U.S. patent number RE48,087 [Application Number 15/487,730] was granted by the patent office on 2020-07-07 for position adjusted guard time interval for ofdm-communications system.
This patent grant is currently assigned to Tamiras Per Pte. Ltd., LLC. The grantee listed for this patent is TAMIRAS PER PTE. LTD., LLC. Invention is credited to Peter Karlsson, Rickard Ljung.
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United States Patent |
RE48,087 |
Ljung , et al. |
July 7, 2020 |
Position adjusted guard time interval for OFDM-communications
system
Abstract
A device and a method within a communications system where at
least some part of the transmission is executed by means of radio
waves, and where symbols are transmitted by means of Orthogonal
Frequency Divisional Multiplexing, so called OFDM-technology,
between a transmitting unit and a receiving unit, at which the
symbol transmission is executed over a transmission channel in
blocks of binary digits with a guard interval GI between said
blocks, where transmitting unit is equipped with means to control
the length of the guard interval (GI) with regard to the physical
conditions for/of the transmission channel, so that the guard
interval can be reduced without the disturbance susceptibility
being increased, but instead increasing the capacity/throughput of
the transmission channel by the time that is set free/made
available being used to transmit information. One embodiment of the
invention includes a guard interval adjustment unit connected to
other OFDM-equipment in transmitting and/or receiving unit.
Inventors: |
Ljung; Rickard (Malmo,
SE), Karlsson; Peter (Lund, SE) |
Applicant: |
Name |
City |
State |
Country |
Type |
TAMIRAS PER PTE. LTD., LLC |
Dover |
DE |
US |
|
|
Assignee: |
Tamiras Per Pte. Ltd., LLC
(Dover, DE)
|
Family
ID: |
20290781 |
Appl.
No.: |
15/487,730 |
Filed: |
April 14, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10549846 |
Sep 17, 2013 |
8537759 |
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PCT/SE2004/000390 |
Mar 17, 2004 |
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Reissue of: |
14011188 |
Aug 27, 2013 |
9008026 |
Apr 14, 2015 |
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Foreign Application Priority Data
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Mar 25, 2003 [SE] |
|
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0300824 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04L
27/2605 (20130101); H04L 27/2607 (20130101); H04L
27/2646 (20130101); H04L 27/2607 (20130101); H04L
27/2646 (20130101); H04L 27/2605 (20130101); H04L
25/0224 (20130101); H04L 25/0224 (20130101); H04L
25/0212 (20130101); H04L 25/0212 (20130101) |
Current International
Class: |
H04L
27/26 (20060101); H04L 25/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1 014 639 |
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Jun 2000 |
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EP |
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10-308716 |
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Nov 1998 |
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JP |
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2001-069110 |
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Mar 2001 |
|
JP |
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504897 |
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May 1997 |
|
SE |
|
Other References
Lee, Donghoon et al., "Coarse Symbol Synchronization Algorithms for
OFDM Systems in Multipath Channels," IEEE Communications Letters,
Oct. 2002, vol. 6, iss. 10, pp. 446-448. cited by applicant .
Tonello et al., "Analysis of the Uplink of an Asynchronous
Multi-user DMT OFDMA System Impaired by Time Offsets, Frequency
Offsets, and Multi-path Fading", 52.sup.nd Vehicular Technology
Conference, vol. 3, pp. 1094-1099 (2000). cited by applicant .
Baum, "A Synchronous Coherent OFDM Air Interface Concept for High
Data Rate Cellular System", 48.sup.th IEEE Vehicular Technology
Conference, vol. 3, pp. 2222-2226 (1998). cited by applicant .
Final Rejection on U.S. Appl. No. 10/549,846, mailed May 8, 2012.
cited by applicant .
Final Rejection on U.S. Appl. No. 10/549,846, mailed Aug. 21, 2009.
cited by applicant .
International Preliminary Report on Patentability for
PCT/SE2004/000390, completed Jun. 7, 2005. cited by applicant .
International Search Report and Written Opinion for
PCT/SE2004/000390, mailed Jun. 21, 2004. cited by applicant .
Non-Final Office Action on U.S. Appl. No. 10/549,846, mailed Dec.
26, 2008. cited by applicant .
Non-Final Office Action on U.S. Appl. No. 10/549,846, mailed Aug.
5, 2011. cited by applicant .
