11 Personal Handyphone System (PHS) ∗ 11.1 Development of the Personal Handyphone System in Japan At the end of the 1980s, two cordless telephone systems, CT2 and DECT, had entered the second step of their development process. At that time, Japan had not yet developed any comparable technology; therefore work began in 1989 on a Japanese cordless standard, which has become known as the Personal Handyphone System (PHS). Disadvantages of conventional cellular telephone systems, such as high costs of infrastructure and cell-planning resulting in high communication fees, had motivated the development of a less expensive system. The review by the Telecommunications Technology Council, a consult- ing organization for the Japanese Ministry of Posts and Telecommunications (MPT), and the technical study by the Research and Development Centre for Radio Systems (RCR) of PHS started at the beginning of 1991 in Japan. The PHS air interface was then standardized through the publication of the RCR STD-28 [1], Version 1, in December 1993. Various field trials in the Sapporo area in October 1993, and in the Tokyo area in April 1994, were then conducted to prove the feasibility of PHS for various demands and services, respectively. The PHS service was commercially launched in Japan by three opera- tor groups—(NTT (Nippon Telegraph and Telephone) Personal Group, DDI (Daini Denden Inc.) Pocket Telephone Group and the Astel Group)—in July 1995. The technology gained unprecedent popularity, with the number of subscribers reaching the six million mark by the beginning of 1997—just two years after its introduction. As of August 1998, the number of subscribers was about 6.4 million [4]. Attracted by the success in Japan, other Asian coun- tries, including Thailand and Hong Kong, and some South American countries announced plans to establish PHS networks. ∗ With the collaboration of Matthias Siebert Mobile Radio Networks: Networking and Protocols. Bernhard H. Walke Copyright © 1999 John Wiley & Sons Ltd ISBNs: 0-471-97595-8 (Hardback); 0-470-84193-1 (Electronic) 592 11 Personal Handyphone System (PHS) Table 11.1: Parameters of the Personal Handyphone System Frequency band 1893.5–1919.6 MHz Access method TDMA/TDD Channel assign. method DCA (with restrictions for control channels) Number of RF carriers 87 (incl. 6 control and 4 guard channels) Carrier spacing 300 kHz Frame duration 5 ms Number of slots/frame 8 Modulation π/4 DQPSK Output power (average) CS: 500 mW or less PS: 10 mW or less Traffic channels/transceiver 3 (resp. 4) Transmission rate/carrier 384 kbit/s Net bit rate/channel 32 kbit/s user data 6.8 or 12.4 kbit/s signalling information 11.2 System Overview The aims of PHS span those of cordless and cellular systems, encompassing the idea of a low-cost wireless handset that can be used in both indoor and out- door environments to access fixed network supported services. Like cordless systems (see Chapter 8), PHS provides access to private communication sys- tems for greater flexibility in the office environment or at home. It also forms the basis for public network access by subscribers moving with pedestrian speed. A microcellular structure has been adopted; thus the radio transmit power can be much smaller than that of existing cellular telephone systems. As described for DECT, by means of dynamic channel allocation (DCA), cell station engineering is simplified and a multi-operator environment in the same service area is facilitated. However, some restrictions concerning the fixed control frequencies still require a certain amount of frequency planning in advance. Various services are supported. Apart from telephone services, the PHS air interface supports voiceband data and facsimile communications at rates up to 9.6 kbit/s. Also, there is the capability of data transmission at rates of 32/64 kbit/s. According to [5] the upper limit of data transmission has been raised to 128 kbit/s, using four channels simultaneously. An overview of the parameters of the Personal Handyphone System is given in Table 11.1. Like CT2 (see Section 8.1), PHS defines only two network elements: termi- nal and base station. The nomenclature for a base station is cell station (CS) and a PHS terminal is referred to as a personal station(PS). 