the cellular user (CU) and D2D user (DU). Four scenarios can elaborate on the interference problem in the frequency reuse concept. The first scenario is, BS receiver will receive interference from the DU for up-link transmission. The second scenario is, DU receiver will receive an interference signal from the CU transmitter for uplink transmission. The third scenario is, the CU receiver receives interference from the DU transmitter for downlink transmission, and finally, the fourth scenario is DU receiver will receive interference from the BS transmitter [12]. In this way, frequency reusing can achieve a large coverage area, efficient spectrum utilization, and enhance system capacity. Figure 6 shows the condition of one cell in a cluster when using the frequency reuse concept.

Fig. 6
figure 6

Frequency reuse for D2D communication in uplink and downlink

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The sharing of resources allows frequency reuse between two networks, i.e., D2D and cellular. This technique can increase SE but interference will degrade the performance. Researchers give efforts to improve the SE, gain, SINR, capacity by using the frequency reuse concept for D2D communication. They work for uplink transmission and downlink transmission for the overlay and underlay D2D communication (Table 4).

The same band of spectrum is utilized by different applications/technologies, in spectrum sharing. Spectrum sharing technique is classified as interweaving, underlay, and overlay [74]. Secondary system access the spectrum holes dynamically for interweaving spectrum sharing while secondary user concurrently transmitting to the primary user for underlay spectrum sharing (where interference due to secondary user to primary user must be below the threshold level), and finally, secondary user access the spectrum in frequency, spatial and time domain for the overlay spectrum sharing (overlay spectrum sharing is the sharing of a band which is already licensed to an operator with others). Some researchers also suggest dynamic spectrum sharing [75] and cooperative spectrum sharing [76]. Dynamic spectrum sharing access the spectrum dynamically based on wireless technologies, markets, and regularity policies while in cooperative spectrum sharing secondary users relay the traffic to the primary users by exchanging devoted spectrum access time for secondary own communication.

Table 4 Summary of frequency reuse techniques for D2D
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In-band and Out-band D2D communication is based on the type of spectrum sharing, i.e. cellular and D2D transmitters use orthogonal frequency or time resources for overlay and D2D transmitters use the time or frequency resources engaged by CU for underlay [86].

5G communication adopts three approaches to improve the system: (1) Renew the network architecture, (2) explore more spectrum resources and (3) adoption of new communication techniques [87]. Although new spectrum resources are allotted to 5G, due to emerging mobile traffic, upcoming communication system still has an issue with the available spectrum. So proper resource management is very important. Spectrum sharing is one of the best ways for resource management. Spectrum sharing allows the existing communication system to use idle or underutilized spectrum bands temporally and geographically.

The cognitive radio approach also uses spectrum sharing. The cognitive radio concept has two stages, i.e., spectrum sensing and cognitive transmission. The first stage is the cognitive user sense the radio environment and the second stage is the cognitive transmission which collects the spectrum information. Then the cognitive user selects the best spectrum bands and adapts transmission according to the collected spectrum information.

In this way, spectrum sharing increases efficiency, reduces waste, offers more useful dividends to the government, generates recurring revenue, and ensures that the spectrum is more accessible to more people (Fig. 7).

Fig. 7
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Spectrum sharing of cellular link and D2D communication

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Figure 5 shows that BS is connected with UE by cellular link and two UEs connected with D2D link. Here both links can use the same spectrum depending upon the availability. Table 5 summarizes the work of authors and researchers for the improvement in the performance of D2D communication. They use uplink, downlink, and uplink/downlink transmission for underlay, overlay D2D network.

Table 5 Summary of spectrum sharing scheme and algorithm for D2D
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Resource Block Allocation

Heavy traffic of users in the communication system, generates interference problems. This interference problem can be faded by using the resource block (RB) allocation concept. RB is the smallest unit of resources that can be allocated to a user. Here RB is 180 kHz wide in frequency and 1 slot long in time. LTE performs on various bandwidths (BW), i.e., 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, 20 MHz which contains 6, 15, 25, 50, 75, and 100 RBs, respectively. LTE works for frequency division duplex (FDD) and time division duplex (TDD) [105]. This manuscript presents the LTE FDD frame for 1.4 MHz BW, to understand the concept of RB allocation. Figure 8 shows the position of an RB in a given frame and the number of subcarriers in the RB. Remember that the subcarriers are used to carry information from transmitter to receiver.

Fig. 8
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LTE FDD frame for 1.4 MHz, normal CP

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Table 6 RBs allocation scheme and algorithm for D2D
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There is two way to allocate the resources, i.e. autonomous D2D and BS/eNB controlled D2D [123]. The first approach is a sensing technique to obtain resources for the D2D pair. Here BS/eNB, broadcasts the resource allocation in the cellular network and collects the information. Based on this information resources are allocated to the D2D pair. This is difficult to implement due to frequent changes in resource allocation at the BS/eNB. The second approach is, BS/eNB allocates RB to each D2D pair or cluster and schedules the transmission. The disadvantage is the burden on BS/eNB and the requirement of signaling overhead. In this way, resource allocation improves throughput, sum rate, spectrum utilization, source capacity, number of admissible D2D pairs, energy conservation, time and energy-saving, fairness, outage ratio, SE, EE, data rate, and reduces delay.

Many researchers work for improvement in throughput, spectrum utilization, number of admissible D2D pairs, SE, EE, data rate, sum rate, and reduction in delay by using the resource allocation concept (Table 6).

