Wireless TDMA transmitter with reduced interference

Low-frequency interference components produced by a time division multiple access (TDMA) portable radio transmitter that might potentially interfere with devices sensitive to low frequencies, such as cardiac pacemakers, are substantially produced. In those TDMA frames in which a protocol calls for either a burst of an information-bearing signal to not be transmitted in its designated time slot, or transmitted at a reduced power level, or for a shorter duration, an interference-compensating signal at a compensation frequency other than the transmitter's carrier frequency is transmitted. The interference-compensating signal is transmitted in the time slot in which the transmitter normally transmits the burst at a power level equal to the power level of the burst. If the protocol calls for a reduced power burst, the interference-compensating signal is transmitted within an adjacent time slot at a power level that compensates for the reduced burst power level. If the protocol calls for a shorter burst length, the interference-compensating signal is transmitted within the same time slot to extend the burst to the length of the time slot. The interference caused by the compensating signal is substantially the same as would have been caused by the information-bearing signal, so that the interference pattern remains unchanged. This eliminates low-frequency variations in the interference pattern.

TECHNICAL FIELD 
This invention relates to radio transmitters using time division multiple 
access (TDMA) and, more specifically, to methods and apparatus which 
reduce baseband envelope interference from such transmitters. 
BACKGROUND OF THE INVENTION 
Baseband envelope interference caused by radio transmissions is a 
well-known problem. It differs from conventional radio interference in 
that the device that is interfered with is not intended to receive radio 
signals at the frequency of the signal causing the interference. It occurs 
because virtually all electronic devices contain wires that can act as 
antennas, and semiconductors that can act as rectifiers or detectors. A 
spurious antenna can pick up a radio signal and a spurious rectifier can 
convert it into a voltage (or current) proportional to the instantaneous 
power of the radio signal. This spurious voltage (or current) signal has 
the potential to disrupt the operation of the electronic device if it 
resembles the signals normally handled by the device. For example, if the 
interfering radio signal exhibits envelope variations at frequencies 
within the audible range, it can interfere with devices such as hearing 
aids and audio tape recorders. Similarly, if the envelope variations occur 
at frequencies in the range of a few Hertz, which are typical of the 
signals handled by cardiac pacemakers, or fire and burglar alarms, the 
radio signal has the potential to interfere with such devices. 
While baseband envelope interference is an old problem, it has recently 
become particularly important because of two factors: a) the ubiquity of 
portable communication devices capable of transmitting at high RF power 
levels (most notably, cellular telephones); and b) the advent of digital 
communication standards based on time division multiple access. The second 
factor is important because the major cause of disruption comes from the 
time variations in the transmitted RF power. A transmitted radio signal 
with a constant envelope is relatively harmless. Therefore, conventional 
analog cellular telephones, which use frequency modulation (FM) with a 
frequency division multiples access (FDMA) scheme are not a major source 
of baseband interference because they transmit a signal whose 
instantaneous power level is substantially constant in time. By contrast, 
TDMA is characterized by short bursts of powerful radio transmission 
separated by longer gaps during which no transmission takes place. These 
wide variations in the transmitted radio power are a known cause of 
unwanted interference. 
A mobile radio transmitter in a TDMA system transmits a burst of an 
information-bearing signal, such as a coded speech signal, at a carrier 
frequency within a designated time slot within a defined frame of fixed 
duration. The successive bursts of the information-bearing signal are 
transmitted in the same time slot in successive frames. Both the time slot 
and the frequency used by the transmitter are assigned by a system 
controller. While one transmitter in a system is transmitting in one 
specific time slot, other transmitters are simultaneously operating at the 
same frequency using the other time slots within the frame. Other 
transmitters within the system are also simultaneously operating in a 
similar manner at other carrier frequencies. TDMA transmission techniques 
are used in various systems such as the Global Systems for Mobile 
Communication (GSM), IS-54 of the Telecommunications Industry Association 
(TIA), Japanese Digital Cordless (JDC). Other similar types of time 
division techniques are time division duplexing (TDD) (used, for example, 
in CT-2), and hybrid TDMA/TDD systems such as the personal handy phone 
(PHP) and digital European cordless telephone (DECT). For the purposes of 
this specification, TDMA, TDD and TDMA/TDD systems shall all be referred 
to as TDMA systems. 