Notice of Allowance on U.S. Appl. No. 10/549,846, mailed May 14,
2013. cited by applicant.
|
Primary Examiner: England; David E
Attorney, Agent or Firm: Foley & Lardner LLP
Parent Case Text
CROSS-REFERENCE TO RELATED .Iadd.PATENT .Iaddend.APPLICATIONS
This application .Iadd.is a Broadening Reissue of U.S. Pat. No.
9,008,026 (previously U.S. patent application Ser. No. 14/011,188),
filed Aug. 27, 2013, which .Iaddend.is a continuation of U.S.
application Ser. No. 10/549,846, filed Nov. 29, 2006, which is a
National Stage of PCT/SE04/00390, filed Mar. 17, 2004 and claims
the benefit of Swedish Application No. 0300824-0, filed Mar. 25,
2003. The entire contents of which are incorporated herein by
reference.
Claims
The invention claimed is:
1. A transmitting unit comprising: an electronic processor
configured to: determine a characteristic of a transmission channel
between the transmitting unit and a receiving unit, wherein symbols
are transmitted by Orthogonal Frequency Divisional Multiplexing
(OFDM) between the transmitting unit and the receiving unit,
wherein the symbol transmission is executed over the transmission
channel in blocks of binary digits with a guard interval between
the blocks; determine a size of a cell in which the transmitting
unit transmits based on the determined characteristic of the
channel; determine a guard interval length that causes intersymbol
interference from other transmitting units to be below a
predetermined threshold value based on the determined size of the
cell; and adjust the guard interval of the transmitting unit to be
the guard interval length to reduce the intersymbol interference
from the other transmitting units to be below the predetermined
threshold value.
2. The transmitting unit of claim 1, wherein the electronic
processor is further configured to adjust the length of the guard
interval based upon an adjustable guard interval parameter.
3. The transmitting unit of claim 2, wherein the guard interval
parameter can be changed via .[.handling/managing.]. .Iadd. a
management .Iaddend.system .[.SNMP.]..
4. The transmitting unit of claim 1, wherein the guard interval
length is determined to be a radius of the cell in meters
multiplied by six.
5. The transmitting unit of claim 1, wherein the characteristic of
a transmission channel between the transmitting unit and the
receiving unit takes into consideration .Iadd.a delay of
.Iaddend.an impulse response of the transmission channel.
6. A method comprising: determining a characteristic of a
transmission channel between a transmitting unit and a receiving
unit, wherein symbols are transmitted by Orthogonal Frequency
Divisional Multiplexing (OFDM) between the transmitting unit and
the receiving unit, wherein the symbol transmission is executed
over the transmission channel in blocks of binary digits with a
guard interval between the blocks; determining a size of a cell in
which the transmitting unit transmits based on the determined
characteristic of the channel; determining a guard interval length
that causes intersymbol interference from other transmitting units
to be below a predetermined threshold value based on the determined
size of the cell; and adjusting the guard interval of the
transmitting unit to be the guard interval length to reduce the
intersymbol interference from the other transmitting units to be
below the predetermined threshold value.
7. The method of claim 6, wherein the determining the
characteristic of the transmission channel comprises determining
.Iadd.a delay of .Iaddend.an impulse response of the channel.
8. The method of claim 6, further comprising estimating a received
guard interval.
9. The method of claim 8, wherein the estimating is constituted by
an operator decided guard interval.
10. The method of claim 8, wherein the estimating comprises
calculating an estimate of a difference between a received and an
expected block start point of time.[., the so called "coarse
framing offset".]. .delta..sub.int according to the formula:
.delta..times..times..times. ##EQU00002## where n=0, 1, 2 . . . ,
2G+2N-1 and G indicates the length of a sample at the guard
interval and y.sub.i indicates the received signal for/of the ith
OFDM-symbol in a time domain.
11. The method of claim 6, wherein the determining the guard
interval length comprises multiplying a radius of the cell in
meters by six.
12. A communications system comprising: a transmission unit
configured to: determine a characteristic of a transmission channel
between the transmitting unit and a receiving unit, wherein symbols
are transmitted by Orthogonal Frequency Divisional Multiplexing
(OFDM) between the transmitting unit and the receiving unit,
wherein the symbol transmission is executed over the transmission
channel in blocks of binary digits with a guard interval between
the blocks; determine a size of a cell in which the transmitting
unit transmits based on the determined characteristic of the
channel; determine a guard interval length that causes intersymbol
interference from other transmitting units to be below a
predetermined threshold value based on the determined size of the
cell; and adjust the guard interval of the transmitting unit to be
the guard interval length to reduce the intersymbol interference
from the other transmitting units to be below the predetermined
threshold value.
