U.S. patent application number 10/288041 was filed with the patent office on 2003-05-08 for frequency assignment for multi-cell ieee 802.11 wireless networks.
Invention is credited to Kim, Byoung-Jo J., Leung, Kin K..
Application Number | 20030087645 10/288041 |
Document ID | / |
Family ID | 26964799 |
Filed Date | 2003-05-08 |
United States Patent
Application |
20030087645 |
Kind Code |
A1 |
Kim, Byoung-Jo J. ; et
al. |
May 8, 2003 |
Frequency assignment for multi-cell IEEE 802.11 wireless
networks
Abstract
A frequency planning method for use in an IEEE 802.11 wireless
network is described. The frequency planning method obtains traffic
load information associated with access points belonging to a
multi-cell wireless network and assigns channels to the access
points based on the traffic load information.
Inventors: |
Kim, Byoung-Jo J.; (Jersey
City, NJ) ; Leung, Kin K.; (Edison, NJ) |
Correspondence
Address: |
AT&T CORP.
P.O. BOX 4110
MIDDLETOWN
NJ
07748
US
|
Family ID: |
26964799 |
Appl. No.: |
10/288041 |
Filed: |
November 5, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60337694 |
Nov 8, 2001 |
|
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Current U.S.
Class: |
455/453 ;
455/450; 455/452.1 |
Current CPC
Class: |
H04W 48/16 20130101;
H04W 28/16 20130101; H04W 16/04 20130101; H04W 84/12 20130101; H04W
72/082 20130101 |
Class at
Publication: |
455/453 ;
455/452; 455/450 |
International
Class: |
H04Q 007/20 |
Claims
What is claimed is:
1. A method for frequency planning in wireless networks comprising:
obtaining traffic load information for access points belonging to a
multi-cell wireless network; and assigning channels to the access
points based on the traffic load information.
2. The method of claim 1 wherein the step of assigning comprises:
determining, for each access point, at least one set of interferers
from among the other access points relative to the access
point.
3. The method of claim 2 wherein the step of determining comprises:
determining, for each of the other access points, if any co-channel
interference by the other access point is greater than or equal to
a detection threshold, the detection threshold indicative of a busy
channel according to the CSMA protocol; and if it is determined
that the co-channel interference is greater than or equal to the
detection threshold, identifying the other access point as
belonging to the set of interferers for the access point.
4. The method of claim 3 wherein the co-channel interference is
derived from values of signal path loss between the access point
and the other access point and transmission power of the other
access point.
5. The method of claim 3 wherein the at least one set of
interferers comprises a second set of interferers, and wherein the
step of determining comprises: determining, for each pair of the
other access points, if any combined co-channel interference by
such pair is greater than or equal to a detection threshold, the
detection threshold indicative of a busy channel according to the
CSMA protocol; and if it is determined that the combined co-channel
interference is greater than or equal to the detection threshold,
identifying the other access points in such pair as belonging to
the second set of interferers for the access point.
6. The method of claim 2 wherein the step of assigning further
comprises: generating random channel assignments for the access
points; determining effective channel utilization values for each
access point; modifying the random channel assignment for
interferers in the at least one set of interferers such that the
highest one of the effective channel utilization values is
minimized; repeating such modification until the highest one of the
effective channel utilization values cannot be reduced by further
modification; and saving the modified random channel assignment as
a final assignment.
7. The method of claim 6 wherein the step of assigning further
comprises: providing the final assignment to the access points.
8. The method of claim 2 wherein the step of assigning further
comprises: assigning randomly a channel to each of the access
points; and computing, based on the random channel assignment, an
effective channel utilization value for each access point, the
effective channel utilization value representing the sum of an
offered load associated with the access point and total traffic
load associated with each set of interferers.
9. The method of claim 8 wherein the step of assigning further
comprises: determining which access point has the highest effective
channel utilization value; identifying which channel is assigned to
the access point having the highest effective channel utilization
value; and for each access point in the first set of interferers,
modifying the random channel assignment; recomputing the effective
channel utilization value for the modified random channel
assignment; and repeating modifying and recomputing for each
available channel other than the channel assigned to the access
point having the highest effective channel utilization value;
determining a minimum effective channel utilization from among the
recomputed effective utilization values; comparing the minimum
effective channel utilization and the recomputed effective channel
utilization values; and replacing the highest effective channel
utilization with the determined minimum effective channel
utilization and saving the modified random channel assignment as a
best solution if the determined minimum effective channel
utilization is lower than the highest effective channel
utilization.
10. The method of claim 9 wherein the step of assigning further
comprises: with a pre-specified probability, replacing the highest
effective channel utilization with the determined minimum effective
channel utilization and saving the modified random channel
assignment as a best solution if the determined minimum effective
channel utilization is equal to the highest effective channel
utilization.
11. The method of claim 6 wherein the step of assigning further
comprises: computing an effective utilization value for each access
point based on the final assignment; and determining if the
effective utilization value for each access point is less than a
value of one.
12. The method of claim 1 wherein the channels comprise
non-overlapping channels.
13. The method of claim 1 wherein the channels comprise overlapping
and non-overlapping channels.
14. The method of claim 1 wherein the access points operate in
accordance with the IEEE 802.11 standard.
15. The method of claim 1 wherein the step of assigning comprises:
seeking to minimize effective channel utilization of a most heavily
loaded of the access points.
16. The method of claim 1 wherein the step of assigning comprises:
seeking to minimize total effective channel utilization of all
access points.
17. The method of claim 1 wherein the step of assigning comprises:
seeking to maximize network throughput.
18. An article comprising: a storage medium having stored thereon
instructions that when executed by a machine result in the
following: obtaining traffic load information for access points
belonging to a multi-cell wireless network; and assigning channels
to the access points based on the traffic load information.
19. An apparatus comprising: a processor; and a memory storing a
computer program product residing on a computer-readable medium
comprising instructions to cause a computer to: obtain traffic load
information for access points belonging to a multi-cell wireless
network; and assign channels to the access points based on the
traffic load information.