FIG. 1 shows the steady-state output power versus time relationship for 
TDMA transmission as might occur in a portable radio transmitter that 
complies with the GSM standard. As can be noted, time is subdivided into 
TDMA frames, which are then further subdivided into eight transmission 
time slots. A particular radio transmitter transmits at a particular power 
level (P.sub.0 for example), in a particular time slot (slot #3, for 
example), and at a particular frequency (f.sub.n, for example). Thus, in 
accordance with the GSM standard, up to eight independent portable TDMA 
radios in separate wireless terminals are able to time-share a single 
frequency channel for simultaneous calls. Each TDMA radio is in a 
"transmit mode" during its assigned time slot and in a "quiet mode" during 
the remaining time slots. 
Based on the pattern of FIG. 1, the frequency spectrum of the baseband 
envelope interference caused by a TDMA mobile radio that complies with the 
GSM standard can be calculated. Inasmuch as there is a signal component 
occurring during one time slot each frame, there is a strong component at 
a fundamental frequency equal to the TDMA frame rate (which is 
approximately 217 Hz for GSM). There is also a somewhat weaker component 
at the second harmonic frequency equal to twice the TDMA frame rate, and 
progressively weaker components at all the harmonic multiples of the TDMA 
frame rate. 
A prior art method for eliminating the interference from these potentially 
interfering signal components at the frame rate and its multiples in such 
a TDMA transmission system is disclosed in UK Patent Application GB 2 238 
449 A, published May 29, 1991. As disclosed therein, an additional signal 
is transmitted by the TDMA transmitter in all of the other seven time 
slots at a different frequency but at the same power level as the signal 
transmitted in the assigned time slot. In that way the transmitter is 
always transmitting a signal of continuous output power, thereby 
eliminating the signal components at the fundamental frequency and at all 
of its harmonics. Disadvantageously, however, transmitting a signal of 
continuous power throughout the entire TDMA frame each and every frame 
requires substantially larger battery consumption and requires a 
transmitter amplifier capable of dissipating much larger average power 
levels. Both factors would disadvantageously lead to a larger and heavier 
transmitter. Furthermore, the tunable oscillator (or frequency 
synthesizer) in the TDMA transmitter would need to switch quickly between 
the frequency of the information-bearing burst during the assigned slot 
and the frequency used for transmission during the rest of the frame. This 
requirement may greatly increase the cost of the oscillator and thus the 
transmitter. For these reasons, such a method for reducing the signal 
components at the fundamental frequency and its harmonics is thus not 
likely to find widespread adoption in such TDMA radio transmitters. 
As previously noted, interference at 217 Hz and its multiples may cause 
interference with certain devices such as hearing aids and audio tape 
recorders. Interference at much lower frequencies in the order of a few 
Hertz, also as previously noted, may cause interference with devices such 
as cardiac pacemakers and burglar and fire alarms. TDMA transmission under 
the GSM standard during steady-state communication conditions does not 
have signal components at such low-frequencies. However, there are 
situations, such as during call setup, where the GSM protocol prescribes 
that the mobile transmitter not transmit in certain occurrences of the 
TDMA frame. When that happens, the frequency spectrum of the baseband 
envelope interference exhibits components at frequencies lower than the 
TDMA frame rate. In particular, it is known that a mobile radio terminal 
complying with the GSM standard can, at times, cause baseband envelope 
interference at low frequencies around 2 Hz and 8 Hz. These frequencies 
are known to cause disruption in the operation of cardiac pacemakers and 
sensors of various types. (see, e.g. V. Barbaro et al., "Do European GSM 
Mobile Cellular Phones Pose a Potential Risk to Pacemaker Patients?", 
Pace, June 1995, pp. 1218-1224). 
An object of the present invention is to reduce the potentially interfering 
low-frequency signal components caused by TDMA mobile radio transmitters. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a TDMA portable radio transmitter 
selectively transmits within at least one time slot, but in less than all 
the time slots, a non-information-bearing interference-compensating signal 
at a predetermined compensation frequency designated for such purposes 
that is other than the transmitting frequency assigned to the transmitter, 
in order to compensate for variations in the signal power in the 
transmitter's designated time slot imposed by a communications protocol of 
the radio system. Thus, if the communications protocol requires, for 
example, that the radio transmitter omit or skip transmitting an 
information-bearing signal burst in one or more TDMA frames, the 
transmitter transmits in such frames the non-information-bearing 
interference-compensating signal at the compensation frequency and at the 
same signal power used in the other frames to transmit information bursts. 