13. The communications system of claim 12, further comprising a
receiving unit configured to adjust the receiving unit according to
the current guard interval in the cell.
14. The communication system of claim 13, wherein the receiving
unit is further configured to estimate a length of a received guard
interval.
15. The communication system of claim 14, wherein the receiving
unit is configured to calculate an estimate of a difference between
a received block and an expected block start point of time.[., the
so called "coarse framing offset".]. .delta..sub.int according to
the formula: .delta..times..times..times. ##EQU00003## where n=0,
1, 2 . . . , 2G+2N-1 and G indicates the length of a sample at the
guard interval and y.sub.i indicates the received signal for/of the
ith OFDM-symbol in a time domain.
.Iadd.16. A transmitting unit comprising: an electronic processor
configured to: determine a characteristic of a transmission
channel, wherein a symbol transmission is executed over the
transmission channel in blocks of binary digits with a guard
interval between the blocks; determine a size of a cell in which a
transmitting unit transmits based on the determined characteristic
of the transmission channel; determine a guard interval length that
causes intersymbol interference from other transmitting units to be
below a predetermined threshold value based on the determined size
of the cell; and adjust the guard interval of the transmitting unit
to be the guard interval length to reduce the intersymbol
interference from the other transmitting units to be below the
predetermined threshold value..Iaddend.
Description
TECHNICAL FIELD
The present invention relates to a transmission method and a
transmission device, and a reception method and a reception device,
and a system using them. More particularly is related to such ones
within communications systems which are using OFDM (Orthogonal
Frequency Divisional Multiplexing).
PRIOR ART
An information transmission system generally transmits symbols,
where each symbol for instance can be a sequence of ones and zeros
in succession over a transmission channel, and there occupies a
frequency band which of necessity must be wider/larger than the
inverse of the time length of a symbol.
When the transmission speed is increased it finally will be
impossible to guarantee that the transmission channel retains
identical amplitude and phase characteristics over the whole
frequency range which constitutes the pass band. These in this way
developed distortions in the channel give rise to interference
between symbols, which interference can be fought against by means
of an equalizing device, a so called equalizer. Such systems,
however, are rather complex.
One technology to handle/manage this problem includes that the
signal which shall be transmitted is spread/distributed over a
large number of carriers in a parallel way, individually modulated
with/by low speed. Because the speed is low, the pass band width
which is needed is smaller, and therefore it is more probable that
amplitude and phase characteristics will be identical for all
frequencies which constitute this band. This technology is known to
the expert as "Orthogonal Frequency Divisional Multiplexing" or
OFDM. Frequency spectra of the signals which modulate the carriers
overlap in such a way that they fulfill the conditions for
orthogonality, which makes elimination of interference between
modulated sub-carriers possible and also makes it possible to
achieve much larger spectral benefit.
The space between two adjacent sub-carriers corresponds to the
inverse of the time length of a symbol.
The OFDM-modulation is usually incorporated with a
Fourier-transform, so that it can be implemented by means of FTT
(Fast Fourier Transform). The main steps to implement transmission
of a message by means of OFDM-modulation is specified below.
First of all the binary data which constitute the message which
shall be transmitted in data blocks are grouped. Each one of these
blocks is transmitted independent of each other and constitutes,
after base band modulation, an OFDM-signal. In each data block also
the binary digits are grouped in subset. Each subset after that is
subject to a "bijective mapping" over a discrete amount of points
in the Fresnel-space, where each point represents a possible phase
and amplitude. If, for instance, a message consisting of the
following series of bits (00001110010001111000 . . . ) is
considered, it will be possible to from that extract a block of 16
bits 0000111001000111, with which are associated, by mapping, the
following amount of points in the complex plane:
1+j, 1+j, -1-j, 1-j, -1+j, 1+j, -1+j, -1-j.
This consequently gives an amount consisting of eight complex
elements, which define a vector V.
An inverse discrete Fourier Transform with a matrix A then is
allowed to be active/influence on the vectors V which have been
obtained from the original message, which gives rise to an
OFDM-signal consisting of a series of complex amplitudes. Each
transmitted/transferred symbol then is received, after having
passed the transmission channel, by a demodulator, from which there
is extracted a vector V which holds complex elements, by
multiplying the amplitudes which describe the symbol by/with a
direct discrete Fourier-transform matrix A' so that A*A'=I, where I
indicates the unit matrix.
