Telecommunications – Receiver or analog modulated signal frequency converter – Noise or interference elimination
Reexamination Certificate
2001-02-21
2004-02-17
Trost, William (Department: 2682)
Telecommunications
Receiver or analog modulated signal frequency converter
Noise or interference elimination
C455S192100
Reexamination Certificate
active
06694131
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to wireless communication devices and, more particularly, relates to adaptive image rejection.
BACKGROUND OF THE INVENTION
Wireless communication systems are an integral component of the ongoing technology revolution. In fact, mobile radio communication systems, such as cellular telephone systems, are evolving at an exponential rate. In a cellular system, a coverage area is divided into a plurality of “cells.” A cell is the coverage area of a base station or transmitter. Low power transmitters are utilized, so that frequencies used in one cell can also be used in cells that are sufficiently distant to avoid interference. Hence, a cellular telephone user, whether mired in traffic gridlock or attending a meeting, can transmit and receive phone calls so long as the user is within a “cell” served by a base station.
One implementation of a cellular network
100
is depicted in block form in FIG.
1
. The network
100
is divided into four interconnected components or subsystems: a Mobile Station (MS)
106
, a Base Station Subsystem (BSS)
102
, a Network Switching Subsystem (NSS)
104
, and an Operation Support Subsystem (OSS)
118
. Generally, MS
106
is the mobile equipment or phone carried by the user. And BSS
102
interfaces with multiple mobiles to manage the radio transmission paths between the mobiles and network subsystem. In turn, NSS104 manages system-switching functions and facilitates communications with other networks such as the PSTN and the ISDN. The OSS
118
facilitates operation and maintenance of the network.
MS's
106
communicate with BSS
102
across a standardized radio air interface
108
. BSS
102
is comprised of multiple base transceiver stations (BTS)
110
and base station controllers (BSC)
114
. A BTS
110
is usually in the center of a cell and consists of one or more radio transceivers with an antenna. It establishes radio links and handles radio communications over the air interface with MS's
106
within the cell. The transmitting power of the transceiver defines the size of the cell. Each BSC
102
manages transceivers. The total number of transceivers per a particular controller could be in the hundreds. The transceiver-controller communication is over a standardized “Abis” interface
112
. BSC
102
allocates and manages radio channels and controls handovers of calls between its transceivers.
BSC
102
, in turn, communicate with NSS
104
over a standardized interface
116
. For example, in a GSM system, which will be discussed infra, the interface uses an SS7 protocol and allows use of base stations and switching equipment made by different manufacturers. A Mobile Switching Center (MSC)
122
is the primary component of NSS
104
. MSC
122
manages communications between mobile subscribers and between mobile subscribers and public networks
130
. Examples of public networks
130
that the mobile switching center may interface with include Integrated Services Digital Network (ISDN)
132
, Public Switched Telephone Network (PSTN)
134
, Public Land Mobile Network (PLMN)
136
, and Packet Switched Public Data Network (PSPDN)
138
.
MSC
122
typically will interface with several databases to manage communication and switching functions. For example, MSC
122
may interface with Home Location Register (HLR)
124
that contains details on each subscriber residing within the area served by the mobile switching center. There may also be a Visitor Location Register (VLR)
126
that temporarily stores data about roaming subscribers within a coverage area of a particular mobile switching center. An Equipment Identity Register (EIR)
120
that contains a list of mobile equipment may also be included. Further, equipment that has been reported as lost or stolen may be stored on a separate list of invalid equipment that allows identification of subscribers attempting to use such equipment. Finally, there may be an Authorization Center (AuC)
128
that stores authentication and encryption data and parameters that verify a subscriber's identity.
There are several technologies in use today for different implementations of cellular network
100
. When wireless telecommunications began in North America back in the 1950's, an analogue standard called Advanced Mobile Phone Service (AMPS) was used. AMPS operated in the frequency spectrum from 824 to 894 MHz. This spectrum was then divided into 30 kHz channels for use by MS's
106
within cellular network
100
. In order to allow full duplex operation, a 30 Khz channel is reserved for each MS
106
to transmit on, and a 30 kHz channel is reserved for each MS
106
to receive on. These two channels are separated within the frequency spectrum by 45 MHz. Thus, a MS
106
transmitting on a channel at 831.21 MHz would receive at 876.21 MHz.
Dividing the frequency spectrum into multiple equally spaced channels is called Frequency Division Multiple Access (FDMA) and is illustrated in FIG.
2
A. As can be seen, there is a limited number of channels
202
that can be used within the fixed frequency spectrum from 824 to 894 MHz. As a result, new technologies were developed in order to increase the capacity (number of channels) that could be supported by a cellular network
100
. The first of these technologies was called Narrowband Advanced Mobile Phone Service (NAMPS). The key difference between NAMPS and AMPS is the use of a 10 Khz channel in the former. Thus, the capacity in an NAMPS system is three times the capacity of an AMPS system.
Eventually, digital technologies evolved to address the capacity issue and to improve the quality and functionality of the services provided by cellular network
100
. The major difference between digital and analogue is the method used to transmit data between MS
106
and BSS
102
. In an analogue scheme, the information is encoded as proportional variations in a frequency modulation (FM). In a digital scheme, the information is first digitized and then encoded using various complex modulation schemes. The modulated signal is then transmitted to BSS
102
. Additionally, as a result of the digital schemes and the enhanced features they enable, the frequency spectrum from 1.85 GHz to 1.99 GHz has been allocated for new cellular type services called Personal Communications Service (PCS).
The primary digital technologies used in North American are Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). There are several TDMA technologies currently available in the United States. One is the North American-TDMA system (NA-TDMA), also known as Digital-AMPS (D-AMPS). TDMA employs time slots to put multiple calls on the same channel. As illustrated in
FIG. 2B
, NA-TDMA uses the same channel scheme as AMPS; however, each channel is divided into six time slots
204
a
-
204
f.
Each slot is then assigned to a different user, thus the capacity of a NA-TDMA system is six times the capacity of an AMPS system and twice the capacity of an NAMPS system. Before 1995, NA-TDMA was governed by the IS-54 standard. IS-54 is being replaced, however, by IS-136, which incorporates implementation in the PCS band, a new Digital Control Channel (DCCH), and new user services.
Another TDMA system that developed in Europe, where a similar transition from analog to digital technologies took place, is the GSM system. GSM has been adopted for use in the United States as PCS1900, which is now offered in the PCS band.
CDMA, on the other hand, is a completely different type of multiple access scheme. In CDMA, channels are not allocated by dividing the spectrum in frequency or time. Instead, a 1.25 MHz channel is used for all users within a cell. The transmission signal is prepared by first digitizing the data and then multiplying the digitized data by a wide-bandwidth pseudo noise code (pn)-sequence. Thus, as illustrated in
FIG. 2C
, each transmission
206
a
,
206
b
,
206
c
, and
206
d
appears as noise to all other transmissions. In order to recover the signal at a receiver, each user is given a specific (pn)
Ly Nghi
Mitsubishi Electric Corporation
Oblon, Spivak, McClelland, McClelland, Maier & Neustadt, P.C.
Trost William
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