11.3 PHS Radio Characteristics 593 11.2.1 Personal Station (PS) Similarly to DECT (see Chapter 9), personal stations transmit with an aver- age (peak) RF power of 10 mW (80 mW). As a special feature, comparable to the TETRA system (see Section 6.3), PHS also specifies direct radio com- munication between a pair of terminals. The direct mode of communication does not need the intervention of cell stations (CSs). As shown in Figure 11.4, ten frequency carriers are designated for such a use. However, since it is only a supplementary service that should not restrict the ordinary use of these frequencies, the direct communication must end within a time limit of three minutes. As PHS is a system for private and public use, PSs support two modes of operation, namely public and private operation modes. The public operation mode enables the PS to access the public PHS service areas. The private operation mode enables a PS to access private systems like a wireless PBX or the home digital cordless system. The same PS can be used at home and in the office by selecting a relevant private mode within the pre-registered private systems. Terminal mobility allows the PS a smooth transition between public and private PHS services by selecting the appropriate mode on the PS. This se- lection can also be done automatically (Automatic Selection Mode). When the PS is within the range of both services, it will operate in a predetermined mode. Optionally, an automatic dual-mode operation is possible, where the PS can be paged from both the public system and the private system when it is within the range of both. 11.2.2 Cell Station (CS) A cell station consists of CS equipment and antennas. These two parts to- gether create a microcell service area with a radius of several hundred metres. Cell stations control most of the tasks concerning the air interface. These in- clude dynamic channel allocation (DCA), superframe establishment, diversity support, and many more. What is unique is the use of diversity, both for the uplink (by means of post-detection selection) and for the downlink (by means of transmitter antenna selection), controlled by the CS. The average (peak) RF power depends on the kind of CS. An outdoor standard-power CS, for example, transmits at an RF power of 20 mW (160 mW). 11.3 PHS Radio Characteristics 11.3.1 Speech Coding Similarly to DECT, a 64 kbit/s full-rate voice coding is employed first; then the signal is transcoded into 32 kbit/s ADPCM (Adaptive Differential Pulse Code Modulation) based on the ITU-T Recommendation G.726. 594 11 Personal Handyphone System (PHS) Half-rate and quarter-rate voice coding methods are not specified at present, but a superframe configuration allows multiple codecs with lower rates, down to 8 kbit/s, to be incorporated in the system when they become available. The ADPCM compresses speech data without degrading speech quality. Performance of voiceband data transmission via modems is not significantly degraded, and non-speech service can be supported. 11.3.2 Modulation PHS employs a π/4-shifted DQPSK modulation with a roll-off factor of 0.5. This modulation scheme permits a variety of demodulation techniques to be used, such as delay detection, coherent detection and frequency discrimination detection. Furthermore, the use of the DQPSK modulation method enables a higher spectrum utilization efficiency compared with GMSK modulation. 11.3.3 Access Method In common with DECT, the access method applied in PHS is hybrid time-division/frequency-division multiple-access (TDMA/FDMA) with time- division duplexing (TDD). The use of TDD also makes it possible to modify service bit rates. The latest ARIB Standard, RCR STD-28 Version 3, issued in November 1997, lists the specifications for PHS 64 kbit/s digital data trans- mission. It enables high-speed wireless access to the ISDN network from the PS by assigning a second TCH in parallel. A TDMA frame has a length of 5 ms and carries 8 slots. The first four slots are downlink; the other four slots are uplink slots (see Table 11.1 and Figure 11.1). The TDMA/TDD technology allows deviation from the allocation of paired transmit/receive channels, which is usually required in order to accomplish symmetric two-way communication and is able to support asymmetric com- munication relationships. TDMA/TDD is flexible, because it does not need paired bands and both the lower and upper ends of the spectrum can easily be expanded to accommodate needs as in RCR STD-28 Version 3. 