U.S. patent application number 11/974382 was filed with the patent office on 2008-02-14 for wireless communication method and apparatus for compensating for interference.
This patent application is currently assigned to InterDigital Technology Corporation. Invention is credited to Christopher Cave, Angelo Cuffaro, Paul Marinier, Vincent Roy.
Application Number | 20080037439 11/974382 |
Document ID | / |
Family ID | 34743049 |
Filed Date | 2008-02-14 |
United States Patent
Application |
20080037439 |
Kind Code |
A1 |
Cave; Christopher ; et
al. |
February 14, 2008 |
Wireless communication method and apparatus for compensating for
interference
Abstract
A method and apparatus which determine the minimum transmission
power level of an access point (AP) for reliably communicating with
at least one wireless transmit/receive unit (WTRU) in a wireless
communication system. The apparatus obtains the range of the AP and
estimates the interference to the AP by executing a slow
interference estimation process and a fast interference estimation
process. A required received power (RRP) of the WTRU is obtained
from the interference estimate. The minimum transmission power
level of the AP is determined by summing the range of the AP and
the RRP.
Inventors: |
Cave; Christopher; (Candiac,
CA) ; Cuffaro; Angelo; (Laval, CA) ; Roy;
Vincent; (Montreal, CA) ; Marinier; Paul;
(Brossard, CA) |
Correspondence
Address: |
VOLPE AND KOENIG, P.C.;DEPT. ICC
UNITED PLAZA, SUITE 1600
30 SOUTH 17TH STREET
PHILADELPHIA
PA
19103
US
|
Assignee: |
InterDigital Technology
Corporation
Wilmington
DE
|
Family ID: |
34743049 |
Appl. No.: |
11/974382 |
Filed: |
October 12, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11485033 |
Jul 12, 2006 |
7286486 |
|
|
11974382 |
Oct 12, 2007 |
|
|
|
10926892 |
Aug 26, 2004 |
7079494 |
|
|
11485033 |
Jul 12, 2006 |
|
|
|
60535043 |
Jan 8, 2004 |
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Current U.S.
Class: |
370/252 |
Current CPC
Class: |
H04W 52/367 20130101;
H04W 52/245 20130101; H04W 52/243 20130101; H04W 52/143 20130101;
H04W 16/14 20130101; H04W 52/246 20130101 |
Class at
Publication: |
370/252 |
International
Class: |
G06F 11/00 20060101
G06F011/00 |
Claims
1. An access point (AP) comprising: (a) a receiver configured to
receive a received signal strength indicator (RSSI); (b) a
measuring unit electrically coupled to the receiver, the measuring
unit configured to measure external interference at the AP based on
data received via the receiver; and (c) an analyzing unit
electrically coupled to the measurement unit, the analyzing unit
configured to perform a noise floor measurement based on the RSSI,
determine whether the noise floor measurement is available in the
absence of carrier lock, set a required received power (RRP) from
uplink interference (RRP.sub.UI) to the value of a previous
activation, if there is no available noise floor measurement
available in the absence of carrier lock, and set the RRP.sub.UI in
accordance with the following equation,
RRP.sub.UI=I.sub.AP+(C/I).sub.req.sub.--.sub.low, if the noise
floor measurement is available, wherein I.sub.AP is an estimate of
the noise floor at the AP, and (C/I).sub.req.sub.--.sub.low is a
configurable parameter representing the typical required
carrier-to-interference ratio to have a reasonable probability of
success at a low rate.
2. The AP of claim 1 further comprising: (d) a transmitter
configured to transmit transmission parameters to a wireless
transmit/receive unit (WTRU); and (e) a function generator
electrically coupled to the analyzing unit and the transmitter, the
function generator being configured to provide the RRP and minimum
transmission parameters to the transmitter, as determined by the
analyzing unit.
3. An access point (AP) comprising: (a) a receiver configured to
receive a received signal strength indicator (RSSI); (b) a
measuring unit electrically coupled to the receiver, the measuring
unit configured to measure external interference at the AP based on
data received via the receiver; and (c) an analyzing unit
electrically coupled to the measuring unit, the analyzing unit
configured to perform a noise floor measurement based on a received
signal strength indicator (RSSI), and set the required received
power (RRP) from uplink interference (RRP.sub.UI) in accordance
with the following equation,
RRP.sub.UI=I.sub.AP+(C/I).sub.req.sub.--.sub.low, wherein I.sub.AP
is an estimate of the noise floor at the AP, and
(C/I).sub.req.sub.--.sub.low is a configurable parameter
representing the typical required carrier-to-interference ratio to
have a reasonable probability of success at a low rate.
