WO1998008348A2

WO1998008348A2 - Temporally-oriented subscriber configuration in a fixed wireless system

Info

Publication number: WO1998008348A2
Application number: PCT/IB1997/001035
Authority: WO
Inventors: Shun Hua Zhou; Fabio Marco Marchetti; Robert Lee Hicks, Jr.; Gokul V. Subramanian; Ramanathan Balachander; Balaji S. Holur; Chhian Ling; Louis Perez
Original assignee: Northern Telecom Limited
Priority date: 1996-08-16
Filing date: 1997-08-11
Publication date: 1998-02-26
Also published as: AR008303A1; WO1998008348A3

Abstract

While setting up a fixed wireless access system, each subscriber radio-telephone is configured to a permanent home cell. Subscribers located at the border of a cell may be configured to multiple cells for better coverage. The signal levels at the border subscriber's position are received and averaged over time. The time averaged strongest radio control channel signal levels are listed in descending order. The cell that gives the strongest signal level is listed at the home cell. If the second strongest radio control channel signal level is far behind the strongest, the subscriber is configured only for the home cell. If there is only a close margin between the strongest and second strongest, the subscriber is configured as a border subscriber to the second strongest cell.

Description

TEMPORALLY-ORIENTED SUBSCRIBER CONFIGURATION IN A FIXED WIRELESS SYSTEM

BACKGROUND OF THE INVENTION

I. FIELD OF THE INVENTION

The present invention relates to radio communications. More particularly, the present invention relates to temporally configuring a subscriber in a fixed wireless access system.

II. DESCRIPTION OF THE RELATED ART

A cellular radio communication system is typically comprised of a number of cells covering a geographic region. Each cell is allocated a number of radio channels. The cell may also be divided up into sectors with each sector having a number of different channels.

In a typical mobile system, a subscriber is not configured to a specific cell. In these systems, a radio voice channel is allocated to the mobile subscriber for call initiation based on the Received Signal Strength Indicator (RSSI), RSSI being well known in the art (see Electronic Industries Association Interim Standard IS-54 for a detailed explanation of the RSSI), and the availability of the radio voice channel. As the mobile subscriber moves through a cellular system's different cells, radio voice channels are dynamically allocated to the mobile as the channel's signal strength changes.

In a fixed wireless communication system, also known in the art as a wireless local loop system, each subscriber is typically configured to a specific cell on a permanent basis. This type of system uses radiotelephones that are not mobile. A fixed wireless system is an attractive solution to implement communications in developing countries or in rural areas of developed countries where the telecommunications infrastructure is inadequate. For a subscriber radiotelephone located near the interior of a cell, identified as the home cell to the subscriber radiotelephone, the probability of receiving signal coverage is high. However, a radiotelephone located near the border of the cell may not receive signal coverage due to objects, such as terrain or large vehicles, shadowing the antenna from the subscriber radiotelephone. This effectively shrinks the size of the cell. This phenomenon is illustrated in FIG. 6.

The upper portion of FIG. 6 shows a subscriber (600) being configured to a home cell, Cell A. In this drawing, the home cell is providing normal coverage to the subscriber (600). The lower portion of FIG. 6 illustrates the shrinking of Cell A and the resulting loss of coverage experienced by the subscriber (600).

If a subscriber radiotelephone was initially configured to a home cell and the home cell shrinks, the received signal level could drop below the minimum signal level required to keep the connection. This minimum signal level is the radio's minimum sensitivity - the signal level below which the radio cannot detect a signal. In this case, the call is dropped or not set up, if it is a new call, since the subscriber is configured to only one home cell in wireless local loop systems. There is a resulting previously unknown need to provide improved coverage to subscribers located on the border of a cell in a wireless local loop system.

SUMMARY OF THE INVENTION

The process of the present invention assigns cellular radio service to a radiotelephone in a wireless local loop system. The process averages, over time, received radio control channel signal levels for each of the plurality of cells within communication distance of the radiotelephone. The cell that has the largest time averaged control channel signal is configured as the radiotelephone's home cell. Each of the averaged signal levels is listed in descending order. The difference between the strongest and second strongest is determined. If this difference is greater than or equal to a predetermined threshold, the radiotelephone is not configured to additional cell sites.