The use of a decision criterion based on "Maximum likelihood" on
the real part and on the imaginary part of each vector V' makes
regain of the original symbol sequence possible and further
reconstruction of the to that associated binary elements.
The different symbols in each block are linked up due to the linear
combination which is obtained by multiplying the elements in the
transmitted vector V by the inverse discrete Fourier-transform
matrix A. This linear combination guarantees a certain degree of
hardiness and protects the symbols against interference between
complex symbols within one and the same OFDM-symbol.
On the other hand, this protection/guard effect does not extend
from one OFDM-symbol to another, that is, not from one block to
another.
In order to prevent interference between blocks, it is known that a
technology can be used which includes to arrange a time period of
silence or non-transmission, also called guard (time) interval,
between to successive symbols.
In prior art, however, the guard interval preceding current symbol
is decided pragmatically, usually after an evaluation by an expert,
of the time period which is necessary to attenuate the echo of the
transmission of preceding OFDM-symbol.
Some variants including adjustment of the guard interval are also
described below. U.S. Pat. No. 6,115,354-A shows a method which
adapts the "guard intervals for the OFDM symbols" to those
differences in delay which exist in the network. The first guard
interval for a frame, however, is adjusted to "worst case" (see
column 2, line 9-column 3, line 9). According to this document, the
flexibility of the guard interval results in that the OFDM-system
can be optimized both from implementation and network planning
perspective (see column 3, lines 36-40).
U.S. Pat. No. 6,175,550-B1 shows an OFDM-system in which a "guard
time interval" is adjusted dynamically depending on the
communication conditions in the environment (see column 3, lines
3-65), column 6, lines 24-32, and independent patent claims).
EP-1065855-A1 shows adjustment of "cyclic extensions" in an
OFDM-system. The length of the cyclic extension is adjusted to the
delays which are existing at/in the channel. (See abstract)
WO97/30531-A1 says that a "guard space" can be varied so that a
minimal guard space is used (see patent claims).
EP-1061687-A1 shows automatic adjustment of "guard interval"
depending on the quality of received signal.
EP-1014639-A2 shows an OFDM-transmitter/receiver for which an
optimal selection of guard interval is decided.
SUMMARY OF THE INVENTION
At construction of a communications system it is in most cases
adjusted to a "worst case". This results in that in all other cases
than the worst possible, capacity will be wasted. This invention
solves a large part of above mentioned waste of/for an OFDM-system,
where the capacity goes down proportionally with the guard interval
against the time delay of the channel, "Guard Interval GI". The
problem is solved by adjustment of the basic OFDM-structure for
each transmitter/base station so that not utilized time between
symbols will be negligible and the larger part of the transmitted
power can be utilized by the terminals.
The invention relates to a communications system where at least
some part of the transmission is executed by means of radio waves,
and where symbols are transmitted by means of Orthogonal Frequency
Divisional Multiplexing, so called OFDM-technology, between a
transmitting unit and a receiving unit, at which the symbol
transmission is executed over a transmission channel in blocks of
binary digits with a guard interval GI between said blocks, where
transmitting unit is equipped with means to control the length of
the guard interval with regard to the physical conditions for/of
the transmission channel.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in more details in the following
with reference to the enclosed drawings, of which:
FIG. 1a shows symbol start points of time and guard intervals in a
symbol transmission sequence;
FIG. 1b shows a block diagram over a system solution comprising
guard interval adjustment according to one embodiment of the
present invention;
FIG. 2 shows geographical distribution of cells and to that
associated guard interval;
FIG. 3 shows a block diagram over a two-way communications system
where the guard interval adjustment is based on current channel
estimate from a WCDMA-receiver;
FIG. 4 shows a block diagram over a system solution according to
another embodiment of the present invention; and
FIG. 5 shows a block diagram of a guard interval designed as a copy
of the last symbols in each block and are also inserted first in
each block before transmission.
DESCRIPTION OF PREFERRED EMBODIMENTS
An OFDM-system is defined by certain basic parameters such as the
number of FTT-points, the size of the so called guard interval GI,
sampling speed, bandwidth etc. Several of these parameters are
selected for the worst case, that is, for the most difficult
possible communications condition in which the system shall be
functioning satisfactorily. The guard interval means that power and
time between OFDM-symbols are not utilized. The guard interval is
decided for such a system so that all receivers shall have
possibility to receive and detect symbols without intersymbol
interference ISI occurring. The worst conceivable propagation delay
in the radio channel therefore will be dimensioning for the length
of the guard interval.
