Pulse or digital communications – Transceivers – Transmission interface between two stations or terminals
Reexamination Certificate
1999-07-30
2002-10-08
Le, Amanda T. (Department: 2734)
Pulse or digital communications
Transceivers
Transmission interface between two stations or terminals
C341S118000, C341S120000, C341S140000
Reexamination Certificate
active
06463093
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to multi-channel wireless communications systems. In particular, the present invention relates to a technique for reducing spurious and inter-modulation products through static non-linearity correction in a wireless broadband digital multi-carrier transceiver.
2. Background of the Invention
A conventional cellular phone system
100
is shown in FIG.
1
. As illustrated in
FIG. 1
, the cellular phone system
100
includes a plurality of cells
110
a,
110
b,
a mobile unit
120
, a plurality of base station transceivers (BTS)
105
a,
105
b,
communication lines
140
, a mobile telecommunications switching office (MTSO)
130
, an interface
150
and a switched telephone network
160
.
The cellular phone system
100
has a fixed number of channel sets distributed among the BTS
105
a,
105
b
serving a plurality of cells
110
a,
110
b
arranged in a predetermined reusable pattern. The mobile unit
120
, in a cell
110
a
or
110
b,
communicates with the BTS,
105
a
or
105
b,
respectively, via radio frequency (RF) means. The BTS
105
a,
105
b
communicate with the MTSO
130
via communication lines
140
. The MTSO
130
communicates with the switched telephone network
160
via the interface
150
.
In the cellular phone system
100
, the cell areas typically range from 1 to 300 square miles. The larger cells typically cover rural areas, while the smaller cells typically cover urban areas. Cell antenna sites utilizing the same channel sets are spaced by a sufficient distance to assure that co-channel interference is held to an acceptably low level.
The mobile unit
120
in a cell
110
a
has radiotelephone transceiver equipment which communicates with similar equipment in BTS
105
a,
105
b
as the mobile unit
120
moves from cell to cell.
Each BTS
105
a,
105
b
relays telephone signals between mobile units
120
and a mobile telecommunications switching office (MTSO)
130
by way of the communication lines
140
.
The communication lines
140
between a cell site,
110
a
or
110
b,
and the MTSO
130
, are typically T1 lines. The T1 lines carry separate voice grade circuits for each radio channel equipped at the cell site, and data circuits for switching and other control functions.
The MTSO
130
in
FIG. 1
includes a switching network (not shown) for establishing call connections between the public switched telephone network
160
and mobile units
120
located in cell sites
110
a,
110
b
and for switching call connections from one cell site to another. In addition, the MTSO
130
includes a dual access feeder (not shown) for use in switching a call connection from one cell site to another. Various handoff criteria are known in the art and utilize features such as delay ranging to indicate the distance of a mobile unit from a receiving cell site, triangulation, and received signal strength to indicate the potential desirability of a handoff. Also included in the MTSO
130
is a central processing unit (not shown) for processing data received from the cell sites and supervisory signals obtained from the switched telephone network
160
to control the operation of setting up and taking down call connections.
As the popularity of cellular systems grow, the performance of the cellular system needs to improve as more users subscribe to cellular systems. The performance of the BTS is one area of cellular systems that has been the focus of improvement.
A block diagram of a modern wireless broadband digital multi-carrier transceiver system (BTS)
200
is illustrated in FIG.
2
. The BTS
200
includes a network interface module
205
, a digital signal processing (DSP) module
210
, a combiner module
215
, a channelizer module
220
, and a broadband RF transceiver
225
.
The network interface module
205
provides an interface between the BTS
200
and a MTSO. In this particular example, the network interface module
205
provides ninety-two (92). 16 kbps subrate voice channels.
The network interface module
205
is also interfaced with the DSP module
210
. The DSP module
210
provides for channel coding and modulation of thirteen—(13) kbps voice channel data from the network interface module
205
. The DSP module
210
multiplexes eight (8) channels into a single baseband signal for upconversion and combining with other RF carriers by the combiner module
215
. The DSP module
210
also provides the equalization, demodulation and channel decoding from received channels of RF carriers that have been downconverted to a baseband signal by the channelizer module
220
.
The combiner module
215
provides for receiving the baseband RF carriers from the DSP module
210
. Each RF carrier is filtered and upconverted to a unique intermediate frequency (IF). All of the RF carriers in a five—(5) megaHertz (MHz) bandwidth are simultaneously combined into a single composite IF signal. This digital IF signal is then transferred to the broadband RF transceiver
225
.
The channelizer module
220
provides for receiving a digital composite IF signal from the broadband RF transceiver
225
. The digital composite IF signal consists of all twenty-five (25) of the 200-kiloHertz (kHz) RF carriers in a 5 MHz bandwidth. The channelizer module
220
provides for filtering and downconverting each RF carrier to a baseband signal for processing by the DSP module
210
.
The broadband RF transceiver
225
provides for conversion of the digital signals for transmission to mobile users. The broadband RF transceiver
225
also provides for conversion of received analog signals to digital signals for processing by the BTS
200
.
The broadband RF transceiver
225
includes an upconverter
230
, two downconverters
235
a,
235
b,
and a broadband converter
245
.
Each downconverter,
235
a
or
235
b,
accepts at least a 5 MHz wide block of RF frequencies and downconverts to an IF center frequency, and then passed to the broadband converter module
245
. The broadband converter module
245
digitizes the incoming analog signal and then passes the digital signal to the channelizer module
220
.
The broadband converter module
245
also receives a digital broadband signal from the combiner module
215
. The broadband converter module
245
converts the digital broadband signal into an analog IF signal. After filtering, the analog IF signal is upconverted to RF, and transmitted to a mobile user by the transmitter
230
.
The broadband converter module
245
includes low pass analog filters
250
a,
250
b,
a transmit digital half-band filter
255
a,
receive digital half-band filters
255
b,
255
c,
a digital-to-analog converter (DAC)
260
, and an analog-to-digital converter (ADC)
265
a,
265
b.
The low pass filters
250
a,
250
b
provide filtering for the incoming analog signals from a mobile user.
The transmit digital half-band filter
255
a
provides interpolation-by-two on the transmitted digital data from the combiner module
215
.
The receive digital half-band filter
225
b,
255
c
provides for decimation-by-two and filtering of the received digitized analog signals from a mobile user.
The DAC
260
provides for a conversion of digital signals to analog signals in preparation for transmission to a mobile user.
The ADC
265
a,
265
b
provides for a conversion of received analog signals from a mobile user into digital signals to be processed by the BTS
200
.
In using a conventional digital multi-carrier BTS, the ADC and/or DAC of the BTS must operate as linearly as possible in order to avoid adjacent channels and/or harmonic distortion in the transmitted signal. However, the operational characteristics of the ADC and/or DAC may hinder in achieving the goal of linear operation.
Although designed for linear operation, the typical ADC and/or DAC are not perfectly linear. This is due to imperfect transfer functions of the converters. The imperfection may be caused by errors in resistor ladders, which are used to establish the conversion thresholds in the ADC and/or DAC.
The imperfect transfer functions of the ADC an
Komara Michael A.
Noll John R.
Airnet Communications Corporation
Le Amanda T.
LandOfFree
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