METHOD AND APPARATUS FOR EXTENDING COVERAGE AT THZ FREQUENCY USING COOPERATIVE COMMUNICATION

Abstract

Disclosed is a coverage extension method and apparatus in a terahertz (THz) region using cooperative communication. A coverage extension method in a THz region using cooperative communication performed by a computer device includes setting, by an access point (AP) in a downlink system that supports a plurality of user equipments (UEs), a UE that requires assistance due to a blocked line-of-sight (LoS) link as a destination UE and setting a remaining UE as a medium UE among the plurality of UEs; transmitting, by the AP, data to the medium UE; and cooperatively transmitting, by the medium UE, data to the destination UE.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Korean Patent Application No. 10-2022-0141999, filed on Oct. 31, 2022, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND 1. Field of the Invention

The following example embodiments relate to a coverage extension method and apparatus in a terahertz (THz) region using cooperative communication and more particularly, to a coverage extension method and apparatus in a THz region using cooperative communication that may support electronic devices for which data supporting is absent using cooperative communication.

2. Description of the Related Art

Many candidates are being discussed to support sixth-generation (6G) technology that is a future wireless communication system. Among the candidates, terahertz (THz) band wireless communication is being selected as influential technology. THz communication refers to communication in the band of 0.1 THz to 10 THz and a very high information transfer rate may be expected using a wide bandwidth. Here, THz communication has characteristics of being very weak in signal strength and being significantly vulnerable even to blockage due to high propagation attenuation and molecular absorption according to a frequency. As described above, THz communication has various advantages and disadvantages and is currently in an early stage of research in the academic world.

To consider THz communication, both THz propagation characteristics and hardware characteristics need to be considered. The propagation characteristics of THz communication include weak signal strength and blockage. In terms of the hardware characteristics, since equipment that implements a THz signal has very low energy efficiency, output signal strength is very low. Therefore, a current THz communication system may not be readily used for long-distance communication. If securing line-of-sight (LoS) fails, communication becomes difficult. Accordingly, THz communication may be expected to have coverage as a big vulnerability.

Reference material is as follows: Y. Mao, B. Clerckx, J. Zhang, V. O. K. Li and M. A. Arafah, “Max-Min Fairness of K-User Cooperative Rate-Splitting in MISO Broadcast Channel With User Relaying,” in IEEE Transactions on Wireless Communications, vol. 19, no. 10, pp. 6362-6376, October 2020.

SUMMARY

Example embodiments describe a coverage extension method and apparatus in a terahertz (THz) region using cooperative communication and more particularly, provide technology that may extend coverage in a THz region using a cooperative communication technique based on rate-splitting multiple access (RSMA).

Example embodiments provide a new cooperative communication structure called extraction-based cooperative rate splitting (eCRS) to increase coverage and provide a coverage extension method and apparatus in a THz region using cooperative communication that may support electronic devices for which data supporting is absent using cooperative communication by designing and solving an issue based on the new cooperative communication structure and thereby increasing coverage.

According to an example embodiment, there is provided a coverage extension method in a THz region using cooperative communication performed by a computer device, the coverage extension method including setting, by an access point (AP) in a downlink system that supports a plurality of user equipments (UEs), a UE that requires assistance due to a blocked line-of-sight (LoS) link as a destination UE and setting a remaining UE as a medium UE among the plurality of UEs; transmitting, by the AP, data to the medium UE; and cooperatively transmitting, by the medium UE, data to the destination UE.

The transmitting of the data to the medium UE may include transmitting, by the AP, data requested by the destination UE and data requested by the medium UE to the medium UE.

The transmitting of the data to the medium UE may include splitting, by the AP, data requested by the medium UE into private data and a portion of common data; generating, by the AP, common data that includes information of the plurality of UEs by combining portions of the split common data of the medium UEs; encoding, by the AP, the private data to a private stream and encoding the common data to a common stream; and precoding, by the AP, the private stream and the common stream and transmitting a signal to the medium UE.

The transmitting of the data to the destination UE may include decoding, by the medium UE, the common stream; removing, by the medium UE, the common stream and then decoding the private stream that includes its own information; encoding, by each of the medium UEs, data requested by the destination UE to a data stream; and precoding, by the medium UE, the data stream and transmitting a signal to the destination UE.

The transmitting of the data to the medium UE may include splitting, by the AP, data requested by the medium UE without splitting data requested by the destination UE and transmitting the same signal to all the medium UEs based on the data requested by the destination UE and the data requested by the medium UE.

The transmitting of the data to the medium UE may include splitting, by the AP, data requested by the destination UE and data requested by the medium UE and transmitting different signals to all the medium UEs based on the data requested by the destination UE and the data requested by the medium UE.

The transmitting of the data to the destination UE may include splitting a channel using orthogonal frequency division multiplexing (OFDM) to correspond to the channel.

The transmitting of the data to the destination UE may include having a form of a multiple access channel.

According to another example embodiment, there is provided a coverage extension apparatus in a THz region using cooperative communication, the coverage extension apparatus including a setter configured to set, in a downlink system that supports a plurality of user UEs, a UE that requires assistance due to a blocked LoS link as a destination UE and to set a remaining UE as a medium UE among the plurality of UEs; and a transmitter configured to transmit data to the medium UE. The medium UE is configured to cooperatively transmit data to the destination UE.

The transmitter may be configured to transmit data requested by the destination UE and data requested by the medium UE to the medium UE.

The transmitter may include a data splitter configured to split data requested by the medium UE into private data and a portion of common data; a common data combiner configured to generate common data that includes information of the plurality of UEs by combining portions of the split common data of the medium UEs; an encoder configured to encode the private data to a private stream and to encode the common data to a common stream; and a precoder configured to precode the private stream and the common stream and to transmit a signal to the medium UE.

The medium UE may include a decoder configured to decode the common stream and to remove the common stream and then decode the private stream that includes its own information; an encoder configured to encode data requested by the destination UE to a data stream; and a precoder configured to precode the data stream and to transmit a signal to the destination UE.

The transmitter may be configured to split data requested by the medium UE without splitting data requested by the destination UE and to transmit the same signal to all the medium UEs based on the data requested by the destination UE and the data requested by the medium UE.

The transmitter may be configured to split data requested by the destination UE and data requested by the medium UE and to transmit different signals to all the medium UEs based on the data requested by the destination UE and the data requested by the medium UE.

According to still another example embodiment, there is provided a coverage extension apparatus in a THz region using cooperative communication, the coverage extension apparatus including an AP configured to set, in a downlink system that supports a plurality of UEs, a UE that requires assistance due to a blocked LoS link as a destination UE and to set a remaining UE as a medium UE among the plurality of UEs and then to transmit data to the medium UE; and the medium UE configured to cooperatively transmit data to the destination UE based on data received from the AP.