4. The AP of claim 3 further comprising: (d) a transmitter
configured to transmit transmission parameters to a wireless
transmit/receive unit (WTRU); and (e) a function generator
electrically coupled to the analyzing unit and the transmitter, the
function generator being configured to provide the RRP and minimum
transmission parameters to the transmitter, as determined by the
analyzing unit.
5. An access point (AP) comprising: (a) a receiver configured to
receive a received signal strength indicator (RSSI); (b) a
measuring unit electrically coupled to the receiver, the measuring
unit configured to measure external interference at the AP based on
data received via the receiver; and (c) an analyzing unit
electrically coupled to the measuring unit, the analyzing unit
configured to perform a noise floor measurement based on the RSSI,
and set a required received power (RRP) from uplink interference
(RRP.sub.UI) to the value of a previous activation, if there is no
available noise floor measurement available in the absence of
carrier lock.
6. The AP of claim 5 further comprising: (d) a transmitter
configured to transmit transmission parameters to a wireless
transmit/receive unit (WTRU); and (e) a function generator
electrically coupled to the analyzing unit and the transmitter, the
function generator being configured to provide the RRP and minimum
transmission parameters to the transmitter, as determined by the
analyzing unit.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/485,033, filed on Jul. 12, 2006, which is a
continuation of U.S. patent application Ser. No. 10/926,892 filed
on Aug. 26, 2004, which issued as U.S. Pat. No. 7,079,494 on Jul.
18, 2006, which claims the benefit of U.S. Provisional Application
No. 60/535,043 filed Jan. 8, 2004, which are incorporated by
reference as if fully set forth herein.
FIELD OF INVENTION
[0002] The present invention relates to power control in shared
wireless networks. More particularly, the present invention relates
to a method and apparatus which determine the minimum transmission
power level of an access point (AP) to compensate for prevailing
interference conditions between the AP and at least one wireless
transmit/receive unit (WTRU).
BACKGROUND
[0003] Wireless local area networks (WLANs) have become more
popular because of their convenience and flexibility. Such networks
typically include an AP and a plurality of WTRUs which wirelessly
communicate with one another. When transmitting information in a
WLAN, it is found that interference is produced in other nearby
networks using the same frequency band.
[0004] As new applications for such networks are being developed,
their popularity is expected to significantly increase. Institute
of Electrical and Electronics Engineers (IEEE) working groups have
defined an IEEE 802.11 baseline standard having extensions which
are intended to provide higher data rates and other network
capabilities.
[0005] In accordance with the IEEE 802.11 baseline standard, WLANs
use a carrier-sense multiple access/collision avoidance (CSMA/CA)
medium access scheme in which the WTRU's transmissions are not
distinguished from each other by means of different modulation
codes. Rather, each WTRU, (and AP), transmits packets containing
the sender and destination addresses in their headers. In order to
avoid reception errors, the WTRUs attempt to avoid transmitting
simultaneously by sensing the wireless medium prior to
transmitting.
[0006] The goal of the power control process is to determine the
transmission power of an AP to the most appropriate value. The
power control process must adequately serve associated WTRUs that
are within a certain region (coverage area) around the AP, taking
into account possible interference experienced by these WTRUs. This
may be accomplished by determining the minimum power level at which
the AP transmission power. Furthermore, the power control process
must minimize the interference to WTRUs and APs in neighboring Base
Service Sets (BSSs) which results in an excessive number of lost
packets and/or deferrals in these BSSs. This may be accomplished by
selecting a power level between the minimum power level and a
maximum power level.
[0007] A method and system for reliably and accurately determining
the minimum power level of AP transmissions is desired.
SUMMARY
[0008] The present invention is a wireless communication method and
apparatus for determining the minimum transmission power level of
an AP for reliably communicating with at least one WTRU in a
wireless communication system. The apparatus may be a wireless
communication system, an AP or an integrated circuit (IC). The
apparatus obtains the range of the AP and estimates the
interference to the AP by executing a slow interference estimation
process and a fast interference estimation process. A required
received power (RRP) of the WTRU is obtained from the interference
estimate. The minimum transmission power level of the AP is
determined by summing the range of the AP and the RRP.