If the difference is less than the predetermined threshold, a second cell site's coverage is necessary. In this case, the radiotelephone is configured as a border subscriber to access the second cell site also.

If the difference is equal to the predetermined threshold, it is up to the service provider to best balance the quality of service and the resources.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a typical fixed, wireless radiotelephone system of the present invention.

FIG. 2 shows a typical cell layout of the radiotelephone system of FIG. 1.

FIG. 3 shows a flowchart of a preferred embodiment process of the present invention.

FIG. 4 shows a distribution of a local mean in a two-neighboring cell situation.

FIG. 5 shows the situation of inner border and outer region of a home cell. FIG. 6 shows a typical cell layout experiencing the cell breathing phenomenon.

FIG. 7 shows a fixed wireless access system's RF coverage using the present invention.

FIG. 8 shows the estimated location of subscriber radiotelephone based on the value of Δ in a two-neighboring cell situation.

FIG. 9 shows a flowchart of an alternate embodiment process of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The wireless local loop radiotelephone system of the present invention enables a wireless radiotelephone user to access the public switched telephone network. By averaging, over time, the mean value of the signals received by the radiotelephone, the system can optimally configure the radiotelephone to additional neighboring cells beyond the radiotelephone's home cell.

FIG. 1 shows a block diagram of the fixed wireless radiotelephone system of the present invention. This cellular system is comprised of multiple base stations (101-103) that communicate radiotelephone signals with the fixed subscriber radiotelephone (110). Each base station (101-103) is comprised of multiple receivers and transmitters that operate in any RF band. However, the 800 and 900 MHz bands are typical for time division multiple access (TDMA) cellular service. This system can be installed in office buildings, parts of cities, or other places requiring telephone access without requiring telephone wiring to be installed.

The radio base stations (101-103) are connected to a fixed wireless controller (105). The controller (105) keeps track of which subscribers are configured to a particular base station (101, 102, or 103) and routes any calls from the public switched telephone network (PSTN) (120) to the appropriate base station (101, 102, or 103) for transmission to the subscriber radiotelephone (110). The fixed wireless controller is well known in the cellular art.

FIG. 2 illustrates the cell layout of the present invention. In this example, there are three base stations: BS_A, BS_B, and BS_C (201-203). The fixed subscriber radiotelephones (210, 220, and 230) each communicate with the appropriate base station (210, 220, and 230). The processes of the present invention determine to which base station (201-203) each subscriber radiotelephone (210, 220, and 230) is configured. One subscriber radiotelephone (210) may be configured to a single base station (202). Another radiotelephone (220) may be configured to two base stations (201 and 202) while yet another radiotelephone may be configured to all three stations (201-203). The power received by a radiotelephone in the above wireless system is expressed as:

where: _R(^r) ^~ the received power at a distance r from the base station.

P₀ = the received power at a close-in reference distance ro. r = the distance between the base station and the radiotelephone. r_Q =^'the close-in reference distance. α = the path-loss slope. In the preferred embodiment, the close-in reference distance is greater than 1.39'GΗ where G is the antenna gain and H is the antenna height. The path-loss slope, α, also referred to as the propagation constant, is well known in the art. The path-loss slope is the rate of decay of signal strength as a function of distance and is typically in the range of 2-5 depending on the terrain and conditions. For example, a path-loss slope of 2 is without any obstructions while a path-loss slope of 5 would be a dense urban environment. r

In the above equation, the component P₀ is the mean value of the

received power, subsequently referred to as m. The component M(r) is the local mean, also referred to in the art as the long-term fading component. M(r) observes lognormal distribution around the mean value, m. M(r) varies due to the terrain contour between the base station and the radiotelephone. The component R(r) is the multipath fading, or short-term fading, component. The variation of R(r) is due to the radio waves being reflected from man-made structures.