The AP may be configured to transmit data requested by the destination UE and data requested by the medium UE to the medium UE.

The AP may include a data splitter configured to split data requested by the medium UE into private data and a portion of common data; a common data combiner configured to generate common data that includes information of the plurality of UEs by combining portions of the split common data of the medium UEs; an encoder configured to encode the private data to a private stream and to encode the common data to a common stream; and a precoder configured to precode the private stream and the common stream and to transmit a signal to the medium UE.

The medium UE may include a decoder configured to decode the common stream and to remove the common stream and then decode the private stream that includes its own information; an encoder configured to encode data requested by the destination UE to a data stream; and a precoder configured to precode the data stream and to transmit a signal to the destination UE.

The AP may be configured to split data requested by the medium UE without splitting data requested by the destination UE and to transmit the same signal to all the medium UEs based on the data requested by the destination UE and the data requested by the medium UE.

The AP may be configured to split data requested by the destination UE and data requested by the medium UE and to transmit different signals to all the medium UEs based on the data requested by the destination UE and the data requested by the medium UE.

According to example embodiments, it is possible to provide a coverage extension method and apparatus in a THz region using cooperative communication that may extend coverage in a THz region using a cooperative communication technique based on RSMA.

According to example embodiments, it is possible to provide a new cooperative communication structure called eCRS to increase coverage and to provide a coverage extension method and apparatus in a THz region using cooperative communication that may support electronic devices for which data supporting is absent using cooperative communication by designing and solving an issue based on the new cooperative communication structure and thereby increasing coverage.

Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects, features, and advantages of the invention will become apparent and more readily appreciated from the following description of embodiments, taken in conjunction with the accompanying drawings of which:

FIG. 1 illustrates a terahertz (THz) downlink system that operates through two phases according to an example embodiment;

FIG. 2 illustrates a first phase of a cooperative communication structure of identical extraction-based cooperative rate splitting (IeCRS) according to an example embodiment;

FIG. 3 illustrates a second phase of a cooperative communication structure of IeCRS according to an example embodiment;

FIG. 4 illustrates a first phase of a cooperative communication structure of distinct eCRS (DeCRS) according to an example embodiment;

FIG. 5 illustrates a second phase of a cooperative communication structure of DeCRS according to an example embodiment;

FIG. 6 illustrates an achievable rate according to the number of medium user equipments (mUEs) according to an example embodiment;

FIG. 7 illustrates an achievable rate according to the number of mUEs when power of an access point (AP) and mUEs is low according to an example embodiment;

FIG. 8 is a flowchart illustrating a coverage extension method in a THz region using cooperative communication according to an example embodiment; and

FIG. 9 is a diagram illustrating a coverage extension apparatus in a THz region using cooperative communication according to an example embodiment.

DETAILED DESCRIPTION

Hereinafter, some example embodiments will be described in detail with reference to the accompanying drawings. The following detailed structural or functional description of example embodiments is provided as an example only and various alterations and modifications may be made to the example embodiments. Accordingly, the example embodiments are not construed as being limited to the disclosure and should be understood to include all changes, equivalents, and replacements within the technical scope of the disclosure. Also, the example embodiments are provided to fully explain the present disclosure to those skilled in the art. Shapes and sizes, etc., of components in drawings may be exaggerated for clarity of description.

Cooperative communication refers to a collective term for technology that allows an electronic device in an environment for which data supporting is difficult to receive data supporting through cooperation between electronic devices in addition to a base station when performing communication in the electronic device.

The example embodiment proposes a cooperative communication system to complement weakness of THz communication. In detail, provided is a cooperative communication structure called extraction-based cooperative rate splitting (eCRS) using the concept of rate-splitting multiple access (RSMA) and a problem may be designed and solved by using the eCRS structure to be suitable for a THz band. Here, although THz band communication is considered, the proposed eCRS structure refers to a technique that may be employed regardless of a frequency domain.

The proposed eCRS may include two cases, IeCRS and DeCRS. Example embodiments may represent a model capable of well representing a THz region and, based on this, may design and solve a problem related to cases of IeCRS and DeCRS, verifying that coverage increases. Here, although two cases, IeCRS and DeCRS, are intensively explored, eCRS that is the proposed cooperative communication structure may be a broad concept that may encompass many cooperative communication structures. According to example embodiments, through a new cooperative communication structure, it is possible to solve coverage problems that may emerge in a future environment, such as an Internet of things (IoT) environment.

FIG. 1 illustrates a terahertz (THz) downlink system that operates through two phases according to an example embodiment.

A communication system considered in the example embodiment refers to a multi-user downlink system. An access point (AP) 110 with N t antennas supports user equipments (UEs) (120 and 130) with (K+1) single antennas. Here, one UE that requires assistance due to a blocked line-of-sight (LoS) link is set as a destination UE (dUE) 130 and remaining K UEs are set as medium UEs (mUEs) 120 that perform cooperative communication. The mUEs 120 are expressed as a set ={1,2, . . . , K}. The dUE 130 requests data W_d, and the k-th mUE 120 requests data W_k. Here, when data requested by the mUEs 120 is set to 0, it is equivalent to a multi-relay system. Therefore, a communication structure proposed in the example embodiment is a system that includes a relay system.

To support cooperative communication, the following example embodiment proposes a cooperative communication structure called an eCRS structure. The cooperative communication structure includes two stages in total. In a first stage, the AP 110 transmits the data W_d and the data W_k to the mUEs 120. In a second stage, the mUEs 120 cooperatively transmit the data W_d to the dUE 130.

Referring to FIG. 1, the cooperative communication structure may be verified.

In the first phase, the AP 110 splits the data W_d into ≠∅, and splits the data W_k into private data W_p,k and k∈_i. Then, the AP 110 generates common data by combining data with the same superscript . Here, the common data refers to data that includes information of a plurality of UEs, for example, the mUEs 120 and the dUE 130. The AP 110 encodes W_p,k to private stream s_k and encodes to common stream . For example, if ={1,3}, W_c^{1,3} includes W_c,1^{1,3}, W_c,3^{1,3}, W_d^{1,3} and is encoded to s_c,{1,3}. Then, the k-th mUE 120 decodes the private stream s_k that includes its own information and that is k∈ among a plurality of common streams. Here, for the mUE 120 to decode a plurality of signals, a plurality of successive interference cancellation (SIC) layers is required. That is, (n−1) SIC layers are required to decode n signals.

