Designation of communication physical slots is performed by a signal linkchannel assignment message on a Signalling Control Channel SCCH seeSection 11.4.1.3, sent from CS.. Basically, th
Trang 1on a Japanese cordless standard, which has become known as the PersonalHandyphone System (PHS) Disadvantages of conventional cellular telephonesystems, such as high costs of infrastructure and cell-planning resulting inhigh communication fees, had motivated the development of a less expensivesystem.
The review by the Telecommunications Technology Council , a ing organization for the Japanese Ministry of Posts and Telecommunications(MPT), and the technical study by the Research and Development Centrefor Radio Systems (RCR) of PHS started at the beginning of 1991 in Japan.The PHS air interface was then standardized through the publication of theRCR STD-28 [1], Version 1, in December 1993 Various field trials in theSapporo area in October 1993, and in the Tokyo area in April 1994, were thenconducted to prove the feasibility of PHS for various demands and services,respectively
consult-The PHS service was commercially launched in Japan by three tor groups—(NTT (Nippon Telegraph and Telephone) Personal Group, DDI(Daini Denden Inc.) Pocket Telephone Group and the Astel Group)—in July
opera-1995 The technology gained unprecedent popularity, with the number ofsubscribers reaching the six million mark by the beginning of 1997—just twoyears after its introduction As of August 1998, the number of subscribers wasabout 6.4 million [4] Attracted by the success in Japan, other Asian coun-tries, including Thailand and Hong Kong, and some South American countriesannounced 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)
Trang 2Table 11.1: Parameters of the Personal Handyphone System
Channel assign method DCA (with restrictions for control channels)Number of RF carriers 87 (incl 6 control and 4 guard channels)
Transmission rate/carrier 384 kbit/s
6.8 or 12.4 kbit/s signalling information
The aims of PHS span those of cordless and cellular systems, encompassing theidea of a low-cost wireless handset that can be used in both indoor and out-door environments to access fixed network supported services Like cordlesssystems (see Chapter 8), PHS provides access to private communication sys-tems for greater flexibility in the office environment or at home It also formsthe basis for public network access by subscribers moving with pedestrianspeed A microcellular structure has been adopted; thus the radio transmitpower 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 thesame service area is facilitated However, some restrictions concerning thefixed control frequencies still require a certain amount of frequency planning
Trang 3com-a supplementcom-ary service thcom-at should not restrict the ordincom-ary use of thesefrequencies, the direct communication must end within a time limit of threeminutes.
As PHS is a system for private and public use, PSs support two modes ofoperation, namely public and private operation modes The public operationmode enables the PS to access the public PHS service areas The privateoperation 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-registeredprivate systems
Terminal mobility allows the PS a smooth transition between public andprivate PHS services by selecting the appropriate mode on the PS This se-lection can also be done automatically (Automatic Selection Mode) Whenthe PS is within the range of both services, it will operate in a predeterminedmode 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 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, diversitysupport, and many more What is unique is the use of diversity, both for theuplink (by means of post-detection selection) and for the downlink (by means
to-of transmitter antenna selection), controlled by the CS The average (peak)
RF power depends on the kind of CS An outdoor standard-power CS, forexample, transmits at an RF power of 20 mW (160 mW)
11.3.1 Speech Coding
Similarly to DECT, a 64 kbit/s full-rate voice coding is employed first; thenthe signal is transcoded into 32 kbit/s ADPCM (Adaptive Differential PulseCode Modulation) based on the ITU-T Recommendation G.726
Trang 4Half-rate and quarter-rate voice coding methods are not specified atpresent, but a superframe configuration allows multiple codecs with lowerrates, down to 8 kbit/s, to be incorporated in the system when they becomeavailable.