In a similar manner, if the communications protocol requires that the 
radio transmitter transmit an information-bearing signal burst that is 
shorter than the time slot during its designated time slot in one or more 
TDMA frames, then the interference-compensating signal is transmitted 
adjacent to the information-bearing burst within the same or a nearby time 
slot at the compensation frequency and at the power of the 
information-bearing signal burst.

DETAILED DESCRIPTION 
With reference to FIG. 2, information-bearing signals are transmitted 
during time slot 3 of frames 1, 3 and 4 of a mobile TDMA radio transmitter 
operating under the GSM protocol. The transmitter transmits at a 
particular power level (P.sub.0) and at a particular frequency (f.sub.n). 
The protocol may require that no information-bearing signal be transmitted 
at frequency f.sub.n in certain frames, such as frame 2 in FIG. 2. In 
accordance with the present invention, in order to minimize the 
low-frequency signal components that would arise from a pattern in which 
no signal power is transmitted during certain frames, the transmitter is 
modified to transmit during each of such time slots a 
non-information-bearing interference-compensating signal at substantially 
the same signal power, P.sub.0, but at a compensation frequency, f.sub.c, 
designated for such transmissions. Thus, when the protocol requires that a 
particular radio not transmit in certain TDMA frames, the radio instead 
transmits in those frames at substantially the same signal power as in the 
other frames where it is required to transmit an information-bearing 
signal, using the same time slot but at the frequency of the 
interference-compensating channel. A particular frequency channel is 
designated as a communal interference-compensating channel that is shared 
by all TDMA radio transmitters on a system. In a typical application, the 
frequency of the interference-compensating channel is sufficiently close 
(within a few percent, for example) to that of the information-bearing 
channel such that the behavior of the transmitting antenna and the 
parasitic receiving antenna in a unit suffering the interference are 
essentially the same. As a result, the amount of baseband envelope 
interference will be substantially the same and there will be no frequency 
components below the TDMA frame rate. 
The need to sacrifice one frequency channel for use as an 
interference-compensating channel means that the total system capacity is 
somewhat reduced. Since the interference-compensating channel is shared 
among all transmitters and all cells of a cellular system, however, only 
one interference-compensating channel is required. For example, in the GSM 
standard there are 124 frequency channels. If a single channel is 
designated as an interference-compensating channel, the reduction in 
system capacity is only 0.8%. If an entire frequency channel is not 
designated as the interference-compensating channel, in another embodiment 
of the present invention a subset of the TDMA frame time slots are 
designated as interference-compensating time slots to be used at a 
particular compensation frequency, f.sub.c. Thus, only certain TDMA time 
slots in the interference-compensating channel are designated for 
compensation purposes. The remaining slots at the compensation frequency 
are available for information-bearing signals. As in the previously 
discussed embodiment of the invention, the TDMA radio transmits a burst in 
each and every TDMA frame. However, in this embodiment, when the protocol 
calls for an information-bearing burst to not be transmitted in a certain 
frame, an interference-compensating burst is transmitted in the 
interference-compensating slot nearest the position of the slot carrying 
the information-bearing bursts at the compensation frequency. With this 
variant of the invention, the desired reduction in frequency components 
below the TDMA frame is not as great. The extent of the reduction is 
dependent upon how many slots in the TDMA frame are designated as 
interference-compensating time slots, with more being better. 
It should be noted that, in either case, it is not necessary for all 
transmitters in a TDMA system to implement the interference-reduction 
technique. For example, the technique might be implemented as an option in 
the transmitters used by wearers of pacemakers. In this case, the system 
controller would instruct these select transmitters to use a particular 
compensating frequency that can be assigned dynamically, as needed. Most 
of the time, when only phones that do not require the technique are in 
use, the compensating frequency is available for normal communications 
use. 
In another embodiment, if a particular protocol calls for the transmission 
of an information-bearing burst at a reduced power level during its 
assigned time slot in certain frames, then to compensate for the 
low-frequency signal components that would otherwise arise, an 
interference-compensating burst is transmitted during such frames at the 
compensation frequency in one or more adjacent or near-by time slot at a 
power level that compensates for the reduced power level of the 
information-bearing burst. Thus, as shown in FIG. 3, the output power of 
the information-bearing-burst transmitted in slot 3 is reduced by half to 
P.sub.0 /2 in frame 2 in accordance with a protocol. Thus, an 
interference-compensating signal is transmitted at the compensation 
frequency f.sub.c in time slots 2 and 4 at the power level P.sub.0 /4. 