The most general case of a communication structure proposed in the example embodiment is a structure that uses all common streams. In the example embodiment, the following is performed with the assumption that each of the mUEs 120 has a single SIC layer. If each mUE 120 has a single SIC layer, a single common stream may be decoded for each mUE 120 and a process of determining a common stream to be used is required. Here, to set all the mUEs 120 each to decode a single common stream, a mathematical decision process is performed as follows.

∩{}=∅, ∪{}=, _i⊂.  [Equation 1]

Here, _i represents selecting common stream . Here, although various methos may be used to select common streams, an example embodiment describes that a total of two cases, a case in which {}= that is called identical eCRS (IeCRS) and a case in which {}_i=1^K={{1}, . . . , {K}} that is called distinct eCRS (DeCRS). Here, IeCRS refers to a case of transmitting the same signal to all the mUEs 120 and DeCRS refers to a case of transmitting different signals to all the mUEs 120.

IeCRS

FIG. 2 illustrates a first phase of a cooperative communication structure of IeCRS according to an example embodiment, and FIG. 3 illustrates a second phase of a cooperative communication structure of IeCRS according to an example embodiment.

In the case of IeCRS, a common stream is used as {}=. In this case, without splitting data W_d for a dUE, data for a k-th mUE is split into W_k={W_p,k, W_c,k}. In the case of IeCRS, since distinguishment is not difficult, subscript in equation is omitted. Referring to FIG. 2, a data communication process that is the first phase of IeCRS may be verified. Then, common data is combined into W_c={W_d, W_c,1, W_c,K} and then W_c, W_p,k are encoded to s_c, s_k, respectively. An AP precodes (K+1) streams and transmits a signal as follows.

$\begin{array}{cc}x=\sum _{k=1}^K{f}_k{s}_k+f_c{s}_c.& \left[\mathrm{Equation}2\right]\end{array}$

Here, f_c, f_k are precoders for s_c, s_k, respectively.

Based on this, a received signal of the k-th mUE is expressed as follows.

y _k =h _k ^H Fs+z _k.  [Equation 3]

Here, h_k denotes a channel from the AP to the k-th mUE, F denotes a precoding matrix, s denotes a vector of streams, and z_k denotes additive white Gaussian noise (AWGN).

After receiving a signal, the k-th mUE needs to decode s_k that is a private stream and s_c that is a common stream. The mUE may initially decode s_c, and an achievable rate thereof may be expressed as follows.

$\begin{array}{cc}{R}_{c,k}={\log }_2\left(1+\frac{{❘h_k^H{f}_c❘}^2}{{❘h_k^H{f}_k❘}^2+I_k+1}\right).& \left[\mathrm{Equation}4\right]\end{array}$

Here, I_k=Σ_i≠k|h_k^Hf_i|² represents interference of another signal. After removing the decoded s_c, a private stream may be acquired and an achievable rate thereof may be expressed as follows.

$\begin{array}{cc}{R}_{p,k}={\log }_2\left(1+\frac{{❘h_k^H{f}_k❘}^2}{I_k+1}\right).& \left[\mathrm{Equation}5\right]\end{array}$

Here, since all the mUEs need to decode a common stream, common data W_c for all the mUEs need to satisfy an achievable rate condition as follows.

$\begin{array}{cc}{C}_d+\sum _{i=1}^K{C}_i\le R_{c,k}\forall k\in 𝒦.& \left[\mathrm{Equation}6\right]\end{array}$

Here, C_k denotes a rate for common part W_c,k and C_d denotes a rate for W_d. Then, the mUE acquires W_c,k and W_d from the common stream and acquires W_k by combining W_c,k and W_p,k. Expressing this as equation, an achievable rate for the k-th mUE is R_k=R_p,k+C_k.

In the second phase, all the mUEs transmit the same data W_d to the dUE. Referring to FIG. 3, a data communication process that is the second phase of IeCRS may be verified. Each mUE encodes W_d to data stream s d and then performs precoding and performs communication as follows.

x_d,k=f_d,ks_d.  [Equation 7]

If transmission is performed as above, a signal received by the dUE at an m-th time is expressed as follows.

$\begin{array}{cc}\begin{array}{c}{y}_d\left[m\right]=\sum _{k=1}^K{g}_k{x}_{d,k}\left[m-{\tau }_k\right]+z_d\left[m\right]\\ =\sum _{k=1}^K{\stackrel{\_}{g}}_k{s}_d\left[m-{\tau }_k\right]+z_d\left[m\right].\end{array}& \left[\mathrm{Equation}8\right]\end{array}$

Here, g_k and τ_k denote a channel value and a delay value between the k-th mUE and the dUE, respectively, z_d denotes AWGN, and g_k=g_kf_d,k denotes a valid channel value that considers a precoder. In an example embodiment, a characteristic of a THz system is reflected and signals received through mUEs are modeled to have different delay values. This has a form of a frequency selective channel and a different communication strategy needs to be applied.

To correspond to a channel, in an example embodiment, the channel is split into N_c sub-carriers using orthogonal frequency division multiplexing (OFDM). The channel to which OFDM is applied may be represented as follows.

{tilde over (y)}={tilde over (g)} _n {tilde over (s)} _n +{tilde over (z)} _n.  [Equation 9]

Here, {tilde over (s)}_n denotes a transmission signal and {tilde over (z)}_n denotes AWGN. A channel value may be represented according to OFDM, as follows.

$\begin{array}{cc}{\tilde{g}}_n=\sum _{k=1}^K{\stackrel{\_}{g}}_k\exp \left(-\frac{j2\pi {\tau }_{k}n}{N_c}\right).& \left[\mathrm{Equation}10\right]\end{array}$

If all data acquired from a total of N_c sub-carriers is added, an achievable rate may be expressed as follows.

$\begin{array}{cc}\begin{array}{c}{R}_d^{\left(2\right)}=\frac{1}{N_c+L}\sum _{n=1}^{N_c}{\log }_2\left(1+{❘{\tilde{g}}_n❘}^2\right)\\ \approx \frac{1}{N_c}\sum _{n=1}^{N_c}{\log }_2\left(1+{❘{\tilde{g}}_n❘}^2\right).\end{array}& \left[\mathrm{Equation}11\right]\end{array}$

Here, L denotes a length of a cyclic prefix and is sufficiently short compared to N_c and cannot be ignored accordingly. Combining contents of the first phase and the second phase, an achievable rate of the dUE in IeCRS may be expressed as follows.

R _d=min{C_d, R_d⁽²⁾}  [Equation 12]

DeCRS

FIG. 4 illustrates a first phase of a cooperative communication structure of DeCRS according to an example embodiment, and FIG. 5 illustrates a second phase of a cooperative communication structure of DeCRS according to an example embodiment.