The ADPCM compresses speech data without degrading speech quality.Performance of voiceband data transmission via modems is not significantlydegraded, 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 beused, such as delay detection, coherent detection and frequency discriminationdetection 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 hybridtime-division/frequency-division multiple-access (TDMA/FDMA) with time-division duplexing (TDD) The use of TDD also makes it possible to modifyservice 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 mission It enables high-speed wireless access to the ISDN network from the
trans-PS by assigning a second TCH in parallel
A TDMA frame has a length of 5 ms and carries 8 slots The first fourslots are downlink; the other four slots are uplink slots (see Table 11.1 andFigure 11.1)
The TDMA/TDD technology allows deviation from the allocation of pairedtransmit/receive channels, which is usually required in order to accomplishsymmetric two-way communication and is able to support asymmetric com-munication relationships TDMA/TDD is flexible, because it does not needpaired 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
Designation of communication physical slots is performed by a signal (linkchannel assignment message) on a Signalling Control Channel (SCCH) (seeSection 11.4.1.3), sent from CS The slot designation position is indicated bythe slot number, counted relatively to the first slot starting 2.5 ms after thesignal (link channel assignment message) has been received by the PS
An example is given in Figure 11.1: here the link channel assignment sage is transmitted in the first TDMA slot The PS waits 2.5 ms after thereception, and then starts to count the following slots until the relative valuemaps with that given in the message As a bidirectional channel is always
Trang 5mes-11.3 PHS Radio Characteristics 595
8 7 6 5 4 3 2 1
8 7 6 5 4 3 2 1 8 7
8 7 6 5 4 3
2
1 9 10 11 12 13 14 15 16 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 framelength (2.5 ms) later, corresponding to the TDD scheme
Thus the physical time slot number is specified by a combination of lute and relative slot number by the CS The TDMA slot number (SN) of acommunication carrier is obtained from the following equation:
The minimum cycle of the downlink logical control channel (LCCH) thatspecifies the slot position of the first repeated LCCH elements is specified asthe LCCH superframe All transmission/reception timing of physical slotsfor controlling intermittent transmission and SCCH uplink slot designation isgenerated based on the superframe structure
Elements (subchannels) of the downlink LCCH (see Section 11.4.1 for a tailed description) are the Broadcast Control Channel (BCCH), Paging Chan-
Channel (SCCH) and an optional User-Specific Control Channel (USCCH).The BCCH must be transmitted in the first slot of the LCCH superframewhereby the lead position of the superframe is reported In detail, this isdone by means of profile data contained in the radio channel information
temporarily steal LCCH elements, except for BCCH, and send some otherLCCH elements
The downlink logical control channel (LCCH) has the superframe structureshown in Figure 11.2 After each n TDMA frames, the CS intermittentlytransmits an LCCH slot (as discussed in Section 11.4.1, the CS does nottransmit control signals in each TDMA frame) The parameter m definesthe number of LCCH elements that have to be transmitted until all kinds ofinformation have been conveyed once
Trang 61 8
CRC 16 Guard 16
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 mit their first signalling message by using the Slotted Aloha protocol (seeSection 2.8.1), using an uplink SCCH slot, if they want a connection to beestablished
trans-11.3.4 Slot Structure
As a frame in PHS lasts 5 ms and each frame consists of 8 slots, each slot has
time-slot formats corresponding to different logical channels Basically, thereare 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 effective hardware, certain slots (and related traffic channels) might not beused (see Figure 9.50), PHS makes use of all its time slots Therefore allslot formats start with a 4-bit ramp time in which the PS or CS turns onits 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 commondata field for performing an ITU-T 16-bit CRC The start symbol is followed
Trang 7cost-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 physicalslots, 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 thereceiver, which is different for the downlink and uplink channels It also differsfor control physical slots and communication physical slots, so that it helps todistinguish between them Moreover, together with the CRC check, it is usedfor error detection As it is known in advance, the receiving party listeningfor 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 0001indicates that the time slot carries a fast associated control channel (FACCH).Slow associated control channels (SACCH) do not have a special channelidentifier since they are part of each communication physical slot, with theexception of an optional user-specific packet channel (USPCH), described inSection 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
