GB2313253A - Radio telephone - Google Patents

Abstract

A receiver for a radio telephone is disclosed, which is capable of receiving information signals comprising data slots disposed in a predetermined time relationship such as in a Time Division Multiplexed information signal for example. The receiver comprises a timer for determining when to receive a data slot intended for the receiver, and a data slot identification unit which identifies the received data slot as being a data slot intended for the receiver. The receiver also comprises a synchronisation initiation unit which initiates synchronisation of the timer with the information signal data structure when the data slot identification unit identifies the received data slot as not being intended for the receiver.

Description

RADIO TELEPHONE The present invention relates to a receiver and method for operation thereof for a radio telephone.

Radio telephones, in particular battery powered hand portable telephones, have been designed to be smaller and more lightweight than ever before. This is due to user requirements for small, lightweight mobile stations. However, users also require longer periods of use of the mobile stations before the battery requires replacing or recharging. This requirement is in conflict with the requirement that mobile stations are small and lightweight, since a battery's current capacity is proportional to its weight and/or volume. Thus, in order to increase the time period of operation of a mobile station between recharging or replacing of a battery, effort has been made to reduce the power consumption of the mobile station, as well as attempting to increase the power to weight (volume) ratio of the batteries for the mobile station.

For a mobile station there are two distinct modes of operation, each characterised by different power consumption levels. In the first mode, known as an "in call" mode, power consumption is high since the mobile will be periodically transmitting up to 0.8 watts of power for a Class 111 PDC hand portable for example. The second mode is the "standby" mode. In this mode the mobile station operates on a low current consumption for internal housekeeping of the mobile station such as a clock, only periodically "waking up" to receive information signals such as paging information from a base station of the radio telephone network. Even when receiving signals the power consumption of the mobile station is much lower than when it is transmitting.

In addition to periodically "waking up" to receive paging messages, the mobile station is also required to keep in synchronisation with signals transmitted by the the radio telephone network with which it is communicating or logged onto. In order to maintain synchronisation the mobile station periodically "wakes up" during standby mode and looks for synchronisation signals transmitted by the radio telephone network. This results in a further use of power during standby mode, and can impact significantly on the length of time a mobile station can operate in standby mode.

Typically, a user of a mobile telephone does not make many calls, and if they do they accept high power consumption. However, users are concerned with the length of standby time, and this is an important marketing feature of a mobile station.

Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows a schematic diagram of a superframe structure for the ARIB 27 system; Figure 2 shows a schematic diagram for a control channel structure; Figure 3 shows a schematic diagram for the control channel slot structure for the ARIB 27 system; Figure 4 shows message unit structures for the ARIB 27 system; Figure 5 shows a paging message bit pattern for the ARIB 27 system; Figure 6 is a schematic diagram of a radio telephone.

Figure 6 shows a typical radio telephone. The portable radio telephone shown in Figure 6 is a cellular telephone 1 powered by a rechargeable battery pack.

The telephone 1 includes a transceiver 8 comprising a receiver and transmitter and all the other features 9 conventionally found in a cellular telephone. Also, since these aspects of the telephone are not directly relevant to the instant invention no further details will be given here, except to say that a single microprocessor 4 is employed to control all the basic functions of the telephone 1 and to control the keypad and display functions.

Optionally, however, the telephone functions may be controlled by a master microcomputer, while the keypad and display functions are under the control of a separate slave microcomputer coupled to communicate with the master microcomputer.

The user-interface of telephone 1 comprises a display, e.g. a liquid crystal display 5, itself well-known in the art and a keypad 6 on the front of the telephone 1. The display is coupled to and regulated by the microprocessor 4 in the usual manner. The keypad 6 essentially comprises two main sets of keys, namely alpha numeric keys 6a associated with alpha numeric data especially for dialling telephone numbers, but also (optionally) for entering alphanumeric data into the telephone memories, e.g. a subscriber number index, and a set of function keys 6b for enabling various predetermined functions or operations.

The keys 6a are arranged in four rows of three keys each. As is conventional for the numeric key layout of a telephone, the top row comprises keys for numbers 1, 2 and 3 respectively, the second row down for numbers 4, 5 and 6 respectively, the next row down for numbers 7, 8 and 9 respectively, and the bottom row for *, 0 and # respectively. Some or all of these keys may also be associated with alphabet information, as again is quite conventional. The alphabetic rather than numeric data is selected for example by preceding the alphanumeric keystroke with another predetermined keystroke or set of keystrokes, specifically using the function keys. Hence the alphabetic data mode may be enabled for example by preceding the particular keystroke with previously depressing a "MEMORY" or "STORE" key disposed among the function keys 6b.

