Key Measures and Technologies for Advanced PHS

  As reported in the latest issue of the PHS MoU Group News (No.34, August 2001), the Advanced PHS Committee, subordinate of the Telecommunications Council (an advisory body to the Japanese Government), compiled the key findings of its study on advancing the PHS systems as a report in June 2001. Key measures and technologies for advanced PHS in the report are introduced in this and the following issues.


Addition of modulation systems

    1. Outline

Upon consideration of advanced PHS, it would be desirable to add to the existing (π/4 shift-QPSK (Quadrature Phase Shift Keying) i) highly efficient modulation system for improving efficiency of the frequency use per band and ii) modulation system with high resistance against interference, but which is less efficient in frequency use per band.

    2. Modulation systems to be added

Desirable modulation systems to be added are as follows:
In addition to π/4 shift-QPSK,



Public system:	BPSK, QPSK, 8PSK, 12QAM, 16QAM, 24QAM, 32QAM
Private system:	BPSK, QPSK, 8PSK, 12QAM, 16QAM, 24QAM, 32QAM, 64QAM, 256QAM

    3. Features and Signal Space Diagram of each modulation system

Features and Signal Space Diagram for above listed BPSK (Bi-Phase Shift Keying), π/4 shift-QPSK, 8PSK (8-positions Phase Shift Keying) and 16QAM (16-positions Quadrature Amplitude Modulation) are as follows:

(1) BPSK

[Features]
Transmits 1 bit/symbol information. In comparison with the existing /4 shift-QPSK, resistance against noise and interference has been improved.


Fig. 1 Signal Space Diagram of BPSK


(2) π/4 shift-QPSK

[Features]
Although resistance against noise and interference is high and circuit design is easy, this can only transmit 2bits/symbol information. A symbol at a given time takes any one of four signal points (Fig. 2 (a)), however, the next symbol takes any one of four signal points shifted by π/4 (Fig. 2 (b)), this resembling the Signal Space Diagram of 8PSK at first glance. By differential encoding for deciding signal points using phase differences, a delay detection is enabled.


Fig2. Signal Space Diagram of π/4 shift-QPSK


(3) 8PSK

[Features]
This can transmit 3 bits/symbol information, 1.5 times larger than those of the π/4 shift-QPSK, namely, the efficiency of frequency use per band can be improved by 1.5 times.


Fig. 3 Signal Space Diagram of 8PSK


(4) 16QAM
[Features]
This can transmit 4 bits/symbol information, 2 times larger than those of the π/4 shift-QPSK, namely, the efficiency of the frequency use per band can be improved by 2 times. However, upon making positions multiple, resistance against noise and interference may be deteriorated.


Fig. 4: Signal Space Diagram of 16QAM

Adaptive modulation system

    1. Outline

Improvement in frequency use efficiency for land mobile communications is becoming one of the most important issues for the future.
The π/4 shift-QPSK system currently used for PHS etc. has advantages, including easy circuit design, fading resistant etc.; however, it can transmit only 2 bits/symbol information.
On the other hand, the 16QAM system currently used for digital MCA, land mobile communications systems, etc. can transmit 4bits/symbol information. Compared with the π/4 shift-QPSK system, the 16QAM has improved efficiency in frequency use per band.
However, if the number of bits is increased, signal intervals of Signal Space Diagram become narrower, resulting in weak resistance against fading, interference waves and noise. Accordingly, service areas will become limited. (See Fig. 5)


Fig. 5 Adaptive Modulation System

    2.Introduction of adaptive modulation system

Thus, when a mobile station is close to the cell station and the radio wave propagation conditions are good, data communication is conducted at a high speed using highly efficient modulation system like the 16QAM system. As the radio wave propagation conditions become poor, modulation system that can achieve good transmission characteristics even in more severe environments shall be selected, such as 8PSK, π/4 shift-QPSK, BPSK, etc.

Although data transmission volume per second decreases when using a modulation system with lower efficiency, which transmits less data volume than those of the π/4 shift-QPSK (e.g., BPSK), effective data transmission volume (throughput) can be maintained at a high level in an environment with poor radio wave propagation conditions (e.g., indoors or in a shadow of the building), thanks to resistance against interference/noise and reduced frame errors.

The adaptive modulation system which selects an optimum modulation system according to the radio propagation conditions can use frequency resources more effectively than in the case where the π/4 shift-QPSK is fixed to be used, because the throughput can be improved without widening frequency bandwidths or increasing transmission power. Fig. 6 shows an example in which the adaptive modulation system is employed.

Fig. 6 Service Area Employing Adaptive Modulation System

Fig. 7 shows a comparison of the adaptive modulation system and the π/4 shift-QPSK fixed modulation system. In both cases for the ƒÎ/4 shift-QPSK fixed modulation system where the reception level is high with fluctuations (shown in Fig.7(a)) and conditions for radio wave propagation paths are very excellent (shown in Fig.7 (b)) throughputs cannot be improved, however, the adaptive modulation system (shown in Fig.7 (c)) can improve throughputs.

Under poor radio wave propagation conditions with low reception level, data errors may occur in case of Fig.7 (b) and, as a result, data could be lost. In case of Fig.7 (c), through use of the BPSK with low efficiency, although data transmission speeds are slow, data are not lost and throughput deterioration can be maintained at a minimum level.

Fig. 7 Adaptive Modulation System



   The adaptive modulation system can improve throughputs when the radio wave propagation conditions are excellent, and prevents loss of data by lowering data transmission speeds when the radio wave propagation conditions become poor. As shown above, the adaptive modulation system, which can maintain high throughputs, can be said as effective means for improving the efficiency of the frequency use.