BRIEF DESCRIPTION OF THE DRAWING(S)
[0009] A more detailed understanding of the invention may be had
from the following description, given by way of example and to be
understood in conjunction with the accompanying drawings
wherein:
[0010] FIG. 1 is a block diagram of a wireless communication system
in accordance with the present invention;
[0011] FIG. 2 is a flowchart of a power control process for
determining the minimum transmission power of an AP in the system
of FIG. 1;
[0012] FIG. 3 is a flowchart of a slow interference estimation
process used by the power control process in accordance with the
present invention; and
[0013] FIG. 4 is a flowchart of a fast interference estimation
process used by the power control process in accordance with the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0014] Hereafter, the terminology "WTRU" includes but is not
limited to a user equipment (UE), mobile station, fixed or mobile
subscriber unit, pager, or any other type of device capable of
operating in a wireless environment.
[0015] When referred to hereafter, the terminology "AP" includes
but is not limited to a base station, a Node-B, site controller or
any other type of interfacing device in a wireless environment. The
invention is particularly applicable to wireless local area
networks (WLAN).
[0016] The present invention will be described with reference to
the drawing figures wherein like numerals represent like elements
throughout. The present invention applies as an add-on to the WLAN
IEEE 802.11 standards (802.11 baseline, 802.11a, 802.11b, and
802.11g), and also applies to IEEE 802.11e, 802.11h and 802.16.
[0017] The present invention may be further applicable to Time
Division Duplex (TDD), Frequency Division Duplex (FDD), and Time
Division Synchronous CDMA (TD-SCDMA), as applied to a Universal
Mobile Telecommunications System (UMTS), CDMA 2000 and CDMA in
general, but is envisaged to be applicable to other wireless
systems as well.
[0018] The features of the present invention may be incorporated
into an integrated circuit (IC) or be configured in a circuit
comprising a multitude of interconnecting components.
[0019] This present invention computes a minimum transmission power
level for an AP for providing a desired coverage area and to
provide satisfactory performance of downlink transmissions from the
AP to the WTRU.
[0020] The desired coverage area is defined in terms of a maximum
path loss (hereinafter referred to as the "range of the AP")
between the AP and a WTRU associated to this AP, such that the WTRU
enjoys satisfactory performance. The range may be automatically
computed based on measurements of path loss to other neighboring
APs, or be specified manually as a configuration parameter.
[0021] The satisfactory performance of downlink transmissions from
the AP to the WTRU is provided by computing a minimum RRP based on
statistics of packet errors in the downlink direction in the AP's
own BSS, as well as the level of interference directly perceived by
the AP. The minimum transmission power level (in dBm) is then
obtained by summing the range of the AP (in dB) with the RRP (in
dBm).
[0022] FIG. 1 is a block diagram of a wireless communication system
100 according to the present invention. The system 100 includes an
AP 105 and at least one WTRU 110. The AP 105 includes a receiver
115, a transmitter 120, a measuring unit 125, an analyzing unit
130, and a function generator 135. The measuring unit 125 measures
the interference at the AP 105 based on data received via receiver
115. The analyzing unit 130 analyzes measurement data provided by
the measuring unit 125. Furthermore, the analyzing unit 130
receives and uses information on data packets transmitted by the
transmitter 120 to the WTRU 110. The function generator 125
provides RRP and minimum transmission power parameters to the
transmitter 120 as determined by the analyzing unit 130. The
parameters are then transmitted from the AP 105 to the WTRU
110.
[0023] FIG. 2 is a flowchart of a power control process 200
including method steps which are executed in system 100. In step
205, the range of the AP 105 is determined. In step 210, an
estimate of the interference to the AP 105 is performed by the
analyzing unit 130. In step 215, the RRP of the WTRU 110 is
obtained from the interference estimate of step 210. As a result,
the minimum transmission power level of the AP 105 is obtained by
summing the range of the AP 105 and the RRP of the WTRU 110 (step
220).
