TECHNICAL FIELD

The invention is a novel multi-user design for effective future multiple access communication systems and is proposed as an alternative technique to Power Domain Non-Orthogonal Multiple Access (PD-NOMA) which was proposed for 5G systems and was studied from release 13 to release 16 of 3GGP (3 ^rd Generation Partnership Project) before being eliminated from study items in release 17 of 3GPP due to various reasons such as the complexity of the receiver and the associated risks of security and thus, the presented design in this patent termed as Multi-User Auxiliary Signal Superposition Transmission (MU-AS-ST) is proposed as an alternative to PD-NOMA where the problems of conventional NOMA are solved through the use of specifically designed auxiliary signals superimposed on top of the users’ data resulting in interference-free and eavesdropping-free communication while freeing the receiver from any complex processing tasks by carrying all of the processing at the base station making from this design a solid candidate for processing-restricted scenarios such as Internet of Things (loT) devices.

PRIOR ART

The techniques designed for future wireless systems are expected to serve a very huge interconnection of data-hungry devices due to the explosion of mobile devices, the emergence of the Internet of things (loT) where billions of devices which are supported with sensing and communication capabilities are connected, and due to many other technological advancements such as extended reality (XR) services including augmented, mixed, and virtual reality (AR/MRA/R), haptics (tactile Internet), real-time gaming, telemedicine, brain-computer interfaces, flying vehicles, connected autonomous systems, etc.

This massive network which shaped the 5G era and will increase in the era of future generations of wireless systems imposes on wireless designers the development of novel techniques which consider the scarcity of the spectrum allocated for wireless communications and use it in a very effective manner to serve the needs of this massive network.

In this regard, one of the most solid and promising candidates for radio access techniques in next generation wireless communications is the use of non-orthogonal multiple access where users are served using the same resources of time, frequency, or code. Moreover, OMA (Orthogonal Multiple Access) techniques which were deployed in before-5G generations were proven incapabable of addressing the requirements of 5G communication systems including eMBB (enhanced Mobile Broadband), URLLC (Ultra Reliable Low Latency Communications), mMTC (massive Machine Type Communications), and thus Non-Orthogonal Multiple Access (NOMA) techniques were proposed for 5G communications due to benefits of high throughput, massive connectivity and coverage, low latency and signaling overhead, and relaxed channel feedback.

Many multiple access designs with varying degrees of success in addressing the requirements of 5G systems have been proposed from the part of both academia and industry such as multi-user shared access (MUSA), pattern division multiple access (PDMA), resource spread multiple access (RSMA), interleave-grid multiple access (IGMA), interleave division multiple access (IDMA), sparse-code multiple access (SCMA), and many others that were included in different releases of the 3rd Generation Partnership Project (3GPP).

Particularly, the most powerful candidate among all of this designs is power domain non-orthogonal multiple acces (PD-NOMA) which is a technique that gained too much interest from both the part of wireless academia and industry and was considered a work item from release 13 till release 16 of 3GPP under the name MUST (Multi-User Superposition Transmission); howevere, it was eliminated from the work items in release 17 due to some limitations and shotcomings.

The limitations of conventional power domain NOMA can be summarized as follows:

• Working only in scenarios where the superimposed users must have different distances from the base station where in the transmission a far user and a near user are combined while if the users have the same distance from the base station NOMA fails. • Increasing the complexity of the design through the use of successive interference cancellation (SIC) which is performed by the near user to eliminate the dominating signal of the far user and decoding its own signal.

• Having significant interference at the far user where the signal of the near user is not cancelled but treated as noise.

• Having security issues at the near user due to SIC and at the far user due to the non-cancellation of the near user’s signal, and also there is a need for securing this design against external eavesdropping activities taking into account the broadcast nature of the wireless channels and the sensitivity of the data in this era such as bank account details and transactions, private messages and documents, etc.

BRIEF DESCRIPTION OF THE INVENTION

The invention aims to provide an alternative multiple access design to power domain NOMA which is suitable for serving future wireless systems and where the above-mentioned limitations of conventional power domain NOMA are solved.

The presented design is termed as Multi-User Auxiliary Signal Superposition Transmission (MU-AS-ST) where, due to the superposition of specifically designed auxiliary signals with the users’ data as shown in the system model displayed in Figure 1 , the following benefits and contributions points are achieved:

• Working in general scenarios where, unlike conventional NOMA where only users having different distance from the base station can be super-imposed, this design works for the combination of any two users regardless of their distances from the base station,

• Ensuring perfect information secrecy against both external and internal eavesdropping,

• Ensuring reduced receiver complexity where, unlike conventional NOMA where successive cancellation at the receiver side leads to a complexity of the system, in this technique the specific design of the auxiliary signals solves this problem effectively making it a strong candidate for processing-restricted applications and low complexity application scenarios such as loT devices,

• Ensuring reduced transmission latency where this design is featured with its simple transceiver design, and since at the transmitter the design of the auxiliary signals employs diagonal matrices this makes the computational cost lower, while at the receiver simply a conventional OFDM receiver is used and thus freeing the receiver from any complex processing.