DeCRS refers to a case in which {}_i=1^K={{1}, {2}, . . . , {K}} is selected as a common stream in the eCRS structure. Referring to FIG. 4, a data communication process that is the first phase of DeCRS may be verified. In DeCRS, K private streams and a common stream may be used and a transmission signal of an AP may be expressed as follows.

$\begin{array}{cc}x=\sum _{k=1}^K{f}_{c,k}{s}_{c,k}+f_k{s}_k.& \left[\mathrm{Equation}13\right]\end{array}$

Here, f_c,k and f_k denote precoders for common stream s_c,k and private stream s_k, respectively.

Based on this, a received signal of a k-th mUE may be represented as follows.

y _k =h _k ^H Fs+z _k.  [Equation 14]

Here, variables may be defined in the same manner as in IeCRS.

After receiving a signal, the k-th mUE needs to decode s_k that is the private stream and s_c,k that is the common stream. The mUE may initially decode s_c,k and an achievable rate thereof may be expressed as follows.

$\begin{array}{cc}{R}_{c,k}={\log }_2\left(1+\frac{{❘h_k^H{f}_{c,k}❘}^2}{{❘h_k^H{f}_k❘}^2+I_k+I_{c,k}+1}\right).& \left[\mathrm{Equation}15\right]\end{array}$

Here, each of I_k=Σ_i≠k|h_k^Hf_i|² and I_c,k=Σ_i≠k|h_k^Hf_c,i|² represents interference of a different signal. After removing the decoded s_c,k, a private stream may be acquired and an achievable rate thereof may be represented as follows.

$\begin{array}{cc}{R}_{p,k}={\log }_2\left(1+\frac{{❘h_k^H{f}_k❘}^2}{I_k+I_{c,k}+1}\right).& \left[\mathrm{Equation}16\right]\end{array}$

Similar to the case of IeCRS, the following condition needs to be satisfied to successfully decode a common stream.

C _d,k +C _k ≤R _c,k ∀k∈ .  [Equation 17]

Here, C_k denotes a rate for common part W_c,k^k and C_d,k denotes a rate for W_d^k. Then, the mUE acquires W_c,k^k and W_d^k from the common stream and acquires W_k by combining W_c,k^k and W_p,k. Expressing this as equation, an achievable rate for the k-th mUE is R_k=R_p,kC_k.

In the second phase, the k-th mUE transmits W_d,k to the dUE. Referring to FIG. 5, a data communication process that is the second phase of DeCRS may be verified. The k-th mUE encodes W_d,k^k to data stream s_d,k and then performs precoding and performs communication as follows.

x_d,k=f_d,ks_d,k.  [Equation 18]

If transmission is performed as above, a signal received by the dUE at an m-th time is expressed as follows.

$\begin{array}{cc}{y}_d\left[m\right]=\sum _{k=1}^K{g}_k{x}_{d,k}\left[m-{\tau }_k\right]+z_d\left[m\right]=\sum _{k=1}^K{\stackrel{\_}{g}}_k{s}_{d,k}\left[m-{\tau }_k\right]+z_d\left[m\right].& \left[\mathrm{Equation}19\right]\end{array}$

While the case of IeCRS uses a form of a frequency selective channel in the second phase, DeCRS uses a form of a multiple access channel. With the assumption that all signals are simultaneously received, an achievable rate may be computed as follows.

$\begin{array}{cc}{R}_d^{\left(2\right)}={\log }_2\left(1+\frac{{❘{\stackrel{\_}{g}}_k❘}^2}{J_k+1}\right).& \left[\mathrm{Equation}20\right]\end{array}$

Here, J_k=Σ_i≠k|g_i|² denotes interference. Collecting this, an achievable rate of the dUE in DeCRS may be expressed as follows.

$\begin{array}{cc}{R}_d=\sum _{k=1}^K\min \left\{C_{d,k},R_{d,k}^{\left(2\right)}\right\}.& \left[\mathrm{Equation}21\right]\end{array}$

Optimization Technique in IeCRS

The example embodiment maximizes minimum rate values (achievable rates) of mUEs and a dUE as a single implementation example. A problem may be solved based on other objectives.

The whole problem may be expressed as follows.

$\begin{array}{cc}\left[\mathrm{Equation}22\right]& \\ \underset{F,c,\stackrel{\_}{g}}{\max }\min \left\{R_1,...,R_K,R_d\right\}& \left(\mathrm{P1}\right)\end{array}$ $\begin{array}{cc}s.t.R_k=R_{p,k}+C_k,\forall k\in 𝒦,& 1a)\end{array}$ $\begin{array}{cc}{C}_d+\sum _{i=1}^K{C}_i\le R_{c,k},\forall k\in 𝒦,& 1b)\end{array}$ $\begin{array}{cc}{R}_d=\min \left\{C_d,R_d^{\left(2\right)}\right\},& 1c)\end{array}$ $\begin{array}{cc}\mathrm{tr}\left({\mathrm{FF}}^H\right)\le P_{\mathrm{AP}},& 1d)\end{array}$ $\begin{array}{cc}{❘{\stackrel{\_}{g}}_k❘}^2\le {❘g_k❘}^2{P}_k,\forall k\in 𝒦.& 1e)\end{array}$

Here, P_AP, P_k denote maximum power values of an AP and a k-th mUE, respectively, and c and g are defined as c=[C₁, . . . , C_K, C_d]^T and g=[g₁, . . . , g_K]^T, respectively. (P1) does not have a form of a convex optimization technique and thus, is directly unsolvable. Therefore, after dividing the problem into a problem for the first phase and a problem for the second phase, a minimum value of rates acquired from the respective phases may be considered as a result.

The problem in the first phase may be expressed as follows.

$\begin{array}{cc}\left[\mathrm{Equation}23\right]& \\ \underset{F,c}{\max }\min \left\{R_1,...,R_K,C_d\right\}& \left(\mathrm{P2}\right)\end{array}$ $s.t.\left(1a\right),\left(1b\right),\left(1d\right).$

Also, the problem in the second phase may be expressed as follows.

$\begin{array}{cc}\left[\mathrm{Equation}24\right]& \\ m\underset{\stackrel{\_}{g}}{a}x{R}_d^{\left(2\right)}& \left(\mathrm{P3}\right)\end{array}$ $s.t.\left(1e\right).$

Then, the problem in the first phase and the problem in the second phase may be transformed and solved using a weighted minimum mean square error (WMMSE) technique and a successive convex approximation (SCA) technique, respectively.

To use the WMMSE technique in the first phase, the following variables are defined.

