The Personal Handyphone System (PHS) is a wireless technology that offers high-quality, low-cost mobile telephone services using a fully digital system operating in the 1.9- GHz range. Originally developed by NTT, the Japanese telecommunications giant, PHS is based on the Global System for Mobile (GSM) Telecommunications standard. PHS made its debut in Japan in July 1995, where service was initially offered in metropolitan Tokyo and Sapporo. Although PHS was originally developed in Japan, it is now considered a pan-Asian standard.
Advantages over Cellular
PHS phones (Handyphones) operate at 1.9 GHz, whereas cellular phones operate at 800 MHz. To achieve high voice quality, PHS uses a portion of its capacity to support a highperformance voice-encoding algorithm called Adaptive Differential Pulse Code Modulation (ADPCM). With this algorithm, PHS can support a much higher data throughput (32 kbps) than a cellular-based system, enabling PHS to support fax and voice-mail services and emerging multimedia applications such as high-speed Internet access and photo and video transmission.
PHS supports the handoff of calls from one microcell to the next during roaming. However, PHS goes a step farther than cellular by giving users the flexibility to make calls at home (just like conventional cordless phones), at school, in the office, while riding the subway, or while roaming through the streets. PHS also gives subscribers more security and complete privacy. And unlike cellular phones, PHS phones cannot be cloned for fraudulent use.
Another key advantage of PHS over cellular is cost—PHS can provide mobile communications more economically that cellular. Through its efficient microcell architecture and use of the public network, startup and expansion costs for operators are minimized. As a result, total per-subscriber costs tend to be much lower than with traditional cellular networks.
Because PHS is a “low tier” microcellular wireless network, it offers far greater capacity per dollar of infrastructure than existing cellular networks, which results in lower calling rates. In Japan, the cost of a 3-minute call using a PHS handset is comparable to the cost of making that same call on a public phone.
Handsets
Not only are PHS handsets extremely small and lightweight— almost half the size and weight of cellular handsets—but the battery life of PHS handsets is superior to that offered by cellular handsets. PHS phones output 10 to 20 milliwatts, whereas most cellular phones output between 1 and 5 watts. Whereas the typical cellular handset has a battery life of 3 hours talk time, the typical PHS handset has a battery life of 6 hours talk time.
Whereas the typical cellular handset has a battery life of 50 hours in standby mode, the typical PHS handset has a battery life of 200 hours in standby mode (more than a week). The low-power operation of PHS handsets is achieved through strict built-in power management and “sleep” functions in individual circuits.
Applications
There are a number of applications of PHS technology. In the area of mobile telecommunications, users can establish communications through public cell stations, which are installed throughout a serving area. PHS phones also can be used with a home base station as a residential cordless telephone at the Public Switched Telephone Network (PSTN) tariff.
When used in the local loop, PHS provides the means to access the PSTN in areas where conventional local loops— consisting of copper wire, optical fiber, or coaxial cable—are impractical or not available. PHS also can be adopted as a digital cordless PBX for office use, providing readily expandable, seamless communications throughout a large office building or campus.
Users carry PHS handsets with them and are no longer chained to their desks by their communications systems. As a digital system, PHS provides a level of voice quality not normally associated with a cordless telephone. In addition, the digital signal employed by PHS provides security for corporate communications, and the system’s microcell architecture can be reconfigured easily to accommodate increases or decreases in the number of users.
Another benefit of PHS is the ease and minimal expense with which the entire network can be dismantled and set up again in another facility if, for example, a business decides to relocate its offices. For personal use as a cordless telephone at home, PHS is a low-cost mobile solution that allows the customer to use a single handset at home and out of doors with a digital signal that provides improved voice quality for a cordless phone and enough capacity for data and fax transmissions that are increasingly a part of users’ home communications.
Network Architecture
The PHS radio interface offers four-channel Time Division Multiple Access with Time Division Duplexing (TDMA/TDD), which provides one control channel and three traffic channels for each cell station. The base station allocates channels dynamically and is not constrained by a frequency reuse scheme, thus deriving the maximum advantage of carrier-switched TDMA.
This means that PHS handsets communicating to a base station may all be on different carrier frequencies. The PHS system uses a microcell configuration that creates a radio zone with a 100- to 300-meter diameter. The base stations themselves are spaced a maximum of 500 meters apart. In urban areas, the microcell configuration is capable of supporting several million subscribers.
This configuration also makes possible smaller and lighter handsets and the more efficient reuse of radio spectrum to conserve frequency bands. In turn, this permits very low transmitter power consumption and, as a result, much longer handset talk times and standby times than are possible with cellular handsets.
A drawback of the lower operating power level is the smaller radius that a PHS base station can cover: only 100 to 300 meters, versus at least 1500 meters for cellular base stations. The extra power of cellular systems improves penetration of the signals into buildings, whereas PHS may require an extra base station inside some buildings. Another drawback of PHS is that the quality of reception can diminish significantly when mobile users are traveling at a rate greater than 15 miles per hour.
Since PHS uses the public network rather than dedicated facilities between microcells, the only service startup requirements are handsets, cell stations, a PHS server, and a database of services to support PHS network operation. With no separate transmission network needed for connecting cell stations and for call routing, carriers can introduce PHS service with little initial capital investment.
Service Features
PHS is a feature-rich service, giving Handyphone users access to a variety of call-handling features, including:
- Call forwarding To a fixed line, to another PHS phone, or to a voice mail box.
- Call waiting Alerts the subscriber of an incoming call.
- Call hold Enables the subscriber to alternate between two calls.
- Call barring Restricts any incoming local or international call.
- Calling line identification (CLI) Displays the number of the incoming call, informing the subscriber of the caller’s identity.
- Voice mail The subscriber receives recorded messages, even when the phone is busy or turned off.
- Text messaging Enables the subscriber to send and receive text messages through the PHS phone.
- International roaming Enables subscribers to use their PHS phones in another country but be billed by the service provider in their home country.
Depending on the implementation progress of the service provider, the following value-added data services also may be available to Handyphone users:
- Virtual fax Enables subscribers to retrieve fax messages anywhere, have fax messages sent to a Handyphone, or have them redirected to any fax machine.
- Fax By attaching the Handyphone to a laptop or desktop computer, subscribers can send and receive faxes anywhere.
- E-mail/Internet access Allows subscribers to retrieve email from the Internet through the Handyphone.
- Conference calls Enables subscribers to talk to as many as four other parties at the same time.
- News, sports scores, and stock quotes Enables subscribers to obtain a variety of information on a real-time basis.
Many other types of services can be implemented over PHS. There is already the world’s first consumer-oriented videophone service in Japan. Kyocera Corp. offers a mobile phone able to transmit a caller’s image and voice simultaneously. Two color images are transmitted per second through a camera mounted on the top of the handset. The recipient can view the caller via a 2-inch active matrix liquid- crystal display (LCD). Since the transmission technology sends data at only 32 kbps, however, this makes for jerky video images.
While 32-kbps channels are now available in Japan, research is now under way to achieve a transmission rate of 64 kbps through the combined use of two channels. With this much capacity, PHS can be extended to a variety of other services in the future, including better-quality video.
In combination with a small, lightweight portable data terminal, PHS also may be used to realize the concept of “mobile network computing,” whereby users would access application software stored on the Internet. With the limited memory and disk storage capacity of the PHS terminals, the applications and associated programs would stay on the Internet, preventing the PHS devices from becoming overwhelmed.