Note that in FIG. 3, the transmitter needs to quickly switch from the 
frequency f.sub.c of the compensating signal to the frequency f.sub.n of 
the information-bearing burst between slots 2 and 3, and again from the 
frequency f.sub.n of the information-bearing burst, to the frequency 
f.sub.c of the compensating signal between slots 3 and 4. A small gap 
(shown somewhat exaggerated in FIG. 3), during which no transmission 
occurs, is therefore required at each transition in order to avoid 
spurious transmission at unwanted frequencies while the transmitter 
switches from the compensating signal to the information-bearing burst and 
from the information-bearing burst to the compensating signal. This gap 
can be made as long as necessary to allow the tunable oscillator (or 
frequency synthesizer) in the transmitter to switch frequencies. By 
keeping the gap small compared to the duration of a burst, the 
advantageous reduction of baseband envelope interference will be 
substantially the same. In order for the total signal energy 
(power.times.duration) of the reduced-power burst plus the compensating 
signal to equal the signal energy of a burst in a frame in which a full 
power information-bearing burst is transmitted, the compensating signal is 
extended beyond the slot boundaries to compensate for the necessary gap. 
Thus, in FIG. 3, the compensating signal in slot 2 commences in slot 1, 
and in slot 4, extends into slot 5. 
In a further embodiment of the invention, the interference-compensating 
signal may have a higher instantaneous power over a shorter duration of 
time than the power and duration, respectively, of the information-bearing 
burst it is replacing. Similarly, the interference-compensating signal may 
have a lower instantaneous power over a longer duration of time than the 
power and duration, respectively, of the information-bearing burst it is 
replacing. Essentially, the energy of the interference-compensating signal 
should substantially equal the energy of the information-bearing burst it 
is intended to replace. 
In a similar manner, when a portable radio is first turned on and starts 
transmitting at full power in its assigned time slot in every frame, the 
abrupt change from zero power output to full power output creates 
low-frequency components. In accordance with the invention, an 
interference-compensating signal is transmitted at the compensation 
frequency in the initial frames, at increasing power levels in successive 
frames, prior to the commencement of transmission of the 
information-bearing bursts. As shown in FIG. 4, an 
interference-compensating signal is transmitted at the power level of 
P.sub.0 /3 at compensation frequency f.sub.c during time slot 3 of frame 
1, and at power level 2P.sub.0 /3 at frequency f.sub.c during time slot 3 
of frame 2. In frame 3, the information-bearing burst is transmitted at 
full power, P.sub.0, at its assigned frequency f.sub.n during its assigned 
time slot 3. The gradual introduction of transmitted power thus reduces 
the low-frequency signal components produced. 
It has been heretofore assumed that all transmission bursts have the same 
duration, i.e., the length of a time slot. In some system this in fact may 
not be true. For example, in the GSM standard, the so-called "random 
access channel" bursts are shorter than regular bursts. Two techniques of 
the present invention can be applied to such systems. 
In accordance with the first technique, when transmitting in the 
interference-compensating channel, the duration of the 
interference-compensating burst is adjusted to match the duration of the 
corresponding information-bearing bursts. A system can be envisioned where 
the protocol requires the simultaneous use of several bursts of different 
sizes and power levels in the same TDMA frame. In such a case, in those 
frames where some or all of the information-bearing bursts are supposed to 
be omitted, the omitted bursts are replaced with interference-compensating 
bursts at the compensating frequency that match the energy contents of the 
corresponding missing information-bearing bursts. 
In accordance with the second technique, information-bearing bursts that 
are shorter than others are extended in length so that all bursts become 
equal in size. The extension can be accomplished by transmitting at the 
compensating frequency, f.sub.c, at substantially the same power level, 
P.sub.0, as that of the burst being extended, for a period of time equal 
in duration to the desired extension, immediately following (or preceding) 
the burst being extended. FIG. 5 illustrates this embodiment. As in the 
embodiment of FIG. 3 discussed herein above, the transmitter needs to 
quickly switch from the frequency f.sub.n of the information-bearing 
burst, to the frequency f.sub.c of the compensating signal during the 
extension period. Thus, as can be noted in FIG. 5, for the reasons 
previously discussed for the embodiment of FIG. 3, a small gap, during 
which no transmission occurs, is required at the transition. Also, as in 
FIG. 3, the compensating signal extends beyond the slot boundary for a 
duration equal to the gap to maintain the total signal energy of the 
reduced-length information burst plus the compensating signal at the same 
energy level of a full slot-width information burst. With all bursts 
having the same duration due to the extension, the invention can be 
applied as previously discussed to achieve the advantageous reduction in 
baseband envelope interference. The advantage of this second more 
complicated technique rather than the first technique described above in a 
system with variable burst lengths is that, by effectively making the 
length of all bursts the same, the reduction of the unwanted low-frequency 
components of the baseband envelope interference can be made more 
complete. This is particularly important if the protocol requires 
frequent, rapid switching from bursts of one length to bursts of a 
different length. 