ε_c,k=|w_c,k|²T_c.k−2Re{w_c,kh_k^Hf_c}+1,

ε_p,k=|w_p,k|²T_p.k−2Re{w_p,kh_k^Hf_k}+1,

ξ_c.k=μ_c,kε_c,k−ln(μ_c,k),

ξ_p.k=μ_p,kε_p,k−ln(μ_p,k),

Here, T_c,k=|h_k^Hf_c|²+Σ_i=1^K|h_k^Hf_i|²+1 and T_p,k=T_c,k−|h_k^Hf_c|². Also, (P2) may generate the following problem through transformation.

$\begin{array}{cc}\left[\mathrm{Equation}26\right]& \\ \underset{{𝒱}_{\mathrm{IeCRS}}}{\min }{t}_0& \left(\mathrm{P2}\text{.1}\right)\end{array}$ $\begin{array}{cc}s.t.X_k+\frac{\left({\xi }_{p,k}-1\right)}{\ln 2}\le t_0,\forall k\in 𝒦,& \left(2.1a\right)\end{array}$ $\begin{array}{cc}\frac{{\xi }_{c,k}-1}{\ln 2}\le X_d+\sum _{i=1}^K{X}_i,\forall k\in 𝒦,& \left(2.1b\right)\end{array}$ $\begin{array}{cc}{X}_d\le t_0,& \left(2.1c\right)\end{array}$ $\left(1d\right).$

Here, x=[X₁, . . . , X_K]=—c and t₀ denotes a slack variable to expressmin{R₁, . . . , R_K, C_d}. A set of variables is _IeCRS={F, x, w, μ, t₀}, and remaining variables are w=[w_p,1, . . . , w_p,K, w_c,1, . . . , w_c,K] and μ=[μ_p,1, . . . , μ_p,K, μ_c,1, . . . , μ_c,K].

Here, if variables {w, μ} are fixed, (P2.1) may satisfy a form of an optimization problem with respect to {F, x, t₀} and the problem may be solved using the optimization technique accordingly. Through this, with variables {F, x, t₀} being fixed, variables {w, μ} may be updated with

$w_{c,k}=f_c^H{h}_k{T}_{c,k}^{-1},w_{p,k}=f_k^H{h}_k{T}_{p,k}^{-1},{\mu }_{c,k}={\epsilon }_{c,k}^{-1},{\mu }_{p,k}={\epsilon }_{p,k}^{-1},$

and with variables {w, μ} being fixed, {F, x, t₀} may be updated using the optimization technique. By repeating this, a local optimum solution for the problem (P2) may be acquired.

In the second phase, (P3) may be generated into the following SCA technique problem through transformation.

$\begin{array}{cc}\left[\mathrm{Equation}27\right]& \\ \underset{\stackrel{\_}{g},a,b,u}{\max }{R}_d^{\left(2\right)}=\frac{1}{N_c}\sum _{n=1}^{N_c}{\log }_{2}1+u_n)& \left(\mathrm{P3}\text{.1}\right)\end{array}$ $s.t.u_n\le {\tilde{a}}_n^{\left(\ell \right)}+{\tilde{b}}_n^{\left(\ell \right)},\forall n\in 𝒩,$ $a=\mathrm{Re}\left\{\tilde{g}\right\},b=\mathrm{Im}\left\{\tilde{g}\right\},\forall n\in 𝒩,$ $\left(1e\right).$

Here, {tilde over (g)}=[{tilde over (g)}₁, . . . , {tilde over (g)}_Nc]^T and , is defined as follows.

=()²+2{(a_n−)},

=()²+2{(a_n−)}.  [Equation 28]

In addition, a=[a₁, . . . , a_Nc]^T, b=[b₁, . . . , b_Nc]^T, u=[u₁, . . . , u_Nc]^T are all slack variables, and , denote local points in an -th iteration. If problem (P3.1) is repeatedly solved using the optimization technique and , are updated with an (−1)-th solution, a local optimum solution for the problem (P3) may be acquired.

If a value C_d of the first phase is acquired through (P2) and a value R_d⁽²⁾ of the second phase is acquired through (P3), a final minimum rate value R_d may be acquired using R_d=min{C_d, R_d⁽²⁾}. An algorithm for IeCRS may be verified from the following Table 1.

Table 1 shows a minimum rate maximization algorithm in IeCRS.

TABLE 1
Algorithm 1 Pseudo code for minimum rate
maximization in IeCRS
1:  Initialization: Set    1 = 0,    , and    .
2:  repeat
3:  Set    1 =    1 + 1.
4:  Solve (P2.1) with fixed w and μ.
5:  Update     and    .
6:  until t0 decreases by a fraction below a predefined
    threshold.
7:  Cd = −t0.
8:  Set    2 = 0,    , and    .
9:  repeat
10: Set    2 =    2 + 1.
11: Solve (P3.1).
12: Update the solution of (P3.2) as     and    .
13: until Rd(2) increases by a fraction below a predefined
    threshold.
14  Rd = min {Cd, Rd(2)}.

Low Complexity Technique in IeCRS

The example embodiment proposes a low complexity algorithm for acquiring a solution in a closed form in addition to a method using an optimization technique for a solution of the second phase in IeCRS.

Since devices operate with low power in THz band, R_d⁽²⁾ may be approximated as follows by assuming a low signal-to-noise ratio (low SNR)

$\begin{array}{cc}\sum _{n=1}^{N_c}{\log }_2\left(1+{❘{\tilde{g}}_n❘}^2\right)\approx \sum _{n=1}^{N_c}\frac{{❘{\tilde{g}}_n❘}^2}{\ln \left(2\right)}.& \left[\mathrm{Equation}29\right]\end{array}$

Also, by modifying Equation 10, an OFDM channel may be expressed as {tilde over (g)}_n=Ω_n^Hg. Here, Ω_n=[Ω_n,1, . . . , Ω_n,K]^T and

${\Omega }_{n,k}=\exp \left(\frac{j2{\mathrm{\pi \tau }}_{k}n}{N_c}\right).$

Using the modified form, R_d⁽²⁾ may be simplified as follows.

$\begin{array}{cc}\sum _{n=1}^{N_c}{❘{\tilde{g}}_n❘}^2={\stackrel{\_}{g}}^H\Omega \stackrel{\_}{g},\Omega =\sum _{n=1}^{N_c}{\Omega }_n{\Omega }_n^H.& \left[\mathrm{Equation}30\right]\end{array}$

Then, through mathematical derivation, if delay values of all mUEs are different, Ω=N_cI_K may be acquired and, through this, the problem of the second phase may be expressed as follows.

$\begin{array}{cc}m\underset{\stackrel{\_}{g}}{a}x{\stackrel{\_}{g}}^2& \left[\mathrm{Equation}31\right]\end{array}$ $s.t.\left(1e\right).$

Through this, it can be known that it is optimal for all mUEs to communicate with maximum power at the low SNR. Also, when there are mUEs having the same delay value, it can be easily derived in the similar manner as above that it is optimal for all the mUEs to communicate with the maximum power and optimal to adjust signals coming from the mUEs with the same delay to come in the same phase.

