Suppressed side lobe direct-sequence-spread-spectrum radio communication method and device

A direct sequence spread spectrum (DSSS) radio communication system of the present invention comprises a transmitter and a matching receiver. The transmitter includes a pseudorandom number (PRN) code generator, a chip clock generator and a 16.times. clock generator that runs at sixteen times the chip clock rate. The output of the PRN code generator and chip clock generator are exclusive-OR'ed to derive a Manchester encoding of the DSSS spreading code. A tri-state buffer is used to deliver such Manchester encoded DSSS spreading code to a mixer to spread a biphase shift keyed (BPSK) radio carrier before being transmitted. The tri-state buffer can be a part of the exclusive-OR logic and is connected to place its output in a high impedance state one sixteenth of every chip clock period. This provides for a suppression of the spurious sidebands that otherwise limit adjacent channel packing.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates generally to radio communication methods and systems 
and more specifically to direct sequence spread spectrum wireless 
telephone sets. 
2. Description of the Prior Art 
Conventional direct sequence spread spectrum (DSSS) wireless communication 
has now become very popular to use in cordless telephones and other 
consumer electronics. But at the very high carrier frequencies used, e.g., 
UHF and microwave, the communication links often suffer from multipath 
interference. 
Typical wireless telephone sets operate in full duplex mode, meaning that 
they apparently send and receive at the same time. Both time division and 
frequency division techniques have been used to support such full duplex 
channels. Since direct sequence spread spectrum signals can put a high 
proportion of the total energy far away from the carrier, it has been 
difficult, without allowing severe overlap of spectra, to pack more than 
two such DSSS carriers in the twenty-six megahertz band of 902-928 MHz 
allowed for unlicensed use by the US Federal Communications Commission 
(FCC). The number of non-overlapping channels is inversely proportional to 
the spreading bandwidth. For illustrative purposes we assume a spreading 
rate of one to two Mcps (mega chips per second). If the spreading rate is 
much less, then the number of channels can increase, but the interference 
rejection, also known as spreading gain, is proportionately reduced. Thus 
it is always desired to spread as much as possible, limited by allowed 
out-of-band emissions, and by the need to have minimal overlap of 
channels. The out-of-band signal energies are strictly limited in one 
square step to -20 dBr. Therefore, DSSS methods and equipment that hold 
their out-of-band signal energies to lower levels would allow packing as 
many as four or five DSSS carriers in the twenty-six megahertz band 
allocated for cordless telephone set use. The same arguments apply to 
other bands where spread spectrum signals are permitted. 
Conventional DSSS transmissions produce one main energy lobe centered on 
the carrier frequency and dozens of wide side lobes at fairly constantly 
diminishing energies shoulder-to-shoulder marching away from the carrier 
frequency. 
The prior art does not offer a solution to providing DSSS transmissions 
with specially attenuated side lobes beyond the band edges, other than 
sharper conventional bandpass filtering which is considered obvious and 
has a significant cost disadvantage. Beyond the band edges are where the 
law requires that the transmitted energies be limited in a square step 
down from the maximum-permitted in-band signal levels to much lower 
out-of-band signal levels, e.g., twenty decibels down. 
SUMMARY OF THE PRESENT INVENTION 
It is therefore an object of the present invention to provide a DSSS radio 
communication system with lower out-of-band energies that permits the use 
of more channels in band. 
It is another object of the present invention to provide a DSSS modulation 
method that nulls the largest side lobes of a DSSS transmission in the 
frequency demarcation areas defined by regulatory agencies for out-of-band 
signal amplitude levels. 
Briefly, a DSSS radio communication system of the present invention 
comprises a transmitter and a matching receiver. The transmitter includes 
a pseudorandom number (PRN) code generator, chip clock generator and a 
16.times. clock generator that runs at sixteen times the chip clock rate. 
The output of the PRN code generator and chip clock generator are 
exclusive-OR'ed to derive a Manchester encoding of the DSSS spreading 
code. The proposed technique works equally well without Manchester coding. 
Manchester coding is part of the implementation but not a new invention. A 
tri-state buffer is used to deliver such Manchester encoded DSSS spreading 
code to a mixer that uses it to spread a biphase shift keyed (BPSK) radio 
carrier before being transmitted. We require that the mixer can shut off 
the signal and will do so if the input current is zero. This is indeed the 
case for many passive mixers. For active mixers it may be necessary to 
bias the high impedance state to a voltage halfway between the minimum and 
maximum PN-junction voltages. The tri-state buffer can be a part of the 
exclusive-OR logic and is connected to place its output in a high 
impedance state one sixteenth of every chip clock period. One can move the 
position of the spectral null closer to the center frequency by increasing 
the duration of the high-impedance (signal off) state. This generates a 
spectral null which provides for a suppression of the largest out-of-band 
sidebands that otherwise limit adjacent channel packing. 
An advantage of the present invention is that a DSSS radio communication 
system is provided with lower out-of-band energies that permits the use of 
more channels in band because channels can be placed closer to the band 
edge than would be possible without this technique. 
Another advantage of the present invention is that a DSSS modulation method 
is provided that nulls the largest side lobes of a DSSS transmission in 
the frequency demarcation areas defined by regulatory agencies for in band 
and out-of-band signal amplitude levels. 