[0024] Table 1 illustrates exemplary parameters involved in the
determination of the AP transmission power. Other parameters and
values may be used, in addition to, or in place of these parameters
and values. TABLE-US-00001 TABLE 1 Symbol Description Value RNG
Range of the AP Manually configured or calculated using other
processes RRP Required Received Power Calculated by the AP
P.sub.max Maximum AP transmission power P.sub.min Minimum AP
Transmission power P = min (P.sub.max, RNG + RRP)
[0025] Control of the RRP based on the estimation of interference
according to the present invention will now be described with
reference to FIGS. 3 and 4. Adjustment of the optimal RRP is
implemented by executing a slow interference estimation process and
a fast interference estimation process. Each of these processes use
different measurements to evaluate the most appropriate value of
the RRP at a given time.
[0026] The role of the slow interference estimation is to obtain
the RRP with a reasonable degree of accuracy. The slow interference
estimation produces a value designated RRP from downlink statistics
(RRP.sub.DS) obtained from the transmission of packets to the WTRUs
110 during normal operation. The slow interference estimation is
performed with a period, T.sub.DS, which is relatively long, (e.g.,
approximately 1 minute).
[0027] The slow interference process is not executed every time a
packet is transmitted by the WTRU 110, in the sense that a new
value of the RRP would be computed. Rather, statistics are
collected over the multiple packets transmission that take place in
between the activations (i.e., over T.sub.DS). Upon execution of
the slow interference estimation, the statistics are processed and
the value of RRP is updated on a periodic basis (i.e., it is not
triggered by a specific event). However, there could be a random
component (jitter) between executions of the slow interference
process.
[0028] The role of the fast interference estimation is to ensure
that the AP 105 can rapidly determine the transmission power to
enable fast compensation of external interference, and allow at
least some packets to be transmitted successfully after a sudden
increase of interference. The fast interference estimation process
produces a value designated RRP from Uplink Interference
(RRP.sub.UI). This value is obtained by measuring external
interference at the AP 105 obtained by measurement of a received
signal strength indicator (RSSI) associated with one or more WTRU
transmitted packets in the absence of carrier lock. The fast
interference estimation runs with a period (T.sub.UI), which is
relatively short, for example, on the order of a second. The RRP is
obtained by summing RRP.sub.DS and RRP.sub.UI at any desired
time.
[0029] FIG. 3 is a flowchart showing the method steps of a slow
interference estimation process 300 in accordance with the present
invention. The slow interference process 300 is activated on a
periodic basis. During a particular period T.sub.DS, a plurality of
packets are transmitted to one or more WTRUs 110 and statistics of
the packets are collected (step 310). The analyzing unit 130 in the
AP 105 analyzes the transmitted packets and records the result of
the analysis in two separate histograms (step 320). The two
histograms have the RSSI perceived by the WTRU 110 as the category
axis. Bins of approximately 1 or 2 dB, preferably, may be used.
[0030] A first histogram H.sub.r records the average data rate of
successfully transmitted packets and a second histogram H.sub.s is
the percentage of successfully transmitted packets. The histogram
H.sub.r is used only if rate control is enabled. Thus, the AP 105
may use different data rates for different WTRUs 110, according to
the signal-to-noise ratio (SNR) perceived by the WTRU 110. The AP
105 would use a higher bit rate for WTRUs 110 enjoying a high SNR
and a lower bit rate for WTRUs 110 with lower SNRs by performing a
rate control process. Such a process may be based on downlink
performance statistics for individual WTRUs 110.
[0031] Every time a packet is transmitted to the WTRU 110, a
success or a failure event is recorded in the H.sub.s histogram
according to whether or not the packet is received successfully. In
the event that the packet is received successfully, it is recorded
as a success event in the H.sub.s histogram. If the packet is not
received successfully, it is recorded as a failure in the H.sub.s
histogram. The data rate of the packet sent from the AP 105 is also
recorded in the Hr histogram.