20. An access point for use in a multi-cell wireless network
comprising: logic configured to obtain traffic load information for
access points belonging to the multi-cell wireless network; and
logic configured to assign channels to the access points based on
the traffic load information.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application Ser. No. 60/337,694 (Attorney Docket No.
2001-0531), filed Nov. 8, 2001, which is incorporated herein by
reference in its entirety for all purposes.
BACKGROUND
[0002] The invention relates to frequency planning for wireless
networks.
[0003] To meet the growing demand for wireless data services, many
companies have started deploying wireless local area networks
(WLANs) in airports, hotels, convention centers, coffee shops and
other locations in which network access by the public is desirable.
Many of these WLANs support the popular IEEE standard for wireless
Local Area Network (LAN) protocol, known as the IEEE 802.11
standard. The IEEE 802.11 standard includes a medium access control
(MAC) layer and several physical layers, including a
frequency-hopping spread spectrum (FHSS) physical layer and a
direct sequence spread spectrum (DSSS) physical layer. Versions of
the IEEE 802.11 standard include the IEEE 802.11a standard, which
describes a physical layer based on orthogonal frequency division
multiplexing (OFDM), and the IEEE 802.11b standard, which specifies
a high-rate DSSS layer. Because of its maturity and low cost, IEEE
802.11b capability has been included as standard equipment in many
laptop computers and hand-held devices. Thus, IEEE 802.11b products
make up the bulk of the installed base of IEEE 802.11 systems. The
IEEE 802.11 WLANs support data rates up to 11 Mbps, albeit over
short ranges, far exceeding that to be offered by the third
generation (3G) cellular wireless networks.
[0004] The IEEE 802.11 WLANs and 3G networks (or conventional
cellular wireless networks) have major differences in their design
at physical (PHY) and medium access control (MAC) layers to meet
different needs. In general, the IEEE 802.11 design is much simpler
than that of the 3G network because the IEEE 802.11 standard was
devised to serve a confined area (e.g., a link distance of at most
several hundred meters) with stationary and slow-moving users,
while the 3G specifications were developed for greater flexibility
in terms of geographical coverage and mobility, even providing for
users traveling at a high speed. As a result, the IEEE 802.11
network can support data rates higher than those by the 3G
networks. In addition, the cost of IEEE 802.11 equipment is much
lower than that for 3G equipment because of the simple and open
design of IEEE 802.11 networks, coupled with competition among WLAN
vendors.
[0005] In terms of operations, the 3G spectrum (such as the
Personal Communications System (PCS) band at 1.9 GHz) is licensed
and very expensive. As a result, every effort has been directed
toward optimizing the spectral efficiency while maintaining the
quality of service in terms of coverage and data rate for a limited
spectrum allocation. In contrast, the IEEE 802.11b networks operate
in the unlicensed Industrial, Scientific and Medical (ISM) band at
2.4 GHz. Since the frequency band is free, there is apparently no
pressing need to optimize the spectral efficiency. Rather,
simplicity and achieving low cost for the equipment are more
important. Despite the relatively abundant spectrum (i.e., a total
of 75 MHz in the 2.4GHz Band) at the ISM band, as IEEE 802.11b
networks are deployed widely, they start to interfere with each
other. Such interference leads to a degradation in network
throughput.
[0006] Frequency planning, i.e., allocation of a limited number of
frequencies, for an IEEE 802.11b network is different from that for
a traditional cellular network. Frequency planning techniques for
cellular wireless networks are well known. In typical cellular
wireless networks, such as those based on the Global System for
Mobile Communications (GSM) and Enhanced Data GSM Evolution (EDGE)
standards, two separate radio channels, namely the traffic and
control channels, are used to carry user data and control traffic,
respectively. For example, terminals access the control channels to
send control information via some contention mechanism. After the
information is successfully received and processed by a base
station (BS), the terminal is assigned with a specific traffic
channel for transmitting its data traffic. Existing frequency
assignment or radio-resource allocation schemes were devised mainly
for such traffic channels. Such schemes seek to avoid mutual
interference among various terminals or BSs using the same
frequency. In practical networks, there is no real-time
coordination among BSs in the assignment of traffic channels to
terminals in different cells. Thus, frequency assignment or
radio-resource allocation is based on statistical averages or worst
cases, e.g., 90% chance of acceptable link quality, across multiple
co-channel cells. Typically, frequency planning mechanisms for
traditional cellular networks tend to assign the same frequency to
cells that are a sufficient distance apart.
[0007] There is no such distinction between control and traffic
channels in the IEEE 802.11b network. Instead, all user data and
control information (in both directions between terminals and APs)
are carried on the same physical channel. The access to the channel
by multiple transmitters is coordinated by the MAC protocol, e.g.,
the well-known, Carrier Sense Multiple Access (CSMA) protocol with
collision avoidance feature. Under that protocol, a transmitter can
transmit only if it senses that the channel is currently idle. As a
result, even if two closely located APs are allocated with the same
frequency channel, much of the mutual (co-channel) interference can
still be avoided by the CSMA protocol, and the available bandwidth
is shared implicitly between the two cells served by the two APs.
In a sense, the MAC protocol provides an effective, distributed
mechanism to "coordinate" the channel access among terminals and
APs. In the worst case, both APs behave as if they share the same
frequency. Nevertheless, the IEEE 802.11 protocol still works
properly, thus demonstrating the robustness of its design, at the
expense of increased delay (due to backoff when sensing channel
busy) and degraded network throughput.
[0008] Consequently, existing frequency allocation mechanisms that
do not consider the combined effect of physical channel and MAC
protocol are not directly applicable to the IEEE 802.11 networks.
The MAC CSMA protocol helps to avoid much of co-channel
interference in large multi-cell IEEE 802.11 networks, but does so
at the potential expense of network performance.
SUMMARY
[0009] The invention provides for frequency planning in wireless
networks. Traffic load information is obtained for access points
belonging to a multi-cell wireless network. Channels are assigned
to the access points based on the traffic load information.
[0010] Embodiments of the invention may include one or more of the
following features.
