Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-10-08
2002-04-30
Mullen, Thomas (Department: 2632)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C370S347000, C375S132000
Reexamination Certificate
active
06381053
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to Fast Frequency Hopping Spread Spectrum (FFHSS) Code Division Multiple Access (CDMA) communications. More particularly, the invention relates to the transmission and reception signal processing methods and devices. The invented method avoids the requirement of fast frequency hopping synthesis in the FFHSS transmitter and receiver previously used In mobile radio communications or the like. Preferred embodiment of the invented method Is particularly suitable for fiber optical implementation of the FFHSS-CDMA technique.
BACKGROUND OF THE INVENTION
Code Division Multiple Access (CDMA) communications is a technique presently used in wireless applications. CDMA accommodates a large pool of subscribers, while providing dynamic simultaneous access to an arbitrary subset of them. In a typical CDMA network, a number of K users simultaneously communicate sharing the same communication medium. This is achieved by assigning a unique code to each individual user. The assigned codes are selected so as to minimize the interference or the cross-talk between users and to reduce the synchronization loop complexity in the receiver.
In the fields of satellite and mobile communications, spread spectrum (SS) signals served as a basis of the development of CDMA network systems. SS techniques are very popular in a wide variety of fields such as satellite communications, mobile communications, naval and avionics communication systems, distance or range measurement, high resolution target and direction finding systems. There are two categories of SS systems: direct sequence (DS) system, in which each information bit is multiplied by a temporal pseudo-random sequence, and a frequency hopping (FH) system, in which the carrier frequency of a narrow-band information transmitted signal is switched (or hopped) at a random and discrete method. Slow frequency hopping (SFH) means that only one frequency-hop is achieved per bit, however, fast frequency hopping (FFH) means that a number of frequency hops are achieved for every information bit.
The Prior Art FFHSS Transmission System
In a conventional FFHSS transmitter, as shown in
FIG. 1
, the data modulated signal is multiplied by the output of the frequency hopping synthesizer
104
using the first multiplier
102
1
. The frequency synthesizer
104
output signal is a wide band time periodic deterministic signal with time period equal to the duration of a one data bit modulated signal (Tb). In the following, it is assumed that only two kinds of information will be transmitted 1 and 0;
FIG. 3A
shows a sequence of four bits; 1010. Each bit period Tb is divided into an integer number (M) of time intervals Tc=Tb/M called chips. During every chip interval no more than one discrete frequency (or frequency band) from an available set of M frequencies (or frequency bands) is used in the frequency synthesizer
104
output signal. The M available frequencies are assigned to the M chip intervals as prescribed by the selected code from the code generator
1052
FIG. 3D
shows an example where the integer M is equal to 5, hence the code is composed from 5 frequencies f
1
, f
2
, f
3
, f
4
and f
5
. The order of frequencies In the selected code is f
3
, f
1
, f
4
, f
2
and f
5
; which means that the frequency f
3
is transmitted during the first chip interval, f
1
is transmitted during the second chip interval, . . . , and the last frequency f
5
is transmitted during the fifth chip interval. The modulation operation using the first multiplier
102
1
spreads the data modulated signal energy over a bandwidth, called spread spectrum bandwidth Wss=M*Wb, which is M times larger than the data modulated signal bandwidth Wb. Hence, the FFHSS encoding operation cuts the modulated signal energy in time into M pieces, and shifts the frequency band of each piece by an amount corresponding to the FFH code. The frequency domain of the first multiplier
102
1
output signal Is usually referred as the intermediate frequency. The second modulation, achieved using multiplier
102
2
, shifts all the SS signal to the carrier frequency fixed by the oscillator
103
. The multiplier
102
2
output signal is fed to the emitting or transmitting antenna
106
.
Receiver System
In the conventional FFHSS receiver, as shown in
FIG. 2
, a receiving antenna
151
provides the received FFHSS signal. A local oscillator
153
generates a signal for shifting a frequency band of a received signal to a band of an intermediate frequency. A multiplier
152
1
multiplies the received signal by the local oscillator
153
output signal for shifting the frequency band to the base band domain if the synchronization is well established between the received frequencies and the locally generated frequencies. The band pass filter (BPF)
161
limits the band of the output signal of the first multiplier
152
1
to the SS bandwidth Wss=M*Wb. A hopping synthesizer
154
outputs an SS signal similar to the transmitter hopping synthesizer
104
corresponding to the selected FFH code. The second multiplier
152
2
multiplies the BPF
161
output signal with the hopping synthesizer
154
output signal, the product inputs the low pass filter LPF
162
which limits the band of the output signal of the second multiplier
152
2
to the original data modulated signal bandwidth Wb. A power measuring device
157
measures a detection power for a one bit portion from the low pass filter
162
; on the basis of this power measurement, the hopping sequence phase control equipment
166
controls the hopping synthesizer via a code generator
155
to continuously shift its hopping sequence until full synchronization is established between the received signal hopping sequence and the locally generated hopping sequence. A decision circuit
158
receives the output of the low pass filter
162
and decides about the received output.
Illustrative Example
FIG. 3
illustrates the signal evolution through the various major signal processing steps in the prior art FFHSS transmitter and receiver.
FIG. 3A
depicts a sequence of 4 data bits (
1010
) in the logical state.
FIG. 3B
shows the data modulated signal at the first multiplier
102
1
input. In FFHSS systems, frequency shift keying (FSK) and phase shift keying (PSK) are the most popular modulation techniques in mobile radio communications. Amplitude shift keying (ASK) is less robust in wireless communications. Since only two types of information are considered, 1 and 0, only the binary cases of the modulation schemes, (binary ASK, FSK and PSK), are considered.
FIG. 3C
shows the time (Tb) versus frequency bandwidth (Wb) allocated to the data modulated signal in the first multiplier
102
1
input during each data bit.
FIG. 3D
shows the time (Tb) versus frequency bandwidth (Wss=
5
*Wb) allocated to the spread spectrum signal in the first multiplier
102
1
output during each data bit. Each bit energy is distributed in 5 pieces, each of which is of Wb frequency bandwidth and TcT=b/5 chip time duration. The time and frequency distribution of the band pass filter (BPF)
161
output signal is similar to the time versus frequency bandwidth allocated to the spread spectrum signal in the first multiplier
102
output depicted by
FIG. 3D
in absence of multiple access interference. The time and frequency distribution (or occupancy) of the low pass filter (LPF)
162
output signal is depicted by FIG.
3
E.
Discussion of Some Points
In prior art FFHSS techniques, the frequency synthesizer hopping rate is usually considered the major limitation. The FFH encoding/decoding stages require chip rate frequency hopping synthesizers which substantially increases the system cost. Before effective data transmission or reception, the frequency hopping synthesizer (FHS) output is determined (or fixed). During the transmission or reception process the FHS output is a deterministic signal. In the transmission system, only the data signal is random. In principle, only the data rate limits the transmitter minimum rate. Howe
Fathallah Habib
La Rochelle Sophie
Rusch Leslie Ann
Anglehart James
Mullen Thomas
Renault Ogilvy
Universite Laval
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