Optimization Technique Solution in DeCRS

The example embodiment maximizes a minimum rate value even in DeCRS, which is similar as in the case of IeCRS. If a problem is generated similar to the IeCRS problem, the problem may be represented as follows.

$\begin{array}{cc}\left[\mathrm{Equation}32\right]& \\ \underset{F,c,\stackrel{\_}{g}}{\max }\min \left\{R_1,...,R_K,R_d\right\}& \left(\mathrm{P4}\right)\end{array}$ $\begin{array}{cc}s.t.C_{d,k}+C_k\le R_{c,k},\forall k\in 𝒦,& \left(4a\right)\end{array}$ $\begin{array}{cc}{R}_d+\sum _{k=1}^K\min \left\{C_{d,k},R_{d,k}^{\left(2\right)}\right\},& \left(4b\right)\end{array}$ $\left(1a\right),\left(1d\right),\left(1e\right).$ $\mathrm{Here},c=\left[C_1,...,C_K,C_{d,1},...,C_{d,K}\right].$

In the case of DeCRS, the problem may be solved at a time using a WMMSE technique without dividing the problem into two problems, which differs from the case of IeCRS. Transforming problem (P4) to be suitable for the WMMSE technique, the problem (P4) may be expressed as follows.

$\begin{array}{cc}\left[\mathrm{Equation}33\right]& \\ \underset{{𝒱}_{\mathrm{DeCRS}}}{\min }{t}_0& \left(\mathrm{P4}\text{.1}\right)\end{array}$ $\begin{array}{cc}s.t.X_k+\frac{\left({\xi }_{p,k}-1\right)}{\ln 2}\le t_0,\forall k\in 𝒦,& \left(4.1a\right)\end{array}$ $\begin{array}{cc}\frac{{\xi }_{c,k}-1}{\ln 2}\le X_{d,k}+X_k,\forall k\in 𝒦,& \left(4.1b\right)\end{array}$ $\begin{array}{cc}{X}_{d,k}\le t_k,& \left(4.1c\right)\end{array}$ $\begin{array}{cc}\frac{{\xi }_{d,k}-1}{\ln 2}\le t_k,\forall k\in 𝒦,& \left(4.1d\right)\end{array}$ $\begin{array}{cc}\sum _{i=1}^K{t}_k\le t_0,& \left(4.1e\right)\end{array}$ $\begin{array}{cc}\left(1d\right),& 1e).\end{array}$

Here, a set of variables is _DeCRS={F, x, w, μ, t, t₀}. Similar to the case of IeCRS, if this optimization problem is solved alternately using the WMMSE technique, a local optimum solution for (P4) may be acquired. An algorithm for DeCRS may be verified from the following Table 2.

Table 2 shows a minimum rate maximization algorithm in DeCRS.

TABLE 2
Algorithm 2 Pseudo code for minimum rate
maximization in DeCRS
1: Initialization: Set     = 0,    , and    .
2: repeat
3: Set     =     + 1.
4: Solve (P4.1) with fixed w and μ.
5: Update     and    .
6: until t0 decreases by a fraction below a predefined
   threshold.
7: Rd = −t0.

Performance Comparison

Since a cooperative communication system disclosed in the example embodiment is absent in the related art, performance comparison is performed between proposed techniques. A case of using only the optimization technique in the case of IeCRS is referred to as IeCRS, a case of using the low complexity technique is referred to as LOW, and a case of using the optimization technique in the case of DeCRS is referred to as DeCRS. Also, IeCRS and DeCRS in a case in which the concept of common data is not used are referred to as identical extraction-based cooperative nonorthogonal multiple access (IeC-NOMA) and distinct extraction-based cooperative nonorthogonal multiple access (DeC-NOMA), respectively. Also, a case in which all the signals are simultaneously received while using IeCRS is referred to as single tap (ST).

FIG. 6 illustrates an achievable rate according to the number of mUEs according to an example embodiment.

Referring to FIG. 6, achievable rates for the number of mUEs are compared. It can be seen from results that each technique has a point at which performance peaks. This is since the proposed structure includes two phases. In the second phase, since cooperation increases according to an increase in the number of mUEs, performance increases according to the increase in the number of mUEs. Conversely, in the first phase, since power of an AP is split more according to the increase in the number of mUEs, performance decreases. Therefore, if the number of mUEs is small, the performance in the second phase may be bottlenecked and as the number of mUEs increases, the performance may also increase. However, later, the performance in the first phase may be bottlenecked and as the number of mUEs increase, the performance may decrease. Also, overall, it can be seen that IeCRS has better performance than that of DeCRS and LOW has similar performance to that of IeCRS despite its low complexity. Also, it can be seen that IeC-NOMA and DeC-NOMA are special cases of IeCRS and DeCRS and have the same or degraded performance accordingly. Also, it can be seen that in the case of ST in which all the taps come at a time unrealistically, performance increases and in IeCRS, performance decreases due to fast sampling.

FIG. 7 illustrates an achievable rate according to the number of mUEs when power of an AP and mUEs is low according to an example embodiment.

Referring to FIG. 7, achievable rates for the number of mUEs when power of an AP and mUEs is very low as −10 dBm are compared. Due to operation in two phases, each technique has a point at which performance peaks in the middle, which is the same as in FIG. 6. Here, contrary to FIG. 6, it can be seen that, when the number of mUEs is large, DeCRS has better performance than that of IeCRS. Since DeCRS transmits different signals to mUEs, cooperative communication may be performed by selecting specific mUEs when power is insufficient. Conversely, it can be seen that IeCRS has low power but communicates with all the mUEs and performance decreases accordingly. Through this, it can be seen that IeCRS and DeCRS have different advantages and both have excellent performance.

FIG. 8 is a flowchart illustrating a coverage extension method in a THz region using cooperative communication according to an example embodiment.

Referring to FIG. 8, a coverage extension method in a THz region using cooperative communication performed by a computer device according to an example embodiment may include operation S110 of setting, by an AP in a downlink system that supports a plurality of UEs, a UE that requires assistance due to a blocked LoS link as a destination UE and setting a remaining UE as a medium UE among the plurality of UEs, operation S120 of transmitting, by the AP, data to the medium UE, and operation S130 of cooperatively transmitting, by the medium UE, data to the destination UE.

Here, operation S120 of transmitting the data to the medium UE may include splitting, by the AP, data requested by the medium UE into private data and a portion of common data, generating, by the AP, common data that includes information of the plurality of UEs by combining portions of the split common data of the medium UEs, encoding, by the AP, the private data to a private stream and encoding the common data to a common stream, and precoding, by the AP, the private stream and the common stream and transmitting a signal to the medium UE.