The local mean, M(r), is a Gaussian distributed random variable with a standard deviation of σ around the m. Both σ and M(r) are measured in dB. Typically, in an urban environment, σ = 8dB but could range between 4 dB and 12 dB, depending on the conditions of the actual terrain. The probability density function of M(r) can be expressed as:

An example of the signal coverage in a border cell of the present invention is illustrated in FIG. 4. FIG. 4 shows the signal level contours and distribution of local mean values in the two-neighboring cell embodiment. This figure illustrates that in a home cell, Cell A, a subscriber (400) located in the interior of Cell A has a much higher mean signal level, m , received from its home cell, Cell A, than the mean signal level, m_B, received from its neighboring cell, Cell B. The two probability density functions of the signals from the two cells do not have much overlap. In otherwords, the probability that the signal from Cell B is higher than that from Cell A due to signal fluctuation is low. In this case, the subscriber should be configured to Cell A only.

A subscriber located at the border of Cell A (405), the overlapping region between Cells A and B, experiences a mean signal level from Cell A, m_A, only slightly higher than from Cell B, m_B. In this case, the two probability density functions have large overlapping areas. This means that even though m_A > m_n, there is a certain probability that the signal from Cell B is higher than that from Cell A due to signal fluctuations. The process of the present invention, illustrated in FIG. 3, is based on the time averaged radio control channel RSSI measurements of the received signal levels. The received signals are from the neighboring cells that are within communication distance of the subscriber radiotelephone. The preferred embodiment process begins with the power-up of the subscriber's radiotelephone. The radiotelephone unit will attempt registration on the strongest radio control channel at this time (300). Upon registration, the system verifies that the subscriber's radiotelephone is legitimate for billing purposes. The radiotelephone registration process is well known in the art. RSSI measurements, indicators that are based on signal power levels received by the radiotelephone, are taken by the radiotelephone (305). The radiotelephone is directed by the system, following registration, to take repeated RSSI measurement on control channels of the home cell and the nearby cells that are within communication distance of the radiotelephone. The radiotelephone constantly scans the radio control channel signal levels received from the neighboring cells and sends these signal strength indicators to the base station of the cell to which the radiotelephone is registered (310). The base station then forwards the indicators to the system's fixed wireless controller (315). These radio control channel RSSI measurements from the different cells are then averaged over time (320) per radio control channel. The time taken depends on system traffic. The greater the number of RSSI samples per radio control channel, the more accurate the time average per radio control channel. In a fixed, wireless access environment, the multipath fading is small. Therefore, it is possible to use the time-averaged strongest radio control channel signal level as a typical value and identify a subscriber based on this value.

The time averaged radio control channel signal levels are then listed in descending order (325). For example, the mean values could be listed as follows: r_A: Cell A gives the strongest radio control channel signal level. r_n: Cell B gives the 2^nd strongest radio control channel signal level.

r : Cell K gives the weakest radio control channel signal level.

Since Cell A has the strongest radio control channel signal level, it is identified as the home cell to the subscriber (330). This is the first cell that the radiotelephone will try to register with upon power-up.

The process next determines the difference between the time averaged radio control channel signal levels from the home cell, Cell A, and that from the second strongest cell, Cell B, i.e., between r_A and r_fl (335). This difference is subsequently referred to as Δ. This difference is used to distinguish a border cell subscriber from a home cell subscriber and is defined as:

A = ^ ^χ l00(%)

The differences between r_A and r_c, between r_A and r_β, and between τ_A and r_κ are also determined. Those differences are subsequently referred to as Δ_m. with m = C, ..., K. They are calculated if more than two serving cells are to be configured to a border cell subscriber. Δ_m is defined as:

Δ,„ = ^ — ^- χ l00(%) where m = C, , K

Referring again to FIG. 3, Δ is next compared with a pre-set parameter, α (340). The parameter, α, is pre-set by the service provider. This value is a tradeoff between the voice quality and the system cost, α can be modified later as the system requirements change.

As a starting point, α can be set at 20%. If it is set too low, such as at 5%, then those subscribers located at the outer region of the home cell, as illustrated in FIG. 5, are leniently identified as only home cell subscribers with no additional serving cells being configured. This could lead to unsatisfactory voice quality.

If α is set too high, such as at 60%, those subscribers located at the outer border of the home cell, as illustrated in FIG. 5, are unnecessarily identified as border cell subscribers. These subscribers will be configured with an additional serving cell. This leads to unnecessary radio resource allocation, thus costing the service provider money to provide the unused resources.