11.3.3.1 Communication Physical Slot Designation Method Designation of communication physical slots is performed by a signal (link channel assignment message) on a Signalling Control Channel (SCCH) (see Section 11.4.1.3), sent from CS. The slot designation position is indicated by the slot number, counted relatively to the first slot starting 2.5 ms after the signal (link channel assignment message) has been received by the PS. An example is given in Figure 11.1: here the link channel assignment mes- sage is transmitted in the first TDMA slot. The PS waits 2.5 ms after the reception, and then starts to count the following slots until the relative value maps with that given in the message. As a bidirectional channel is always 11.3 PHS Radio Characteristics 595 87654321 8765432187654321 87654321 161514131211109 87654321 161514131211109 Relative slot number from personal station’s point of view uplink reception Slots for downlink/ Link channel assignment message (first slot) The slot designation for the CS corresponding to the PS’s slot position is shifted half a frame Absolute TDMA slot number Relative slot number from cell station’s point of view Slots for downlink/ uplink transmission 2.5 ms Figure 11.1: Example of relative slot numbers made up of a pair of slots, the second slot is defined to follow half a frame length (2.5 ms) later, corresponding to the TDD scheme. Thus the physical time slot number is specified by a combination of abso- lute and relative slot number by the CS. The TDMA slot number (SN) of a communication carrier is obtained from the following equation: TDMA SN = {(absolute SN + relative SN − 2)mod4} + 1 (11.1) 11.3.3.2 PHS Superframe Structure The minimum cycle of the downlink logical control channel (LCCH) that specifies the slot position of the first repeated LCCH elements is specified as the LCCH superframe. All transmission/reception timing of physical slots for controlling intermittent transmission and SCCH uplink slot designation is generated based on the superframe structure. Elements (subchannels) of the downlink LCCH (see Section 11.4.1 for a de- tailed description) are the Broadcast Control Channel (BCCH), Paging Chan- nels (PCH, P 1 − P k : number of paging groups = k), the Signalling Control Channel (SCCH) and an optional User-Specific Control Channel (USCCH). The BCCH must be transmitted in the first slot of the LCCH superframe whereby the lead position of the superframe is reported. In detail, this is done by means of profile data contained in the radio channel information broadcasting message (see Section 11.5.5.1). If necessary, it is possible to temporarily steal LCCH elements, except for BCCH, and send some other LCCH elements. The downlink logical control channel (LCCH) has the superframe structure shown in Figure 11.2. After each n TDMA frames, the CS intermittently transmits an LCCH slot (as discussed in Section 11.4.1, the CS does not transmit control signals in each TDMA frame). The parameter m defines the number of LCCH elements that have to be transmitted until all kinds of information have been conveyed once. 596 11 Personal Handyphone System (PHS) 187 2 81 65 332 54 1 2 1 4 2 3 4 1 1 3 3 1 2 5 ms TDMA frame 2 65 Slots used by CS Downlink Uplink LCCH superframe ms downlink intermittent transmission cyclen ms Downlink logical control channel (LCCH) m Uplink logical control channel (LCCH) n nm 5 5 Figure 11.2: PHS superframe 240 bit, 625 PR R CRC 4 Guard 1616 32 µs (b) Control physical slot (a) Communication physical slot (traffic slot) SS 2 R 4 CI 4 CRC 16 Guard 16 SS 2 Unique 16 4 CI SACCH 16 Information 160 Word Word Unique 42 or 70 Header 62 or 34 6 PR Information 62 Figure 11.3: PHS time-slot formats For the uplink, no superframe structure is defined. Personal stations trans- mit their first signalling message by using the Slotted Aloha protocol (see Section 2.8.1), using an uplink SCCH slot, if they want a connection to be established. 11.3.4 Slot Structure As a frame in PHS lasts 5 ms and each frame consists of 8 slots, each slot has a length of 625 ➭s in which 240 bits can be arranged. PHS specifies several time-slot formats corresponding to different logical channels. Basically, there are two categories: control physical slots used by common control channels (CCHs), and communication physical slots used by traffic channels (TCHs); see Figure 11.3. In contrast to the DECT standard, where, as a concession to more cost- effective hardware, certain slots (and related traffic channels) might not be used (see Figure 9.50), PHS makes use of all its time slots. Therefore all slot formats start with a 4-bit ramp time in which the PS or CS turns on its transmitter and a 2-bit start symbol for establishing the phase of the re- mote demodulator. At the end of each slot format, there is another common data field for performing an ITU-T 16-bit CRC. The start symbol is followed 11.3 PHS Radio Characteristics 597 by a preamble, a layer-1 signal pattern used to establish bit synchronization. Its number of bits depends on the kind of slot format. Together, start sym- bol and preamble are repetitions of the pattern 1001. With control physical slots, a preamble of 62 bits is used to allow synchronization for each slot in- dependently; with communication physical slots, a preamble of 6 bits serves to update the