Also, operation S130 of transmitting the data to the destination UE may include decoding, by the medium UE, the common stream, removing the common stream and then decoding the private stream that includes its own information, encoding, by each of the medium UEs, data requested by the destination UE to a data stream, and precoding, by the medium UE, the data stream and transmitting a signal to the destination UE.

Hereinafter, the coverage extension method in the THz region using the cooperative communication according to an example embodiment is further described.

The coverage extension method in the THz region using the cooperative communication according to an example embodiment may be further described using, as an example, a coverage extension apparatus in a THz region using cooperative communication according to an example embodiment.

FIG. 9 is a diagram illustrating a coverage extension apparatus in a THz region using cooperative communication according to an example embodiment.

Referring to FIG. 9, a coverage extension apparatus 900 in a THz region using cooperative communication according to an example embodiment may include a setter 910 and a transmitter 920. Here, the coverage extension apparatus 900 in the THz region using cooperative communication according to an example embodiment may represent an AP or may include the AP.

In operation S110, in a downlink system that supports a plurality of UEs, the setter 910 of the AP may set a UE that requires assistance due to a blocked LoS link as a destination UE and may set a remaining UE as a medium UE among the plurality of UEs.

In operation S120, the transmitter 920 of the AP may transmit data to the medium UE. Therefore, the medium UE may cooperatively transmit data to the destination UE.

The transmitter 920 may transmit data requested by the destination UE and data requested by the medium UE to the medium UE. In detail, the transmitter 920 may include a data splitter configured to split data requested by the medium UE into private data and a portion of common data, a common data combiner configured to generate common data that includes information of the plurality of UEs by combining portions of the split common data of the medium UEs, an encoder configured to encode the private data to a private stream and to encode the common data to a common stream, and a precoder configured to precode the private stream and the common stream and to transmit a signal to the medium UE.

For example, in the case of IeCRS, the transmitter 920 may split data requested by the medium UE without splitting data requested by the destination UE and may transmit the same signal to all the medium UEs based on the data requested by the destination UE and the data requested by the medium UE.

As another example, in the case of DeCRS, the transmitter 920 may split data requested by the destination UE and data requested by the medium UE and may transmit different signals to all the medium UEs based on the data requested by the destination UE and the data requested by the medium UE.

Also, a coverage extension apparatus in a THz region using cooperative communication according to another example embodiment may include the AP 110 and the medium UE 120 as illustrated in FIG. 1. Also, depending on example embodiments, the coverage extension apparatus may further include the destination UE 130.

The coverage extension apparatus in the THz region using cooperative communication according to another example embodiment may include the AP 110 configured to set, in a downlink system that supports a plurality of UEs (120, 130), a UE that requires assistance due to a blocked LoS link as the destination UE 130 and to set a remaining UE as the medium UE 12 among the plurality of UEs (120, 130) and then to transmit data to the medium UE 120, and the medium UE 120 configured to cooperatively transmit data to the destination UE 130 based on data received from the AP 110.

The AP 110 may transmit data requested by the destination UE 130 and data requested by the medium UE 120 to the medium UE 120. In detail, the AP 110 may include a data splitter configured to split data requested by the medium UE 120 into private data and a portion of common data, a common data combiner configured to generate common data that includes information of the plurality of UEs by combining portions of the split common data of the medium UEs 120, an encoder configured to encode the private data to a private stream and to encode the common data to a common stream, and a precoder configured to precode the private stream and the common stream and to transmit a signal to the medium UE 120.

In the case of IeCRS, the AP 110 may split data requested by the medium UE 120 without splitting data requested by the destination UE 130 and may transmit the same signal to all the medium UEs 120 based on the data requested by the destination UE 130 and the data requested by the medium UE 120. Also, in the case of DeCRS, the AP 110 may split data requested by the destination UE 130 and data requested by the medium UE 120 and may transmit different signals to all the medium UEs 120 based on the data requested by the destination UE 130 and the data requested by the medium UE 120.

The medium UE 120 may include a decoder configured to decode the common stream and to remove the common stream and then decode the private stream that includes its own information, an encoder configured to encode data requested by the destination UE 130 to a data stream, and a precoder configured to precode the data stream and to transmit a signal to the destination UE 130.

In the case of IeCRS, to correspond to a channel, the medium UE 120 may divide the channel using OFDM. Also, in the case of DeCRS, the medium UE 120 may have a form of a multiple access channel.

As described above, example embodiments propose a cooperative communication system to compensate for limited coverage in consideration of disadvantages of THz band communication. To this end, the example embodiments propose a system model that specifically considers a THz band and also propose a new cooperative communication structure called an eCRS structure to support cooperative communication. Also, an optimization problem is designed and solved with respect to cases of IeCRS and DeCRS that are special two cases of the eCRS structure. Performance comparison shows that it is possible to successfully support a dUE out of coverage through the eCRS structure and it is verified that both IeCRS and DeCRS have excellent performance.

The apparatuses described herein may be implemented using hardware components, software components, and/or a combination of the hardware components and the software components. For example, the apparatuses and the components described herein may be implemented using one or more general-purpose or special purpose computers, for example, a processor, a controller, an arithmetic logic unit (ALU), a digital signal processor, a microcomputer, a field programmable array (FPA), a programmable logic unit (PLU), a microprocessor, or any other device capable of responding to and executing instructions in a defined manner. The processing device may run an operating system (OS) and one or more software applications that run on the OS. The processing device also may access, store, manipulate, process, and create data in response to execution of the software. For purpose of simplicity, the description of the processing device is used as singular; however, one skilled in the art will be appreciated that a processing device may include multiple processing elements and/or multiple types of processing elements. For example, a processing device may include multiple processors or a processor and a controller. In addition, different processing configurations are possible, such as parallel processors.

The software may include a computer program, a piece of code, an instruction, or some combinations thereof, for independently or collectively instructing or configuring the processing device to operate as desired. Software and/or data may be embodied in any type of machine, component, physical equipment, virtual equipment, a computer storage medium or device, to be interpreted by the processing device or to provide an instruction or data to the processing device. The software also may be distributed over network coupled computer systems so that the software is stored and executed in a distributed fashion. The software and data may be stored by one or more computer readable storage media.

