Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
2001-08-02
2002-04-30
Jeanpierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C327S133000, C368S277000, C348S571000
Reexamination Certificate
active
06380875
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a transmitter of a radio system using burst transmission, and particularly to a ramp generator for shaping the rise and the fall of a burst and for controlling the power level of the burst.
BACKGROUND OF THE INVENTION
A burst is the transmission quantum of numerous digital radio systems based on the principle of time division duplex (TDD), frequency division duplex (FDD), and code division duplex (CDD). The transmission takes place during a short time window. Within this time interval the emission rises from the starting power level to the nominal power level. The signal is then modulated to transmit a packet of bits. After that, the power level decreases until it reaches the minimum power level. The time mask of the burst, during which the bits are transmitted, is called a useful part or a payload part. Modulation is performed in the transmitter analogically or digitally, either at a base band frequency or at an intermediate frequency (IF). The modulated IF signal is then mixed up to the radio transmission frequency.
FIG. 1
depicts the rising portion of the power envelope of a burst comprising a digitally modulated intermediate frequency signal. In this example, during the rise of the burst the envelope should track a raised sine curve. After a predetermined period the power has reached its selected nominal level, whereupon modulation starts. This instant is denoted as 0 in the horizontal time axis. Usually the transmission power is adjustable according to the requirement of the system concerned. For that reason the nominal power level value can vary between the maximum power level and the minimum power level, as shown by the dotted line and the dashed line in FIG.
1
. The nominal power level between those levels can usually attain one of several discrete power levels. For example, the downlink dynamic power control in the GSM system uses 16 power levels with 2 dB separations.
FIG. 2
depicts the falling portion of the envelope of a digitally modulated intermediate frequency signal. The signal envelope during the fall of the transmission burst should track a raised cosine curve.
The power level can be controlled burst by burst. Control is realized by scaling the ramp curve which follows the raised sine/cosine curve. Hence, the ramp-up curve starts from the minimum power level, but settles at the level specified by the power level indication as shown in FIG.
1
. At the end of the burst the ramp-down curve starts from the nominal power level, but settles at the minimum power level as shown in FIG.
2
.
Conventionally, power ramping and control of the output power level are performed in the analog domain. One problem with analog solutions is inaccuracies caused by aging and by variations in operation temperature and components. Furthermore, the analog solutions are complex, and stability is a problem.
Today the tendency is to perform power ramping and output power level control digitally in order to avoid the afore-mentioned problems. Some basic solutions are presented below.
FIG. 3
illustrates in broad outline the formation of a modulated IF signal into a shape as shown in FIG.
1
and FIG.
2
. Data symbols arrive at digital modulator
31
that carries out modulation according to the modulation scheme of the system concerned. A ramp generator in block
32
generates the rising and falling edges according to the raised sine/cosine curve and a flat portion between the curves. Digital output signals from the ramp generator and the modulator are then converted to analog signals in digital-to-analog converters
33
and
36
: For removing the high frequency sampling components the analog signals are then filtered in low pass filters
37
and
38
, whereupon the analog modulated signal is multiplied in analog multiplier
35
by the analog ramp signal in order to smoothen out the rise and fall of the burst. The output from the multiplier is the analog modulated IF signal with ramped power.
FIG. 4
illustrates another digital modulator. Data symbols arrive at digital modulator
41
, which carries out modulation according to the modulation scheme of the system concerned and produces I and Q signals. Said signals are then converted into analog signals by DA converters
43
and
44
. For removing the high frequency sampling components the analog signals are filtered in low pass filters
410
and
411
, whereupon both the analog I signal and the analog Q signal are transformed into an intermediate frequency by mixer
45
and mixer
46
, accordingly. After mixing the sum of I signal and Q signal are added up in analog adder
47
to form the sum signal. A ramp generator in block
42
generates the ramp signal, i.e. the rising and falling edges according to the raised sine/cosine curve and the flat portion between them. The digital ramp signal is then converted into an analog signal in converter
48
. The high frequency sampling components are filtered in low pass filter
412
, whereupon the analog modulated sum signal is multiplied in multiplier
49
by the analog ramp signal in order to smooth out the rise and fall of the IF burst. The output from the multiplier is the analog modulated IF signal.
Common to both prior art solutions described above are the performance of both the modulation and the generation of the ramp signal digitally but conversion of the digital result signals into the analog domain before multiplying. However, there is a tendency in the art to carry out all processes within the digital domain. In order to better understand one possible realization of the digital ramp generator, a short review of the mathematical background is of assistance.
The burst signal can be considered as a product of an original modulated signal m(t) and a periodical switching signal sw(t). The spectrum of the burst signal is the convolution of the spectra of these two signals in the frequency domain.
For rectangular switching, i.e. without raised cosine/sine shaping, formula (1) is valid for frequency response:
W
(
f
)
=
M
(
f
-
f
C
)
*
Sw
(
f
)
=
K
∑
n
=
-
∞
∞
M
(
f
-
f
C
-
nf
g
)
sin
π
nf
g
τ
π
nf
g
τ
(
1
)
where
* denotes convolution,
f
c
is the carrier frequency,
f
g
is the burst gating rate,
&tgr; is the burst length and
K is a proportional constant.
For raised cosine/sine switching, i.e. with raised cosine/sine shaping, formula (2) is valid for frequency response:
W
(
f
)
=
H
∑
n
=
-
∞
∞
M
(
f
-
f
C
-
nf
g
)
(
τ
-
T
r
)
sin
c
(
nf
g
(
τ
-
T
r
)
)
cos
(
π
T
r
nf
g
)
1
-
(
2
T
r
nf
g
)
2
(
2
)
where T
r
indicates the ramp duration, and H is proportional constant.
The spectrum of the periodic burst signal consists of infinite numbers of secondary spectral lobes having the same shape as M(f) separated by the burst gating rate f
g
, and having decreasing amplitudes. Since the secondary spectral lobes resulting from formula (2) decay faster than those resulting from formula (1), the raised cosine/sine switching is used.
The following function is used to smooth out the rise of the burst:
(
A
-
dc
)
sin
(
π
t
2
T
r
)
2
+
dc
,
(
3
)
where
T
r
indicates the ramp duration,
t is [0 T
r
],
A is the envelope of the modulated signal, and
dc is the dc offset which settles the starting power level in FIG.
1
.
The following function (4) is used to smooth out the fall of the burst:
(
A
-
dc
)
cos
(
π
t
2
T
r
)
2
+
dc
.
(
4
)
FIG. 5
illustrates a ramp generator and an output power controller known in the art which are based on formulas (3) and (4). The raised sine values of formula (3) or the raised cosine values of formula (4) are stored in the read only memory (ROM)
51
. Digital multiplier
52
is used to control the amplitude level, i.e. value (A−
Honkanen Mauri
Vankka Jouko
Altera Law Group LLC
Jeanglaude Jean Bruner
Jeanpierre Peguy
Nokia Networks Oy
LandOfFree
Digital ramp generator with output power level controller does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Digital ramp generator with output power level controller, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Digital ramp generator with output power level controller will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2932822