LIST OF FIGURES

Figure 1. General system model of MU-AS-ST.

Figure 2. Transmitter of MU-AS-ST.

Figure 3. Receiver of MU-AS-ST.

Figure 4. Bit error rates of MU-AS-ST in the legitimate and eavesdropping scenarios.

Figure 5. MU-ASST vs PD-NOMA; under different power reallocation scenarios.

Figure 6. Data rate results of MU-AS-ST for the legitimate and eavesdropping nodes

Figure 7. Peak to average power ratio (PAPR) results of MU-AS-ST compared to conventional OFDM.

Figure 8. MU-AS-ST compared to power domain NOMA in terms of system model.

DETAILED DESCRIPTION OF THE INVENTION

The invention is a novel physical layer security technique for effective, reliable, and secure multi-user communication systems proposed as an alternative technique which solves the limitations and avoids the risks found in the core design of power domain NOMA which suffers from security risks and increased design complexity and thus latency.

To clearly illustrate the contributions that the proposed technique brings, its design principles are studied in detail and then it is compared to conventional power domain NOMA.

A- System Model

1. Transmitter Design

As shown in the transmitter design in Figure 2, the data processing passes through the following set of steps;

• Modulating the data of two different users UE ₁ with data and UE ₂ with data x ₂ using binary phase shift keying (BPSK), • Combining the modulated data of both users which is known as users’ superposition,

• Adding the specifically designed auxiliary signals on top of the superimposed data (i.e., and thus the resulting signal can be expressed as

• Performing conventional OFDM operations to signals u ₁ and u ₂ such as taking the Inverse Fast Fourier Transform (IFFT), Adding the cyclic prefix (CP) and converting the signal from digital to analog.

• Transmitting each of the signals from a separate antenna where the signal is sent from antenna A ₁ and the signal u ₂ is sent from the antenna A ₂ .

2. Receiver Design

The receiver of the proposed design is shown in Figure 3, where the below steps are followed for detecting the transmitted data;

• Performing the analog to digital conversion,

• Removing the cyclic prefix

• Taking the Fast Fourier Transform (FFT)

• Performing equalization to remove the channel effect by dividing the received samples by the effective channel for each legitimate user

• Demodulating the data using BPSK.

- Received signal at UE1

The transmitted signals u ₁ and u ₂ are received simultaneously at this user node and thus the received signal, written in the frequency domain, can be expressed as follows by summing the signals coming from antenna A ₁ and antenna A ₂ : where, is the additive white gaussian noise, and H _11, H ₁₂ are, respectively, the channels between the receiving user equipment and antenna A ₁ and antenna A ₂ . - Receiver signal at UE2

Similarly, the received signal at the second user can be expressed by the following equations: where, n ₂ is the additive white gaussian noise, and H ₂₁ , H ₂₂ are, respectively, the channels between the receiving user equipment and antenna and antenna

- Design of the Auxiliary Signals

The auxiliary signals are designed such that the interference that the users create to each other is eliminated and such that legitimate user device receives its own signal and thus eliminating the risks of internal eavesdropping in the case of the presence of an untrusted legitimate user among the superimposed users.

For user 1 , by looking at the received signal in equation (3), it can be seen that the first term of this equation (i.e., is the desired term with respect to this user while the other terms can be set to zero to ensure that this user receives only its own data, yielding the following equation:

Similarly, for user 2, by looking at equation (6), the only desired term with respect to user 2 is while the other terms correspond to interference and noise and thus, for this user to receive only its own data, the following equation can be written:

By solving (7) and (8) together, the values of the auxiliary signals that guarantee full inter-user interference cancellation are found as follows: When the above values for r ₁ and r ₂ are used then, then the received signals at UE _lt UE ₂ expressed by equation (3) and (6) can be written, respectively, as follows:

Then, as shown in the receiver of MU-AS-ST, when the equalization by the effective channels in (13), (14) is done the transmitted data is recovered at each legitimate user end.

- Received signal at the external eavesdropper

The eavesdropper user equipment UE _X receives the broadcasted signals from antenna A ₁ and antenna A ₂ and, as such, the received signal at the eavesdropper device can be written as follows: where, n _x is the additive white gaussian noise, and H _X1 , H _X2 are, respectively, the channels between the receiving user equipment and antenna A ₁ and antenna A ₂ . Unlike the legitimate users, this received signal does not contain the data of only one user but the data of both users and more than that, it is very extremely challenging to be decoded due to the many unknowns that this received signal in (17) contains with respect to an external eavesdropping device and thus this makes from this design a robust design against external eavesdropping scenarios.