At normal use of a communications system, however, there sometimes
will be considerably better conditions, which means that the
parameters which have been selected at the design/construction of
the system are too resource exacting because they are not adjusted
to at present existing conditions. The inventors have realized
that, by adequate measures, it will be possible to, by reducing the
guard interval from 1/4 to 1/8, 1/16, and 1/32, increase the system
capacity correspondingly. In a system for distribution
"one-to-many" (point-to-multipoint), however, it is not practical
to change the guard interval GI for each separate receiver because
all OFDM-symbols are transmitted to all receivers within the
coverage area.
One of the ideas behind the invention includes to by, in access
points AP or base stations BS in a communications system,
designing/constructing the OFDM-transmitter in such a way that the
guard interval GI can be adjusted as an installation parameter, the
guard interval of the OFDM-signal can be adjusted to existing
channel conditions from the transmitter within each separate
coverage area, also called cell. When a new receiver is connected
in this cell, it will be possible to, for instance, via an
adjustment algorithm, for instance such as is described by Kim et
al, "Frame Selection Algorithm with Adaptive FFT input for OFDM
Systems", at ICC, the International Chamber of Commerce 2002,
automatically change to the guard interval selection of this
cell.
The algorithm is functioning in such a way that it identifies where
the OFDM-symbol really starts, that is, estimates how wide/large
guard interval that is used. In FIG. 1 a consequently is visualized
that the algorithm identifies the occasions which have been marked
with arrows A1, A2, B1, B2 at the time axis, that is, where
OFDM-symbols start. The figure includes examples of two different
places, Place 1 and Place 2, where on the one hand a long and on
the other a short guard interval is used. By the algorithm, which
is built-in in receiving unit, here called the terminal,
identifying where the OFDM-symbol starts, the guard interval can be
selected by the operator based on what each specific cell looks
like, without any setting needed to be changed in the terminal when
the terminal is roved between different cells. In other words, the
algorithm executes frame synchronization by in receiving unit
calculating an estimate of the guard interval GI by using the in
the time domain received signals, and by calculating an estimate of
the difference between received and expected frame start time, the
so called "coarse framing offset", {circumflex over
(.delta.)}.sub.int expressed as
.delta..times..times..times. ##EQU00001## where n=0, 1, 2 . . . ,
2G+2N-1 and G indicating the sample length at the guard
interval.
The guard interval GI then can, as indicated above, in a flexible
way be adjusted to/for each cell in the communications system, and
the capacity/throughput can be improved in the system. This is
illustrated in FIG. 1b A communications system consists of a
network core 101 which via a connection 151 is in connection with
two adjacent/nearby located transmission units 111, 112. The
physical distance between the transmission units 111, 112 is
.alpha.. Each transmission unit has, via ether communications 151,
152, contact with at least one terminal 121, 122. A network monitor
unit 131 monitors the system and handles system parameters. The
network monitor unit 131, together with the OFDM-modules 115, 116,
attends to that the guard interval GI is adjusted to the distance a
between the transmission units in such a way that the guard
interval is adjusted to the cell radius. Suitable guard interval
parameter GI is applied to/in OFDM-module 115, 116, and further
respective terminal 121, 122 is arranged to adjust itself to the
guard interval GI of the cell. The guard interval GI should be
selected so that it corresponds to the maximum time dispersion a
received signal can experience in respective coverage area. For
instance, if there in a cellular system is about 100 m cell radius,
the distance for a reflected signal can be up to about 200 m. The
flexible guard interval GI consequently is adjusted to handle the
delay 200 m, which corresponds to a guard interval GI of about 600
ns. For a cell with a radius of 200 m, about 1200 ns is selected,
that is, so that the length of the guard interval in nanoseconds is
set to, on the whole, six times the cell radius in meters. The
principle for selection of size of the guard interval GI is
illustrated in FIG. 2.
In one preferred embodiment, the parameter GI can be preset by the
operator or the system administrator via the ordinary interfaces
for setting/adjustment of a radio channel, modulation, etc., in
each access point AP and each base station BS. Setting of the
parameter guard interval GI by/from a centrally located
administrator, operator or separate user results in that one
continuously achieves optimal performance without need to change
hardware at the place of respective access point AP or base station
BS. The invention by that includes the possibility to improve
capacity performance for an OFDM-system both initially and when the
system is extended by more access points AP or base stations
BS.