[0011] The channels may be assigned by determining, for each access
point, at least one set of interferers from among the other access
points relative to the access point. The at least one set of
interferers may be determined by determining, for each of the other
access points, if any co-channel interference by the other access
point is greater than or equal to a detection threshold and, if it
is determined that the co-channel interference is greater than or
equal to the detection threshold, identifying the other access
point as belonging to the set of interferers for the access point.
The detection threshold is indicative of a busy channel according
to the CSMA protocol.
[0012] The co-channel interference may be derived from values of
signal path loss between the access point and the other access
point and transmission power of the other access point.
[0013] Particular implementations of the invention may provide one
or more of the following advantages. The frequency planning
mechanism serves as a valuable tool for frequency planning of
large-scale multi-cell IEEE 802.11 WLANs by focusing on
interactions among devices such as access points based on their
traffic loads and radio propagation. Thus, collision of signals in
a frequency band that would otherwise occur among the APs are
minimized or avoided while throughput of information is optimized.
The frequency planning tool can be deployed in a number of
different applications, e.g., as part of managed wireless LAN
services for business customers or, alternatively, as part of an
access point product for an automatic and adaptive frequency
planning.
[0014] Other features and advantages of the invention will be
apparent from the following detailed description and from the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is block diagram of a wireless network having
multiple access points (APs).
[0016] FIG. 2 is a block diagram showing an internal architecture
of an AP configured with a tool for performing a frequency
assignment process.
[0017] FIG. 3 is an illustration of different classes of co-channel
interferer APs relative to a given AP.
[0018] FIG. 4 is a flow diagram of one exemplary embodiment of the
frequency assignment process (of FIG. 2).
[0019] FIG. 5 is an illustration of an exemplary frequency
assignment produced by the frequency assignment process (of FIG. 4)
for a wireless network with 7 cells and 21 APs.
[0020] FIG. 6 is an illustration of an exemplary frequency
assignment produced by the frequency assignment process (of FIG. 4)
for a wireless network with 37 cells and 111 APs.
DETAILED DESCRIPTION
[0021] Referring to FIG. 1, a wireless network 10 includes a wired
network 12 (e.g., a Local Area Network or "LAN") having multiple
wireless access points 14 coupled thereto. The network 10 further
includes wireless stations or terminals 16 associated with the
different APs 14 to form infrastructure basic service structures
(or cells) 18. The AP 14 and terminals 16 served by that AP 14
(collectively referred to as a "cell") in a given infrastructure
basic service set (BSS) 18 communicate with each other over a
common channel that is assigned to the AP. In the embodiment
described herein, the AP 14 and terminals 16 communicate with each
other according to the wireless protocol provided by the IEEE
802.11 standard. The IEEE 802.11 standard specifies the medium
access control (MAC) and the physical (PHY) characteristics for
WLANs. The IEEE 802.11 standard is defined in International
Standard ISO/IEC 8802-111, "Information
Technology-Telecommunications and Information Exchange Area
Networks," 1999 Edition, which is hereby incorporated by reference
in its entirety. The APs 14 thus provide for communications between
the terminals 16 and any devices that may be connected to the wired
network 12.
[0022] Adjacent access points (APs) in IEEE 802.11 networks can be
assigned with the same channel or frequency, which is shared by
those APs and their associated terminals according to the multiple
access protocol (MAC), namely, the Carrier Sensing Multiple Access
with Collision Avoidance (CSMA/CA) protocol. Although the CSMA/CA
protocol can coordinate the bandwidth sharing of the same radio
frequency in IEEE 802.11 networks, traffic load for the APs has to
be considered so that there is enough link capacity for the
expected traffic load.
[0023] In accordance with the present invention, therefore, the
network 10 employs a frequency planning mechanism that considers
the combined effects of radio propagation, the IEEE 802.11 MAC
protocol and traffic load, so as to mitigate the impact of
co-channel interference on the performance of an IEEE 802.11
network.
[0024] Referring to FIG. 2, an exemplary AP 14 is shown. The AP 14
includes a processor 20, coupled to the network 12 by way of a
network interface 22. The network interface 22 permits the
processor 20 to send and receive units of data, such as packets,
over the network 12 using conventional techniques. The processor 20
is also coupled to memory 24. The memory 24 stores firmware 26
that, when executed by the processor 20, causes the access point 14
to operate as described herein. In particular, when the AP 14 is
designated to serve as a "master" AP, the firmware 26 includes a
frequency planning (or assignment) process 28 that allows the AP 14
to generate channel assignments for all of the APs 14 in the
network 10. In an alternative embodiment, with appropriate
synchronization, each AP 14, with its own copy of the frequency
assignment software, could perform the process to determine channel
assignment in a distributed manner. Also stored in memory 24 is a
parameter store 30 which stores, among other information, AP
configuration 32, including channel assignment information and
possibly AP traffic load information and radio parameter data. The
AP 14 can also include an I/O interface 33 to allow the AP to be
connected to other peripherals.
[0025] It will be appreciated that the functionality of the AP 14
may reside in a computer system such as a PC or workstation, with a
user interface for manually configuring the access point with
information, e.g., channel assignment, or, in the case of the AP
running the channel assignment process 28, parameter data to be
used by the channel assignment process, or can be connected to a
management console for such purpose.
[0026] Alternatively, the entire channel assignment process can be
installed and executed on a separate system such as a network
management system. Once the network management system or AP
responsible for the channel assignment has generated the assignment
information, AP configuration information including the channel
assignment can be provided to the APs over the network, or the APs
can be configured with the appropriate channel assignment
manually.
[0027] The process 28 can be implemented as an automated process
that is performed when an initial site "layout" is being defined.
At such a stage, the process runs after some pre-determined time
interval during which initial loading information is collected.
Preferably, it can execute whenever an access point joins or is
removed from the network, or whenever AP loading conditions have
changed.