point-to-multipoint channels such as the broadcasting control channel (BCCH)and the paging channel (PCH) only need to transmit a 42-bit CS identificationcode 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 Thisimplies that the information rate for logical control channels is either
62 bit/frame
34 bit/frame
16 bits, i.e., a burst carried in a slot is 224 bits in length
Trang 811.3.5 Radio-Frequency Band
The PHS band spans 26.1 MHz from 1893.5–1919.6 MHz Originally the frequency band allocated for PHS service spanned 23.1 MHz in the range of1.895–1918.1 MHz Because it was working close to capacity in hot-spot areas,
radio-an extension became necessary There are 87 carriers in the PHS bradio-and, with
a spacing of 300 kHz Figure 11.4 shows the relationship between the PHSfrequency band and the carrier numbers Control carriers for private use areassigned to 1898.450 MHz and 1900.250 MHz for Japan, and 1903.850 MHzand 1905.650 MHz for other countries In Japan, four control carriers arereserved for public use One control channel is assigned to each of the threePHS 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 sharedwith public use In addition, the first 10 carriers of private use (1895.15–1897.85 MHz) are also designated for direct communication between personalstations (Transceiver mode, Walkie Talkie)
11.3.6 Frequency Allocation
As Figure 11.4 shows, the PHS frequency band is not separately allocatedfor each operator, but is shared, with the exception of the channels that arepre-assigned as control channels dynamically by all PHS operators by means
of dynamic channel assignment (DCA) This is an autonomous decentralizedradio channel control technology, which enables efficient and flexible use offrequencies and time slots according to the local interference levels of CS andPS
In contrast to the DECT system, where it is the mobile’s task to select
channel establishment request or a TCH switching channel request (see tion 11.5.5.1), the PS asks for the assignment of a channel The cell station canautomatically pick up carriers at random and select an available carrier thathas no interference problem As a result of checking signal strength when acall is established, the CS renews an internal two-dimensional frequency–timematrix with available channels
Sec-If no carrier is available, the CS refuses the request The PS will thenautomatically request again This can be done up to three times; then the PShas to wait a certain time before another try is possible
11.3.7 Microcellular Architecture
PHS applies a microcellular architecture, which permits efficient spectrumutilization and the use of low-power handsets Thus long standby/talk timescan 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
Trang 911.3 PHS Radio Characteristics 599
18
(Japan) (Private Use) *2
50
60
40 38
69 70 71 72 73 74 75 76 78 79 80 81
Control Channel Direct
(Private Use) *2 (Japan)
82
Carrier number
Communication between PSs (Private Use)
1906.850 1906.250
*2 Used for communication carrier, outside Japan
*1 Frequency band for private use (public use possible)
87 frequencies Frequency spacing 300 kHz
(Private Use) *3 (Outside Japan) Control Channel
*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 000standard 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 inthe field, based on factors such as traffic demand and the presence of otheroperators serving the same area Cell stations with 10 mW of average powerare deployed at about 200 m spacing in downtown areas Wider separationbetween cell stations is possible using CSs with higher-power transmitters andextremely low-noise amplifiers
Trang 1011.3.8 Handover
In PHS, a handover is called channel switching During a call, both networkelements—the cell station and the personal station—observe the channel qual-ity This is done by evaluating the radio signal strength indicator (RSSI) andthe frame-error ratio The latter is indicated by detected errors in the cyclicredundancy check (CRC) in every time slot, and also unique word errors ifthere is a call in progress In response to deteriorating quality, either networkelement can initiate a handover
PHS distinguishes between two types of handover: one is called type and the other traffic channel switching-type
recalling-TCH switching-type If a high frame-error ratio is detected but the receivedsignal strength indicates that the terminal is still in the vicinity of theserving cell station, this is mostly due to rising interference Remedialmeasures can be taken by switching to another slot or frequency; thusthe serving cell station does not have to be changed To do so, theterminal transmits a TCH switching request message on the fast associ-ated control channel (FACCH) The cell station responds with a TCHswitching indication message, directing the PS to a new physical chan-nel Then the two stations exchange synchronization bursts on the newchannel and, if successful, resume communication In other systems thisfunction is called intracell handover
