Revelation: freely-controlled and highly efficient next-generation signals

LANGUAGE ≫ Japanese


Specialized field

signal theory, modulation and demodulation technology

For Details

Finding ideal signals by slightly modifying OFDM

With the spread of smartphones and wireless LANs, users want to be able to communicate comfortably at high speed, so there is always a demand for a communication method that offers greater frequency utilization efficiency. Professor Hamamura conducts research towards high speed, high quality next-generation communications. He describes his mission as "using radio waves to speed up communications as much as possible."

A communication method called OFDM (orthogonal frequency-division multiplexing) is widely used today in smartphones and wireless LANs. Prof. Hamamura aims to create signal waveforms that are more efficient and stronger against interference than OFDM.

Prof. Hamamura says, "To improve efficiency, it is possible to create new signals in a completely different way from what we're using now--but if we could use our current OFDM hardware just as it is, it would be cost effective. That's why I have been pursuing ways of achieving high-efficiency communications without significantly changing the structure of commonly used signal transmitters and receivers."

OFDM is a form of multicarrier transmission using multiple orthogonal subcarriers. On the other hand, in order to further improve the frequency utilization efficiency to increase communication speed or narrow the occupied bandwidth, attention has recently been focused on non-orthogonal signals rather than orthogonal ones. In the past 15 years, Prof. Hamamura has achieved some ground-breaking results in his ongoing trial and error research focusing on non-orthogonal multicarrier (NOMC) signals using non-orthogonal subcarriers.

Prof. Hamamura explains, "I found out that one NOMC signal, called discrete prolate spheroidal wave function, can be generated easily by means using a slight modification to the signal transmission method used in OFDM and creating a transmission signal using a sequence called Slepian sequences. It turns out that this is a very efficient signal."

It has also become clear that the NOMC signal obtained in that way cannot only increase frequency utilization efficiency but also freely control the waveform of the radio waves. This enables various designs appropriate to any particular communication channel. You could call this an innovative discovery.

Realizing signals that can make the best use of frequency band

Information communication signals, like those used by telecommunications carriers such as docomo, KDDI, and SoftBank, use a fixed range of frequencies for many different purposes. Therefore, communication speed within that fixed frequency range is important. In the Slepian sequence-based NOMC signal developed by Prof. Hamamura, it is possible to easily generate spectra that can be used to the maximum without wasting bandwidth within the given frequency range. What are those spectra?

"In the world of information communication, 'spectrum' refers to the frequency characteristics of a signal. Using a rectangular spectrum whose first and last waveform amplitudes are small, a signal can be utilized without loss in the given frequency range. This is a state in which the signal utilizes the range of frequency to the maximum, but the radio waves do not leak out. That is, the range of the frequency can be fully utilized without causing or incurring any interference."

(Image from The Institute of Electronics, Information and Communication Engineers, Fundamentals Review Vol. 11, No.1)

A signal with a large amplitude at the beginning and end of the waveform may cause radio waves to leak out and interfere with adjacent frequency bands. In addition, the frequency range in which radio waves leak is wasted because it cannot be used properly. It is very advantageous to be able to easily generate a signal with a spectrum that solves such problems.

Steep notches prevent mutual interference

By making further improvements to such highly efficient signal waveforms, it may be possible to innovate diverse properties even more. After additional trial and error studies, Prof. Hamamura found that this signal can be given new properties, i.e., it's possible to make a sharp notch in the spectrum. So, how does notching affect the signal?

"A notch is an amplitude drop in a narrow frequency range. In a notch the frequency band does not emit radio waves. This enables us to avoid causing interference to people who are using the radio waves in that frequency, and incurring interference ourselves. By using notches as a tool we can prevent spectrum leakage to bands which should never be interfered with, such as the band for emergency broadcasting and the band allocated for radio astronomy, which handles subtle radio waves," explains Prof. Hamamura.

When we want to avoid incurring interference from others or causing interference, we can prevent it before it happens by putting a notch in that part of the spectrum. We have demonstrated that a notch can be freely placed anywhere in the NOMC signal.

(Image from The Institute of Electronics, Information and Communication Engineers, Fundamentals Review Vol. 11, No.1)

In addition, by modifying the Slepian sequence in different ways, we can easily generate a signal in a spectrum with strong or weak radio waves, depending on the frequency band.

"Although a sharply cut notch is needed to achieve absolutely no interference, if small leaks are permissible over a certain range, a shallow cut is suitable. The placement of a notch and the degree of its strength can be adjusted freely to meet specific requests such as: not emitting radio waves much over a given range within a given frequency band; only emitting radio waves slightly over a certain range; or using only a certain range."

(Image from The Institute of Electronics, Information and Communication Engineers, Fundamentals Review Vol. 11, No.1)

Improving frequency utilization efficiency by using multiple bands simultaneously

Furthermore, it is also possible to use multiple frequency bands simultaneously by placing notches over a wide range in the NOMC signal. What would be the benefit of that?

Consider the case where a band is used for multiple applications within a certain frequency range. If multiple bands are used separately, spectrum leaks may occur in adjacent bands, or leaks between bands may cause mutual interference. Therefore, we can place a guard interval to smooth the waveform, or widely place a guard band as a preventive measure, but this causes waste in the use of the frequency band, reducing frequency utilization efficiency.

On the other hand, in the case of a signal that simultaneously uses multiple bands, any leak from the left or right band also has its own spectrum, but since it does not cause interference, no countermeasure is necessary. By using multiple bands at the same time, it is possible to improve overall frequency utilization efficiency.

Prof. Hamamura continues, "It is an epoch-making discovery that the spectrum of an NOMC signal can be freely controlled by making minor modifications to OFDM. We are creating signals of higher efficiency by working from various viewpoints to extract new properties from the signal."

Prof. Hamamura has focused solely on information and communication since college. "It is simply very interesting to think about signals. You don't need more reason than that," he says.

"As I eagerly pursue my research to see what lies ahead, I often encounter new discoveries by chance."

Along with precisely calculated theory, a refined sense is also important. Also, the feeling that there is something ahead will often lead to unexpected discovery.

"There is an ongoing demand for faster and faster information communication, which is our chance to show our skills. So far, our research has shown that we are working towards the creation of an ideal form that is fast and robust against interference, and enables free control of the spectrum."