Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1999-03-23
2002-09-17
Chin, Stephen (Department: 2634)
Pulse or digital communications
Transceivers
Modems
C375S219000, C375S262000, C370S252000
Reexamination Certificate
active
06452964
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an adaptive modulation method which can be applied to a mobile communications system, and more particularly to an adaptive modulation method for changing a modulation level based on a channel power gain.
2. Description of the Related Art
In digital communications, particularly in mobile communications systems, it is widely known that line quality (such as a bit error rate) significantly degrades by variations in the received signal-to-noise ratio (SNR) due to fading. This phenomenon occurs when signal waves are affected by delayed signal waves including scattered signal waves. That is, the amplitude and the phase of a signal varies with time. The fading varies occur in the range of several tens of dBs.
The modulation method which is not adaptive to a fading channel (hereinafter referred to as a non-adaptive modulation method) is a method with which a modulation level (and a transmission power) is fixed. If this modulation method is applied, a fairly large link margin must be secured in order to maintain the channel quality allowed by the system in a time period during which the signal level drops due to fading (deep fades). To secure the link margin, for example, a strong error correction which trades off a transmission bandwidth must be applied. Namely, the system to which the non-adaptive modulation method is applied is effectively designed for the worst case channel conditions. Accordingly, with such a system, the channel capacity over fading channels cannot be efficiently utilized.
In recent years, an adaptive modulation method intended to efficiently utilize the capacity over a fading channel has been actively studied and developed.
The bit error rate performance of a modulation method depends on the distance between signal points implemented with the modulation method, that is, on Eb/No (or Es/No). This is common to any modulation methods.
Also an error correction technique intended to improve channel quality is a technique for regularly extending the distance between signal points by using redundant bits.
In fading channels, the probability of occurrence of the worst conditions is considerably small. That is, communications are made under the condition better than the maximum bit error rate allowed by the system in most of the total time. Accordingly, an optimum transmission can be realized by performing control so that the required bit error rate is maintained with an adaptive change of a transmission power level, a transmission symbol rate, a modulation level, a coding rate, or their combination, depending on a channel condition. This is the principle of an adaptive modulation method.
Adaptive modulation techniques provide high average spectral effectively by transmitting at high data rates with higher modulation levels under favorable channel conditions, and reducing throughput via lower modulation levels or transmission-off as the channel degrades.
The measurement of the channel condition can be performed by the instantaneous received SNR and the average received SNR.
FIG. 1A
shows the principle of the adaptive modulation method. In this figure, x(i) is a transmission signal output from a transmitter
1001
at a station A which is a first station, y(i) is a received signal received by a receiver
1003
at a station B which is a second station opposing the station A, g(i) is a time-varying gain due to fading, and n(i) is an additive white Gaussian noise (AWGN).
For a bidirectional communications system, two sets of the system shown in
FIG. 1A
, which has reverse transmission directions, are included.
If the channel
1002
is a channel on which the fading of the link in the first direction (from the station A to the station B) correlates with that of the link in the second direction (from the station B to the station A), for example, a TDD (Time Division Duplex) channel, the following control operations are performed.
A channel estimating unit
1006
within the receiver
1003
at the station B on the first direction link estimates the power gain of the channel
1002
, and notifies of the estimated channel information a demodulator/decoder unit
1007
within the receiver
1003
at the station B. The demodulator/decoder unit
1007
demodulates/decodes the received signal y(i) received from the first direction link after equlization based on the estimated channel information. Furthermore, the channel estimating unit
1006
within the receiver
1003
at the station B notifies an adaptive modulator/encoder unit
1004
within the transmitter
1001
at the station B on the second direction link, of the estimated channel information (or the estimated information obtained by applying an extrapolation-interpolation to the estimated power gain). The adaptive modulator/encoder unit
1004
sets the modulation level information according to the notified estimated channel information, and sends the transmission signal x(i) along with the modulation level information to the second direction link on the channel
1002
.
The channel estimating unit
1006
within the receiver
1003
at the station A on the second direction link estimates the power gain of the channel
1002
, and notifies of the estimated channel information the demodulator/decoder unit
1007
within the receiver
1003
at the station A. The demodulator/decoder unit
1007
demodulates/decodes the received signal y(i) received from the second direction link after equlization based on the estimated channel information. Additionally, the channel estimating unit
1006
within the receiver
1003
at the station A notifies the adaptive modulator/encoder unit
1004
within the transmitter
1001
at the station A on the first direction link, of the estimated channel information (or the estimated information obtained by applying an extrapolation-interpolation to the estimated power gain). The adaptive modulator/encoder unit
1004
sets the modulation level information according to the notified estimated channel information, and sends the transmission signal x(i) along with the modulation level information to the first direction link on the channel
1002
.
In this way, a reciprocating transmission of the modulation level information can be implemented.
In the meantime, if the channel
1002
is a channel on which the fading of the first direction link does not correlate with that of the second direction link, for example, an FDD (Frequency Division Duplex) channel, the following control operations are performed.
First of all, the channel estimating unit
1006
within the receiver
1003
at the station B on the first direction link estimates the power gain of the channel
1002
, and notifies of the estimated channel information the demodulator/decoder unit
1007
within the receiver
1003
at the station B. The demodulator/decoder unit
1007
demodulates/decodes the received signal y(i) received from the first direction link after equlization based on the estimated channel information. Additionally, the channel estimating unit
1006
within the receiver
1003
at the station B feeds back its estimated channel information (or the estimated information obtained by applying an extrapolation-interpolation to the estimated power gain) to the adaptive modulator/encoder unit
1004
within the transmitter
1001
(shown in
FIG. 1A
) at the station A on the first direction link by using a feedback channel
1008
for the first direction link. The adaptive modulator/encoder unit
1004
sets the feedback modulation level information, and sends the transmission signal x(i) along with the modulation level information to the first direction link on the channel
1002
.
Also the second direction link requires exactly the same feedback mechanism as that described above.
In
FIG. 1A
, a transmitting power control unit
1005
within the transmitter
1001
implements the above described power adaptation process.
FIG. 1B
exemplifies the signal point arrangements implemented with respective modulation methods which can be selected by the adaptive modulator/encoder un
Chin Stephen
Katten Muchin Zavis & Rosenman
Kim Heechul
LandOfFree
Adaptive modulation method does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Adaptive modulation method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Adaptive modulation method will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2885168