A View of PHS being standarized by PHS MoU Group
By Hiroshi Kawai, NWG Chairperson, PHS MoU Group
(Director, Corporate Planning & Development, Development Headquarters, WILLCOM,
Inc.)
1.Introduction
PHS MoU Group is a nonprofit organization (NPO) to conduct the PHS
diffusion, promotion, standardization activities etc. It's so-called standardization organization with total
74 member companies at home and abroad and trying to make more advanced technologies available at more lower
cost by diffusing the PHS to increase the scale merit.
At the TWG (Technical Working GroupjKunming, China Meeting held in November
2005, standardization of high-speed and large capacity PHS (Next Generation PHS) towards the wireless
broadband era was approved to discuss and at the 19th PHS MoU Group General Meeting held in Fukuoka, Japan
in March 2006, it's approved to newly establish an exclusive working group NWG (Next Generation PHS Ad-hoc
Working Group) for discussing about the Next Generation PHS.
This time, we'll introduce an outline and the future view of the Next Generation PHS which is now being
standardized.
2.Worldwide PHS Diffusion
PHS has already been introduced in more than thirty countries/areas (including trial)
worldwide as the public, private or FWA (Fixed Wireless Access) network. (Refer to Fig.1)
China is the country where the PHS has been diffused most in the world. As of
October 2006, already a little more than 93 million people are using PHS in China.
Japan is next to China in terms of the PHS subscribers and there are a little
less than 4.9 million users.
The decisive factor for PHS's success in China is its low price. PHS service
is being provided at as low price as the fixed telephone and its tariff is settled lower than other mobile
telephones (GSM etc.) due to the oneway billing system etc.
Total PHS worldwide subscribers have already exceeded 100 million mainly in Asian
countries/areas such as China, Japan, Taiwan etc. In Japan, WILLCOM launched flat-rate voice service and its
subscribers are still steadily growing and further, according to the Chinese statistics, 500 million people
will move from rural parts to urban parts from now on so that further diffusion of the PHS is expected.
3.Concept and Features of PHS
PHS (Personal Handyphone System) has originally been developed based on digital
cordless telephone taking the FMC (Fixed Mobile Convergence) concept prior to its practical use so that it
can be used as handset unit of the digital cordless telephone or wireless PBX (Private Branch eXchange)
indoors and as mobile telephone outdoors. Further, it can also be used in rural area as FWA in substitution
for fixed telephone as well.
PHS features different from the mobile telephone are as follows.
Both PS (Personal Station) and CS (Cell Station) have small radio emission power so
that the reach of radio waves from CS to PS (= cell radius) is also small, generally, cell radius is 500meters
or so and it's considerably small compared with the mobile telephone system. Mobile telephone system is
basically utilizing macrocell system which enables economic and early expansion of service area, however, it
may take quite many years to establish PHS nationwide covering area because of its microcell system.
Further, as for the mobile telephone system, site location is strictly designed
according to each area condition such as antenna location, frequency bandwidth, radio emission power level
etc. Therefore, antenna relocation, addition and destruction to deal with the traffic change based on
subscriber increase and decrease are not so easy. On the other hand, PHS is utilizing the self-dispersion
system which basically doesn't require site locating design. Therefore, CS relocation, addition and
destruction can easily be done according to the traffic change because its DCA (Dynamic Channel Assignment)
system enables highest quality channel assignment by judging the whole channels' vacant situation and
interference situation instantly when PS is connecting with CS (CS receives PS's CR (Connection Request)).
And further, an availability of overlapped site locating for multiple CSs in the same area is peculiar to PHS.
In case of macrocell system, data speed per user is apt to decrease due to its large
cell radius and concentration of many traffics on one CS. On the other hand, microcell system has small cell
radius and self-dispersion system so that data speed per user can be maximized and stabilized because the
traffics are dispersed among multiple CSs.
Microcell/Self-dispersion system has overwhelmingly high frequency utilization
efficiency compared with macrocell system so it must be the only one technology which can establish a large
capacity network.
