1. Field of the Invention
This invention relates to a method of and apparatus for reducing power consumption of portable radio communication systems, such as mobile telephones, in standby mode to increase the time between necessary battery charges and recharges.
2. Discussion of Related Art
Cellular mobile telephone systems comprise a network of base stations, each covering a particular geographical area or cell, that communicate with the plurality of mobile or hand-portable phones (hereinafter "mobile phones"). Such systems contain means to ensure as far as possible that the nearest base station is used to communicate with each mobile phone, thus minimizing the transmitter power needed in the mobile phone.
When a mobile phone is in a standby mode, i.e., when it is neither originating nor receiving a call, it must listen to the nearest base station for calls. Not all of the receiver section need be operative in this standby mode and none of the transmitter section need be operative for the mobile phone does not transmit in the standby mode. This results in longer battery life, typically around eight hours as compared to the one or two hours of battery life when the mobile phone is in active conversation and both the transmitter and receiver operate.
The demand to obtain ever longer battery operating life is clear. European Patent No. EP 0 473 465 to Harte describes a method of reducing standby current consumption. This method is based on calling channel transmission formats used by base stations in conformance with the British ETACS standard or to the U.S. EIA-553 cellular standard. In the latter system, 40-bit calling messages to mobile phones in the standby mode are transmitted by base stations in a format that includes five repeats of the 40-bit blocks. A calling message identifies a called mobile phone by including its telephone number, also known as the Mobile Identification Number (MIN), in the message. Each message also contains a cyclic redundancy check (CRC) code, whose value depends on the data bits, that can be used to verify the correct decoding and even to correct single bit errors in the message. Each 40-bit message consists of 28 data bits and 12 CRC bits. Because the MIN is 34 bits long, it takes two such calling messages to identify the phone uniquely. 40-bit blocks conveyed at one time are renamed "control words" or just "words," and a calling message consists of two such words. Whenever any message consists of more than one word, a continuation bit is set in all words except the last to indicate that further words will follow.
In the aforementioned European patent, Harte proposes a phone that would decode each message repeat independently as it is received, carry out a check for correct decoding using the CRC, and if correct decoding is indicated by the CRC check, check whether the MIN of the receiving mobile phone is contained in the received word. If the MIN is not in the received word, as is the case for the vast majority of messages, then the mobile phone powers down until the next set of five message repeats is due, according to the European patent. Thus, according to the European patent, the mobile phone could enter a reduced power mode for the remaining four message repeats whenever the first of the repeats is deemed not to contain the mobile's MIN, representing a potential savings of at least 80% of the battery energy consumed in the standby mode. Additionally, when the mobile phone has already identified from the first of the two control words of a message that the MIN does not match that of the receiving mobile phone, the mobile phone does not need to process the second word, and can power down for all five repeats of the second word. This results in a potential power savings of 90% in the standby mode.
On the other hand, a disadvantage of the device disclosed in the European patent is that when a message is intended for the mobile phone, the probability of detecting the message correctly is reduced if the mobile phone is at maximum range, such as on the periphery of a cell where signal levels are likely lowest. The reason for this is that the method of decoding used in the disclosed system requires three out of the five message repeats to be correctly decoded. The power savings method results in a lower probability of decoding the intended message by using just one decoded message out of the five repeated messages. In fact, the method disclosed in the European patent reduces the probability of decoding an intended message accurately below that of mobile phones not implementing the method. Thus, a trade-off between the intended message reception reliability and power consumption in the standby mode is made when using the method disclosed in the European patent.
A reason for the reduced performance is that the five-message repeat structure transmitted according to the U.S. EIA-553 cellular standard is not optimally designed for decoding in the manner disclosed in the aforementioned European patent. The normal method of decoding messages in a U.S. EIA-553 cellular system requires reception of all five message repeats in order to implement bit-wise majority vote decoding, and thus the scope for implementing the power saving method disclosed in the European patent is not clear.
When using redundant coding such as repeat coding with majority vote decoding, it is desirable to space repeat bits as far as apart as possible in time so that they are subject to uncorrelated fading. This known as bit interleaving. The furthest apart each repeat can be theoretically placed by a bit-interleaver is the length of one complete message. Therefore, maximally bit-interleaved repeat coding results in a transmission that appears to be five repeats of the message in succession. However, the optimum decoding is not to decode each message, repeat independently, as used in the prior art, but to decode each bit by means of a 5-fold majority vote process before performing CRC checking on the majority-decoded bits.
Therefore, the five message repeats sent in accordance with the EIA-553 standard should in fact be regarded as five repeats of each bit, a 5-fold majority vote being employed to decode each bit before attempting to perform the CRC check on each 40-bit word of the message. However, this requires that all five word-repeats be received, which conflicts with the basic method disclosed in the European patent for reducing power consumption.