A subscriber is identified as a home cell subscriber or a border cell subscriber based on the value of Δ. This value represents how close r_β is to r . This, in turn, implies the relative location of a subscriber in relation to the base stations, that is d_A> the difference between the subscriber and the base station of Cell A, and d_B, the distance between the subscriber and the base station of Cell B.

If Δ > α, meaning r_β is much smaller compared to r_A, it is implied that the subscriber is closer to Cell A compared to the second or other cells. This is the case where the subscriber is located at the interior of Cell A, identified as a home cell subscriber. Since the home cell gives sufficiently strong RF coverage, no additional serving cell is required (345).

If Δ < α, meaning r_β is in a close margin with respect to r_A, it is implied that the subscriber is equally distant from the home cell, Cell A, and the second or other cells. In this case, the subscriber is located at the border of several neighboring cells and the subscriber is identified as a border cell subscriber. The home cell, Cell A, may not give sufficiently strong RF coverage by itself. The cell having the second strongest signal level, Cell B, is configured as a second serving cell (350).

If Δ = α, means that r_β is in a close margin with respect to r_A. It is up to the service provider to best balance service quality and resources, by deciding whether or not to include Cell B as an additional serving cell to the subscriber.

As the location of the subscriber radiotelephone unit changes from close to the home cell, Cell A, to away from Cell A, the value Δ changes accordingly as illustrated in FIG. 8.

Referring ^σ to FIG. 8A, r A » r B , leads to a Δ of close to 100%. This implies that the subscriber is very close to the base station of Cell A, meaning d « d .

Here, the two probability density functions of the local mean have very small overlapping areas, meaning m A » m B . The probability is very low that the signal from Cell B is higher than that from Cell A due to signal fluctuation. The subscriber is identified as a home cell subscriber and is to be configured to home cell, Cell A, only.

In FIG. 8B, r > r by a solid margin leads to a Δ of approximately 60%.

This implies that the subscriber is closer to Cell A than to Cell B, meaning d < d by a solid margin. Here, the two probability density functions have small overlapping areas, meaning m A > m B by a solid margin. The probability that a signal from Cell B is higher than that from Cell A due to signal fluctuation is low. The subscriber is identified as a home cell subscriber. It is not necessary to configure Cell B to the subscriber as an additional serving cell.

In FIG. 8C, r > r by a small margin leads to a Δ of approximately 20%.

A B

This implies that the subscriber is relatively closer to Cell A than to Cell B, meaning d A < d B by a small margin. Here, the two probability density functions have relatively large overlapping areas, meaning m A > m B by a small margin.

The probability that a signal from Cell B is higher than that from Cell A due to signal fluctuation is relatively high. The subscriber is identified as a border cell subscriber. It is necessary to configure Cell B to the subscriber as an additional serving cell.

In FIG. 8D, r > r by a very small margin leads to a Δ of approximately

A B 5%. This implies that the subscriber is closer to Cell A than to Cell B, meaning d < d by a very small margin. Here, the two probability density functions have very large overlapping areas, meaning m A > m B by a very small margin.

The probability that a signal from Cell B is higher than that from Cell A, due to signal fluctuation, is very high. The subscriber is identified as a border cell subscriber. It is necessary to configure Cell B to the subscriber as an additional serving cell.

In FIG. 8E, r = r leads to a Δ of approximately 0%. This implies that the subscriber is equally distant from Cell A and Cell B, meaning d = d . Here, the two probability density functions totally overlap, meaning m = m . The probability that a signal from Cell B is higher than that from Cell A due to signal fluctuation is extremely high. The subscriber is identified as a border cell subscriber. It is highly recommended to configure Cell B to the subscriber as an additional serving cell.

An example of one benefit of the present invention is illustrated in FIG. 7. The upper portion of this figure shows a subscriber (700) configured to both Cells A and B, with Cell A being the home cell. If the coverage of Cell A shrinks, as illustrated in the lower portion, coverage from Cell A is lost. However, since the subscriber (700) is still within Cell B's coverage area, communication ability is not lost. The process then enters the subscriber's status, i.e., home cell subscriber or border cell subscriber, together with the serving cells, into the system data base. The subscriber has now been configured to the fixed wireless access system. Alternate embodiments of the process of the present invention configure the radiotelephone to multiple border serving cells. An example of such an embodiment is illustrated in FIG. 9.