4.PHS Function Expansion
As described above, PHS has many features different from other mobile telephone
systems and positive discussion towards further expansion of the PHS function is being conducted in TWG by
operators and vendors at home and abroad.
(1)Raising Data Speed up to 1Mbps
PHS is utilizing 4 slot TDMA (Time Division Multiple Access) system and its π/4
Shift QPSK (Quadrature Phase Shift Keying) modulation system's data speed is 32Kbps/slot. WILLCOM has
realized 128Kbps/carrier data speed by using all 4 slots (4x). Further, PHS has been designed to be realized
by relatively low cost circuits compared with other mobile telephones placing stress on low consumption power,
easiness of terminal development and taking those advantages, 256Kbps data speed has been realized by using 8
slots (8x) with multiple transceivers in single terminal.
Recently, data speed competition with the mobile telephones is becoming keen so that
PHS's data speed has also been raised up to 400Kbps equivalent/carrier by introducing new modulation systems
(8PSK (Phase Shift Keying), 16QAM (Quadrature Amplitude Modulation), 32QAM, 64QAM) in addition to π/4 Shift
QPSK.
Data speed-up more than 1Mbps can also be realized by further multi-carrierization and
adoption of multi-value modulation system. (Refer to Fig.2)
(2)Microcell Standardization Is in Progress
In case of BPSK (Binary Phase Shift Keying) modulation system, data volume per symbol
becomes smaller than π/4 Shift QPSK, however, cell radius can be increased because it's robust against
interference. By adopting BPSK etc., link budget improvement by over 10 dB and nearly same cell radius with
other mobile telephone systems are expected to be realized.
WILLCOM is already promoting the introduction of new CSs with this modulation system
or new technology for microcellization. Besides Japan, microcellization trend is the same in China as well.
New standard ″Turbo PHS″ was proposed by WILLCOM and now being discussed
in TWG. This ″Turbo PHS″ is covering problems specific to the Chinese PHS such as the network
congestion measures, especially, paging channel congestion measures for SMS or microcell realization system
etc.
Towards the realization of microcell, BPSK, halfrate technologies etc. are now being
standardized led by WILLCOM, UTStarcom etc. in TWG.
(3)China Will Finally Be Introducing the Packet Communication Service
Packet communication service launched by WILLCOM in Japan in 2001 will finally be
introduced in China as well. As for packet communication itself, it has already been standardized by TWG,
however, an exclusive standards study group was reestablished in TWG and the discussion of optimizing it to
the Chinese PHS network is being conducted in order to make it developed in China. This standards study
group is led by three major Chinese PHS system integrators, UTStarcom, Lucent Technologies and ZTE and
further, other domestic vendors were also joined and the real Japan-China joint standardization is being
progressed at a fever pitch. Voice and SMS (Short Messaging Service) are the mainstream of the present Chinese
PHS service, however, when introducing packet communication service, new business developments such as MMS
(Multimedia Messaging Service), contents distribution etc. seem to be also promising so that a promotion of
such services is now being discussed in BWG (Business Working Group) as well.
5.What Is the Next Generation PHS
In final report of the Japanese Ministry of Internal Affairs and Communications′
Wireless Broadband Promotion Study Group released in December 2005, the Next Generation PHS is placed as one
of the MBWA (Mobile Broadband Wireless Access) candidate systems.
MBWA is defined as the system enabling IP (Internet Protocol) level continuous
connection as well as effective high-speed communications instantaneously, and besides having middle-speed
mobility.
Next Generation PHS is a large capacity and high-speed communication system utilizing
OFDMA (Orthogonal Frequency Division Multiple Access) technology adopted in many wireless broadband systems
after 3G (3rd Generation mobile telephone system) while keeping various features different from other mobile
telephone systems.
Next Generation PHS′s features are as follows.