Another advantage of the present invention is that a grossly nonlinear 
power amplifier may be used without compromising the desired spectrum 
because the input radio frequency to the amplifier is turned completely 
off for short periods and this causes the output to turn off even for a 
very nonlinear amplifier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a radio communication system embodiment of the present 
invention, referred to herein by the general reference numeral 10. The 
system 10 comprises a direct sequence spread spectrum (DSSS) radio 
transmitter 12 and a matching DSSS radio receiver 14 that communicate over 
a radio signal 16. For example, the radio signal 16 may be in the 
twenty-six megahertz band of 902-928 MHz to allow unlicensed operation in 
the United States. 
In the following description, the use of Manchester coding is in no way 
necessary to the generation of the spectrally-tailored signal which 
constitutes the proposed invention. The radio transmitter 12 comprises a 
digital clock generator 18 that operates at sixteen times a basic "chip" 
rate for DSSS modulation. A tri-state output controlled exclusive-OR logic 
gate 20 is connected to receive a chip clock and a pseudorandom number 
(PRN) from a chip-clock and PRN code generator 22. The exclusive-OR part 
of the logic gate 20 produces Manchester encoding, e.g., one-zero, 
zero-one, from the PRN modulating code for the DSSS and has the effect of 
creating a double main lobe at the transmission carrier frequency. A 
tri-state controller 24 combines such 16.times. clock and chip clock to 
produce a digital control signal with a 15/16 duty cycle synchronized to 
the chip clock rate that is used to control the output of the logic gate 
20. The tri-state output control part of the logic gate 20 forces the 
output to a mixer 26 to go to a high impedance state (hi-Z) for one 
sixteenth of every chip period at the time of chip transition. This turns 
off the mixer 26 during such hi-Z periods and has the overall effect of 
specially attenuating the transmitted side lobes in a range removed from 
the carrier frequency, e.g., .+-.fclock/2 MHz of the carrier in the 
902-928 MHz band. If the hi-Z duration is 2/16ths of a chip then the nulls 
are located at .+-.fclock/4. The carrier frequency and data to be 
communicated are applied to a biphase shift keyed (BPSK) modulator 28. The 
composite is amplified by a RF power amplifier stage 29 and filtered by a 
RF-bandpass filter 30 after mixing with the PRN code. An antenna 32 
couples the radio signal 16 to the ether. 
The RF-bandpass filter 30 is a full-band filter type and need not be 
specially adapted to each in-band channel being used. A power amplifier 
will typically follow the mixer 26 in practical designs. Additional 
frequency up-conversion may also be used. The logic gate 20 may comprise 
discrete exclusive-OR and tri-state buffering that are implemented in 
separate semiconductor devices. The mixer 26 is conditioned to assume a 
proper, stable off state when its input from the logic gate 20 goes to its 
high impedance state, e.g., the input is not simply allowed to float and 
cause problems with oscillation or false triggering. A voltage divider on 
the input may provide such input conditioning. Other digital techniques, 
that will be readily apparent to artisans, can be used to combine the PRN 
code, chip clock and BPSK modulated carrier, e.g., to effect a side lobe 
suppression that tracks in frequency from the carrier in relation to the 
pulse width of the gating of the PRN code. 
The receiver 14 includes an antenna 40, a RF-bandpass filter 42, a mixer 44 
for PRN code removal, a narrow-band filter 45 and a demodulator 46 for 
carrier removal. A chip clock generator 48 is connected to a PRN code 
generator 50 and an exclusive-OR logic gate 52. The Manchester coding that 
results is used to strip off the Manchester-encoded PRN code from the 
transmitter 12 in the mixer 44. The demodulator 46 provides a detected 
amplitude control signal to a code acquisition and tracking processor 54. 
This, in turn, provides frequency and phase control to the chip clock 
generator 48 to match the local code phase to the received code phase. 
Embodiments of the present invention suppress spectral side lobe emissions 
and place a broad extra null at approximately f=fcarrier.+-.fchip clock/2. 
Changing the width, e.g., duty cycle, of the pulse coming out of the logic 
gate 20 will change the frequency of the null. 
FIG. 2 shows how the carrier can be placed closer to the band edge without 
violating FCC spectral emission requirements using the techniques of the 
present invention. Without this method the carrier can usually be no 
closer than 8.7 MHz from the band edge. The present invention allows the 
carrier to be placed as close as 4.8 MHz from the band edge. This allows 
space for more carriers in band, e.g., more frequency channels that do not 
overlap on the main lobe peaks. 
FIG. 3 is a functional block diagram of a second radio communication system 
embodiment of the present invention that differs from that shown in FIG. 1 
by the connection of the on/off control 24 to the RF power amplifier stage 
29 in the transmitter 12. The element numbers are repeated from FIG. 1 
because the elements are essentially the same. The RF amplifier 29 is 
periodically turned off and this achieves the same result as described in 
connection with FIG. 1. 
Although the present invention has been described in terms of the presently 
preferred embodiments, it is to be understood that the disclosure is not 
to be interpreted as limiting. Various alterations and modifications will 
no doubt become apparent to those skilled in the art after having read the 
above disclosure. Accordingly, it is intended that the appended claims be 
interpreted as covering all alterations and modifications as fall within 
the true spirit and scope of the invention.