[0032] The RSSI of the latest successfully received packet from the
WTRU 110 is used to determine which bin of the histogram is
utilized. The RSSI measured at the AP 105 is translated to an RSSI
perceived by the WTRU 110. This translation is obtained by adding
to the RSSI at the AP 105 the difference between the transmission
power of the AP 105 and the assumed transmission power of the WTRU
110, which can be described as follows:
RSSI(WTRU)perceived=TxPower(AP)-TxPower(WTRU)+RSSI(AP) Equation 1
Equation 1 is determined based on the following two equations:
RSSI(AP)=TxPower(WTRU)-PathLoss(WTRU-AP) Equation 2
RSSI(WTRU)=TxPower(AP)-PathLoss(WTRU-AP) Equation 3 where TxPower
(WTRU) is the transmission power of the WTRU 110, TxPower (AP) is
the transmission power of the AP 105, and PathLoss (WTRU-AP) is the
path loss between the WTRU 110 and the AP 105. An age limit
(A.sub.maxRSSI) may be imposed on the latest RSSI measurement, to
avoid biasing the statistics when a WTRU 110 wanders away from the
AP 105.
[0033] Table 2 shows an exemplary result that could be observed as
a result of the computation of the average data rate Hr as well as
the percentage of successful transmissions Hs for each RSSI bin.
The Hr and Hs histograms will not be perfectly monotonous in
practice because of the different levels of interference
experienced by the WTRUs 110. TABLE-US-00002 TABLE 2 H.sub.r
Average H.sub.s Percentage RSSI Number of data rate of successes
(dBm) packets (Mbps) (%) -93 243 1.0 20 -92 17 1.2 23 -91 204 1.7
20 -90 100 1.5 18 -89 87 2.1 25 -88 127 1.9 23 -87 83 2.7 35 -86
462 4.4 50 -85 303 4.2 51 -84 298 5.1 59 -83 74 5.3 62 -82 193 5.8
64 -81 382 7.0 68 -80 584 7.4 70
[0034] Table 2 is a good example of a histogram. A bin corresponds
to a row in Table 2. The RSSI column in Table 2 shows the value
corresponding to the center of the bin. For example, the row where
RSSI=-89 dBm provides the bin collecting statistics of packets for
which RSSI values range from -89.5 dBm to -88.5 dBm.
[0035] A new histogram is built based on the packets transmitted
over a period of duration T.sub.DS. Instead of computing the
RRP.sub.DS based on this last histogram only, it is preferable to
combine the data from this last histogram to the data from past
(N.sub.hav-1) histograms in order to get better statistical
significance and smoother behavior. First, the "number of packets"
from all histograms are summed to get a total number of packets.
Then, the data rates of packets from all histograms are summed, and
the result is divided by the total number of packets from all
histograms to get the average data rate (H.sub.r). Finally, the
number of packets successfully transmitted from all histograms are
summed, and the result is divided by the total number of
transmitted packets from all histograms to get the percentage of
success (H.sub.s).
[0036] Referring still to FIG. 3, after the transmitted data is
analyzed the results are recorded in the histograms in step 320, it
is determined whether there is at least one low quality bin (step
330). It is preferable to determine the above criteria according to
the following definitions of "a significant bin" and "a low quality
bin". The significant bin is an RSSI bin for which a minimum amount
of packets (N.sub.DSmin) were received. The low quality bin is a
significant bin satisfying one of the following conditions: average
data rate of successful transmissions is below a threshold
(R.sub.min) and rate control is enabled; or the percentage of
successful transmissions is below a threshold (S.sub.min) (i.e.,
the percentage of non-successful transmissions is above a threshold
F.sub.min). The value of RRP.sub.DS is then calculated according to
whether or not there is at least one low quality bin, as determined
in step 330. In the example set forth in Table 2, RRP.sub.DS would
be set to -84 dBm if the R.sub.min and the S.sub.min were set to 5
Mbps and 50% respectively.
[0037] If in step 330 it is determined that there is no low quality
bin, RRP.sub.DS is set to the same value that was used in a
previous activation, minus N.sub.b times the bin width (step 340).
N.sub.b is a parameter and may be set to 1 by default. However, it
is preferable that the RRP.sub.DS does not fall below the minimum
value RSSI.sub.min used in the histograms. In case there was no
previous activation, it is set to a minimum value, RRP.sub.min.
This value is configurable parameter.
[0038] If in step 330 it is determined that there is at least one
low quality bin, RRP.sub.DS is set to the RSSI value corresponding
to the first bin above the highest low quality bin (step 350).