(1)Next Generation PHS′s Features
• Transmission Speed Higher than 3G
‾ To achieve over 20Mbps data speed
• Coexistence with The Existing PHS
‾ TDD (Time Division Duplex) System
‾ Vertically Symmetric Frame Structure (Refer to Fig.3)
‾ 19.2MHz (Crystal Oscillator Frequency)
• Succession of the Existing PHS′s Advantages
‾ Microcell (For high frequency utilization efficiency)
‾ Self-dispersion System (For high system expansion flexibility)
• Maximum Utilization of Smart Antenna Technology
‾ Adaptive Array Antenna
‾ Space Diversity Technology (SDMA (Space Division Multiple Access), MIMO
(Multiple Input Multiple Output))
(2)Adoption of OFDMA Technology
Next Generation PHS is utilizing OFDMA technology which is watched with interest as
the next generation wireless technology and also utilized in wireless broadband systems after 3G etc. as
well.
OFDMA technology is to divide high-speed signal into multiple carriers (each divided
carrier is called as sub-carrier) and to transmit them in parallel after having transformed them into
low-speed signals so that the frequency utilization efficiency can be increased because those sub-carriers
can be arranged closely without interference even some of them may be overlapped. The narrower the sub-carrier
interval is, the lower the multi-path influence, however, it needs a large-scale hardware and complicated
calculation due to the increased sub-carrier numbers.
In case of the Next Generation PHS, sub-carrier interval is to be decided based on
crystal oscillator frequency (19.2MHz) of the existing PHS′s PS in order to be able to coexist with
that. By this, PHS/Next Generation PHS Dual-Mode PS using one crystal oscillator can easily be developed.
An example of the Next Generation PHS's OFDMA parameters is as follows. (Refer to
Fig.4)
Sub-carrier interval of 37.5KHz is slightly wide compared with other OFDMA based
wireless broadband systems. This is to minimize the FFT (Fast Fourier Transform) number for small hardware
circuit size and as a result, power consumption can be reduced.
Guard interval of 3.33μs is narrow compared with other OFDMA based wireless
broadband systems. This is because the other wireless broadband systems are based on macrocell system,
however, the Next Generation PHS is on microcell.
(3)NWG′s Role and Standardization Trends
NWG is composed of the following three SWGs (Sub Working Groups).
• SWG1 : Standardization of the Next Generation PHS
• SWG2 : Preparation for validation testing of the Next Generation
PHS
• SWG3 : Investigation of Wireless Broadband Technology
SWG1 is proceeding with standardization of the Next Generation PHS and has already
completed ″Next Generation PHS Standards-Ver.01″ and it has been approved officially at the
20th PHS MoU Group General Meeting held in Hanoi, Vietnam in August 2006. Further, this officially
standardized Next Generation PHS system has also been added to ITU Recommendation as one of the wireless
broadband systems. SWG1 is now actively conducting meetings to standardize the MAC (Media Access Control)
layer based on Next Generation PHS network architecture proposed by WILLCOM, to lower the PS′s power
consumption, to discuss about the domestic LSI vendors' proposal on low cost PS standards etc.
SWG2 is now preparing for validation testing of the Next Generation PHS based on its
approved standards. In Japan, WILLCOM has already started such validation testing from January 2006. At
present, primary testing has been completed and the secondary testing is ongoing. At the primary testing,
propagation characteristics of the newly adopted OFDMA were mainly validated and at the secondary testing,
parameters of the Next Generation PHS standards based on PHS/OFDMA merged technology such as TDMA,
Self-dispersion, Smart Antenna etc. are being validated. Further, transmission speed of over 20Mbps has
already been achieved at the secondary testing. However, validation testing of the Next Generation PHS led by
the PHS MoU Group will certainly be significant to the countries which have already introduced or are planning
to introduce the PHS for their considering the future migration to the Next Generation PHS, so that SWG2′s
role for accomplishing its aim is greatly expectable.
5.Conclusion
Japan originated PHS has achieved brilliant results in China and those two countries
are getting more harmonized to aim at leaping towards the future world market. The PHS MoU Group will
actively be conducting diffusion promotion and standardization of the PHS aiming at several hundred million
potential PHS subscribers worldwide. The PHS MoU Group's role should be quite important in order for the PHS
to become a real global standard system.
♣♣♣The Above Document Will Be Formally Released On 2007 New Year Issuance Of ARIB Circulation.♣♣♣