[0028] The AP 14 includes a wireless interface 34 that includes one
or more wireless transceivers 36. In the described embodiment, the
transceivers 36 are radio frequency (RF) transceivers. Typically,
each transceiver 36 includes its own receiver for receiving
wireless RF communications from a terminal, a transmitter for
transmitting wireless RF communications to a terminal, and a
microprocessor to control the transceiver. Wireless communications
are received and transmitted by the transceivers 36 via respective
antennas 38, which are connected to the transceiver. Each of the
transceivers 36 and antennas 38 are conventional in configuration
and operation.
[0029] Frequency planning for IEEE 802.11 networks has two distinct
characteristics. First, according to the spectrum allocation in
North America, there are three overlapping channels for allocation
in the IEEE 802.11b networks and eight overlapping channels for
IEEE 802.11a networks. Thus, one has to adopt a tight frequency
reuse strategy for the 802.11 networks.
[0030] The original IEEE 802.11 specification allows for several
different kinds of physical layers, including direct sequence
spread spectrum (DSSS), frequency hopping spread spectrum (FHSS)
and infrared (IR). In particular, the DSSS design supports data
rates of 1 and 2 Mbps. Subsequently, while maintaining backward
compatibility to the DSSS 802.11, the IEEE 802.11b was adopted to
support data rates of 5.5 and 11 Mbps, operating in the 2.4 GHz ISM
band. As a result, the IEEE 802.11b network can support 1, 2, 5.5
and 11 Mbps, depending on radio conditions. Another extension is
IEEE 802.11a, which uses a different physical layer known as
orthogonal frequency division multiplexing (OFDM) to support data
rates ranging from 6 to 54 Mbps, operating in the 5.5 GHz band (the
U-NII band).
[0031] Although the channel assignment technique of the process 28
is described with respect to IEEE 802.11b networks, it will be
understood that the technique can be applied to other IEEE
802.11-based networks as well. The IEEE 802.11 MAC protocol
supports the independent basic service set (IBSS), which has no
connection to wired networks (i.e., an ad-hoc wireless network), as
well as an infrastructure BSS, which includes an AP connecting to a
wired network (as shown in FIG. 1). While the present invention
also applies to the IBSS case, only the infrastructure BSS will be
considered.
[0032] A brief description of the IEEE 802.11 MAC protocol follows.
The IEEE 802.11 specification defines five timing intervals for the
MAC protocol. Two of them are considered to be basic ones that are
determined by the physical layer: the short interframe space (SIPS)
and the slot time. The other three intervals are defined based on
the two basic intervals: the priority interframe space (PIFS) and
the distributed interframe space (DIFS), and the extended
interframe space (EIFS). The SIFS is the shortest interval,
followed by the slot time. The latter can be viewed as a time unit
for the MAC protocol operations, although the IEEE 802.11 channel
as a whole does not operate on a slotted-time basis. For IEEE
802.11b networks (i.e., with a DSSS physical layer), the SIFS and
slot time are 10 .mu.s and 20 .mu.s, respectively. The PIFS is
equal to SIFS plus one slot time, while the DIFS is the SIFS plus
two slot times. The EIFS is much longer than the other four
intervals and is used if a data frame is received in error.
[0033] The IEEE 802.11 MAC supports the Point Coordination Function
(PCF) and the Distributed Coordination Function (DCF). The PCF
provides contention-free access, while the DCF uses the carrier
sense multiple access with collision avoidance (CSMA/CA) mechanism
for contention-based access. The two modes are used alternately in
time.
[0034] The DCF operates as follows. An AP (or station) with a new
packet ready for transmission senses whether or not the channel is
busy. If the channel is detected idle for a DIFS interval (i.e., 50
.mu.s for IEEE 802.11b networks), the AP starts packet
transmission. Otherwise, the AP continues to monitor the channel
busy or idle status. After finding the channel idle for a DIFS
interval, the AP: a) starts to treat channel time in units of slot
time, b) generates a random backoff interval in units of slot time,
and c) continues to monitor whether the channel is busy or idle. In
the last step, for each slot time where the channel remains idle,
the backoff interval is decremented by one. When the interval value
reaches zero, the AP starts packet transmission. During this
backoff period, if the channel is sensed busy in a slot time, the
decrement of the backoff interval stops (i.e., is frozen) and
resumes only after the channel is detected idle continuously for
the DIFS interval and the following one slot time. Again, packet
transmission is started when the backoff interval reaches zero. The
backoff mechanism helps avoid collision since the channel has been
detected to be busy recently. Further, to avoid channel capture, an
AP must wait for a backoff interval between two consecutive new
packet transmissions, even if the channel is sensed idle in the
DIFS interval.
[0035] The IEEE 802.11 standard requires a receiver to send an
acknowledge message (ACK) for each packet that is successfully
received. Furthermore, to simplify the protocol header, an ACK
contains no sequence number and is used to acknowledge receipt of
the immediately previous packet sent. That is, APs and stations
exchange data based on a stop-and-go protocol. The sender is
expected to receive the ACK within the 10 .mu.s SIFS interval after
the packet transmission is completed. If the ACK does not arrive at
the sender within a specified ACK-timeout period, or it detects
transmission of a different packet on the channel, the original
transmission is considered to have failed and is subject to
retransmission by the backoff mechanism.
[0036] In addition to the physical channel sensing, the IEEE 802.11
MAC protocol implements a network allocation vector (NAV), whose
value indicates to each station the amount of time that remains
before the channel will become idle. All packets contain a duration
field and the NAV is updated according to the field value in each
decoded packet, regardless of the intended recipient of the packet.
The NAV is thus referred to as a virtual carrier sensing mechanism.
The MAC uses the combined physical and virtual sensing to avoid
collision.
[0037] The protocol described above is called the two-way
handshaking. In addition, the MAC also contains a four-way protocol
that requires the transmitter and receiver to exchange
Request-to-Send (RTS) and Clear-to-Send (CTS) messages before
sending actual data, as a way to resolve the so-called hidden
terminal problem.
[0038] The available number of non-overlapping channels for IEEE
802.11 WLAN systems depends on the underlying PHY layer. In North
America, the ISM band at 2.4 GHz is divided into eleven channels
for the IEEE 802.11 network where adjacent channels partially
overlap each other. Nevertheless, among these eleven channels,
there are three completely non-overlapping ones, separated by 25
MHz at their center frequency. In principle, all eleven channels
are available for allocation in a given IEEE 802.11 network.
