Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1998-06-09
2003-02-25
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S509000, C370S335000
Reexamination Certificate
active
06526035
ABSTRACT:
BACKGROUND
The invention relates to cellular radio telephone communication systems, and more particularly, to identifying the source of a digital signal in a cellular system.
The cellular telephone industry is growing exponentially in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is outgrowing system capacity. If this trend continues, the effects of rapid growth will reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as to maintain high quality service and avoid rising prices.
Throughout the world, one important change in cellular systems is the transition from analog to digital systems. Equally important is the choice of an effective digital transmission scheme. Channel access is often achieved using frequency division multiple access (FDMA) and time division multiple access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signal's transmission power is concentrated. Interference with adjacent channels is limited by the use of bandpass filters which only pass signal energy within the specified frequency band. Thus, with each channel being assigned a different frequency, system capacity is limited by the available frequencies as well as by limitations imposed by radio channels.
In TDMA systems, a channel consists of a time slot in a periodic train of time intervals over the same frequency. Each period of time slots is called a frame. A given signal's energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, the portion of the interference from different relative signal strength levels is reduced. However, to support more users, the information has to be transmitted in shorter time slots at a faster bit rate.
With FDMA or TDMA systems, or a hybrid FDMA/TDMA system, it is desirable to avoid the case where two potentially interfering signals occupy the same frequency at the same time. In contrast, code division multiple access (CDMA) allows signals to overlap in both time and frequency. Thus, all CDMA signals share the same frequency spectrum. In either the frequency or the time domain, the multiple access signals appear to be on top of each other.
In principle, the information data stream to be transmitted is first coded or spread using a unique spreading code and then combined with a long PN-sequence or a shorter scrambling-sequence. In the latter case, the scrambling-sequences are planned from cell to cell so that neighboring cells use different scrambling-sequences or scrambling-masks. The information data stream and the PN-sequence or the scrambling sequence can have the same or different bit rates. The information data stream and the PN-sequence or the scrambling-sequence are combined by multiplying the two bit streams together. The bits of the unique spreading code and long PN-sequence are usually referred to as chips.
A plurality of coded information signals are transmitted on radio frequency carrier waves and jointly received as a composite signal at a receiver. Each of the coded signals overlaps all of the other coded signals, as well as noise related signals, in both frequency and time. By correlating the composite signal with one of the unique spreading codes, a corresponding information signal is isolated and decoded.
FIG. 1
illustrates the use of base stations to transmit radio waves to mobile users (mobile stations) in a cellular system. Base station
10
transmits a signal
12
that has a maximum signal strength that is limited so as to reduce interference with other base stations. The maximum signal strength of the base station's transmission creates a foot print or a region within which mobile stations can easily communicate with base station
10
. If base station
10
uses a single omni-directional antenna, the foot print extends in an unlimited direction, i.e. 360 degrees. While each footprint is an irregular shape that overlaps with adjacent foot prints, a foot print is often depicted as a hexagon
18
and is usually referred to as a cell.
In a CDMA system, base station
10
can transmit signals to mobile stations
14
and
15
as a single (composite) signal. The signal directed to mobile station
14
is typically coded with a short code that is orthogonal to a short code that is used to code the signal directed to mobile station
15
. These signals are spread with a code that is sometimes referred to as a long code. The sum of the two coded and spread signals is then transmitted by base station
10
. When mobile station
14
receives the composite signal, mobile station
14
multiplies the spread signal with the long code and the short code to recreate the signal directed to mobile station
14
and the signal directed to mobile station
15
is suppressed as interference noise. Similarly, mobile station
15
multiplies the spread signal with the long code and the short code assigned to mobile station
15
to recreate the signal directed to mobile station
15
and the signal directed to mobile station
14
is suppressed as interference noise. The interference noise is usually not distracting to the user of the mobile station, but as the number of mobile stations increases, so does the level of interference noise. If the omni-directional antenna at base station
10
is replaced with directional antennas, it is possible to divide cell
18
into smaller sectors and thereby reduce system interference. The use of directional antennas increases the capacity of a cellular system and is usually referred to as sectoring.
FIG. 2
a
illustrates the use of three directional antennas to divide a cell into three 120° sectors. Cell
20
has three sectors
21
,
22
, and
23
.
FIG. 2
b
illustrates the use of six directional antennas to divide a cell into six 60° sectors. Cell
30
has six sectors
31
,
32
, . . . , and
36
. If base station
10
uses directional antennas, base station
10
can transmit more than one composite signal. When a base station uses directional antennas, each directional antenna transmits to a smaller number of mobile stations than a single antenna would. As a result, the amount of interference decreases and the base station can support a larger number of mobile stations without exceeding an acceptable level of interference noise.
It is sometimes advantageous to transmit the same signal to the same mobile station via more than one source, that is, to provide diversity reception. Sometimes the best source is the source that provides the best signal to noise ratio. Other times the best source is the source that minimizes the interference experienced by the other mobile stations in the system.
As a mobile station moves away from a source (antenna), the quality of the received signal usually decreases. When the quality of the received signal decreases to the point that another source can provide a better signal or the system determines that it can decrease the amount of interference experienced by other mobile stations in the system, the system should perform a handoff. The base station or base stations can perform what is referred to as a soft handoff. A soft handoff occurs when the original source and the new source transmit substantially the same information to the mobile station at the same time and, subsequently, the original source terminates its transmission. If the mobile station is using a RAKE receiver, the signal from the new source appears as additional multipaths, and the RAKE receiver can process the two signals as a single signal.
In some cases, the original base station continues to serve the mobile station, but handsoff the mobile station to a better directional antenna. In other cases, the original base station hands off the mobile station to a neighboring base station. If the neighboring base station has directional antennas, the base station should not only handoff the mobile stati
Atarius Roozbeh
Eriksson Håkan B.
Henningsson Häkan
Östberg Christer
Burns Doane Swecker & Mathis L.L.P.
Elallam Ahmed
Kizou Hassan
Telefonaktiebolaget LM Ericsson (publ)
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