Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
2000-03-10
2004-01-20
Vanderpuye, Kenneth (Department: 2661)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S509000, C370S516000, C375S226000, C375S362000
Reexamination Certificate
active
06680932
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of wireless communications, more particularly to synchronizer performance within a wireless modem.
2. Description of the Related Art
Explosive growth in the market for internet and intranet related applications has provided the impetus for a greater demand for fixed wireless networking services and systems. A wireless internet access system (WIAS) illustrated in
FIG. 1
is composed; of four major parts: (a) multiple data base stations (BS)
100
(
a
) and
100
(
b
) which provide wireless connectivity and gain coverage to subscriber units
102
(
a
)-(
d
) of a large geographical area (for example, residential and corporate terminal equipment as illustrated in FIG.
1
); (
b
) wireless modems
170
(
a
)-(
c
) (hereinafter “WM”) which are connected to BS
100
(
a
) or
100
(
b
) via wireless links
115
(
a
)-(
c
); (c) a data switching center (DSC)
125
with integrated management functions; and (d) a backbone transmission network
135
interconnecting (a)-(c) above.
As can be seen from
FIG. 1
, corporate terminals
102
(
c
) and
102
(
d
) can be, and many times are, connected to WM
170
(
c
) via a local area network (LAN) and a wireless router or firewall (not shown). Additionally, BS
100
(
a
) and
100
(
b
) may communicate with DSC
125
via frame relays (not shown). Further in conventional wireless internet access systems or networks, DSC
125
is interconnected with backbone transmission network
135
by a router and/or firewall (not shown for clarity).
FIG. 2
illustrates BS
100
(
a
) and
100
(
b
) of
FIG. 1
in an operational mode. Each BS
100
(
a
) and
100
(
b
) provides 360° RF coverage on the order of several gigahertz (preferably operating in the 3.5 GHz spectrum using approximately 5 MHz wide channels), sending and receiving signals over air lines
115
(
a
)-(
c
) between individual subscriber units
102
(
a
)-(
d
) served by BS
100
(
a
) and/or
102
(
b
). More particularly, the designated geographical area of subscribers served by each BS
100
(
a
) and
100
(
b
) is typically called a cell
150
, defined by its coverage area as shown in
FIG. 2
, where BS
100
(
a
) and
100
(
b
) are situated in designated cells
150
(
a
) and
150
(
b
). Within each cell
150
(
a
) or
150
(
b
) reside a plurality of subscribers
102
(
a
)-(
d
) served by the BS
100
(
a
) and/or
100
(
b
) includes a plurality of access points (hereinafter “AP”, not shown in
FIG. 1
) serving as an interface between individual subscribers
102
(
a
)-(
d
) of a cell
150
(
a
)-(
b
) served by BS
100
(
a
)-(
b
). Each access point includes receiver and transmitter circuitry of the base station for communicating with individual subscribers
102
(
a
)-(
d
) within a designated cell
150
(
a
)-(
b
).
Due to the need for increasing frequency spectrum reuse in the gigahertz band, in an effort to conserve this precious resource, the trend has been to reduce cell size even further (to microcells or picocells) which cover an even smaller geographical area, or which can serve hard to reach areas such as gullies and depressions where subscribers reside. Unfortunately this beneficial effect of increasing frequency spectrum reuse is offset by an increasing chance of neighboring cells interfering with each other, causing loss or degradation of the wireless signal. This loss or degradation of the wireless signal may be caused by, for example: (a) Rayleigh fading or delay spread due to multipath propagation; (b) shadow fading due to obstructions from natural and man-made objects around the main transmission path of the subscriber's devices; and (c) interference between co-channels and/or adjacent channels of wireless networks serving the subscriber's devices.
One particular problem related to (a) above could result from the development of signal delay spread in the wireless channel between a WM and an AP. A channel is the wireless link between a WM antenna and an AP antenna. A WM can function in at least five different frequency bands, but it only works in one frequency band, or one channel, at a time for receiving packets of information transmitted by an access point (AP), for example. Within the receiver circuitry of a WM is a synchronizer which perform an algorithm for time and frequency synchronization between received packet information and the receiver. The AP typically uses the same synchronizer algorithm as the WP. The synchronizer determines the starting time of an incoming packet and estimates the frequency offset between the transmitter of the AP and the receiver of the WM, so as to process the detected packet information.
A channel can go bad due to a variety of environmental conditions or changes, such as that due to traffic, temperature, rain, foliage, etc. For example, the terrain of a geographical area served by a wireless network can create multi-path delay spread of radio propagation. Multi-path delay spread in turn creates inter-symbol interference in the receiver detection circuitry, which ordinarily should be remedied by an equalizer component within the receiver. The equalizer, as well as the various components of a receiver are discussed further below.
For mobile systems, severe delay spread channels may be avoided by moving the mobile systems from place to place. However fixed wireless systems, employing a Time Division Multiple Access (TDMA) air interface for example, do not have the flexibility to be moved around in order to reduce the effects of severe delay spread channels. Thus the receivers within these fixed WMs need to be as robust as possible in order to handle delay spread channels and the effects thereof, which are discussed below.
A severe delay spread channel can usually be determined by examining the impulse response of the channel, or its h(z) function. If the frequency response of h(z) is relatively flat, the channel is a good channel in the sense that the inter-symbol interference is not so severe, or may be adequately handled by the equalizer within the receiver. However, if the frequency response exhibits a deep null, this is indicative of a bad channel, and the inter-symbol interference resulting from this spread will be difficult to equalize. For example, a bad channel could develop if the receiver is receiving from more than one strong signal source, and these two signal sources are separated by some time delay longer than “one symbol” time due to the multi-path effect described above.
To understand how the current synchronizer works, and also to comprehend the effects of delay spread on modem performance, the following terms are defined. Each detected packet is divided into segments allocated to various components within the receiver. The synchronizer segment of an incoming packet contains
17
symbols. There are eight time samples per symbol allocated in the synchronizer segment. A bin is a storage location for storing a corresponding one of the eight samples for each sequentially processed symbol; thus there are eight bins in the synchronizer, bins b
1
to b
8
.
The synchronizer first wants to determine if there is any differential phase error (DFE) test failure in each bin. Because these
17
transmitted sync symbols are always known to the receiver beforehand, the receiver compares the phases of the received samples in a certain bin with the phases of those known sync symbols. Suppose that the phases of sync symbols are ∠x(
1
), ∠x(
2
), . . . , ∠x(
17
). The receiver checks if bin
1
has a DFE test failure by first looking at the difference between |∠x(
1
)−∠x(
2
)| and |∠s
1
−∠s
9
|. This difference is called DFE. If the absolute value of DFE is larger than 90°, a DFE test failure occurs. Next the receiver checks the difference between |∠x(
2
)−∠x(
3
)| and |∠s
9
−∠s
17
|, and so on. If all 16 DFEs are <90°, then bin
1
does not have a DFE test failure.
In the current algorithm, the synchronizer chooses a bin locati
Hsuan Yi
Monsen Peter Thomas
Lucent Technologies - Inc.
Phunkulh Bob A.
Vanderpuye Kenneth
LandOfFree
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