However, it may be that overlapping channels can cause enough
interference that it is not beneficial to assign overlapping
channels to APs. Therefore, only the assignment of non-overlapping
channels is considered. The approach to frequency planning
described herein can be extended to the allocation of overlapping
channels with proper weighting of the overlapped spectrum,
proportional to their overlaps, however.
[0039] The frequency assignment process 28 described herein focuses
on transmission by the APs because the bandwidth consumption for
downlink (i.e., from AP to terminal) transmission is much higher
than that for uplink (i.e., from terminal to AP) transmission for
typical office environment and Internet applications.
[0040] The frequency assignment process 28 takes into account the
radio-path signal loss between every pair of APs in the network 10
and uses that information to define sets or classes of interferers
for each i-th AP (or "AP.sub.i"). Based on the interferer
classification and the expected traffic utilization (load)
associated with each AP, the effective channel utilization as seen
by each AP can be determined. The effective channel utilization
represents the sum of the traffic load of the AP and that "induced"
by its interferers because of channel sensing. In one embodiment,
the problem of frequency planning is formulated as a non-linear
zero-one integer programming problem, where one of the objective
functions is to minimize the effective utilization of the
"bottleneck" channel (i.e., the AP with the most highly loaded
channel). A heuristic algorithm is used to solve the problem.
[0041] For a network having M APs, indexed from 1 to M, and in
accordance with the CSMA protocol, an AP with traffic ready for
transmission determines if the assigned channel (frequency) is busy
or idle. For example, if the AP detects that the received power of
co-channel interference is equal to or greater than a channel-busy
detection threshold .alpha. (in units of mW), which corresponds to
about -80 dBm in the IEEE 802.11b standard, the channel is
considered to be busy. Otherwise, it is idle.
[0042] It is possible that the channel busy status is due to a
single transmitting AP or a group of multiple APs transmitting
simultaneously. For efficient frequency assignment, the interferers
for each AP can be classified as follows. Specifically, for each
AP.sub.i, C.sub.i(1) denotes a set of interfering APs where
transmission by any one AP in the set can cause enough interference
for AP.sub.i to detect channel busy. The APs in the set C.sub.i(1)
are called class-1 interferers for AP.sub.i. Likewise, C.sub.i(2)
denotes a set of pairs of two interfering APs where transmission by
any pair of APs in the set can cause AP.sub.i to sense channel
busy. The APs in C.sub.i(2) are referred to herein as class-2
interferers. It can be noted that transmissions by any single AP in
C.sub.i(2) are not sufficient to cause AP, to sense channel busy.
Further, the APs in any AP pair in C.sub.i(2) are not class-1
interferers to each other.
[0043] Referring to FIG. 3, an example of interferer class
definition 40 for a given AP is shown. The C.sub.i(1) and
C.sub.i(2) interferers for each AP.sub.i 16a can be determined by
measuring or estimating signal path loss between each pair of APs
in the network. Letting P.sub.j and h.sub.ij denote the
transmission power at AP.sub.j 16b and the signal path loss from
AP.sub.j to AP.sub.i, respectively, the classification of AP.sub.j
16b as a C.sub.i(1) interferer requires that
h.sub.iP.sub.j.gtoreq.a Eq. (1)
[0044] where h.sub.ijP.sub.j represents, for AP.sub.i, the
co-channel interference contributed by AP.sub.j, (indicated in the
figure by reference numeral 42a) and .alpha. is the power threshold
to detect channel busy.
[0045] Similarly, where P.sub.m and P.sub.n denote the transmission
power at AP.sub.m 16b and AP.sub.n 16c, respectively, and h.sub.im
and h.sub.in denote the signal path loss from AP.sub.m 16b to
AP.sub.i 16a and AP.sub.n 16c to AP.sub.i 16a, respectively, the
pair AP.sub.m and AP.sub.n belongs to C.sub.i(2) if
h.sub.imP.sub.m+h.sub.inP.sub.n.gtoreq..alpha. Eq. (2)
[0046] where h.sub.imP.sub.m+h.sub.inP.sub.n represents the
co-channel interference of the AP pair AP.sub.m and AP.sub.n
(indicated in the figure by reference numeral 42b).
[0047] It is assumed the transmission power in Equations (1) and
(2) is fixed in this disclosure. However, the channel assignment
mechanism could be adapted to support dynamic power control as
well.
[0048] It is possible to define class-3 or even higher classes of
interferers as well. Due to the contention-oriented nature of the
CSMA protocol, however, the traffic load on each channel (i.e., the
probability of transmission at a given AP) cannot be too high.
Thus, the probability of having interferers of class-3, which
require simultaneous transmission at all three interfering APs, is
much smaller relative to that of the class-1 and class-2
interferers. Hence, for simplicity, only class-1 and class-2
interferers are considered by the process 28. The process 28 also
takes into account AP traffic load, denoted generally by .rho..
[0049] Measurement of known RF parameters such as transmission
power and signal path loss can be carried out by a dedicated
hardware device, such as a handheld measurement device, or a site
survey software tool running on a network manager console or PC, or
even on the AP device itself. Many wireless LAN equipment vendors
bundle such tools with their access point hardware. Traffic load
can also be measured or modeled by commercially available network
management software.
[0050] Once measured, modeled or estimated, such parameter data
(measurements or estimates, as discussed above) is stored in the
memory 24 for use by the process 28.
[0051] There are a total of N (non-overlapping) channels, indexed
by 1 to N, available for allocation. As pointed out above, N=3 for
the IEEE 802.11b network for non-overlapping channels. With such a
small N, it is assumed that each AP is assigned one and only one
channel. An effective channel utilization U.sub.i is defined as the
fraction of time at which the channel can be sensed busy or is used
for transmission by AP.sub.i. That is, 1 U i = i + k = 1 X ik [ j
Ci ( 1 ) N j X jk + ( m , n ) Ci ( 2 ) m n X mk X nk ] . Eq . ( 3
)
[0052] where assignment indicator (or weight) X.sub.ij is equal to
`1` if AP.sub.i is assigned with channel.sub.j and is equal to `0`
otherwise.