synchronization established in the previously transmitted slots. Additionally, all time slots carry a unique word, known in advance by the receiver, which is different for the downlink and uplink channels. It also differs for control physical slots and communication physical slots, so that it helps to distinguish between them. Moreover, together with the CRC check, it is used for error detection. As it is known in advance, the receiving party listening for its dedicated slot either detects it or not (unique word detection error). The channel identifier follows right after the unique word. It is similar to the flag bits in GSM bursts. For example, the channel identifier sequence 0000 indicates that the time slot carries user information (TCH), while 0001 indicates that the time slot carries a fast associated control channel (FACCH). Slow associated control channels (SACCH) do not have a special channel identifier since they are part of each communication physical slot, with the exception of an optional user-specific packet channel (USPCH), described in Section 11.4.2.4. The most important data field in Figure 11.3 (a) is the information field, which carries the user data. For telephone services such as voice transmission it consists of 160 bits from an adaptive differential pulse code modulation (ADPCM) encoder. Therefore, in common with DECT, the bit rate carried by communication physical slots (with the exception of USPCH) is 160 bit/frame 0.005 s/frame = 32 kbit/s (11.2) Control physical slots have headers containing addresses. Broadcasting point-to-multipoint channels such as the broadcasting control channel (BCCH) and the paging channel (PCH) only need to transmit a 42-bit CS identification code. Thus the information field contains 62 bits. On the other hand, bidirec- tional point-to-point channels such as the signalling control channel (SCCH) need to specify a CS identification code (42 bits) and a PS identification code (28 bits). In these channels the length of the information field is 34 bit. This implies that the information rate for logical control channels is either 62 bit/frame 0.005 s/frame = 12.4 kbit/s or 34 bit/frame 0.005 s/frame = 6.8 kbit/s (11.3) Finally, all slot formats have a guard time of 41.7 ➭s, corresponding to 16 bits, i.e., a burst carried in a slot is 224 bits in length. 598 11 Personal Handyphone System (PHS) 11.3.5 Radio-Frequency Band The PHS band spans 26.1 MHz from 1893.5–1919.6 MHz. Originally the radio- frequency band allocated for PHS service spanned 23.1 MHz in the range of 1.895–1918.1 MHz. Because it was working close to capacity in hot-spot areas, an extension became necessary. There are 87 carriers in the PHS band, with a spacing of 300 kHz. Figure 11.4 shows the relationship between the PHS frequency band and the carrier numbers. Control carriers for private use are assigned to 1898.450 MHz and 1900.250 MHz for Japan, and 1903.850 MHz and 1905.650 MHz for other countries. In Japan, four control carriers are reserved for public use. One control channel is assigned to each of the three PHS operators, and the remaining one is set aside as a spare channel. In order to protect the public control channels from adjacent channel interferences, they are enclosed by guard channels. Currently the spectrum for public use is 15 MHz. The spectrum for private use is 11.1 MHz, which can be shared with public use. In addition, the first 10 carriers of private use (1895.15– 1897.85 MHz) are also designated for direct communication between personal stations (Transceiver mode, Walkie Talkie). 11.3.6 Frequency Allocation As Figure 11.4 shows, the PHS frequency band is not separately allocated for each operator, but is shared, with the exception of the channels that are pre-assigned as control channels dynamically by all PHS operators by means of dynamic channel assignment (DCA). This is an autonomous decentralized radio channel control technology, which enables efficient and flexible use of frequencies and time slots according to the local interference levels of CS and PS. In contrast to the DECT system, where it is the mobile’s task to select a suitable channel, in PHS this is done by the cell station. With a link channel establishment request or a TCH switching channel request (see Sec- tion 11.5.5.1), the PS asks for the assignment of a channel. The cell station can automatically pick up carriers at random and select an available carrier that has no interference problem. As a result of checking signal strength when a call is established, the CS renews an internal two-dimensional frequency–time matrix with available channels. If no carrier is available, the CS refuses the request. The PS will then automatically request again. This can be done up to three times; then the PS has to wait a certain time before another try is possible. 