Cordless telephone system having automatic control of transmitter power and frequency in response to changing conditions

A communications system includes at least two communications units, each communication unit including a transmitter capable of transmitting to the other unit at different power levels and on different frequencies, the power levels and frequencies of transmission being controlled by a mode control unit in response to indicators of transmission quality and reliability, wherein the mode control unit initially establishes a minimum power output of the transmitter at a fixed frequency to establish communications and if reliable communications cannot be maintained, increasing the level of output power of the transmitter until reliable communications are established as indicated by signals returned from another unit in the communications system, and wherein the mode control unit changes the output frequency of the transmitter from a single frequency mode to a time division spread spectrum mode if the required power output level of the transmitter exceeds a first predetermined threshold.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to telephone systems and more particularly to 
telephone systems including telephone devices capable of cordless handset 
operation remote from a base station. 
BACKGROUND OF THE INVENTION 
The Federal Communications Commission ("CFCC") has established a cordless 
telephone service in the 900 Mhz frequency band and has established rules 
regarding power levels for transmitters employed in such cordless 
telephone systems and spectrum spreading techniques (frequency hopping) 
depending on preestablished power levels. Under the rules established by 
the FCC, a first power level of a maximum of one milliwatt (50 .mu.v/m 
measured at 3 m) is established for single frequency operation of cordless 
telephone systems in the 900 Mhz band. If the transmitting power of each 
telephone unit does not exceed one milliwatt of power, a single frequency 
within the allocated frequency band may be used for communication between 
the base station and the remote unit. 
If for reliable communication between the base station and the remote unit 
a power level exceeding milliwatt but less than a maximum of one watt is 
required, a spectrum spreading or "frequency hopping" technique must be 
employed to minimize interference with other devices using the frequency 
band. 
A dilemma is presented which requires choices to be made as between low 
power single frequency operation of the cordless telephone system which 
might provide spectrum space for a larger number of simultaneous users of 
the frequency band or a high power spread spectrum technique which 
provides for reliable communication between the base station and the 
remote unit at greater distances but with the disadvantage that fewer 
users can reliably use the allocated frequency band without interference 
from the higher power transmissions. Further, the higher power 
transmissions in the spread spectrum technique may also cause interference 
to users employing a low power single frequency transmission system. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to communicate between 
a base station and a remote unit in a telephone system employing automatic 
control of power level and frequency of transmission based on indicia of 
quality of communication between the remote cordless unit and the base 
station. 
Accordingly, a communications system includes at least two communications 
units, each communication unit including a transmitter capable of 
transmitting to the other unit at different power levels and on different 
frequencies, the power levels and frequencies of transmission being 
controlled by a mode control unit in response to indicators of 
transmission quality and reliability, wherein the mode control unit 
initially establishes a minimum power output of the transmitter at a fixed 
frequency to establish communications and if reliable communications 
cannot be maintained, increasing the level of output power of the 
transmitter until reliable communications are established as indicated by 
signals returned from another unit in the communications system, and 
wherein the mode control unit changes the output frequency of the 
transmitter from a single frequency mode to a time division spread 
spectrum mode if the required power output level of the transmitter 
exceeds a first predetermined threshold. 
Another unit in the communications system has all the features and 
functions of the first unit described above and all units also include a 
receiver for receiving the transmissions from the other unit, the receiver 
having a controller for establishing communications and generating signals 
to be transmitted to the first transmitting unit indicating the quality 
and reliability of communications between the two units. 
The mode control unit of each communications unit in the communications 
system employs a frequency hopping time divided transmission technique to 
maintain communications between two units in the communications system 
when the power level of the transmitter exceeds a predetermined threshold, 
for example, one milliwatt, to maintain reliable communication. The 
frequency hopping frequency control technique employs a narrow band 
frequency modulation (FM) or frequency shift keying (FSK) transmission 
mode for efficient spectrum utilization. 
Thus, a communications system, according to the present invention, has 
communications units which are capable of operating in either low power 
single frequency mode or high power frequency hopping mode without 
modification of the units. 
It is an additional feature of the communications system according to the 
present invention that bit and frame timing recovery may be performed 
independent of the spectrum spreading (frequency hopping) function. 
The foregoing has outlined broadly the features and technical advantages of 
the present invention in order that the detailed description of the 
preferred embodiment of the invention which follows may be better 
understood. The preferred embodiment of the invention will be described 
with reference to the drawing.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
Referring now to FIG. 1, a communications system embodying the present 
invention will be described. 
Communication system 10 includes a base unit 12 and a remote unit 14. 