New Generation Waveform

The improvement in the SE of D2D is very important to incorporate with 5G. New efforts are given by researchers to improve the SE by introducing a new generation waveform for D2D communication, i.e. orthogonal frequency division multiplexing (OFDM) [124], Filtered orthogonal frequency division multiplexing (F-OFDM) [125], CP-OFDM (cyclic prefix OFDM) [126], Filter bank multicarrier (FBMC) [127], Universal carrier multicarrier (UFMC) [128], GFDM (generalized frequency division multiplexing) [129].

OFDM is a popular signaling method in broadband wired/wireless channels. IEEE802.11, IEEE802.16, 3GPP-LTE, and LTE-A (long-term evaluation-advanced) adopt OFDM because of its high efficiency and low complexity. But it requires synchronization with users for uplink, which is very difficult to establish. Another limitation is carrier aggregation [130]. For the OFDM, the transmitter modulated input data by QAPSK (Quadrature amplitude shift key) or M-ary QAM (Quadrature amplitude modulation). Modulated waveform now transmitted after performing N-point IFFT (inverse Fast Fourier transform). Now add CP (cyclic prefix) to generate the OFDM signal. Vice versa operations are required for the reception of OFDM (Fig. 9). F-OFDM is the same as OFDM except for the filtering operation and CP removal (Fig. 10).

Fig. 9
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Generation and detection of OFDM signals

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Fig. 10
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Generation and detection of F-OFDM signals

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FBMC modulation uses a set of synthesis and analysis filters on transmitter and receiver. This is also defined as Offset quadrature amplitude modulation (OQAM) [131]. It’s derived from a prototype filter that can determine stopband attenuation, inter symbol interference (ISI), and inter-channel interference (ICI). FBMC generation and reception are shown in Fig. 11 [127, 132]. The transmitter applies O-QAM, earlier to the serial to parallel (S/P) conversion. Next is frequency spreading before the extended IFFT (E-FFT). At last, the parallel to serial (P/S) conversion. Exact vice-versa operations are performed on the receiver.

Fig.11
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FBMC generation and detection

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FBMC as an alternative to OFDM. It reduces ingress and egress noises that are generated by carrier aggregation (using high-quality filters). It is also able to reduce the synchronization problem [133]. Three types of FBMC systems are defined: (1) CMT, i.e. cosine modulated multi-tone (based on change concept) [134]; (2) SMT, i.e. staggered multi-tone (introduced by Saltzberg) [135]; (3) adopted method of frequency division multiplexing [136].

UFMC is the generalization of the F-OFDM and FBMC. F-OFDM filters the entire band while FBMC filters individual subcarriers. UFMC filters group of subcarriers which reduces the filter length of UFMC. At this juncture, QAM with existing MIMO retains complex orthogonality. Here ‘N’ subcarriers are divided into the various sub-band where each sub-band has a fixed number of subcarriers. Subcarrier interference can be avoided by IFFT. The signals that came from the IIFT operation, go to the band of filter length ‘L’, where each band is output fileted by a band of the filter of length ‘L’. Generally, the Chebyshev window is used for filtering operations. The output of the band filters is added and sent to the receiver by the channel. At this point, an FFT operation is performed which converts data of the time domain to the frequency domain. One point must be noted that the guard intervals of zeros are added to IFFT successive symbols, which will reduce the effect of ISI. This ISI will generate due to transmitter filter delay. Now data is allotted to equalizers before parallel to serial conversion (Fig. 12).

Fig. 12
figure 12

UFMC waveform generation and detection

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Table 7 summarizes the efforts of researchers in D2D communication to improve the system throughput, SE, system capacity by using OFDM, F-OFDM, FBMC, and UFMC. In this way, the new generation waveform in D2D communication improves data rate, removes inter UE time synchronization overhead, capability improvement, reduces inter D2D interference, and spectral leakage. They use overlay, underlay D2D communication for uplink, downlink, and uplink/downlink transmission. Additionally, manuscript [137] presents a comparative analysis of UFMC and FBMC based on offset QAM with 4G system CP-OFDM.

Table 7 Summary of the implementation of new generation waveform on D2D
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Miscellaneous

Many researchers give their efforts to improve SE for D2D communication in various ways i.e., concept, technology, algorithms, network architecture, etc. Here, this article is going to present the summary of the efforts by researchers, where they able to find out improved Quality of Services (QoS), SE, EE, reliability, data rate, and coverage area, while the reduction in computational cost, power, size of overhead, network traffic, and latency (Table 8).

Table 8 Summary of implementation of miscellaneous algorithms and techniques
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Conclusions

Interference is a basic problem for D2D communication. So, researchers are mainly focusing on various types of interference avoidance methods. Here we investigate clustering methods, frequency reusing, spectrum sharing, resource allocation, and implementation of the new generation waveforms to improve SE for D2D communication. Researchers apply these methods with the help of various algorithms, concepts, and network design. These efforts provide improvement in data rate, system capacity, secrecy performance SIR level, sum rate, energy saving, number of admissible D2D pair, SE fairness, outage ratio for various network conditions i.e., uplink/downlink and underlay/overlay D2D communication. Future research challenges for D2D communication, are interference management, network discovery, proximity and context-aware services, network coding, and security. V2V communications, mm-Wave technology, social D2D networks, energy harvesting, and SWIPT (simultaneous wireless and information power transfer), pricing, and incentive are the future direction in the perspective of 5G D2D networks.