As is usual in cellular telephones, the keys 6b include a "SEND" and "END" key for respectively initiating and terminating a telephone call. Another key, specifically located in the top left-hand corner is an "ON/OFF" key for turning the telephone on and off, i.e. by connecting and disconnecting the battery pack power supply. Another of the function keys may be a menu or function key labelled, for example, "MENU" or "FUNCTION" or with a suitable abbreviation thereof. Depression of this key enables a variety of pre-set menus, the related instructions of which are stored in memory, to be viewed and selectively enabled. The various menus are selected by depressing the appropriate alphanumeric keys after depressing the "MENU" or "FUNCTION" key. The relevant menu is shown to the user in words or abbreviations on the display panel 5.

A typical radio telephone system, such as the Japanese ARIB 27 personal digital cellular system, is a Time Division Multiple Access (TDMA) system. In such a system there are allotted a sequence of time slots, each defining a data frame or burst of 6.6ms duration. The sequence format is continuously repeated, and a mobile station or radio telephone is typically assigned to at least one of the time slots in the format during which slot it receives or transmits information, whilst ignoring signals in the other time slots.

Bursts for each of three separate slots are interleaved, together with guard and ramp times to give a repeat rate for frames for a particular channel of 20ms. A further structure is specified called a superframe, which consists of a consecutive string of 36 of these frames. A superframe structure thus has a repeat rate of 720ms.

Figure 1 shows the superframe structure 102 for an ARIB-27 radio telephone system. Each superframe 102 comprises 36 (0 ...35) frames 106 of 20 ms duration and each frame 106 comprises three time slots 104 each of 6.6ms duration. From start 108 to finish 110 of a superframe 102 is 720 ms.

Physical channels, that is to say the actual communication channel, can be designated as either control channels or traffic channels. Traffic channels are used to carry speech data or other data between mobile station (MS) and base station (BS) whilst in a call. Control channels are used to exchange information between a mobile station and base station (both point to point and point to multipoint) whilst out of a call, or during the short period whilst setting up a call before a traffic channel is assigned.

A control channel has a structure which repeats every superframe. In this structure, individual frames are allocated to one of three possible logical channels in the downlink base station to mobile station direction. These three logical channels are called Broadcast Control Channel (BCCH), Signalling Control Channel (SCCH) or Paging Channel (PCH). All frames in the uplink direction are SCCH.

These channels are used to carry messages between a mobile station and base station. These messages are of arbitrary length, and (apart from BCCH) may spread across multiple frames and superframe boundaries. The three logical channels have the following purposes.

BCCH is used in the downlink direction only. It informs a mobile station of the allocation of logical channel frames within a superframe structure, and also other information such as network identity, location areas and neighbouring zones. The message contained within frames allocated as BCCH must be transmitted entirely within one superframe, and repeated every superframe.

SCCH is a Bi-directional channel and is used to send messages between the mobile station and base station when performing location registration procedure or when setting up a call.

PCH is used n the downlink direction only. It is used by the base station to page the mobile station when there is a call for it.

The slots labelled B in Fig. 2 are reserved for transmitting broadcast messages (BCCH) from a base station to radio telephones. The slots labelled P are paging channels (PCH), and those labelled S are Signalling Control Channels (SCCH).

The number of each type of slot and their position in the control channel are determined by the base station parameters Ab, Asl, Ap, As2 and Np which may vary from base station to base station, and network to network.

When a mobile station registers with a system or a new base station, it is assigned one or more slots 104 for receiving paging information. This is known as the paging control channel (PCH), and in Figure 2 is shown as slot 112 labelled P. Only one PCH channel is assigned for each mobile station. During standby mode the mobile station is in its quiescent or sleep state, and only housekeeping functions, such as timers, are operable for the mobile station.

The timing during the sleep state operates to activate the mobile station during each superframe 102 to enter a receive state co-incident with the PCH 11 2.

Typically, the receive state can account for between 50% to 80% of the power consumed during the standby mode.