In a cellular two-way communications system based on OFDM and
according to one embodiment of the invention, consequently a
flexible adjustment of the guard interval GI is achieved. Said
guard interval GI is adjusted to the existing transmission
conditions in each cell. In another embodiment, each terminal 121,
122 is equipped with an automatic adjustment unit 310 which
automatically adjusts the guard interval GI in the cell of current
interest so that mobile units can move in the cellular system and
adjust their reception to the flexible parameter selections of the
different cells.
In tests an OFDM-system according to one embodiment of the
invention and based on IEEE 802.11a has been tested and verified in
an indoor environment. In these comparatively small cells with
maximum distances of about 50 m between transmitter and receiver,
no symbol errors due to time dispersion of the channel have been
measured. The standardized guard interval for OFDM in IEEE 802.11a
is 800 ns, which by use of one embodiment of the present invention
should be possible to reduce to 400 ns in most indoor
environments.
In FIG. 3 a structure for/of the OFDM-module 115, 116 according to
FIG. 1b is described. An automatic guard interval adjustment unit
310 is in connection with other electronics 315 for execution of
OFDM. The guard interval adjustment unit 310 calculates the under
the circumstances best guard interval and transmits this to other
electronics 315 for execution of OFDM.
In one preferred embodiment, the guard interval is adjusted
according to longest delay in the impulse response. Because the
delay, however, maximally can be about the time it takes for the
signal to travel twice the cell radius, this value can be supposed
to be a suitable guard interval, that is (distance between base
stations)/(3*18.sup.8). As a rule of thumb, it can be assumed that
radio waves travel 300 m in 1 microsecond, and if there is this
distance between BS, and consequently 150 m max. between BS and
terminal, the guard interval should be 1 microsecond in this
typical example in city environment. For BS in suburb with 1 km
between BS, one should have 3 microseconds guard interval.
It is not necessary to, in all situations, calculate the guard
interval in an embodiment of the invention. The guard interval can
be calculated before the base station is installed, for instance at
the cell planning occasion, and after that only when changes in the
network planning are made.
The guard interval consequently need not necessarily be calculated
anywhere in the system. If one concentrates with more BS one can,
via handling/managing system SNMP, reduce the guard interval with
corresponding new shorter BS-distance so that more OFDM-symbols are
transmitted within each time frame or block of symbols.
In yet another preferred embodiment there is in a receiver which
receives the OFDM-signal a receiver adjustment module arranged
which adjusts said receiver according to the current guard interval
in the cell. Said adjustment is made by use of an adjustment
algorithm described in "Frame Selection Algorithm with Adaptive FFT
Input for OFDM Systems, ICC 2002", which has been described
above.
In yet another embodiment of the present invention, an adjustable
guard interval GI in a two-way communications system according to
FIG. 4 is provided. In a two-way communications system, where
communication is executed both in an uplink UL and in a downlink
NL, an adjustment of the parameters of the OFDM-signal can be made
to existing communication conditions, which increases the
capacity/throughput of the system. In order to receive a
transmitted signal, an estimation of the qualities of the
transmission channel is made. For instance, the delays and the
changes of amplitude which occur between transmitter and receiver
are measured. This so called channel information from the receivers
of the system by that can be utilized for the generation of the
signals in the OFDM-transmitter, where one, by knowing the impulse
response of the channel, can adjust the guard interval GI of the
OFDM-signal to existing channel conditions from the transmitter
within each separate coverage area, and also for each terminal
within the coverage area. The impulse response is produced in
receiving unit by estimating the channel from a transmitted symbol
in a so called "preamble". The principle is illustrated in FIG. 4,
where the guard interval GI is adjusted based on current channel
estimate from a WCDMA-receiver 430. The channel estimate is
transmitted/transferred to the OFDM-unit 440 where the guard
interval GI is adjusted on basis of current channel estimate.
A radio network control unit RNC is in (wire) connection with a
number of nodes, one of which is node B.
The guard interval should be equal in up and down link; then in
systems using TDD (Time Division Duplex), the up and downlink of
the channel is identical, and the same guard interval should be
used.
The system in FIG. 4 also includes a so called dual mode terminal
450, which is arranged to receive both OFDM- and WCDMA-signals.
This terminal 450 includes a channel estimation unit 460 for
production of channel estimates and an estimation unit 470 for
estimation of impulse responses from WCDMA-training sequences. Said
estimate and estimation then is used to adjust the guard interval
OFDM in downlink.
In yet another preferred embodiment, the guard interval is designed
as a copy of the last symbols in each block. These symbols are
copied and are also inserted first in each block before
transmission.
* * * * *