Base unit 12 receives its power from the alternating current power supply 
by the power utility on lines 16 and is connected to a public switching 
telephone network (PSTN) 18. Also, base unit 12 includes an antenna 19 for 
communications with remote unit 14. Remote unit 14 communicates with base 
unit 12 by transmission and reception of radio frequency signals through 
antenna 22. Remote unit 14 also may include a microphone 24 and a speaker 
or earpiece 26 for conversion of signals between sound and electronic 
form. In addition, remote unit 14 may also include a keypad of the DTMF 
type. 
Referring now to FIG. 2, one unit of communication system 10, for example 
remote unit 14, will be described in greater detail. 
It should be noted that the communications functions, including transmit 
power and frequency control described with reference to remote unit 14 
will be the same for base unit 12. Base unit 12 may or may not include a 
keyboard 28 and most likely will not include a microphone 24 and an ear 
piece or speaker 26 as does the remote unit 14. 
Remote unit 14 communicates with base station 12 through RF transceiver 210 
which receives signals from and transmits signals to base unit 12 through 
antenna 22. The RF transceiver provides a frequency synthesizer, an RF 
receiver, an RF transmitter and modulation and demodulation functions in 
remote unit 14. Burst mode device 212 communicates with transceiver 210 to 
control burst mode operation to recover clock signals and to synchronize 
data frames between the base unit 12 and the remote unit 14. Burst mode 
unit 212 also controls sequencing and outputting of data from the Voice 
CODEC 216. 
Voice CODEC 216 contains a Pulse Code Modulation (PCM) codec-filter. The 
term codec is an acronym from "COder" for the analog-to-digital converter 
(ADC) used to digitize voice and "DEcoder" for the digital-to-analog 
converter (DAC) used for reconstructing voice. A codec is a single device 
used for digitizing and reconstructing the human voice. Typically, the 
voice is quantized with an 8 bit word at a sampling rate of 8 kHz yielding 
a serial data rate of 64 kbps. 
64 kbps PCM codecs are widely known in the art and are readily available 
from manufacturers such as Motorola, OKI of Japan and Texas Instruments. 
The burst mode device (BMD) 212 has as a fundamental frequency control a 
master clock 214 which provides timing signals to permit the burst mode 
device 212 to generate clocking signals to other functional units in 
remote unit 14. BMD 212 provides bit timing and frame timing recovery. A 
digital phase lock loop (DPLL) within the BMD extracts the bit timing from 
the signal output of the receiver. The DPLL estimates the phase of the 
signal by measuring the time interval between zero crossings of the 
receive signal. With bit timing established, a correlator is used to 
detect the presence of a 24 bit unique word sequence embedded in the 
transmission stream. Detection of the 24 bit unique word identifies 
framing boundaries. Also embedded in the transmission stream is a 24 bit 
unique ID which prevents synchronization with an undesired system. BMD 212 
uses the recovered frame timing to correctly position the transmit and 
receive bursts within the frame. 
The operation of burst mode devices in time division duplex (TDD) 
applications is widely known in the art. They are used in second 
generation cordless telephone systems (CT2) and the Digital European 
Cordless Telecommunications (DECT) system. Burst mode devices for these 
systems are manufactured by Motorola, Philips and VLSI Technology. 
Voice CODEC 216 converts sound information received by microphone 24 to 
electrical signals, amplifies the electrical audio frequency signals, and 
converts the audio frequency signals to digital representation by means of 
an analog to digital converter (ADC). Voice CODEC 216 also includes a 
digital to analog converter (DAC) for converting received information in 
digital form to analog form. An audio power amplifier amplifies the 
converted analog information and provides it to speaker 26 for conversion 
to sound for the user. A pulse code modulation technique is used in the 
ADC and in the DAC. The pulse trains are provided to the burst mode device 
214 for storage in a transmit temporary storage device such as a FIFO 
buffer for transmission to transceiver 210 at an appropriate time to be 
transmitted in one or more transmission frames. Conversely, burst mode 
device 212 receives incoming data from RF transceiver 210 and stores the 
incoming data in pulse code format in a receive buffer which may be an 
FIFO buffer for transmission to the Voice CODEC 216 for conversion to an 
analog signal for amplification and conversion to sound in speaker 26. 
The functions described above for remote unit 14 and similarly for base 
unit 12 are controlled by mode control unit 218. 
Mode control unit 218 includes a microprocessor such as a model 6805C8 
commercially available microprocessor, a random access memory 220, and a 
read only memory 222. Mode control unit 218 is connected to keypad 28 for 
entry of DTMF signals and to burst mode device 212, Voice CODEC 216 and to 
RF transceiver 210. MCU 218 controls all the functions in unit 14. For 
example, mode control unit 218 controls the phase lock loop (PLL) 
programming for transceiver 210, the frequency hopping pattern control, 
control channel signaling for synchronization, transmit power control for 
RF transceiver 210, mode control for RF transceiver 210 and other 
telephone features which are not significant to the present invention. 