11.3.7 Microcellular Architecture PHS applies a microcellular architecture, which permits efficient spectrum utilization and the use of low-power handsets. Thus long standby/talk times can be realized. In cooperation with PHS, large numbers of cell stations can be deployed without planning. (In fact, this is only true for traffic channels. 11.3 PHS Radio Characteristics 599 18 (Japan) (Private Use) *2 50 60 40 38 69 70 71 72 73 74 75 76 77 78 79 80 81 Control Channel Direct Public Use (Private Use) *2 (Japan) 82 Carrier number Communication between PSs (Private Use) Control Channel 251 252 255 1893.650 .950 1 5 10 12 30 36 1905,950 1900,250 1898,450 Control Channel 1 Control Channel 2 Control Channel 3 Spare Control-ch. Public Use 1897,850 1896.350 1894.850 1905.650 37 20 1900.850 1903.850 1919.450 1917.950 1917.350 1916.750 1916.150 1915.850 1915.550 1912.850 1909.850 1906.850 1906.250 Public Use *2 Used for communication carrier, outside Japan *1 Frequency band for private use (public use possible) 87 frequencies Frequency spacing 300 kHz Private Use *1 (Private Use) *3 (Outside Japan) Control Channel frequency Carrier [MHz] number Carrier Carrier [MHz] frequency *3 Used for communication carrier within Japan Figure 11.4: PHS frequency table Concerning the fixed control channels, frequency planning still has to be done). For example, NTT launched a PHS service in July 1995 with some 25 000 standard cell stations in Tokyo, using a conventional small-cell structure, 150– 200 m, with antennas mounted on public telephone boxes. The microcellular technology allows easy addition and removal of CSs in the field, based on factors such as traffic demand and the presence of other operators serving the same area. Cell stations with 10 mW of average power are deployed at about 200 m spacing in downtown areas. Wider separation between cell stations is possible using CSs with higher-power transmitters and extremely low-noise amplifiers. 600 11 Personal Handyphone System (PHS) 11.3.8 Handover In PHS, a handover is called channel switching. During a call, both network elements—the cell station and the personal station—observe the channel qual- ity. This is done by evaluating the radio signal strength indicator (RSSI) and the frame-error ratio. The latter is indicated by detected errors in the cyclic redundancy check (CRC) in every time slot, and also unique word errors if there is a call in progress. In response to deteriorating quality, either network element can initiate a handover. PHS distinguishes between two types of handover: one is called recalling- type and the other traffic channel switching-type. TCH switching-type If a high frame-error ratio is detected but the received signal strength indicates that the terminal is still in the vicinity of the serving cell station, this is mostly due to rising interference. Remedial measures can be taken by switching to another slot or frequency; thus the serving cell station does not have to be changed. To do so, the terminal transmits a TCH switching request message on the fast associ- ated control channel (FACCH). The cell station responds with a TCH switching indication message, directing the PS to a new physical chan- nel. Then the two stations exchange synchronization bursts on the new channel and, if successful, resume communication. In other systems this function is called intracell handover. Recalling-type Here, change of the communication channel is done in such a manner that the handset establishes a connection in the same way as a call originating in a new communication cell. This function allows the call to be maintained whilst the traffic channel is switched between cell stations or interfaces as the personal station moves during the call. The stimulus for this kind of handover had been that the received radio signal strength became too weak to maintain communications with the serving cell station. Measuring signal strengths on common control channels received from surrounding cells, the PS picks up a new CS. Therefore it transmits a link channel establishment request message to the chosen cell station on a signalling control channel (SCCH). In answer to this, the CS responds with a link channel assignment message, also transmitted on SCCH, and both stations start synchronizing their operations on the new channel. To perform this kind of intercell handover, both PS and CS need access to a signalling control channel. For the uplink, a Slotted-ALOHA proto- col is applied, and the downlink’s access is restricted by the superframe structure. As a result, seamless handovers cannot take place owing to a short time gap. According to [3], intercell handovers are performed to support con- nections at motorcar speed. However, this is not guaranteed by the providers. If a new cell station is not available when the handover is [...]