Digital mobile unit having a function of issuing a tone level controllable input signal degradation alarm and a method thereof

In order to adaptively control the level of an alarm which warns of imminent or impending communication interruption, an error information signal and data frame energy, which are both derived during the decoding of an incoming signal, are used. If the incoming signal is deteriorated to an extent that communication interruption is imminent, an alarm signal is issued and superimposed on a voice signal. The level of the alarm signal is controlled in accordance with the detected frame energy.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to improvements in a digital mobile 
communications system, and more specifically to improved techniques for 
generating an alarm signal in a mobile unit when an incoming speech signal 
becomes deteriorated to a predetermined level. Still more specifically, 
the present invention relates to an arrangement and method of generating 
such an alarm signal whose level is controllable depending on input speech 
signal strength. 
2. Description of the Related Art 
It is known in the art to provide a mobile unit with means for generating 
an alarm signal if an incoming speech signal deteriorates due to noises 
induced during transmission. The alarm signal is added to or superimposed 
on a distant-end party's speech signal. Thus, a subscriber (viz., a 
near-end party) is advised or alerted, while talking with the distant-end 
party, that the communication interruption is imminent. By way of example, 
such a known technique is disclosed in the Japanese Laid-open Patent 
Application No. 3-182129 (Paper 1). 
The conventional alarm generator, however, is arranged to produce an alarm 
signal which has a level previously fixed to a predetermined value and 
thus, has suffered from the following drawbacks. That is, if an incoming 
speech signal exhibits a high level, the alarm signal may undesirably be 
masked by the speech signal thereby resulting in a subscriber being unable 
to perceive the alarm tone. On the contrary, if an incoming speech signal 
is low in strength, the alarm signal in turn becomes an obstacle to being 
able to hear the distant-end party's voice. In the worse case, the alarm 
signal prevents the distance party's voice from reaching the subscriber. 
The above mentioned two difficulties are respectively exacerbated if the 
alarm level has previously been set to a lower level and a higher level. 
Accordingly, what is desired is to be able to adaptively control the level 
of the alarm signal depending on the strength of an incoming speech 
signal. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a digital 
mobile unit which is able to adaptively control an incoming signal 
degradation indicative alarm depending on the strength of the signal. 
Another object of the present invention is to provide a method of 
generating an alarm which indicates an incoming signal degradation and 
whose level is changeable depending on the strength of the incoming 
signal. 
These objects are fulfilled by a technique wherein in order to adaptively 
control the level of an alarm which warns of imminent or impending 
communication interruption, an error information signal and data frame 
energy, which are both derived during the decoding of an incoming signal, 
are used. If the incoming signal is deteriorated to an extent that 
communication interruption is imminent, an alarm signal is issued and 
superimposed on a voice signal. The level of the alarm signal is 
controlled in accordance with the detected frame energy. 
More specifically, a first aspect of the present invention resides in a 
mobile telephone unit having an alarm signal generating function for 
audibly advising a subscriber of incoming speech signal deterioration, 
comprising: first means for deriving an error information signal and a 
speech energy value during decoding an incoming signal; second means for 
receiving the error information signal and the speech energy value, the 
second means checking the error information signal and issuing an alarm 
signal when the incoming signal is deteriorated to an extent that 
communication interruption is imminent, the second means controlling a 
level of the alarm signal in accordance with the speech energy value; and 
third means for superimposing the alarm signal on a speech signal derived 
from the incoming signal. 
A second aspect of the present invention resides in a method of audibly 
advising a subscriber of incoming speech signal deterioration in a mobile 
telephone unit, comprising the steps of: deriving an error information 
signal and a speech energy value during decoding an incoming signal; 
receiving the error information signal and the speech energy value; 
checking the error information signal and issuing an alarm signal when the 
incoming signal is deteriorated to an extent that communication 
interruption is imminent; controlling a level of the alarm signal in 
accordance with the speech energy value; and superimposing the alarm 
signal on a speech signal derived from the incoming signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before turning to a preferred embodiment of the present invention, it is 
deemed advantageous to briefly discuss a digital cellular system to which 
the instant embodiment is applicable. 
In view of the acute shortage of analog cellular capacity in large cities 
in the United States, the Telecommunication Industry Association (TIA), in 
association with the Electronic Industries Association (EIA), has 
developed a dual mode mobile-station/base-station compatibility standard 
(IS-54, December 1989) wherein a TDMA (time division multiple access) 
technique was adopted. Thereafter, April 1992, the EIA/RIA Interim 
Standard (IS-54-B) was released (hereinafter referred to as Paper 2). The 
proposed specification calls for a dual-mode mobile unit that will be 
allowed to access the base station whether it is an analog or a digital 
cellular system subscriber. Further, the aforesaid dual mode system is 
also discussed in a book entitled "Cellular Radio, Analog and Digital 
Systems" by Asha Mehrotra, published 1994 by Artech House (hereinafter 
referred to as Paper 3). 
Merely for a better understanding of the present invention, the present 
embodiment is described when being applied to the above mentioned dual 
mode system. However, it should be noted that the present invention is in 
no way limited to this particular application. 
Referring now to FIG. 1, there is schematically shown a mobile unit to 
which the present invention is applicable. 
A transmitted carrier, which carries speech and control data, is applied to 
a receiver 10 via an antenna 12 and a duplexer 14. At the receiver 10, the 
transmitted carrier is filtered, down-converted to an IF (intermediate 
frequency) signal, and demodulated using a .pi./4 DQPSK (digital 
quadrature phase shift keying) technique whereby the transmitted data is 
recovered. 