[0039] The slow interference estimation process 300 set forth in
FIG. 3 produces a relatively accurate result during low activity
periods when little data is transmitted. For example, if there is a
single associated WTRU 110 located in the vicinity of the AP 105
and experiencing good quality, RRP.sub.DS will not be raised to the
power received by this WTRU 110. However, one potential problem
with this slow interference estimation process 300 is the
variability of the transmission power amongst WTRUs 110;
particularly those from different manufacturers. Since the AP 105
cannot access the setting of transmission power of a particular
WTRU 110, a value must be assumed that would be used for all WTRUs
110. If a WTRU 110 transmits at a power lower than the assumed
value, its RSSI will be underestimated. As a result, the quality of
service for this RSSI could be overestimated, which will tend to
reduce the RRP and the AP transmission power level.
[0040] In the opposite case, if a WTRU 110 transmits at a power
higher than the assumed value, its RSSI will be overestimated and
the end result is that the AP 105 transmission power will be
increased. This problem would be eliminated if it were possible to
read or control the maximum power of a WTRU 110.
[0041] A prerequisite for the slow interference estimation process
300 is that a reasonable number of downlink transmissions are
successful. This condition may not be met when the interference
conditions in the BSS deteriorate rapidly and/or dramatically due
to the appearance of an external interference source.
[0042] FIG. 4 is a is a flowchart showing the method steps of a
fast interference estimation process 400 in accordance with the
present invention. In step 410, the measuring unit 125 in the AP
105 measures a noise plus interference floor, (hereinafter called
"noise floor" for simplicity). This noise floor consists of a
combination of external interference and of weak signals that
cannot be decoded. The noise floor can be measured as the RSSI when
there is no carrier lock, that is, when the receiver has not
detected the presence of an 802.11 type of signal. Alternatively,
the noise floor may also be estimated during the reception of a
packet depending on the capabilities of the receiver. Measurements
are performed at intervals on the order of T.sub.nf, which is
relatively short, for example 100 ms or less. These measurements
are averaged over a period of time (T.sub.UI), on the order of a
second.
[0043] Referring still to FIG. 4, it is then determined whether a
noise floor measurement is available during the measurement period
(step 420). In step 430, if a noise floor measurement at the AP 105
(I.sub.AP) exists, RRP.sub.UI is determined as follows:
RRP.sub.UI=I.sub.AP+(C/I).sub.req.sub.--.sub.low Equation 4 where
I.sub.AP is the noise floor estimate, and
(C/I).sub.req.sub.--.sub.low is a configurable parameter
representing the typical required carrier-to-interference ratio to
have a reasonable probability of success at a low rate, (e.g., 1 or
2 Mbps). If no measurement of noise floor is available during the
period, RRP.sub.UI is left to the value it was set to at the
previous activation, or RRP.sub.min if this is the first activation
(step 440).
[0044] The RRP is obtained by combining the results of the slow
interference estimation process 300 and the fast interference
estimation process 400, as follows: RRP=max{RRP.sub.DS, RRP.sub.UI}
Equation 5
[0045] If Equation 5 is analyzed according to the relationship
between RRP.sub.DS and RRP.sub.UI, RRP.sub.UI should ideally be set
at a level slightly lower than what would be necessary for the
desired downlink quality, so that the RRP be primarily obtained by
RRP.sub.DS, except just after a sudden increase of external
interference. In this way, the fast interference estimation
sub-process avoids that the downlink quality stays low during many
minutes after the increase of interference.
[0046] After the RRP of the WTRU 110 is obtained, the function
generator 135 in the AP 105 sets the transmission power of the AP
105 in order to make the WTRU 110 receive the previously obtained
RRP.
[0047] In a scenario where the AP 105 sustains a level of
interference much higher than the WTRUs 110 at the range, this
method of obtaining the RRP would make the AP 105 transmit at a
power higher than needed. Alternatively, the RRP may be reset to
RRP.sub.DS (ignoring RRP.sub.UI) after a minimum amount of time,
e.g., several minutes, during which the RRP.sub.UI is constantly
above the RRP.sub.DS. Thereafter, RRP.sub.UI would be taken into
consideration only upon bad downlink performance.
[0048] Although the features and elements of the present invention
are described in the preferred embodiments in particular
combinations, each feature or element can be used alone without the
other features and elements of the preferred embodiments or in
various combinations with or without other features and elements of
the present invention.
* * * * *