The embodiment of the invention in FIG. 6 can also be used if signal 
components at the fundamental frequency of the frame rate cause 
interference with certain devices, but which devices are not susceptible 
to signal components at multiples of the frame rate. In accordance with 
this embodiment, a compensating signal is transmitted at frequency f.sub.c 
within a time slot in every frame. As shown in FIG. 6, the compensating 
signal at the same power level, P.sub.0, of the information-bearing 
signal, is transmitted in slot 7 of each frame. Thus, there will be no 
interference components at the frame rate, but only at twice the frame 
rate and at multiples thereof. 
With reference to the simplified block diagram of a mobile transmitter 700 
in FIG. 7, an information-bearing signal to be transmitted is inputted on 
lead 701 through a selector 702, controlled by a controller 705, to a 
modulator 704, which modulates the signal for transmission with a carrier 
frequency outputted by a controllable oscillator 703. Oscillator 603 is 
controlled in frequency by the controller 705, which determines from a 
system controller (not shown) the frequency f.sub.n at which the 
transmitter 700 is to transmit its bursts of information. Controller 705 
also controls the timing at which the unmodulated carrier output of 
oscillator 703 changes frequency to the frequency f.sub.c of the 
interference-compensating signal. Bursts of the modulated 
information-bearing signal are amplified an amplifier 706, controlled by 
the controller 705, and passed to an antenna 707. The 
interference-compensating signal, outputted by the oscillator 703 at the 
compensation frequency f.sub.c, may be modulated by modulator 704 by a 
non-information-bearing signal inputted on lead 708 to selector 702. 
Alternatively, the oscillator output at compensation frequency f.sub.c may 
pass unmodulated by modulator 704, in response to a "no modulation" signal 
on input lead 709 to selector 702. The output of selector 702 of the 
information-bearing signal on lead 701, the non-information-bearing signal 
on lead 708, or the "no modulation" signal on lead 709 are determined by 
controller 705. Controller 705 has stored in an associated memory (not 
shown) timing and frame pattern information associated with terminal 700. 
Accordingly, it "knows" for each time slot of each frame whether the 
transmitter is in a mode to be 1) transmitting a burst of an 
information-bearing signal, 2) transmitting an interference-compensating 
signal, or 3) is in a quiet mode during which no transmission is to take 
place. Thus it controls oscillator 703 to output a carrier frequency 
appropriate to the each of the three modes and controls selector 702 to 
provide the appropriate modulating or non-modulating signal to modulator 
704. Controller 705 further controls amplifier 706 to turn-off 
amplification to produce a zero output during the quiet mode and to 
amplify its input signal at other times with the amount of amplification 
being adjusted to achieve the desired transmitted power level. Thus, as 
previously described, the interference-compensating signal is selected for 
output during those frames in which the usual steady-state power level of 
the information-bearing signal is not transmitted at all, is transmitted 
at a reduced power level, and/or is transmitted for less than its usual 
time slot duration. Thus, as described, if the protocol requires that no 
information-bearing signal be transmitted during a frame, the frequency of 
oscillator 703 is adjusted to f.sub.c, and the interference-compensating 
signal is transmitted in place of the information-bearing burst in either 
the same or a nearby time slot within the frame. If the protocol requires 
the information-bearing signal be transmitted at a reduced power level, 
then controller 705 adjusts the amplification of amplifier 706 during the 
time slot of the information-bearing burst, adjusts the frequency of 
oscillator 703 to f.sub.c, and selects the interference-compensating 
signal for transmission during the frame at another time slot and at an 
appropriate power level necessary to compensate for the reduced-power 
burst. Similarly, if the protocol requires transmission of an 
information-bearing burst for less than a time slot, controller 705 
adjusts the frequency of oscillator 703 and transmits the 
interference-compensating signal as an extension of the burst during a 
duration necessary to compensate for the shortened burst. 
The above-described embodiments are illustrative of the principles of the 
present invention. Other embodiments could be devised by those skilled in 
the art without departing from the spirit and scope of the present 
invention.