[0053] Referring to Equation (3) above, the first term .rho..sub.i
is the offered traffic load for AP.sub.i in terms of channel
utilization without interference from any source. The first
summation term inside the brackets in Equation (3) represents the
total traffic load of all class-1 interfering APs that are assigned
the same channel as AP.sub.i. As discussed earlier, according to
the CSMA protocol and because of the detection threshold .alpha. in
use, AP.sub.i senses channel busy when any one of its class-1
interferers transmits on the same channel. The last summation term
in Equation (3) represents the total traffic load of all class-2
interferers. The interferer classes can be defined to include
overlapping channels as well. For example, the transmission power
from interferers on overlapping channels can be weighted
proportionally to the spectrum overlap. The weight for
non-overlapping channels is `0`, and for fully overlapping
co-channel cases is `1`. Partially overlapping ones are somewhere
in between depending on their carrier frequency offset, filter
shapes and other factors.
[0054] Channel stability is maintained (i.e., all traffic can be
sent eventually) by requiring that
U.sub.i<S Eq. (4)
[0055] for all AP.sub.i where i=1 to M, and a threshold S is equal
to a value of 1. The value of S can be made less than 1 to account
for overhead of CSMA contention or other source of
interference.
[0056] One objective function for the channel assignment is to
minimize the effective utilization of the "bottleneck" AP, that
is,
minimize max {U.sub.1, U.sub.2 . . . , U.sub.m} Eq. (5)
[0057] over the assignment indicator {X.sub.ij} subject to the
constraints of Equation (4) for all i=1 to M. Clearly, the
objective function in Equation (5) is to assign channels such that
the effective utilization of the most heavily loaded AP is
minimized. This results in more resources available for the most
heavily loaded AP, given offered traffic loads.
[0058] In one embodiment, for the channel assignment process 28
with Equation (5) as the objective function, a heuristic algorithm
is utilized, as described below with reference to FIG. 4. Thus, the
heuristic algorithm attempts to minimize the effective channel
utilization for the bottleneck AP. The heuristic algorithm makes
use of the following parameters: offered traffic load p.sub.i and
interferer sets C.sub.i(1) and C.sub.i(2) for each AP.sub.i.
Preferably, the process 28 is subject to constraints of Equation
(4) for all APs.
[0059] Referring to FIG. 4, the process 28 begins (step 50) by
generating a random (initial) channel assignment for each AP, in
the network (step 52). This assignment is treated as the best
assignment obtained so far. The process 28 determines the effective
channel utilization U.sub.i for each AP.sub.i based on the
generated channel assignment (step 54). The process 28 identifies
the AP (say, the "i-th" AP, or AP.sub.i) with the highest or
maximum effective channel utilization (step 56). This AP is
referred to as the "bottleneck" AP. The maximum effective channel
utilization, that is, max {U.sub.i}, for the assignment is denoted
by V (step 58). In case of a tie, one such AP.sub.i is chosen
randomly as the "bottleneck." For the bottleneck AP.sub.i, the
process 28 identifies its current assigned channel, say channel k
(step 60). For each available channel n from 1 to N with n.noteq.k
and each co-channel AP (say j) in C.sub.i(1) (i.e., those APs in
the set that have been assigned with channel k), the process 28
temporarily modifies the channel assignment by reassigning only
AP.sub.j with channel n, and recomputes the maximum effective
channel utilization, denoted by W.sub.jn, for the new assignment
(step 62). After completing such testing for all such n and j, the
process 28 determines the minimum, denoted by W, from among all the
W.sub.jn's (step 64). The process 28 compares the values of W and V
(step 66). If the process 28 determines that the value of W is less
than that of V, then the process 28 replaces V by W, records the
associated new assignment as the "new" best solution (i.e., to
finalize the channel change for one AP that minimizes the objective
function the most) (step 70), and returns to step 54. If, at step
72, the process 28 determines that W and V are equal, then, with a
pre-specified probability .delta., preferably in the range
1>.delta.>O (to avoid infinite looping, as discussed later),
the process 28 replaces V by W, records the new assignment as the
best solution (step 74) and returns to step 54. If the process 28
determines that W is greater than V, the process 28 saves the
current assignment and associated V value as the best solution
obtained so far (that is, the current assignment is the local
suboptimal assignment) (step 76). The process 28 determines if
there is another random assignment to be considered (step 78). If
so, the process 28 returns to step 52 to repeat the processing for
another random assignment. If no further random assignments are to
be considered, the process 28 selects a final assignment as the
best solution, that is, it is the channel assignment with the
lowest value of V, among the local suboptimal assignments reached
at step 76 (step 80). The process 28 tests the final solution to
determine if constraints of Equation (4) for all APs are satisfied
for the final assignment (step 82). If so, the final assignment is
feasible. Otherwise, it is considered that no feasible solution
exists for the network under consideration. After the feasibility
is tested, the process 28 terminates (step 84).
[0060] While the process 28 as illustrated in FIG. 4 may not
explicitly consider the constraints of Equation (4), minimizing the
maximum U.sub.i implicitly enhances the chance of satisfying
constraints of Equation (4) for all APs.
[0061] There are several characteristics of the heuristic
assignment technique that are worth further consideration. First,
it can be shown that the heuristic assignment technique has a
loop-free property, that is, with 1>.delta.>O in step 74
(FIG. 4), the heuristic algorithm does not have infinite looping.
The proof is as follows. Given that the number of AP's M and
available channels N in the system are finite, steps of identifying
the bottleneck AP and determining W can be completed in a finite
amount of time. The only possibility that the algorithm has an
infinite loop is that the steps of processing a random assignment
are executed repeatedly without stop. Assume, preliminarily, that
such looping can occur, that the V value after the m-th execution
(iteration) is denoted by V.sub.m, and that .delta.=0 in step 74.
To form the infinite looping requires that V.sub.1>V.sub.2> .