... messages needed for radio control, mobility control and call control in link channels as in service channels 11.5.1 Radio- Frequency Transmission Management (RT) RT incorporates radio resources management functions and also signal encryption The radio frequency transmission entity RT has functions related to management of radio resources These functions include radio zone selection, radio line setup, maintenance,... Indication (CS → PS, DL) This message is transmitted from CS to PS to report zone information Messages Pertaining to Connection Release (Transmitted on SACCH/ FACCH) PS Release (CS → PS, DL) This message is transmitted from CS to PS in order to unilaterally release the radio channel Radio- Channel Disconnect (CS → PS, DL) CS transmits this message to PS to release the radio channel Radio- Channel Disconnect... Call proceeding L3 Service channel establishment phase L3 Authentication request Authentication response L2 L2 L1 L1 L3 L3 Communication phase Radio channel disconnect L2 L2 Radio channel disconnect L1 L1 Complete Management Management Figure 11.11: Basic structure of signals Management Network Layer Radio- Freqency Transmission Management RT Mobility Management MM Call Control CC Protocol Discriminator... used to indicate the fact that the radio channel was released and as a response to Radio- Channel Disconnect After PS transmits this message, it enters standby state Messages Pertaining to Connection Establishment (Transmitted on SACCH/FACCH) Condition Inquiry (CS → PS, DL) This message is transmitted from CS to PS for querying about the reception level of the local zone and peripheral zones Condition... end of the communication phase, the control plane (management) is used to disconnect the call; see Figure 11.11 11.5.5 Call Establishment The call connection phase consists of the link channel establishment phase for establishing the link with the radio interface, and the service channel establishment phase for establishing the radio link for telephone service such as voice transmission and non-telephone... transmitted from CS to PS to autonomously report a condition, and from PS to CS to respond to a condition inquiry Encryption Control (CS ↔ PS, DL, UL) This messages is transmitted in either direction to indicate operation or stopping of the encryption function Encryption Control Acknowledgement (CS ↔ PS, DL, UL) This message is transmitted in either direction as an acknowledgement of encryption control... and re-request CS for link channel establishment after a link channel assignment message has been received from CS Messages for Broadcasting (Transmitted on BCCH) Radio Channel Information Broadcasting (CS → PS, DL) CS must broadcast the radio channel structure to PS using this message This includes 11.5 Network Operations 611 information about the downlink LCCH superframe structure (see Section 11.2),... establishment phase to select a channel (henceforth referred to as the service channel ) with the capacity required for providing service and to select the protocol type required in the communication phase Ordinary call control (CC), higher-level mobility management (MM) and radio- frequency transmission management (RT) functions are performed in this phase Messages are transmitted by means of an associated... systems 11.5.3.2 Call Clearing Messages DISConnect (CS ↔ PS, DL, UL) This message is transmitted from PS to request call clearing from CS, or from CS in order to describe that the call was disconnected RELease (CS ↔ PS, DL, UL) This message is transmitted in one direction from either PS or CS It shows that the equipment that transmitted this message has already disconnected the traffic channel Further... similar to DECT and GSM, PHS formally defines three network control protocols: 1 Radio- Frequency Transmission Management (RT) 2 Mobility Management (MM) 3 Call Control (CC) These layer-3 standards specify procedures for establishing, maintaining, switching, releasing network connections, PS location and authentication at the radio interface of the Personal Handyphone System These procedures apply to messages . assignment Setup Call proceeding Authentication request Authentication response Radio channel disconnect Complete Radio channel disconnect Figure 11.11: Basic. related to management of radio resources. These functions include radio zone selec- tion, radio line setup, maintenance, switching and disconnection functions.