Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
2001-02-20
2002-10-22
Vo, Don N. (Department: 2631)
Pulse or digital communications
Equalizers
Automatic
C375S350000
Reexamination Certificate
active
06470047
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to communication systems and more particularly relates to an apparatus for and a method of reducing interference in a communications receiver.
BACKGROUND OF THE INVENTION
In recent years, the world has witnessed explosive growth in the demand for wireless communications and it is predicted that this demand will increase in the future. There are already over 500 million users that subscribe to cellular telephone services and the number is continually increasing. Eventually, in the not too distant future the number of cellular subscribers will exceed the number of fixed line telephone installations. Already, in many cases, the revenues from mobile services already exceeds that for fixed line services even though the amount of traffic generated through mobile phones is much less than in fixed networks.
Other related wireless technologies have experienced growth similar to that of cellular. For example, cordless telephony, two-way radio trunking systems, paging (one way and two way), messaging, wireless local area networks (WLANs) and wireless local loops (WLLs). In addition, new broadband communication schemes are rapidly being deployed to provide users with increased bandwidth and faster access to the Internet. Broadband services such as xDSL, short range high speed wireless connections, high rate satellite downlink (and the uplink in some cases) are being offered to users in more and more locations.
Along with the unprecedented expansion in the field of communications (e.g., radio, mobile, fiber and satellite communications, etc.), comes an increased interest in the problems associated with noise and interference. Radio interference is described as the presence of unwanted RF signals that impair radio reception and can be either accidental or intentional. In addition, interference can be caused by external means (e.g., neighboring communications systems, co-channel and adjacent interference, etc.) or by internal processes utilized in the process of communication (e.g., intersymbol interference, etc. Examples of radio interference include co-channel interference, adjacent channel interference, intersymbol interference, electromagnetic interference (EMI), splatter, harmonics, spurious emissions, spurious response, jamming and band congestion.
In mobile communications, however, four types of interference dominate. These include co-channel interference, adjacent channel interference, intersymbol interference and interference due to system nonlinearities. In co-channel interference, the interfering signal has the same carrier frequency as the desired information signal. The interfering signal usually is at a lower power level than the desired signal making reception difficult but possible. Higher power levels cause the desired signal to be jammed and reception impossible.
Adjacent channel interference occurs when a radio receiver is tuned to a particular frequency and interference is received from a signal on a nearby frequency. This type of interference is likely when an extremely strong interfering signal lies near the desired information signal. In other words, the interfering signal has a carrier frequency adjacent to the frequency of the desired information signal. Using a selective filter having a suitable bandwidth for the signal to be received while making the adjacent channel response as low as possible is one possible approach to reducing the effects of this type of interference.
In intersystem interference, the interfering signal comes from a different system, which operates either within the geographic proximity of the system or comes from another system, typically non-friendly, that generates an interfering signal of sufficient strength to totally disrupt communications. The latter type of interference is known as jamming and is usually used during wartime to disrupt enemy communications, radar, radiolocations and radio navigation. This type of interference is not very common in cellular systems.
Interference can also be caused by nonlinearities in the system and/or other effects of filters such as intermodulation distortion wherein nonlinear system components (especially in analog signal transmission) cause spurious signals, which may play a role in causing interference in adjacent channels. Intersymbol interference (ISI) in digital transmission operates to effectively lower the bit error rate (BER). ISI can be caused by the filtering of neighboring symbols using filters that do not meet the Nyquist conditions or by channel scattering, such as occurs in mobile communication.
In addition, internal circuit noise such as thermal noise, shot noise, hiss, for example, is produced by the active components making up the electronic equipment. At high frequencies, i.e. above 30 MHz, it is important that internal noise be kept as low as possible in the first stages of a multistage amplifier chain because any noise generated in one amplifier will be picked up and amplified along with the desired signals in subsequent stages. This type of noise is always present in communication systems and samples of this type of noise are typically uncorrelated, i.e., they constitute white noise.
Not all the above types of interference require special attention in the receiver of mobile communication systems. In the GSM standard, for example, the types of interference most frequently encountered include co-channel interference, adjacent channel interference and interference due to nonlinearities in the system.
In mobile radio systems, some form of equalization is required to deal with the effects of channel scattering and also with the effects of ISI caused by transmit and receive filters. An example of a commonly used type of equalizer includes the maximum likelihood sequence estimation (MLSE) equalizer that utilizes the well known Viterbi Algorithm (VA). The goal is to limit the ISI as much as possible while maintaining a reasonable complexity for the receiver.
Another important consideration in the design of the system is the degree of whiteness and Gaussian distribution of the noise sources other then ISI. For example, in the case wherein the main noise source is thermal additive white Gaussian noise (AWGN) noise, simple equalization can be used to reduce the noise. Other types of noise and interference, such as co-channel and adjacent channel interference, however, require additional treatment. The goal is to find a receive filter that maximizes the signal to interference ratio (SIR) in this case while keeping ISI at as low a level as possible. In addition, interference suppression (i.e. cancellation) should be applied in an attempt to remove the interference from the received signal before a decision about the data is made.
A diagram illustrating an example prior art communication system employing an inner and outer encoder in the transmitter, inner and outer decoding stages in the receiver and a noise source after the channel is shown in FIG.
1
. The communication system, generally referenced
10
, represents the typical scheme that is used in mobile radio transceivers. In such a system, the transmitter
11
comprises an encoder
14
, symbol generator (i.e. bit to symbol mapper)
16
and modulator
18
. Input data bits
12
to be transmitted are input to the encoder
14
which may comprise an error correction encoder such as Reed Solomon, convolutional encoder, parity bit generator, etc. The encoder functions to add redundancy bits to enable errors in transmission to be located and fixed.
It is noted that both the inner and outer decoders in the receiver have complimentary encoders in the transmitter. The outer encoder in the transmitter comprises the encoder
14
, e.g., Reed Solomon, etc. The inner encoder comprises the channel
20
that often times can be modeled as an L-symbol long FIR-type channel.
The bits output from the encoder may optionally be interleaved wherein the order of the bits are changed so as to more efficiently combat burst errors. The rearrangement of the bits caused by interleaving improves the resistan
Bachar Yuval
Kleinerman Alexander
Navon Edo
Zaltsman Ariel
Comsys Communications & Signal Processing Ltd.
Vo Don N.
Zaretsky Howard
LandOfFree
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