Reference is made to FIG. 2A which shows one TDXA frame format and to FIG. 
2B which shows a slot format from a base station to a mobile unit at 
portion (B). In FIG. 2B, the other slot format from a mobile unit to a 
base station is omitted in that this format is irrelevant to the present 
invention. 
As shown in FIG. 2A, each TDMA frame consists of six equal duration time 
slots 1-6, having 162 (=972/6) symbols (or 324 (=1944/4) bits). Each 
full-rate traffic channel utilizes two equally spaced time slots of the 
frame (slots 1 and 4, slots 2 and 5, or slots 3 and 6). 
In FIG. 2B, each time slot contains 12 bits of signaling message and this 
field is designated as SACCH (slow associated control channel). On the 
other hand, a CDVCC (coded digital verification color code) provides the 
same function as a SAT (Supervisory audio tones) in an analog cellular 
system. The function of the SAT is similar to the closing of a local loop 
in a conventional (viz., land) telephone system. 
Returning to FIG. 1, an FEC (frame error correction) decoder 16 is supplied 
with the baseband signal from the receiver 10. The FEC decoder 16 
implements error correction on the inputted baseband signal using known 
techniques such as CRC (cyclic redundancy checks), convolutional decoding, 
etc. If the FEC decoder 16 is unable to correct the errors, the decoder 16 
generates an error information signal (depicted by EIS). The error 
information signal EIS, which includes frame data correct/incorrect 
information and an estimated number of error bits within each frame, is 
applied to a microcomputer 18. 
As discussed in detail in Papers 2 and 3 which are hereby incorporated by 
reference thereto, a frame energy value (depicted by R(0) or R0) is 
computed and encoded once per frame at a base station. This frame energy 
value R(0) reflects the average signal power in the input speech over 20 
ms. How to compute the frame energy value R(0) at the base station is 
known and described in detail in Paper 2 (pages 33-34). Hence, further 
descriptions thereof is omitted for simplifying the instant disclosure. 
The frame energy value R(0) is encoded into five (5) bits, incorporated 
into the data field DATA in each slot (see FIG. 2B), and transmitted to 
the mobile unit of FIG. 1. The PEC decoder 16 extracts the frame energy 
code R(0) and applies same to the microcomputer 18. 
The FEC decoder 16, after carrying out error correction on the speech data, 
applies same to a VSELP (vector sum-excited linear predictive) speech 
decoder 20. The speech data supplied to the decoder 20 includes various 
parameter codes including the aforesaid frame energy code R(0). 
Subsequently, the output of the VSELP speech decoder 20 is applied to an 
adder (or superimposer) 22. 
As shown in FIG. 1, the microcomputer 18 is coupled to a key pad 24 and a 
suitable display 26 such as an LCD (liquid crystal display). As is well 
known, the key pad 24 functions as an interface between a subscriber and 
the mobile unit of FIG. 1. 
As referred to above, the microcomputer 18 receives the error information 
signal EIS and the frame energy code R(0) both from the FEC decoder 16. 
Further, as mentioned above, the signal BIS includes the frame data 
correct/incorrect information and the estimated number of error data bits 
in each frame. The microcomputer 18 computes an error bit rate using the 
estimated number of error bits. Subsequently, the microcomputer 18 
ascertain, based on the calculated error bit rate and the frame data 
correct/incorrect information, a situation wherein the communication 
interruption is imminent. 
In the above discussion, the frame data correct/incorrect information can 
be omitted from considering the imminent communication interruption. It is 
known in the art that if the bit error rate exceeds about 3%, it is highly 
probable for the communication interruption to occur. However, in view of 
a mobile unit being used in a variety of different environments, the 
threshold for use in determining issuance of the alarm may be settled 
through various field tests. 
In any way, if the communication interruption is threatening, the 
microcomputer 18 issues two control signals CS1 and C92. 
The control signal CS1 indicates that the subscriber should be notified of 
the possibility of communication blockage, while the other control signal 
CS2 indicates a tone alarm level which depends on the above mentioned 
speech data frame energy. 
An alarm generator 28 responds to the control signal SC1 and generates an 
alarm signal having a predetermined level. On the other hand, a level 
controller 30 adjusts the alarm level in response to the control signal 
SC2. The alarm signal issued from the alarm generator 28 is adaptively 
(viz., suitably) controlled in terms of the signal level depending on the 
frame energy. The level controller 30 is well known in the art and 
incorporated in a digital signal processor (DSP) and thus the detailed 
description thereof is deemed unnecessary. 
The alarm signal thus level adjusted is added to or superimposed on the 
speech data from the decoder 20 at the adder 22. The output of the adder 
22 is converted into the corresponding analog signal at an D/A 
(digital-to-analog) converter and passes through an amplifier 32 to a 
speaker 36. 
A transmit section of the mobile unit of FIG. 1 includes a microphone 40, 
an audio amplifier 42, an A/D (analog-to-digital) converter 44, a speech 
encoder 46, an FEC encoder 48, and a transmitter 50, all of which are 
arranged as shown. Since the present invention is irrelevant to the 
transmit section, further description thereof is redundant and accordingly 
omitted for simplifying the disclosure. 
It will be understood that the above disclosure is representative of only 
one possible embodiment of the present invention and that the concept on 
which the invention is based is not specifically limited thereto.