. . >V.sub.m with m increasing towards infinity. With both M and
N being finite, there are only a finite number of all possible
channel assignments. Since each new assignment finalized by step 70
has a unique maximum effective channel utilization, it is thus
impossible that m goes to infinity. That is, step 76 must be
reached after a finite amount of processing.
[0062] Now assume that infinite looping is possible with
1>.delta.>0. Based on the above argument, it is necessary to
have V.sub.1> . . . >V.sub.i=V.sub.i+1> . . .
>V.sub.j=V.sub.j+1&g- t; . . . V.sub.m with m going to
infinity for some i and j. Since the argument above has already
ruled out the possibility of having subsequences of V.sub.i's of
infinite length between two `=` signs on this list, it must contain
an infinite number of `=` signs. Since each `=` sign corresponds to
an execution of the case of W=V with probability .delta., the
probability of executing this step for an infinite number of times
is thus zero. Hence, the infinite looping cannot exist.
[0063] Although it is possible to treat the case of W=V as reaching
a local optimum (like the case of W>V), numerical experience
suggests that the case of W=V helps explore various assignments for
enhanced results, especially when there are multiple bottleneck APs
for the channel assignment under consideration.
[0064] Since heuristics is involved in the process 28 for the
exemplary algorithm illustrated in FIG. 4, achieving the optimal
solution is not guaranteed. It is possible, however, to quantify
the quality of the suboptimal solution generated by the algorithm.
It is observed that the processing--in particular, steps 60, 62 and
64 (FIG. 4)--basically tests out various channel assignments to
identify a better solution. As the algorithm is executed for a
given initial, random assignment, it is possible to let Y.sub.0,
Y.sub.1, Y.sub.2, . . . , Y.sub.m, denote the (random) sequence of
the maximum effective channel utilization associated with the
channel assignments under testing by step 62, with Y.sub.0 denoting
the quantity for the initial, random assignment. Based on the
Y.sub.i sequence, another sequence Z.sub.0, Z.sub.1, Z.sub.2, . . .
, Z.sub.n is constructed as follows: (i) initialize with
Z.sub.0=Y.sub.0 and set i=0; (ii) for each j=1, 2, . . . , m,
compare Y.sub.j with Z.sub.i; and (iii) if Z.sub.i>Y.sub.j, then
set i=i+1 and Z.sub.i=Y.sub.j; otherwise, repeat (ii) for the next
j value.
[0065] In essence, the sequence Z.sub.i is constructed by examining
Y.sub.j one by one, starting with Z.sub.0=Y.sub.0 and adding
Y.sub.j as the last element in the Z.sub.i sequence only if Y.sub.j
is less than Y.sub.i for all i<j (or equivalently, Y.sub.j is
less than Z.sub.i, the last element in the current sequence).
Clearly, the sequence Z.sub.i is monotonic strictly decreasing.
Physically, Z.sub.i represents the sequence of the maximum
effective channel utilization for an improved assignment finalized
by step 70, or step 74 (FIG. 4) that yields a maximum utilization
lower than any assignments examined by the algorithm so far in the
search process.
[0066] The algorithm is repeated for a given number (say K) of
initial random assignments. For each initial assignment, one such
sequence Z.sub.i (as discussed above) can be obtained. It can be
noted that the sequences associated with different initial
assignments have different lengths and are mutually independent of
each other (although elements in the same sequence are dependent).
Furthermore, when the algorithm eventually stops, it is assumed
that it has encountered a total of n improved assignments (i.e.,
improved over those examined earlier and derived from the same
initial assignment), which is the sum of lengths of the sequences
of Z.sub.i minus K.
[0067] One can view that the maximum effective channel utilization
for all possible assignments for the given network has a
probability distribution. Allowing T.sub..pi., to be the maximum
utilization for the top-.pi.-fraction of assignments (e.g., the top
0.001 percentile assignments), a random assignment with its maximum
utilization Z.sub.0, gives
P[Z.sub.0<T.sub..pi.]=.pi. Eq. (6)
[0068] It can be proven that, if the algorithm has encountered a
total of n improved assignments at the completion of its execution,
then
Q.sub..pi.>1-(1-.pi.).sup.n-1 Eq. (7)
[0069] where Q.sub..pi. denotes the probability that the final
suboptimal solution generated by the algorithm falls within the
top-.pi.-fraction of assignments. The proof is as follows. First,
the case of encountering n improved assignments for one initial,
random assignment is examined. By definition, 2 Q = P [ min i Z i T
] = 1 - P [ min i Z i > T ] . Eq . ( 8 )
[0070] The event of (min Z.sub.i>T.sub..pi.) in the above is
identical to having Z.sub.0>T.sub..pi., Z.sub.1>T.sub..pi., .
. . , and Z.sub.n>T.sub..pi.. Given that Z.sub.i is a strictly
decreasing (random) sequence, then
P[Z.sub.0>T.sub..pi..LAMBDA.Z.sub.1>T.sub..pi..LAMBDA. . . .
.LAMBDA.Z.sub.n>T.sub..pi.]<P[Z.sub.0>T.sub..pi..LAMBDA.Z.sub.0.-
sup.1>T.sub..pi..LAMBDA. . . .
.LAMBDA.Z.sub.0.sup.n>T.sub..pi.] Eq. (9)
[0071] where Z.sub.0.sup.i is a random variable independently drawn
from the same distribution for Z.sub.0 for i=1 to n. One can obtain
Equation (9) by replacing Z.sub.i on the left hand side by
Z.sub.0.sup.1 on the right side for one i at a time. Since the
Z.sub.0.sup.i variables are independent,
P[Z.sub.0>T.sub..pi..LAMBDA.Z.sub.0.sup.1>T.sub..pi. . . .
Z.sub.0.sup.n>T.sub..pi.]={P[Z.sub.0>T.sub..pi.]}.sup.n+1 Eq.
(10)
[0072] Using the definition in Equation (6), substituting Equation
(10) into Equation (9) and then Equation (9) into Equation (8)
yields Equation (7). The case with multiple initial random
assignments is proved by exploiting the property that the sequences
Z.sub.i associated with different initial assignments are mutually
independent.