B- Performance Demonstration

The theoretical performance analysis of the idea was performed in detail in “Abewa, M., & Hamamreh, J. M. (2021 ). Multi-User Auxiliary Signal Superposition Transmission (MU-AS-ST) for Secure and Low-Complexity Multiple Access Communications. RS Open Journal on Innovative Communication Technologies, 2(4). (https://doi.org/10.46470/03d8ffbd.92a40b85)” where the performance was studied in terms of the bit error rate, the throughput and peak to average power ratio (PAPR). Moreover, the theoretical derivations were confirmed by simulation results where the necessary simulations to prove the performance of this idea were carried out in the MATLAB simulation environment using the simulation parameters that are specified in Table 1 .

Table 1. simulation parameters

The performance of the proposed design can be seen from the figures:

• Displaying the bit error rates of both the legitimate nodes and the internal external eavesdropping scenarios in MU-AS-ST as shown in Figure 4, where the legitimate users have a good performance while the eavesdroppers experience severe degradation including both types of eavesdropping where internal eavesdropping refers to the case where a legitimate user is trying to listen to the data of another legitimate user and the external eavesdropping case refers to the case where an external illegitimate device is trying to decode the data of the legitimate users,

• Displaying the bit error rates of MU-AS-ST compared to conventional power domain NOMA using different power sharing scenarios for NOMA where it can be seen that MU-AS-ST surpasses NOMA in terms of reliability in all of the investigated cases as shown in Figure 5,

• Displaying the simulations of the data rate performance of MU-AS-ST with two users as shown in Figure 6,

• Displaying the peak to average power ratio (PAPR) which is a crucial metric to be measured for any design that is based on OFDM as it is known that OFDM has higher peaks which are undesired at the input of the power amplifier and thus the PAPR of MU-AS-ST was studied and was shown not to add any degradation to conventional OFDM as shown in Figure 7. C- Comparison between MU-AS-ST and power domain NOMA

1- Design

The proposed system model for MU-AS-ST as shown in Figure 1 and Figure 8 is capable of solving all of the above-mentioned limitations that current conventional NOMA suffers from, by:

• Eliminating the inter-user interference completely through the use of the auxiliary signals unlike NOMA where the far user treats the signal of the near user as noise and decodes its own signal still but this affects the performance as shown in the simulations in Figure 5,

• Providing perfect internal secrecy where the superimposed users cannot listen to each other using MU-AS-ST while in NOMA the near user must perform successive interference cancellation (SIC) in order to decode its own signal and thus there are risks of internal eavesdropping in the presence of an untrusted user in the set of superimposed users,

• Providing external secrecy where external eavesdroppers are extremely challenged while decoding the broadcasted signals in the case of MU-AS-ST since the received signal contains many unknowns which are the combinations of the users’ data, users’ channels and the auxiliary signals which are in turn a very messy combination of the users’ data and users’ channels.

• Providing a simple receiver design where a conventional OFDM receiver is used thus making it a good fit for internet of things (loT) devices which are processing-restricted, this is unlike NOMA where the SIC receiver is one of the most complex receivers especially when the number of users grow it also introduces latency which is undesired in future systems where very low latency requirements are targeted.

• Providing a simple transmitter design where the computations carried in the transmitter use diagonal matrices thus making the computational processing much faster.

• Working in general scenarios where any two users can be superimposed in the case of MU-AS-ST unlike NOMA which fails in power balanced scenarios when the users have the same distance from the base station. 2- Performance

The performance of MU-AS-ST was compared to that of conventional power domain NOMA as shown in Figure 5 where, for the same distance from the base station, and using different power allocation scenarios for NOMA, both designs were compared in terms of the bit error rate performance and, as can be seen, MU-AS-ST can surpass NOMA in all of the investigated cases.

Publication “Abewa, M., & Hamamreh, J. M. (2021). Multi-User Auxiliary Signal Superposition Transmission (MU-AS-ST) for Secure and Low-Complexity Multiple Access Communications. RS Open Journal on Innovative Communication Technologies, 2(4). (https://doi.org/10.46470/03d8ffbd.92a40b85)” offers more details on this comparison and offers a theoretical derivation of the bit error rate of the proposed design and the signal to interference plus noise ratio (SINR) which measures the quality of the communication link in both MU-AS-ST and PD-NOMA where it was shown that MU-AS-ST surpasses conventional power domain NOMA.