The methods according to the above-described example embodiments may be configured in a form of program instructions performed through various computer devices and recorded in computer-readable media. The program instructions stored in the media are specially designed and configured for the example embodiments and may be known to those skilled in the computer software art and thereby available. Examples of the media include magnetic media such as hard disks, floppy disks, and magnetic tapes; optical media such as CD-ROM and DVDs; magneto-optical media such as floptical disks; and hardware devices that are configured to store program instructions, such as read-only memory (ROM), random access memory (RAM), flash memory, and the like. Examples of the program instruction may include a machine code as produced by a compiler and include a high-language code executable by a computer using an interpreter and the like.

Although the example embodiments are described with reference to some specific example embodiments and accompanying drawings, it will be apparent to one of ordinary skill in the art that various alterations and modifications in form and details may be made in these example embodiments without departing from the spirit and scope of the claims and their equivalents. For example, suitable results may be achieved if the described techniques are performed in different order, and/or if components in a described system, architecture, device, or circuit are combined in a different manner, and/or replaced or supplemented by other components or their equivalents.

Therefore, other implementations, other example embodiments, and equivalents of the claims are to be construed as being included in the claims.

Claims

1. A coverage extension method in a terahertz (THz) region using cooperative communication performed by a computer device, the coverage extension method comprising: setting, by an access point (AP) in a downlink system that supports a plurality of user equipments (UEs), a UE that requires assistance due to a blocked line-of-sight (LoS) link as a destination UE and setting a remaining UE as a medium UE among the plurality of UEs;transmitting, by the AP, data to the medium UE; andcooperatively transmitting, by the medium UE, data to the destination UE.

2. The coverage extension method of claim 1, wherein the transmitting of the data to the medium UE comprises transmitting, by the AP, data requested by the destination UE and data requested by the medium UE to the medium UE.

3. The coverage extension method of claim 1, wherein the transmitting of the data to the medium UE comprises: splitting, by the AP, data requested by the medium UE into private data and a portion of common data;generating, by the AP, common data that includes information of the plurality of UEs by combining portions of the split common data of the medium UEs;encoding, by the AP, the private data to a private stream and encoding the common data to a common stream; andprecoding, by the AP, the private stream and the common stream and transmitting a signal to the medium UE.

4. The coverage extension method of claim 3, wherein the transmitting of the data to the destination UE comprises: decoding, by the medium UE, the common stream;removing, by the medium UE, the common stream and then decoding the private stream that includes its own information;encoding, by each of the medium UEs, data requested by the destination UE to a data stream; andprecoding, by the medium UE, the data stream and transmitting a signal to the destination UE.

5. The coverage extension method of claim 1, wherein the transmitting of the data to the medium UE comprises: splitting, by the AP, data requested by the medium UE without splitting data requested by the destination UE and transmitting the same signal to all the medium UEs based on the data requested by the destination UE and the data requested by the medium UE.

6. The coverage extension method of claim 1, wherein the transmitting of the data to the medium UE comprises: splitting, by the AP, data requested by the destination UE and data requested by the medium UE and transmitting different signals to all the medium UEs based on the data requested by the destination UE and the data requested by the medium UE.

7. The coverage extension method of claim 1, wherein the transmitting of the data to the destination UE comprises splitting a channel using orthogonal frequency division multiplexing (OFDM) to correspond to the channel.

8. The coverage extension method of claim 1, wherein the transmitting of the data to the destination UE comprises having a form of a multiple access channel.

9. A coverage extension apparatus in a terahertz (THz) region using cooperative communication, the coverage extension apparatus comprising: a setter configured to set, in a downlink system that supports a plurality of user equipments (UEs), a UE that requires assistance due to a blocked line-of-sight (LoS) link as a destination UE and to set a remaining UE as a medium UE among the plurality of UEs; anda transmitter configured to transmit data to the medium UE,wherein the medium UE is configured to cooperatively transmit data to the destination UE.

10. The coverage extension apparatus of claim 9, wherein the transmitter is configured to transmit data requested by the destination UE and data requested by the medium UE to the medium UE.

11. The coverage extension apparatus of claim 9, wherein the transmitter comprises: a data splitter configured to split data requested by the medium UE into private data and a portion of common data;a common data combiner configured to generate common data that includes information of the plurality of UEs by combining portions of the split common data of the medium UEs;an encoder configured to encode the private data to a private stream and to encode the common data to a common stream; anda precoder configured to precode the private stream and the common stream and to transmit a signal to the medium UE.

12. The coverage extension apparatus of claim 11, wherein the medium UE comprises: a decoder configured to decode the common stream and to remove the common stream and then decode the private stream that includes its own information;an encoder configured to encode data requested by the destination UE to a data stream; anda precoder configured to precode the data stream and to transmit a signal to the destination UE.

13. The coverage extension apparatus of claim 9, wherein the transmitter is configured to split data requested by the medium UE without splitting data requested by the destination UE and to transmit the same signal to all the medium UEs based on the data requested by the destination UE and the data requested by the medium UE.

14. The coverage extension apparatus of claim 9, wherein the transmitter is configured to split data requested by the destination UE and data requested by the medium UE and to transmit different signals to all the medium UEs based on the data requested by the destination UE and the data requested by the medium UE.

15. A coverage extension apparatus in a terahertz (THz) region using cooperative communication, the coverage extension apparatus comprising: an access point (AP) configured to set, in a downlink system that supports a plurality of user equipments (UEs), a UE that requires assistance due to a blocked line-of-sight (LoS) link as a destination UE and to set a remaining UE as a medium UE among the plurality of UEs and then to transmit data to the medium UE; andthe medium UE configured to cooperatively transmit data to the destination UE based on data received from the AP.

16. The coverage extension apparatus of claim 15, wherein the AP is configured to transmit data requested by the destination UE and data requested by the medium UE to the medium UE.

17. The coverage extension apparatus of claim 15, wherein the AP comprises: a data splitter configured to split data requested by the medium UE into private data and a portion of common data;a common data combiner configured to generate common data that includes information of the plurality of UEs by combining portions of the split common data of the medium UEs;an encoder configured to encode the private data to a private stream and to encode the common data to a common stream; anda precoder configured to precode the private stream and the common stream and to transmit a signal to the medium UE.

18. The coverage extension apparatus of claim 17, wherein the medium UE comprises: a decoder configured to decode the common stream and to remove the common stream and then decode the private stream that includes its own information;an encoder configured to encode data requested by the destination UE to a data stream; anda precoder configured to precode the data stream and to transmit a signal to the destination UE.

19. The coverage extension apparatus of claim 15, wherein the AP is configured to split data requested by the medium UE without splitting data requested by the destination UE and to transmit the same signal to all the medium UEs based on the data requested by the destination UE and the data requested by the medium UE.

20. The coverage extension apparatus of claim 15, wherein the AP is configured to split data requested by the destination UE and data requested by the medium UE and to transmit different signals to all the medium UEs based on the data requested by the destination UE and the data requested by the medium UE.