Pulse or digital communications – Transceivers
Reexamination Certificate
2000-02-04
2004-02-10
Tran, Khai (Department: 2631)
Pulse or digital communications
Transceivers
C375S222000
Reexamination Certificate
active
06690717
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a multi-tone transceiver system for the transmission and reception of high-speed digital data at a rate of tens of Mbps through phone lines, and more particularly, to a multi-tone transceiver system having a widened bandwidth that can be interoperable with Asymmetric Digital Subscriber Line (ADSL) transceiver system.
BACKGROUND OF THE INVENTION
Most popular techniques for a digital subscriber line transmitting high-speed digital data are Discrete Multi-Tone-based Asymmetric Digital Subscriber Line (DMT-based ADSL) which is commercialized recently, and Very-high-speed Digital Subscriber Line (VDSL), which is proceeding for a standard.
The DMT transceiver system is a digital transmission technique for transmission and reception of multi-carrier, and the DMT-based ADSL is a system for transmitting high-speed digital data over the common telephone line by locating IFFT on transmission side and FFT on receiving side.
The DMT transceiver system uses 256 tones that are each 4.3125kHz wide in the downstream direction.
FIG. 1
is a block diagram illustrating system architecture.
In the described DMT transceiver System, an input signal of modulator is Quadrature Amplifier Modulation (QAM) symbol type signal completed in channel coding in early phase. Considering one block as an input from 1 to 512, 1
st
and 257th tone signal is zero, 2~256 tone signals are delivered from early phase, and the others are arranged in the type of signals, in Hermitian symmetric relation with 2~256th tone signals. This arranged block of signal input to the side of Inverse Fast Fourier Transform (IFFT), and the output of the side of IFFT becomes real number signal by exponential modulation. These processes are the method of DMT modulation at transmission side.
Next, the modulated symbol block of real signal is converted to serial signal by Parallel to Serial (P/S) transformer
12
, than transmitted over a channel
13
in the type of serial signal. Within this process, cyclic prefix is attached prior to the block in the purpose of removing the effect of transmission channel. For example, in the case of DMT system using the bandwidth of 1.104MHz, one symbol per each tone utilizes the bandwidth of 4kHz and 0.3125kHz exists for the protection bandwidth. As a result, the number of cyclic prefix is 512×(0.3125kHz/4kHz)=40.
In the described DMT transceiver system, a receiving signal on the side of DMT demodulator contains noise in the process of signal transmission over channel from transmitter by a channel attenuation effect. After striping cyclic prefix from the received signal, the S/P transformer unit
15
transforms it to parallel signal. To facilitate this, this real vector of parallel signal transformed to the type of QAM symbol by the FFT unit
16
. This is the method of the DMT demodulation.
The above-described DMT transceiver system divide transmission bandwidth to each tone using the orthogonal characteristic of IFFT/FFT, that is, the transmission bandwidth is divided into plural tones by utilizing filters that is orthogonal each other, and the corresponding filters for each tone performs an exponential modulation. But one acknowledged drawback of this system is that the cross-talk noise is significant between tones, since a difference between main node and sub node for filters, which separate the transmission bandwidth, is just 13 dB. For ideal channel, that is, a response length of impulse is shorter than that of one symbol, the signal transmitted from transmitter can be restored on receiving side completely without interference between tones since symbols of each tone have no effect on others. However, in practical channel environment of transmission, the response length of impulse is relatively long so that symbols of each tone have affected others. This effect is significant in the case of DMT system, the difference between main node and sub node is just 13dB, which divides each tone. In order to eliminate the undesirable effects between symbols, there must be enough protection area to eliminate them on the channel. In general, this protection area is called cyclic prefix, and it should be bigger than “the response length of channel impulse−1” in order to eliminate the interference between symbols. Nevertheless, the cyclic prefix is not a concerned data, the longer its length, the less real data capacity. Additional drawback of DMT system is that it is easily affected by burst noise. That is, even the burst noise has characteristics of narrow bandwidth, the difference between main node and sub node of the transceiver filter, which consists of DMT system, is just 13dB so that it has affected not only tones but also it has affected adjacent tone significantly.
As a solution for that, Time-domain Equalizer (TEQ)
14
is suggested to use illustrated in FIG.
1
. TEQ is used to reduce the response length of the channel impulse, which causes Inter-Symbol Interference (ISI) so that it reduces the length of cyclic prefix ultimately. In case of general transmission channel, the length of impulse including major energy is not significantly long. Therefore if it can be collected within the certain objective length, the response length of channel impulse affected in practice can be reduced. Accordingly, using the adjustment equalizer such as TEA in the process of initializing modem can reduce the response length of the channel impulse affected in practice, so that the length of cyclic prefix can be reduced.
In the preceding description of DMT transceiver system, Frequency-domain Equalizer (FDE)
17
is used to compensate the attenuation and phase transformation caused by channels.
On the other hand, VDSL has submitted for standardization process, which is the technique for transmitting high-speed data over public telephone line faster than ADSL. Comparing to ADSL, VDSL uses broader bandwidth and transmits with better quality of data over the telephone line in relatively short distance.
As well known transmission methods of VDSL are QAM (Quadrature Amplitude Modulation) based SCM (Single Carrier Modulation) and FFT based DMT, however none of those are standardized in VDSL.
FIG. 2
is a block diagram illustrating general concept of DMT-based VDSL (Discrete Multi-Tone-based Very-high-speed Digital Subscriber Line) At the DMT-based VDSL transceiver system, a transmitter includes IFFT unit
21
and P/S transformer unit
22
, and a receiver includes TEQ
24
, P/S transformer unit
22
, FFT unit
26
and FEQ
27
. The operation of each unit in system is similar or same in accordance with FIG.
1
.
Above-described DMT-based VDSL transceiver system is designed so that the bandwidth of each tone is set to 4.3125 kHz similar with ADSL transceiver system, and its utilizing bandwidth is integral number times of bandwidth of tone. In general, the standardization group set the maximum utilizing bandwidth to 17.664MHz that is 16 times of maximum utilizing bandwidth of ADSL transceiver system considering the interoperability with it. Accordingly, the maximum number of tones used in DMT-based VDSL transceiver system is 4096, and modulation/demodulation should be performed using by 8192-point IFFT/FFT.
It should be noted that, what DMT-based VDSL transceiver system is different to DMT-based ADSL transceiver system is that the size of IFFT/FFT should be bigger due to its wider bandwidth. Nevertheless, considering the present technology related to semiconductor and complexity, there is a size limit to implement FFT, consequently the allocated bandwidth cannot be utilized in complete. Another difference is that the number of TEQ tabs and cyclic prefix are big since the bandwidth used in DMT-based VDSL is wide. In addition that the operation speed of TEQ should be also faster, since the more the bandwidth usage, the more the sampling speed. That is, in case of DMT-based ADSL transceiver system using the bandwidth 1.104MHz, it utilizes about 30 tabs of TEQ and its operation speed is 2.208MHz. On the contrary, in case of DMT-based VDSL transceiver system using the ba
Kim Jae Geun
Kim Jung Hak
Lee Hoon
Lee Jeong Jin
Yoo Tae Whan
Electronics and Telecommunications Research Institute
Seed IP Law Group PLLC
Tran Khai
LandOfFree
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