Data related to mode control are stored in random access memory 220, which 
is a part of mode control unit 218, and bootstrap code and basic control 
code for microprocessor 6805C8 is stored in read only memory 222. 
Frequency control coefficients for RF transceiver 210 are stored in random 
access memory 220 in mode control unit 218. A table in RAM 220 stores the 
pattern of frequency hopping which will control transceiver 210. 
MCU 218 also interprets data in the form of received signal strength 
indicator (RSSI). The RSSI signal and signals indicating channel quality 
are used to determine if low power signal frequency transmission is 
sufficient to maintain quality communication or if higher power frequency 
hopping transmission is required to maintain communication over the 
communication channel. 
OPERATION OF PREFERRED EMBODIMENT OF THE INVENTION 
RF transceiver 210 includes a half duplex radio transceiver with variable 
transmitter power level and a received signal strength indicator (RSSI). A 
half duplex radio is required to implement time division duplex (TDD) 
transmission and reception. TDD enables full duplex wireless voice 
communication using a single radio frequency for both transmission and 
reception although at different times. Use of TDD also ensures link 
reciprocity thereby enabling accurate transmit power level adjustments 
based on received signal strength. A programmable synthesized local 
oscillator is incorporated in RF transceiver 210 to enable the operating 
frequency to be changed as often as once per data frame. A frame clocking 
signals is established through synchronization with the call originating 
unit of the communication such as base unit 12. Bit timing and frame 
synchronization are the functions performed by the burst mode device 212. 
MCU 218 controls power level adjustments for RF transceiver 210 based on 
RSSI signals and the accurate reception of frame synchronization bits. The 
combination of the RSSI signal and the frame synchronization bits is used 
to monitor transmission channel quality. 
Using the channel quality determined from the RSSI signal and the frame 
synchronization bits, the MCU 218 employs a variable mode control 
mechanism described in greater detail below with respect to FIG. 4 to 
determine if high power frequency hopping mode is required or if low power 
non-hopping mode can sustain adequate quality communication. Bit and frame 
timing are established independent of radio frequency or frequency 
hopping. To initiate high-power hopping mode, MCU 218 programs RF 
transceiver 210 frequency synthesizer to a different frequency once per 
data frame and increases the transmitter power to a level sufficient to 
maintain communications. Low power non-frequency hopping mode is set up by 
programming the synthesizer to use the same frequency for each data frame. 
The transmit power level in nonfrequency hopping mode is also reduced to a 
level below the one milliwatt limit established by the FCC. 
Referring now to FIG. 3, transmitter power output and frequency control 
will be described with respect to the graphic representations in FIGURES 
3A and 3B. 
FIG. 3A shows a sequence of fifty transmission frames at times T1, T2 . . . 
T50, respectively, at a transmitter frequency of F1 for each flame, 
indicating a single frequency of transmitter output with a power level of 
a maximum of 1 milliwatt as indicated for each of the fifty frames in FIG. 
3A. 
In contrast, FIG. 3B shows fifty transmission frames T1, T2 . . . T50 in 
which a frequency hopping frequency control is used with a maximum power 
output of the transmitter of one watt. For example, during frame T1 the 
transmitter may be operating on frequency F1 at a maximum power of one 
watt, while during frame Tn the transmitter may be operating on frequency 
Fn at a maximum power of one watt. The sequence of frequencies F1, F2 . . 
. Fn are determined by the frequency table stored in random access memory 
220 in MCU 218. 
Control of frequency and power will be described in greater detail with 
respect to FIG. 4. 
Referring now to FIG. 4, mode control, frequency control and power level 
control will be described in greater detail. At a first instance, remote 
unit 14 or base unit 12 is in a receive standby mode awaiting some 
interrupt condition to occur. 
In the preferred embodiment of the present invention, 200 separate channels 
are available for communications or communications set up. Fifty channels 
are used for communications set up in both the high power and the low 
power mode. The designation of set up channels may be fixed or may change 
under control of the base unit. 
Remote unit 14 receiver 210 and base unit 12 receiver 210 scan through each 
of the fifty set up channels and measure the RSSI on each channel. The hop 
time between channels is 100 us. Receiver 210 samples the RSSI for 50 us. 
If RSSI is above the minimum, receiver 210 remains on the channel to 
recover bit timing and verify a matching ID. If the ID does not match, 
receiver 210 proceeds to the next channel in the set up channel hop list 
and repeats this process. To maintain a 10% duty cycle, receiver 210 is 
disabled after 100 ms, for a period of one second in order to conserve 
battery life. 
In addition to scanning the fifty set up channels, base unit 12 performs 
the RSSI measurement on the other 150 channels used by the system. Base 
unit 12 effectively maps the entire frequency band and compiles a hop 
table comprised of vacant channels. 
