Communications: directive radio wave systems and devices (e.g. – Directive – Including directive communication system
Reexamination Certificate
2001-09-20
2003-11-18
Issing, Gregory C. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including directive communication system
C370S345000
Reexamination Certificate
active
06650289
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to point to point wireless data links.
2. Description of Related Art
FIG. 1
shows a block diagram of a prior art system
110
at a site linked to another site by a single point to point wireless data link. In operation in the transmit mode, a data port
112
is connected to a protocol engine
114
to generate the link data stream. Protocol engine
114
adds link management overhead and re-organizes the frame. A coding unit
116
adds redundancy to the data for the purpose of forward error correction. A modulator
118
translates the bits streams to quadrature amplitude modulation (QAM) symbols (mapping). A transmitter
122
up-converts the symbols to high frequency from IF/baseband frequency. In operation in the receive mode, the symbols received are down-converted by receiver
124
to IF/baseband frequency. A demodulator
120
translates the symbols into bit streams. Decoding unit
116
removes redundant data for forward error correction. Protocol engine
114
removes overhead and recovers the original frames. The data stream is outputted through data port
112
. A duplexer or T/R switch
126
allows frequency division duplex (FDD) or time division duplex (TDD). In most cases an antenna
128
has a single polarized feeding network (vertical, horizontal, left hand circular, or right hand circular). A control unit
130
coordinates the link internal and external operations.
There is a fundamental limit on the capacity of a link which can be achieved for a certain bandwidth B. Capacity can be explained as the maximum number of bits that can be transmitted per second with a probability of error arbitrarily close to zero. According to Shannon's well known capacity theorem, the error free capacity C of a noise limited link of bandwidth B is:
C
=
B
·
log
2
(
1
+
SNR
)
bits
second
(
1
)
Where SNR is the signal to noise ratio, which in turn holds all the parameters of the link budget.
In
FIG. 1
, the usage of a dual polarization feeding network for antenna
128
, provides a doubling of the number of links
110
for the same frequency and therefore a doubling of capacity. There is an additional cost in complexity, because most of the hardware has to be doubled and an additional function called cross polar interference canceller (XPIC) has to be added. Because the two polarizations are orthogonal, the two links
110
are independent and the required transmit power for each link remains the same.
However, if it is desired to increase the capacity by more than two-fold other methods must be adopted because there are only two orthogonal polarizations. Based on Shannon's theorem, in order to increase capacity in link(s)
10
, signal power needs to be increased by a much larger amount, assuming other variables such as noise, bandwidth, antenna gain and receiver sensitivity are constant. From equation (1), it can be shown that if the capacity of link(s)
110
is doubled, assuming the other variables are constant, the required signal power must be approximately squared.
In order to provide a more reasonable increase in power for a given increase in capacity, multiple (y) independent links operating on the same frequency can be used. Independent links are links whose signals are differentiable from one another. It can be shown that the capacity of y independent links is proportional to the number of links multiplied by the capacity of one link. See for example “Channel capacity of two antenna BLAST architecture, S. L. Lokya, Electronics Letters, Aug. 19, 1999, Volume 35 No. 17.
The number y can also be thought of as equaling the number of transmitting antennas. As an example if the bit rate of a system using one dual polarized transmitting
C
=
y
·
log
2
(
1
+
SNR
y
)
bits
second
(
2
)
antenna (two links) is 622 Mbits/sec, then for two dual polarized transmitting antennas (four links) the bit rate will be 1244 Mbits/sec
The challenge is therefore to create multiple links operating over the same frequency which are independent of or isolated from each other.
There has been some reporting in the prior art of the usage of the same number of transmitting and receiving antennas for multiple links when the link ends are not in line of sight of each other. For these systems to be efficient, multipath is assumed.
However, for point to point links operating with a clear line of sight between the two ends of the links, in the prior art the number of receiving antennas is larger than the number of transmission antennas in order to separate out the information belonging to each link. See for example,
Advances in Spectrum Analysis and Array Processing
, Volume 3, Chapter 1 by Simon Haykin, Prentice Hall PTR, May 1995, ISBN 130615404. The complexity of the processing is also high.
What is needed in the art is a system and method for multiple point to point line of sight links where the number of receiving antennas can be equal to the number of transmission antennas. What is also needed in the art is an improved beam forming algorithm using multiple statistical values calculated from a constellation or subsets thereof to allow faster convergence.
SUMMARY OF THE INVENTION
According to the present invention there is provided a system for increasing the potential communication capacity between two sites within line of sight of one another, comprising: a plurality of point to point links between the two sites, the plurality of links including at least one beam forming unit and a total of four antennas, wherein a geometrical configuration of the four antennas allows the at least one beam forming unit to substantially differentiate between signals transmitted over the plurality of links, thereby increasing the potential communication capacity compared to a system with only two of the four antennas. According to the present invention, there is also provided a method for increasing the potential communication capacity between two sites within line of sight of one another, including the steps of: providing a total of four antennas at the two sites; selecting a range of values for a function of electrical phase difference for signals to be transmitted between the antennas; and conforming a geometrical configuration of the antennas to a value within the range, wherein the range allows substantial differentiation of transmitted signals and therefore an increase in potential capacity compared to a method providing only two of the four antennas.
According to the present invention, there is further provided a method for increasing the communication capacity between two sites within line of sight of one another, including the steps of: providing two antennas at one site of the two sites, wherein the two antennas at the one site and two antennas at a second site of the two sites have a geometrical configuration which allows substantial differentiation of transmitted signals; and transmitting signals from each of the two antennas at the one site to each of the two antennas at the second site, thereby increasing the capacity compared to a method providing only one of the two antennas at the one site.
According to the present invention, there is still further provided, a method for increasing the communication capacity between two sites within line of site of one another, including the steps of: providing two antennas at one site of the two sites, wherein the two antennas at the one site and two antennas at a second site of the two sites have a geometrical configuration which allows substantial differentiation of signals; each of the two antennas at the one site receiving signals from each of the two antennas at the second site; and performing beam forming to differentiate between signals originating from each of the two antennas at the second site, thereby increasing the capacity compared to a method providing only one of the two antennas at the one site.
According to the present invention, there is still further provided a method for performing beam forming on signals which include symbols enco
Levy Shmuel
Livneh Noam
Saguy Ami
Stern Ori
Issing Gregory C.
Microwave Networks Incorporated
Oliff & Berridg,e PLC
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