[0073] The performance of the process 28 is validated by applying
the process 28 to two settings of multi-cell networks using the
IEEE 802.11 air interface for which the optimal assignment is
known. The settings correspond to settings for a seven (7) cell
network and thirty-seven (37) cell network.
[0074] Referring to FIG. 5, an assignment 90 generated by the
process 28 for a setting that corresponds to a network with 7 cells
is shown. Three adjacent hexagon-shaped sectors 92a, 92b and 92c
form a cell 94. Each sector 92 is served by an AP at the center of
the cell. Each AP antenna has a beamwidth of 60' and points toward
an appropriate direction to serve the associated sector. Thus,
there are 21 APs in the 7 cell network, with 3 APs for each given
cell co-located at the cell center, indicated by reference numeral
96.
[0075] Similarly, and referring to FIG. 6, an assignment 100 for a
setting that corresponds to a network with 37 cells is shown. Three
adjacent hexagon-shaped sectors 102a, 102b and 102c form a cell
104. For this setting, there are 111 APs, with 3 APs for each given
cell co-located at the cell center, indicated by reference numeral
106.
[0076] The antenna gain has a parabolic shape; that is, a 3 dB drop
relative to the front direction occurs at the half beamwidth angle.
Any direction beyond a threshold angle in clockwise or
anti-clockwise direction suffers a given, fixed attenuation
relative to the gain at the front direction, which is called the
front-to-back (FTB) ratio. The FTB is set to be 25 dB.
[0077] It may be recalled that only the AP-to-AP interference is
considered in the current formulation. The radio link between any
pair of APs in the network is characterized by a path-loss model
with an exponential of 3.5. Cell radius is assumed to be 1 Km and
the path loss at 100 m from the cell center is -73 dB. Transmission
power for each AP antenna is 30 dBm (or 1 W). All APs have an
identical amount of offered traffic. It will be noted that the
solution generated by the process 28 in this instance does not
depend on the actual traffic load, but the feasibility of the final
solution does. In order to ensure that the optimal assignment is
known, shadowing and fast fading are not considered. In addition,
the channel-busy detection threshold .alpha. is set to be 2.5e-3
.mu.W (which corresponds to -86 dBm). As pointed out earlier, there
are 3 non-overlapping channels available in the ISM band for
assignment. Based on the parameter settings for both 7 and 37 cell
networks, the optimal assignment is the traditional frequency reuse
of 3. That is, no adjacent sectors (APs) use the same channel.
[0078] When the process 28 is applied to the network with 7 cells
and 21 APs, as shown in FIG. 5, it generates the optimal channel
assignment based on 50 random assignments. The optimal assignment
90 with channels 1 to 3 assigned to the various sectors 92a, 92b
and 92c for each cell 94 is as shown in FIG. 5.
[0079] As for the network with 37 cells and 111 APs, the process 28
was unable to yield the obvious optimal assignment of reuse of 3,
that is, without considering the boundary effect of the cell layout
(which makes the interference conditions non-uniform). The
suboptimal solution for channels 1-3 obtained from the process
using 1,000 random assignments is the assignment 100 shown in FIG.
6. It can be seen from the assignment 100 that most of the sectors
(APs) use a channel different from those in adjacent sectors. In
the worst case, at most two adjacent sectors share the same
channel. The process encountered and finalized a total of 505,363
improved assignments. Based on the analysis set forth above, with a
probability higher than 99.4%, the suboptimal solution, assignment
100, falls within the top 0.001th percentile. This result is quite
acceptable.
[0080] The above two examples have uniform traffic load and uniform
propagation environments with obvious solutions and are only used
to verify the correctness of the algorithm. However, for any
wireless network of considerable size, the traffic load and the
propagation environment are seldom uniform and are usually without
obvious channel assignment solutions. The approach of the frequency
planning process 28 can easily produce a good (albeit suboptimal)
channel assignment solution in such cases, with provable closeness
to the optimal solution. Also, if the traffic load is slowly
fluctuating over time, the approach can be used to generate a
series of channel assignments over time to best accommodate the
changing conditions.
[0081] Other objective functions can be used in the channel
assignment optimization. For example, another objective function
(in addition to objective function of Equation (5)) is to minimize
the overall interference, that is,
[0082] minimize 3 i = 1 U i M ( 11 )
[0083] over the assignment indicator {X.sub.ij} subject to the
constraints of Equation (4) for all i=1 to M. It can be noted that
the sum of all U.sub.i reflects the total effective channel
utilization. Minimizing the sum tends to minimize the overall
interference in the network while maintaining stability of each
channel shared and detectable by multiple neighboring APs.
[0084] For the optimization with Equation (11) as the objective
function, a linear integer programming approach can be used. For a
given network setting, the offered load p.sub.i and the interferer
sets C.sub.i(1) and C.sub.i(2) for each AP, are known. The
programming problem is non-linear due to the cross-products of
X.sub.ij's in U.sub.i, as defined in Equation (3). Using known
techniques--for example, the technique described in the paper by W.
W. Chu entitled "Optimal File Allocation in a Multiple Computer
System, "IEEE Trans. On Computers, C-18, No. 10, pp. 885-889,
October 1969--it is possible to linearize the problem by replacing
X.sub.ikX.sub.mkX.sub.nk by a new term Y.sub.ikmn. Similarly, the
term X.sub.ikX.sub.jk is replaced by a new term Z.sub.ikj. The
resultant problem becomes a linear integer programming problem,
which has been shown to be NP-complete.
[0085] Yet another objective function is to maximize network
throughput.
[0086] Other embodiments are within the scope of the following
claims. For example, the above-described approach may be extended
to consider one or more of the following: non-uniform transmission
power by the APs; upstream traffic; overlapping channels (as
discussed earlier); real-time adaptive channel assignment to meet
the fluctuation of traffic load at various APs over time; inclusion
of path gains for stations; and special frequency constraints for
individual AP's (e.g., AP closest to a Microwave, WLANs of other
carriers).
* * * * *