Recalling-type Here, change of the communication channel is done in such amanner that the handset establishes a connection in the same way as acall originating in a new communication cell This function allows thecall to be maintained whilst the traffic channel is switched between cellstations or interfaces as the personal station moves during the call Thestimulus for this kind of handover had been that the received radio signalstrength became too weak to maintain communications with the servingcell station Measuring signal strengths on common control channelsreceived from surrounding cells, the PS picks up a new CS Therefore ittransmits a link channel establishment request message to the chosen cellstation 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 thenew 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 col is applied, and the downlink’s access is restricted by the superframestructure As a result, seamless handovers cannot take place owing to ashort time gap
proto-According to [3], intercell handovers are performed to support nections at motorcar speed However, this is not guaranteed by theproviders If a new cell station is not available when the handover is
Trang 11con-11.4 PHS Radio Channel Structures 601
required, the communication channel is switched back to the previous
CS Therefore the communication channel is restored when returningquickly to the previous cell
The Personal Handyphone System supports traffic channels (TCHs) and trol channels (CCHs), as shown in Figure 11.5
con-11.4.1 Logical Control Channels (LCCH)
The basic structure for all kind of control slots, with exception of the ated control channel (ACCH), is a control slot as shown in Figure 11.6
ACCH (Associated Control Channel)
FACCH (Fast ACCH) SACCH (Slow ACCH)
(Control Channel)
CCH
Common Channels
Individual Channels UPCH (User Packet Channel)
Figure 11.5: Function channel structure
16 42
4
16 42
42 4 CI
28
62 PR
62
I (BCCH) 34
I (SCCH)
16 CRC
62 PR
identification Called station
R: Ramp (time)
SS: Start Symbol
UW: Unique Word PR: Preamble CAC: Common Access Channel
CI: Channel Identifier
Figure 11.6: Control physical slot structures
Trang 1211.4.1.1 Broadcast Control Channel (BCCH)
This is a one-way downlink channel for broadcasting control information fromthe cell station to the personal station It transmits information related tochannel structure, system information and restrictions The BCCH carriesgeneral system information to all terminals in a cell Cell stations establish asuperframe structure (see Figure 11.2) and inform personal stations by means
of messages transmitted on the BCCH (see Section 11.5.5.1) The transmission
of these paging messages is scheduled in a manner that enables terminals toturn off their receivers for a high proportion of the time Thus, corresponding
to the sleep mode of digital cellular systems, PHS defines a battery savingmode of operation for PSs that do not have a call in progress
This is a one-way downlink point-to-multipoint channel that simultaneouslytransmits identical information to individual cells or a wide area of multi-ple cells (the paging area) from the CS to PS (for more details see Sec-tion 11.5.5.1)
This is a bidirectional point-to-point channel that transmits informationneeded for call connection between CS and PS by means of messages (fordetailed information see Section 11.5.5.1) It transmits independent informa-tion to each cell Personal stations contend for access to the reverse directionrandomly, by means of the Slotted Aloha protocol
A User Packet Channel (UPCH) that can be set up on the control physicalslot is defined as the USCCH (see Figure 11.6) If it satisfies the specifieditems, the usage method is arbitrary This two-way channel for packet datacommunication of user information can be used for transmitting short mes-sages to and from personal stations that do not have calls in progress Theuplink channel is random-access
Trang 13synchro-11.4 PHS Radio Channel Structures 603
VOX Signal (Option)
This is a two-way channel that is associated with a TCH It carries out mission of control information and user packet data necessary for call control.The channel that is ordinarily auxiliary to TCH is called SACCH (Slow Associ-ated Control Channel ), and the channel that temporarily steals TCH capacityand carries out high-speed data transmission is called FACCH (Fast Associ-ated Control Channel ) SACCH and FACCH are mutually independent, andmay exist on the same physical slot
trans-Slow Associated Control Channel (SACCH) The SACCH conveys tional control signals between a cell station and a personal station re-lated to a call in progress
bidirec-Fast Associated Control Channel (FACCH) The FACCH can temporarilysteal slots from the traffic channel (these are the slots carrying the userinformation), in order to exchange time-critical control information, e.g.,for handover support
The UPCH that can be set up on the communication physical slot is defined
as USPCH If it satisfies the specified items, the usage method is arbitrary.The USPCH uses a service channel assigned to a specific terminal The time-slot structure of the USPCH (see Figure 11.7) resembles the communicationphysical slot structure, but with the difference that the USPCH does notcontain a SACCH field Instead, these 16 bits become part of the informationfield, now holding 176 bits