The structure of slot or burst 104 is shown in Figure 3. The slot is 6.6ms in length and at each end 312, 314 there are ramp bits 304 and collision control bits 305 respectively. Additionally, there is a preamble 303. Data is split into two blocks 306, separated by a synchronisation (sync.) word 308 and a colour code 310. The sync. word 308 is a fixed sequence of bits different for respective slots and is used to adjust the timing of the mobile station to the incoming frame or burst 106. The colour code 210 is different for each channel and is again a fixed pattern of bits. The colour code 210 acts to inhibit co-channel interference. When a radio telephone first registers with a base station or system, it looks for a control message and uses the sync. word to sychronise itself with the system snd to match its frame timing with an incoming burst. It also records the colour code as indicative of the channel assigned for the radio telephone. The sync. word is the same for all frames in the superframe except for the first which has a different value.This different value is the superframe sync. word and is used to identify the start of the superframe.

For a paging control channel (PCH) 104, the data in blocks 306 contain information on whether a paging message is present in the PCH, and to which radio telephone the paging message is sent. Respective radio telephones are identified by their Mobile Station Identity (MSI), which is typically transmitted to the network when the radio telephone registers with the network and is assigned a PCH. The data in blocks 306 transmitted over the physical channel has first been encoded, interleaved and then scrambled in a well known manner by the network before being transmitted to the radio telephone. Thus, in order to obtain the paging information the data has to be de-scrambled, deinterleaved and decoded by the mobile station. Since, interleaving and consequently de-interleaving are performed on a burst by burst basis, the whole 6.6ms burst has to be received before the data can be retrieved.

The PCH frames are logically divided into Np groups, and a given mobile will only use the PCH frames that are allocated to its group. The mobile station's unique identifying number called its Mobile Subscriber Identity (MSI) is used by the mobile station to determine which one of the Np groups it belongs to. This assists in determining that there is an even spread of mobiles within each group.

When in standby, the mobile station operates such that it only actually receives the frames designated as PCH for its own particular group. At all other times, the mobile station is in a sleep state.

n normal standby operation, the mobile station ony receives PCH frames. It is able to use the sync. word in these frames to ensure that it is correctly synchronised with the base station. However, as it does not receive superframe sync. words, it must rely on the accuracy of its own clock and control mechanisms to stay in superframe synchronisation. If a slip in superframe sync. occurs, the mobile station starts looking at the wrong frame.

Unfortunately, the mobile station will be unaware of this and will not correct and rectify it. If such a slip occurs, the user of the mobile station may not be able to receive incoming calls whilst the situation persists.

In a typical implementation, this problem may be overcome by the mobile station actually receiving the superframe sync. word at a repetition rate much greater than the superframe time. This has the disadvantage of requiring the mobile station receiver to be on for a longer period, thus reducing the standby time of the mobile station.

In accordance with an embodiment of the present invention, the high level data that is contained within the PCH frames that the mobile station normally receives to determine whether it is being paged is also used to infer whether the mobile station remains in superframe synchronisation with the base station.

No or very little extra receiver current is drawn, resulting in there being no or little noticeable effect on the mobile station standby time.

When a superframe sync. loss condition is detected, the mobile station can take corrective action, which may take the form of resynchronising to the channel it is on, or reselecting the best available channel. In either case, the superframe sync. loss condition will be cleared.

As each frame contains a sync. word 308, the mobile station can be certain that if it slips superframe sync., it can only do so by slipping in multiples of whole frames. Any other time period would result in a frame sync. loss situation, which would be detected as a non-match in the frame sync. word. It will also receive the data contained in the frame that it has slipped to, beiieving it to be data that was contained within its own paging frame.

In order to determine whether a superframe sync loss has occurred, the mobile station detects whether data decoded from what it believes to be the PCH has in reality come from either a BCCH frame, SCCH frame or a PCH frame allocated to a group that differs from the mobiles own.

Each message has a structure as shown in Figure 4. Messages comprise one or more units 402, 404, 406, 408, 410 and 412. Each unit occupies exactly one frame. Messages can occupy one or more units. The most significant two bits 416 of the first octet 414 of each unit indicate whether this unit is the first (header") unit 404 of a message, the last ("tail") unit 408, or an intermediate unit 406. All messages must contain at least one header and one tail unit, although for a message that fits completely inside one unit 410, this one unit can be both head and tail. No message present units 412 are also available.

Multi-unit messages must be transmitted in consecutive frames within the same logical channel.

For a non-head unit the first bit, F1, is 0, and 1 for a head unit. For a non-tail unit the second bit, F2, is 0, and 1 for a tail unit. The remaining 6 bits 418 (WO) indicate the number of valid bytes for F2=0, and the number of remaining units for F2 = 1.