When call set up is requested, the synchronization process must be 
performed. The call originating unit initiates transmission on the fifty 
set up channels with transmitter 210 at the maximum power setting of one 
watt. The transmission sequence consists of a series of dotting bits 
(one-zero transitions) for timing recovery, along with unique ID and a 
frame marking unique word. An acquisition frame consists of this pattern 
repeated for a total duration of 10.8 ms followed by a 3.3 ms receiving 
period used for detection of a response from the call receiving unit. 
Upon detection of the call originating unit transmission, the call 
receiving unit transmits an acknowledgment message at the proper time and 
on the proper frequency in the hop pattern. Both units confirm 
synchronization by remaining on the original detection channel for an 
additional two frames. After successful confirmation, the tracking frame 
is substituted for the acquisition frame. The tracking frame consists of a 
3.3 ms transmit burst and a 3.3 receive burst with guard times between 
each burst to allow for synthesizer settling and propagation delay. For 
each burst, a number of bits are used for control signaling. This control 
channel is used by base unit 12 for transfer to the remote, the status of 
each channel in the 200 channel hop table updated as a result of the 
spectrum mapping routine performed in the receive standby mode. Once this 
transfer is complete, the system initiates the hop sequence defined by the 
200 channel hop table. 
Also during this process, base unit 12 uses the RSSI and channel quality 
status (defined by the correct reception of frame synchronization bits) to 
determine if the remote is sufficiently close to permit low power 
nonfrequency hopping operation. If the RSSI value satisfies the 
requirement that RSSI &gt;X+Y+Z where: 
X equals the minimum allowable signal level that ensures an acceptable 
bit-error-rate (typically less than 1 error in 10,000 bits); 
Y equals the remote unit transmitter power level relative to the 1 mW 
setting; 
and Z equals the margin allowed for multi-path fading (typically 15 dB for 
portable, handheld radio applications), 
with good channel quality, base unit 12 directs the remote to initiate the 
nonfrequency hopping, low power mode. (Note: the value for Y is known to 
the base as a consequence of the base performing the system power control 
function.) Base unit 12 then provides the remote with the location of one 
of 200 voice channels previously determined by base unit 12 to be 
available. 
Once the communications link has been successfully established, MCU 218 
continuously monitors the RSSI signal and receive synchronization bits. 
If the system is operating in nonfrequency hopping mode and remote unit is 
moved out of range, MCU 218 determines that a mode change may be required 
if RSSI falls below a predetermined operating threshold for a period 
longer than a predetermined number of frames such as 10. In this case, the 
power level is incremented in 10 dB steps and frequency hopping mode is 
initiated. The hopping pattern is based upon the spectrum map information 
recorded prior to call set up (above). Both remote unit 14 and base unit 
12 increase the respective transmit power levels until RSSI enters a 
reliable operating range and synchronization bits are correctly received 
on all channels. 
In the case of a sudden loss of synchronization bits with the RSSI signal 
above a minimum level, it is assumed that a jamming signal has been tumed 
on on the frequency of the nonfrequency hopping signal. In the case of a 
sudden loss of the RSSI signal, it is assumed that remote unit 14 has 
rapidly moved out of range. Both units will remain on the channel in an 
attempt to re acquire communications and to characterize the nature of the 
jamming signals. If communications on the link resumes at a reliable level 
after a predetermined number of frames, such as 10, with RSSI in an 
acceptable range, no action need be taken. However, MCU 218 sets a timer 
in an attempt to detect the presence of another frequency hopping device. 
If the other frequency hopping device is a same kind unit, the time 
interval between frame hits will be predictable. Another frame hit 
anywhere within the timer interval will cause base unit 12 and remote unit 
14 to enter frequency hopping mode at full power on the set up channels. 
If channel quality permits, transmit power levels of the base unit 12 and 
remote unit 14 can be reduced in steps, however, the frequency hopping 
mode is maintained for the duration of the call. 
If synchronization is not reestablished during the timer interval, then the 
jammer is assumed to be a continuous carrier signal. 
If no frame hits occur during the timer interval, then the jammer is 
assumed to be a continuous wave signal. Base unit 12 and remote unit 14 
then jump to the next voice channel in the queue that was deemed available 
at the time of call set up. If the system is operating in frequency 
hopping mode, the nonfrequency hopping mode can be resumed only if the 
frequency hopping mode was initiated as a consequence of the remote unit 
14 moving out of range or from an encounter with a nonfrequency hopping 
source. The nonfrequency hopping mode channel is determined by base unit 
12 and is based upon the channel quality measured in the frequency hopping 
mode. 
Although the present invention and its advantages have been described in 
detail, it should be understood that various changes, substitutions and 
alterations can be made herein without departing from the spirit and scope 
of the invention as defined by the appended claims.