Design of Terahertz Wireless Communications Simulator

http://asiafi.net/meeting/2012/summerschool/submissions/AsiaFI2012_Poster/6_Design%20of%20Terahertz%20Wireless%20Communications%20Simulator%20for%20Future%20Wireless%20Internet.pdf
Seongman Min, Wonjung Kim, Taewon Song, and Sangheon Pack

School of Electrical Engineering

Korea University, Korea

{pobi1020, abcxxxx, crazytb, shpack}@korea.ac.kr

ABSTRACT

Terahertz (THz) waves can offer high transmission rates due to

the ultra-broad bandwidth. So THz communications are recently

received attention. However, due to the absence of a proper

simulator, a diversity of attempts are not performed on THz

communications. In order to alleviate this problem and to

contribute the improvement in the performance of THz

communication, we describe the design of THz simulator in this

paper. Specifically, the design of PHY layer and MAC layer are

handled in detail.

Keywords

terahertz, simulator, PHY layer, MAC layer

1. INTRODUCTION

Future wireless communications systems, which supports

ultrahigh data rates, is expected to apply a variety of field

that need more 10GHz. Conventional WLAN, WPAN, and

UWB systems use the bandwidth between multi-MHz and

multi-GHz. Therefore, these have limited in the

requirement of future wireless communication systems. To

address this limitation, terahertz (THz) technologies have

recently gained momentum in the area of research and

development. As THz region covers 100-10000 GHz range,

THz waves support much more bandwidth than millimeter

waves. Due to its characteristic, THz communication

systems with several tens of Gbps data rates can be applied

to many applications such as wireless extension of highspeed

wired local networks, wireless data center networks,

and high-definition and ultra-high-definition television

(HDTV & UHDTV) [1].

In addition to ultra-high bandwidth, THz waves have

several other characteristics. Firstly, THz waves are

inherently more directional than millimeter waves due to

less free space diffraction of the waves. Consequently, THz

communication systems are typically line-of-sight (LOS)

systems. Secondly, THz waves are severely attenuated

along the propagation path. When the propagation distance

or the carrier frequency becomes double, the attenuation

increases 6dB. Finally, in outdoor condition, THz waves

are significantly influenced by weather condition (i.e., rain,

snow, fog, etc.) because they are mainly absorbed in watervapor

[2]. And the degree of attenuation is different

depending on frequency band, so there are available

bandwidths satisfying a standard point [3]. On the contrary,

in indoor condition, the weather condition does not affect

on THz waves but obstacles (i.e., wall, furniture, human,

etc.) absorb or reflect them [4].

Many research projects on THz are being performed

reflecting these characteristics in the industry and academia.

Most of them evaluate a performance of proposing schemes

by relying on experiments. However, it is hard to perform a

diversity of attempts on THz communications because the

method, relying on experiments, has limitations of time,

space, and cost. If we use the THz communications

simulator, we can predict the performance of proposing

schemes by simulations ahead of experiment and perform a

diversity of attempts at the lowest cost. However, there is

no a simulator on THz region yet. Therefore, in this paper,

we address the development of a methodology for THz

communications simulator.

2. THz COMMUNICATIONS SIMULATOR

In the design of the THz communications simulator, we

consider two things; PHY layer and MAC layer. As figure

1 presents a brief architecture of THz communications

simulator, MAC and PHY layer consist of three modules,

respectively. In our PHY layer, THz waves propagation

model is used to predict the received signal power of each

packet. Using this propagation model, the simulator takes

into account the effects of transmission distance, absorption

and reflection due to obstacles, atmospheric conditions, and

directionality of THz waves. In the MAC layer, the

simulations measure the performance at MAC layer such as

throughput, average delay, and packet loss metrics. Since

there is no standard of wireless communications on THz

region yet, we use the MAC layer of IEEE 802.11ad [

which is a standard of WLAN on 60GHz whose

characteristics are similar to THz waves. In our MAC layer,

as network architecture, we consider personal basic service

set (PBSS) which is newly defined in IEEE 802.11ad to

decrease power consumption of mobile terminals and to

support directional transmission. In PBSS, there is PBSS

central point/AP (PCP/AP) which supports QoS and

manages the frequency spectrum.

2.1 Design of PHYSICAL layer

Depending on channel characteristics, PHY layer model

is divided into three modes; free space, outdoor, and indoor.

The free space mode simulates the ideal condition, which

assumes a LOS path between transmitter and receiver. In

the free space mode, free space attenuation is only

considered as a parameter. Therefore, THz waves are

attenuated by the distance between transmitter and receiver

and the carrier frequency based on Friis’ law.

The outdoor mode simulates the realistic condition

which considers atmospheric condition in addition to free

space attenuation. Therefore, water-vapor and atmospheric

gases are also considered as parameters in the outdoor

mode. In the near field wireless communication (under

10m), since the free space attenuation is much higher than

the attenuation due to atmospheric condition, we can

predict the attenuation only considering free space

attenuation. The indoor mode simulates the realistic

condition in a building or house. In an indoor condition,

THz waves are reflected by obstacles (i.e., wall, ground,

ceiling, furniture, human, etc.). As reflection coefficients

depend on material of obstacles, significant reflected waves,

which are once or twice reflected to any materials having

low reflection coefficients, affect on the fading. Therefore,

the indoor THz communication channel can be modeled by

free space attenuation and power delay profile (PDP) [1],

[4]. The PDP refers to the path length difference between

the direct LOS and once or twice reflected waves. Its

important parameters are mean delay and root mean square

(RMS) delay spread. Since RMS delay spread significantly

affects on inter-symbol interference (ISI) so it has a strong

influence on bit-error-rate (BER) of communication system.

2.2 Design of MAC layer

In our MAC layer, three modules are defined; beacon

module, beamforming module, and scheduling module. The

beacon module is used to manage every associated

terminals and connections. And it defines a structure of

beacon frame and the information included in a beacon

frame. Based on it, PCP/AP broadcasts a beacon frame to

every associated terminals periodically.

The beamforming module is used to control the direction

and width of antenna beam. Since THz waves are highly

directional, the antenna direction of terminal should be

adjusted to target terminal for communicating each other.

Also, the beam width of antenna has to be tuned to decrease

power consumption and to reduce interferences. The

scheduling module is used to manage the sequence of

connections and to allocate the resource. Data transfer time

period in a beacon period consists of contention-based

access periods (CBPs) and service periods (SPs). PCP/AP

allocates CBPs and SPs, and it schedules the sequence of

connections on SPs.

3. CONCLUSIONS

In this paper, we present the design of THz

communications simulator with PHY layer channel models

and MAC layer modules. We are currently working on

reflecting THz channel characteristics in ns-3 simulator and

implementing MAC layer protocol by using ns-3 simulator.

THz simulator would allow for further investigation of the

performance of THz communications systems at the low

cost.

4. ACKNOWLEDGMENTS

This research was supported in part by the KCC, Korea,

under the R&D program supervised by the KCA (KCA-

2011-08913-04002), in part by the MKE, Korea, under the

ITRC support program (NIPA-2012-H0301-12-4004)

supervised by the NIPA.

REFERENCES

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