The only valid messages contained within PCH frames are "paging" messages.

Such messages can spread over more than one frame, but the start of the first unit will always have the structure shown in Figure 5. Those bits labelled "1" or "0" are fixed. The presence or absence of these bits in these positions indicate whether the received unit is part of a paging message or not. Those bits labelled "X" are "don't care" for the present invention and may be either "0" or "1" in a paging message.

The above structure can be used to detect whether header units received in the PCH contain data that is valid for the PCH. All header units must match the above pattern. If they do not, this indicates that the mobile station has actually received data from a BCCH or SCCH frame instead, and a superframe sync loss condition has occurred.

If the mobile station slips superframe sync, it is possible that it slips to a frame where the first received frame is not a header unit. To cater for this case, a mechanism is implemented whereby if a non-head unit is encountered, a decision on superframe synchronisation is not made immediately. Instead, if a defined number (in the current implementation 10) of consecutive non-head units is received, it is assumed that superframe sync loss has occurred. The reason behind this is that in a realistic system, the chances of frame losses due to poor signal conditions causing the mobile station to receive 10 consecutive non-head units without a single head unit on the PCH are very low. Such a situation occurring is thus almost certainly indicative of a superframe sync loss condition. If a head frame had been received, the mechanism described previously could be applied.

Where the mobile station slips superframe sync to a frame that contains paging messages for mobiles in a different group, the above cases will not detect the superframe sync loss. Instead the mobile station can use the data normally used to decide if the mobile station under consideration is being paged.

In this instance, the field marked "MSI" in figure 5 is the MSI of the mobile station that the paging message is addressed to. Normally, the mobile station compares its own MSI with the MSl(s) in this message. If a match occurs, the mobile station is being paged.

Paging messages transmitted by the base station in the PCH frames assigned to a particular PCH group will only contain MSls that are part of that group. This fact can be used by the mobile station to determine whether it is iri superframe sync. The mobile station calculates the group number of the MSl(s) in ail received paging messages. It then compares this group number with the group that its own MS is part of. If the two numbers differ, this indicates that the mobile station is no longer receiving PCH frames for its own group, and thus superframe synchronisation loss has occurred.

The above description relates to one particular application of this technique.

However, the general principle that high level data can be used to infer information about low level operations such as synchronisation could have wider application.

The scope of the present disclosure includes any novel feature or combination of features disclosed therein either explicitly or implicitly or any generalisation thereof irrespective of whether or not it relates to the claimed invention or mitigates any or all of the problems addressed by the present invention. The applicant hereby gives notice that claims may be formulated to such features during prosecution of this application or of any such further application derived therefrom.

Claims (12)

1. A receiver for a radio telephone, for receiving an information signal comprising data slots disposed in predetermined time relationship, the receiver comprising; timing means for determining receiving a data slot for the receiver, data slot identifying means for identifying if the data slot is for the receiver, and synchronisation initiating means for initiating synchronisation of the timing means with the information signal responsive to the identifying means identifying the received data slot as not being for the receiver.

2. A receiver according to claim 1, operative in a standby mode.

3. A receiver according to claim 1 or claim 2, wherein the identifying means determines characteristics of data in the received data slot.

4. A receiver according to claim 3, wherein the identifying means is adapted to determine if the data comprises paging information associated with the receiver.

5. A receiver according to claim 3 or 4, wherein the identifying means is adapted to determine if the data comprises a header unit.

6. A receiver according to claim 5, wherein the synchronisation initiating means is adapted to initiate synchronisation of the timing means with the information signal structure for the identifying means identifying a plurality of received data slots not respectively comprising header units.

7. A method for receiving an information signal comprising data slots disposed in predetermined time relationship, the method comprising; determining when to receive a data slot of interest, identifying if a received data slot is of interest; and synchronising to the information signal for the received data slot not being of interest.

8. A method according to claim 7, operative during a standby mode for a receiver.

9. A method according to claim 7 or claim 8, comprising determining characteristics of data in the received data slot.

10. A method according to claim 9, comprising determining if the data comprises paging information of interest and/or determining if the data comprises a header unit.

11. A receiver substantially as hereinbefore described with reference to the drawings.

12. A method substantially as hereinbefore described with reference to the drawings.

1 3. A radio telephone configured in accordance with any preceding claim.