Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-08-18
2004-04-13
Marcelo, Melvin (Department: 2663)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C375S149000, C375S150000
Reexamination Certificate
active
06721299
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to communication systems, and more particularly, cellular communication systems.
2. Background of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of systems are present.
FIG. 1
generally illustrates a system
10
, which uses CDMA modulation techniques in communication between user equipment (UE)
12
a
and
12
b
, each UE including a cellular telephone, and base stations (ITS)
14
a
and
14
b
. A base station controller (BSC)
16
typically includes an interface and processing circuitry for providing system control to the BTS
14
a
,
14
b
. The BSC
16
controls the routing of telephone calls from the public switched telephone network (PSTN) to the appropriate BTS for transmission to the appropriate UE. The BSC
16
also controls the routing of calls from the UEs, via at least one BTS to the PSTN. The BSC
16
may direct calls between UEs via the appropriate BTS since UEs do not typically communicate directly with one another. The BSC
16
may be coupled to the BTS
14
a
and
14
b
by various means including dedicated telephone lines, optical fiber links or by microwave communication links.
The arrows
13
a
-
13
d
define the possible communication links between the BTS
14
a
and UEs
12
a
and
12
b
. The arrows
15
a
-
15
d
define the possible communication links between the BTS
14
ba
and UEs
12
a
and
12
b
. In the reverse channel or uplink (i.e., from UE to BTS), the UE signals is received by BTS
14
a
and/or BTS
14
b
, which, after demodulation and combining, pass the signal forward to the combining point, typically to the BSC
16
. In the forward channel or downlink (i.e., from BTS to UE), the BTS signals are received by UE
12
a
and/or UE
12
b
. The above system is described in U.S. Pat. Nos. 5,101,501; 5,103,459; 5,109,390; and 5,416,797, whose entire disclosure is hereby incorporated by reference therein.
A radio channel is a generally hostile medium in nature. It is rather difficult to predict its behavior. Traditionally, the radio channels are modeled in a statistical way using real propagation measurement data. In general, the signal fading in a radio environment can be decomposed into a large-scale path loss component together with a medium-scale slow varying component having a log-normal distribution, and a small-scale fast varying component with a Rician or Rayleigh distribution, depending on the presence or absence of the line-of-sight (LOS) situation between the transmitter and the receiver.
FIG. 2
illustrates these three different propagation phenomena. An extreme variation in the transmission path between the transmitter and receiver can be found, ranging from direct LOS to severely obstructed paths due to buildings, mountains, or foliage. The phenomenon of decreasing received power with distance due to reflection, diffraction around structures, and refraction is known as path loss.
As shown, the transmitted signal is reflected by many obstacles between a transmitter and a receiver, thus creating a multipath channel. Due to the interference among many multipaths with different time delays, the received signal suffers from frequency selective multipath fading. For example, when the 2 GHz carrier frequency band is used and a car having a UE is travelling at a speed of 100 km/h, the maximum Doppler frequency of fading is 185 Hz. While coherent detection can be used to increase link capacity, under such fast fading, the channel estimation for coherent detection is generally very difficult to achieve. Because of fading channels, it is hard to obtain a phase reference for the coherent detection of data modulated signal. Therefore, it is beneficial to have a separate pilot channel.
Typically, a channel estimate for coherent detection is obtained from a common pilot channel. However, a common pilot channel transmitted with an omnidirectional antenna experiences a different radio channel than a traffic channel signal transmitted through a narrow beam. It has been noticed that common control channels are often problematic in the downlink when adaptive antennas are used. The problem can be circumvented by user dedicated pilot symbols, which are used as a reference signal for the channel estimation. The dedicated pilot symbols can either be time or code multiplexed.
FIG. 3
depicts a block diagram of a transmitter and a receiver for time multiplexed pilot symbols for an improved channel estimation method that works satisfactorily under slow-to-fast fading environments. Known pilot symbols are periodically multiplexed with the sequence of the transmitted data. The pilot symbols and data symbols following pilot symbols constitute a slot, as shown in FIG.
3
.
Further, in a DS-CDMA transmitter, the information signal is modulated by a spreading code, and in the receiver, it is correlated with a replica of the same code. Thus, low cross-correlation between the desired and interfering users is important to suppress the multiple access interference. Good autocorrelation properties are required for reliable initial synchronization, since large sidelobes of the autocorrelation function may lead to erroneous code synchronization decisions. Furthermore, good autocorrelation properties are important to reliably separate the multipath components.
Since the autocorrelation function of a spreading code should resemble, as much as possible, the autocorrelation function of white Gaussian noise, the DS code sequences are also called pseudo-noise (PN) sequences. The autocorrelation and cross-correlation functions are connected in such a way that it is not possible to achieve good autocorrelation and cross-correlation values simultaneously. This can be intuitively explained by noting that having good autocorrelation properties is also an indication of good randomness of a sequence. Random codes exhibit worse cross-correlation properties than deterministic codes.
Such mobile communication system has gone through different stages of evolution, and various countries used different standards. First generation mobile systems in the 1980s used analog transmission for speech services. Advanced Mobile Phone Service (AMPS) in the United States, Total Access Communication System SACS) in the United Kingdom, Nordic Mobile Telephones (NMT) in Scandinavia, Nippon Telephone and Telegraph (NTT) in Japan, etc., belonged to the first generation.
Second generation systems using digital transmission were introduced in the late 1980s. They offer higher spectrum efficiency, better data services, and more advanced roaming than the first generation systems. Global System for Mobile Communications (GSM) in Europe, Personal Digital Cellular (PDC) in Japan, and IS-95 in the United States belonged to the second generation.
Recently, third generation mobile radio networks have been under intense research and discussion and will emerge around the year 2000. In the International Telecommunication Union (ITU), the third generation networks' are called International Mobile Telecommunications—2000 (IMT-2000) and in Europe, Universal Mobile Telecommunication System (UMTS). IMT-2000 will provide a multitude of services, including multimedia and high bit rate packet data.
Wideband CDMA has emerged as the mainstream air interface solution for the third generation networks. Wideband CDMA systems are currently being standardized by the European Telecommunications Standards Institute (ETSI) of Europe, the Association for Radio Industry and Business (ARIB) of Japan, the TIA Engineering Committees TR45 and TR46 and the T1 committee T1P1 of the United States, and the Telecommunication Technology Association TTA I and TTA II (renamed Global CDMA I and II, respectively) in Korea. The above description and a background of various systems can be found in WIDEBAND CDMA FOR THIRD GENERATION MOBILE COMMUNICATIONS by T. Ojanpera et al, published 1998, by Artech House Publishers, whose entire disclosure is hereby i
Do Nhat
Fleshner & Kim LLP
LG Information & Communications Ltd.
Marcelo Melvin
LandOfFree
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