Pulse or digital communications – Transceivers
Reexamination Certificate
1999-09-07
2003-01-07
Pham, Chi (Department: 2631)
Pulse or digital communications
Transceivers
C375S245000, C341S139000, C341S145000, C341S156000
Reexamination Certificate
active
06504863
ABSTRACT:
BACKGROUND
This invention relates to a method and apparatus for adaptive bit resolution. More particularly, this invention relates to a method and apparatus for adaptive bit resolution in a digital receiver and/or a digital transmitter.
Modem communication systems, such as cellular and satellite radio systems, employ various modes of operation (analog, digital, dual mode, etc.), and access techniques such as frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and hybrids of these techniques.
In a FDMA system, each channel is assigned a specific frequency. In a TDMA system, each channel is assigned a specific time slot in a periodic train of time intervals over the same frequency. Each period of time slots is called a frame. In a CDMA system, different users, base stations (BS), and services are separated from each other with unique spreading sequences/codes.
FIG. 1A
is a block diagram of an exemplary cellular mobile radiotelephone system, including an exemplary base station
110
and mobile station
120
. The base station includes a control and processing unit
130
which is connected to a mobile switching center (MSC)
140
which in turn is connected to the public switched telephone network (PSTN) (not shown). General aspects of such cellular radiotelephone systems are known in the art. The base station
110
handles a plurality of voice channels through a voice channel transceiver
150
, which is controlled by the control and processing unit
130
. Also, each base station includes a control channel transceiver
160
, which may be capable of handling more than one control channel. The control channel transceiver
160
is controlled by the control and processing unit
130
. The control channel transceiver
160
broadcasts control information over the control channel of the base station or cell to mobiles locked to that control channel. It will be understood that the transceivers
150
and
160
can be implemented as a single device, like the voice and control transceiver
170
, for use with control and traffic channels that share the same radio carrier.
The mobile station
120
receives the information broadcast on a control channel at its voice and control channel transceiver
170
. Then, the processing unit
180
evaluates the received control channel information, which includes the characteristics of cells that are candidates for the mobile station to lock on to, and determines on which cell the mobile should lock.
In a typical digital cellular transceiver, e.g., a mobile station, a received analog waveform signal is digitized in an analog to digital converter (ADC), and an analog waveform for transmission is generated from a digital signal using a digital to analog converter (DAC).
FIG. 1B
illustrates a conventional receiver which may be included, for example, in the transceiver
170
. The receiver depicted in
FIG. 1B
is shown in a simplified form for ease of understanding. It will be appreciated that a conventional receiver can comprise additional elements which are not shown or described. The receiver shown in
FIG. 1B
includes a frequency selective filter
172
(representing the total selectivity in the receiver), having a bandwidth B, and an ADC
174
. A received signal is filtered through the filter
172
to, among other things, remove interfering signals, resulting in a signal x(t)=s(t)+n
in
(t), where s(t) represents the wanted input signal, and n
in
(t) represents noise. The signal x(t) is converted to digital form in the ADC
174
which includes a Sampler
176
and a Quantizer
178
. The Sampler
176
converts the time-continuous portion of the signal into a time-discrete form, depending on a sample clock frequency, and the Quantizer
178
converts the amplitude-continuous portion of the signal into an amplitude-discrete signal by quantizing the amplitude domain into a fine number of fixed distinguishable levels, each a distance Q apart. The process of quantizing is irreversible, since, regardless of how small the quantization level Q, an unresolvable uncertainty of ±Q/2 is associated with each quantized amplitude value. Thus, a quantization noise is inevitably associated with all quantized signals.
The receiver ADC bit resolution is determined at least partially by how much deterioration of the input signal is acceptable, e.g., by the signal to noise ratio (SNR) or signal to interference ratio (SIR) in the output signal that results in a certain desired bit error rate (BER). For convenience, the abbreviation SNR is used in the following description to represent either thermal noise or interference noise. Referring to
FIG. 1B
, the SNR of the filtered input signal x(t) is SNRin, the SNR of the ADC is SNRadc, and the SNR of the resulting digital signal is SNRtotadc.
