Pulse or digital communications – Equalizers
Reexamination Certificate
1999-07-07
2003-12-23
Fan, Chieh M. (Department: 2634)
Pulse or digital communications
Equalizers
C375S350000, C708S300000
Reexamination Certificate
active
06668013
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a digital filter which can be suitably applied for data communication, and more particularly, to a roll-off filter which can realize a Nyquist filter capable of eliminating intersymbol interference caused by filtering.
BACKGROUND OF THE INVENTION
Generally, it is known that a digital signal has quite a broad spectrum range for its data rate. For example, when a digital signal shown in FIG.
12
(
a
) has a data rate of 1 Mbps, then its spectrum is spread as shown in FIG.
12
(
b
).
On the other hand, a digital signal of images or sounds is transmitted by minutely dividing an available frequency bandwidth in accordance with its use or purposes. In order to send a large volume of information within limited frequency bandwidths, a data transferring rate may be improved, or the bandwidth of a signal having a certain data transferring rate may be narrowed, so that some portions of data are multiplexed by means of frequency dividing. In particular, in the field of a wireless communication, a radio wave source is effectively utilized by narrowing the bandwidth by suppressing an unwanted sideband of a base band signal.
However, when the bandwidth of a spectrum of the digital signal is narrowed, there occurs intersymbol interference, which possibly causes a bit error. In order to solve this problem, a Nyquist filter is extensively used as a filter which does not cause intersymbol interference even when the bandwidth is narrowed.
Characteristics of a roll-off filter R(f) which are given by Nyquist and realize a intersymbol-interference-free Nyquist filter are expressed as Equation below and illustrated in FIG.
13
:
R
(
f
)
=
{
1
…
0
≤
&LeftBracketingBar;
ft
&RightBracketingBar;
≦
1
-
α
2
1
2
{
1
-
sin
[
π
2
α
(
2
ft
-
1
)
]
}
…
1
-
α
2
≦
&LeftBracketingBar;
f
t
&RightBracketingBar;
≦
1
+
α
2
0
…
1
+
α
2
≦
&LeftBracketingBar;
f
t
&RightBracketingBar;
(
1
)
where T is a sign interval, and &agr; is a roll-off ratio defined as 0≦&agr;≦1. The roll-off filter R(f) has been well known as a filter functioning as the Nyquist filter, and in the explanation below, the Nyquist filter is referred to as the roll-off filter.
In
FIG. 13
, a capital letter W denotes a transition period. The transition period W is 0 when the filter characteristics are ideal &agr;=0, and the larger the roll-off ratio &agr;, the longer the transition period W.
FIG. 13
shows the transition period when the roll-off ratio &agr; is 0.5.
As shown in
FIG. 13
, by completing the transition from one sign to the other within a unit sign interval 1T, even there is an interference wave indicated by a broken line in FIG.
14
(
b
), data indicated by a solid line, that is, sign data shown in FIG.
14
(
a
), can be reproduced from data read out at predetermined reading points indicated by circles in FIG.
14
(
b
). Consequently, filter characteristics necessary to establish a intersymbol-interference-free transmission path can be achieved.
Also, an impulse response r(t) of the roll-off filter R(f) is expressed as:
r
(
t
)
=
sin
(
π
t
/
T
)
π
t
/
T
·
cos
(
α
t
/
T
)
1
-
(
2
α
t
/
T
)
2
.
(
2
)
If the roll-off filter R(f) is supplied with a random impulse train &dgr;n (n= . . . , −1, 0, 1, . . . ) having positive and negative polarities, then a resulting output is expressed as:
r
1
(
t
)
=
∑
n
=
-
∞
∞
δ
nr
(
t
-
nT
)
.
(
3
)
If analog elements, such as L, C, and R, are used, a highly sophisticated design using a computer is required to achieve these filter characteristics. However, these characteristics can be achieved relatively easy if an FIR (Finite Impulse Response) filter using delay circuits each equipped with a tap or a non-cyclic filter known as a digital filter is used.
FIG. 15
is a block diagram depicting an electrical arrangement of a typical FIR type conventional digital filter
1
. As shown in the drawing, the typical FIR type digital filter
1
comprises delay devices d
1
, d
2
, . . . , dm cascaded in multiple stages, multipliers g
0
, g
1
, . . . , gm, and an adder circuit
2
. Input data x(n) are delayed sequentially by the delay devices d
1
through dm. Here, the input and output of each of the delay devices d
1
through dm are used as taps, and the data at each tap are multiplied by coefficients h
0
through hm by the multipliers g
0
through gm, respectively. All the multiplication results are added up by the adder circuit
2
, whereby output data y(n) are obtained. For ease of explanation, marks representing multi-bit data are appended only to the input data x(n) and output data y(n), but it should be appreciated that the data processed in the digital filter
1
are also the multi-bit data.
With the above-arranged digital filter
1
, the greater the number m of the taps, the more satisfactory the filter characteristics. However, if the number m of the taps is increased, the number of the delay devices d
1
through dm and the number of elements forming the adder circuit
2
are increased as well. In addition, since the multipliers g
0
through gm occupy a large mounting space, the entire circuit size is undesirably increased with an increasing number of the multipliers g
0
through gm.
On the other hand, a wireless communication, particularly a spectrum diffusion communication, has been receiving a widespread attention in recent years because it can offer advantages as to confidentiality, a volume of transmission data, transmission power, etc. However, the spectrum diffusion communication requires various kinds of digital signal processing including modulation processing to append a diffusion signal to a transmission signal and decoding processing to remove the diffusion signal from a received signal.
Thus, with respect to the digital filters employed for a personal computer designed to transmit information by means of the spectrum diffusion communication through a wireless LAN or a communication unit incorporated in a portable device, there has been an increasing need to reduce the number of components and downsize the circuit to save the mounting space, costs, power consumption, etc. Even when circuit elements are replaced with integrated circuits to reduce the entire circuit size, it is advantageous to use a least necessary number of circuits from the view points of shortening a developing period and saving developing costs.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a digital filter of a simple arrangement which can reduce the number of components and save costs and power consumption while shortening the developing period.
In order to fulfill the above and other objects, a digital filter of the present invention is furnished with:
(a) delay circuits, cascaded in multiple stages and each having a tap, for sequentially delaying actual input data;
(b) a plurality of first adding circuits for adding up outputs from the taps supplied with a same multiplying coefficient among multiplying coefficients used to multiply an output from each tap;
(c) a plurality of multiplying circuits for multiplying an output from each of the first adding circuits with their respective multiplying coefficients;
(d) a second adding circuit for adding up multiplication results from each of the multiplying circuits and outputting an addition result as interpolation data; and
(e) an input data converting section for receiving a transfer clock having a frequency of N·fs, and converting the actual input data in such a manner that the actual input data are outputted to the delay circuits for a 1/N period of a sign interval T of the actual input data and “0” is outputted to the delay circuits for a remaining period,
where N is a multiple of oversampling conducted by computing the in
Fan Chieh M.
Sharp Kabushiki Kaisha
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