FIG. 2
illustrates signal and noise levels in the receiver. In
FIG. 2
, Pn
in
is the power of the input noise signal n
in
(t), Ps is the power of the input signal s(t), Dqadc is the the quantization noise power of the ADC, Pntot is the total noise input power, i.e., Pn
in
+Dqadc, and Px is the power of the filtered signal x(t). Also shown in
FIG. 2
are the SNRin, the SNRadc, and the SNRtotadc. From
FIG. 2
, it can be seen that SNRin is the ratio Ps/Pnin, SNRadc is the ratio Px/Dqadc, and SNRtotadc is the ratio Ps/Pntot. All of these values may be given in decibels (dB).
Note that, in this example, SNRin and SNRtot are negative. In all examples, it is assumed that the minimum SNR to provide an acceptable BER is negative. This is a normal situation in CDMA receivers, in which despreading of the received signal, after the analog-to-digital conversion, increases the SNR by a factor of the processing gain (PG).
The minimum SNR of the signal converted by the ADC (SNRadcmin) which results in a minimum acceptable SNRtotadcmin for a certain performance (BER) can be described by the following equation:
SNR
adcmin
=
10
log
{
10
SNRin
10
+
1
10
SNRin
-
SNRtotadcmin
10
-
1
}
(
1
)
For a large negative SNRin, i.e., for an input signal that is basically Gaussian noise, Equation 1 can be simplified as follows:
SNR
adcmin
=
10
log
{
1
10
Δ
10
-
1
}
(
2
)
where &Dgr; is a degradation of the SNRin, i.e., &Dgr;=SNRin−SNRtotadcmin.
The SNRadc due to quantization of a discrete signal x(k) can be calculated by the following expression:
SNRadc
=
σ
x
2
Dq
adc
(
3
)
where:
Dq
adc
=
∑
i
-
1
M
∫
x
i
-
1
x
i
(
x
-
m
i
)
2
·
p
(
x
)
ⅆ
x
(
4
)
and where &sgr;
x
2
is the power of the signal x(t), M is the number of quantization levels in the ADC (M=2
r
, r=number of bits), m
i
is the quantized level, x
i
is the decision level (wherein if x
i-1
, <x(k) <x
i
then x(k) can be approximated with m
i
), and p(x) is the probability density function for the input signal and can either be approximated by a Gaussian distribution (which is normally the case in a CDMA receiver): X∈ N(0, &sgr;), or the distribution can be continuously estimated.
Table 1 shows exemplary values for SNRadc and corresponding bit resolutions which result in optimum uniform quantization of a Gaussian signal. This is described in by John G. Proakis,
Digital Communications
, p. 116 (3rd ed. 1995).
TABLE 1
Number of bits per sample
SNRadc [dB]
1
4.4
2
9.25
3
14.27
4
19.38
5
24.57
6
29.83
For uniform quantization and a “large” number of quantization levels, Equation 4 can be approximated as follows:
Dq
adc
=
Δ
q
2
12
(
5
)
where &Dgr;
q
is the quantization step size, i.e., &Dgr;
q
=x
i
−x
i-1
.
The required SNRadcmin and the allowed degradation &Dgr; of SNRin according to Equation 2 are plotted in FIG.
3
. For example, from
FIG. 3
, it can be seen that a minimum of 16 dB SNRadc is required if 0.1 dB degradation of the SNRin is acceptable. Based on Table 1, this requires a four bit resolution in the quantizer.
The SNRadc and the SNRin which result in a SNRtotadc of −6 dB (for a BER of 10
Burns Doane Swecker & Mathis L.L.P.
Pham Chi
Telefonaktiebolaget LM Ericsson (publ)
Tran Khanh Cong
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