For the case of a maximum of two serving cells for a border cell subscriber, only Δ, which evaluates the difference between the time averaged radio control channel signal levels from the home cell and that from the second strongest cell is to be compared with α. For a generic case of a maximum of N serving cells for a border cell subscriber, multiple Δ_m each evaluate the difference between the time averaged radio control channel signal levels from the home cell and that from the third, fourth, to the N*^h strongest cell. These deltas are then compared with α. Those cells with Δ_m < α are configured as additional serving cells to the border cell subscriber.

Additionally, embodiments of the process and system of the present invention are not limited to any one cellular technology. The embodiments of the present invention operate with all radio technologies including AMPS, TDMA, and CDMA.

It is obvious from the above description that the process and system of the present invention provides many advantages over the prior art. The present invention offers enhanced probability of receiving service. If the home cell shrinks, the border cell subscriber uses the radio voice channel from another cell to initiate or receive a call.

The present invention also provides reduced probability of blocking. If a subscriber is located on the border of a home cell and the base station is blocked, the subscriber radiotelephone can be directed to the voice channels of neighboring cells.

Additionally, the present invention is implemented automatically. The radio control channel signal level received from neighboring cells at the subscriber's location is collected automatically by the radiotelephone over a period of time. The present invention does not require on-premises RF engineering to set up each subscriber. WE CLAIM:

Claims

1. A method for assigning cellular radio service to a radio unit in a fixed wireless cellular system, the system communicating radio signals through a plurality of cells that are coupled to a fixed wireless controller, the method comprising the steps of: determining received signal strength indicators for received radio control channel signals from each of the plurality of cells; averaging, over time, the received signal strength indicators from each of the plurality of cells; configuring a cell, having a largest time averaged signal strength indicator, as a home cell for the radio unit; determining a difference between a second largest time averaged control channel signal, from a second cell, and the largest time averaged control channel signal; and if the difference is less than a predetermined difference parameter, configuring the second cell to the radio unit.

2. The method of claim 1 and further including the step of the radiotelephone transmitting the received signal strength indicators to the fixed wireless controller.

3. The method of claim 1 wherein the predetermined difference parameter is a function of signal quality and system expense.

4. A method for assigning cellular radio service to a radiotelephone in a fixed wireless cellular system, the system communicating radio signals through a plurality of cells that are coupled to a fixed wireless controller, the method comprising the steps of: the radiotelephone determining received signal strength indicators for received radio control channel signals from each of the plurality of cells; the radiotelephone transmitting the received signal strength indicators to the fixed wireless controller; averaging, over time, the received signal strength indicators; determining, in descending order, a range of averaged indicators comprising a largest indicator to a smallest indicator; configuring a cell, having the largest indicator, as a home cell for the radiotelephone; determining a difference between a second largest indicator, from a second cell, and the largest indicator; and if the difference is less than a difference parameter, configuring the second cell to the radiotelephone.

5. The method of claim 4 wherein the difference is determined by the dividing the largest indicator into the result of subtracting the second largest indicator from the largest indicator.

6. A method for assigning cellular radio service to a radiotelephone in a fixed wireless cellular system, the system communicating radio signals through a plurality of cells that are coupled to a fixed wireless controller, the method comprising the steps of: the radiotelephone determining received signal strength indicators for received radio control channel signals from each of the plurality of cells; the radiotelephone transmitting the received signal strength indicators to the fixed wireless controller; averaging, over time, the received signal strength indicators; determining, in descending order, a range of averaged indicators comprising a largest indicator to a smallest indicator; configuring a cell, having the largest indicator, as a home cell for the radiotelephone; determining a difference between a second largest indicator, from a second cell, and the largest indicator; determining a difference parameter for the fixed wireless system as a function of voice quality and system expense; and if the difference is less than the difference parameter, configuring the second cell to the radiotelephone.

7. A method for assigning cellular radio service to a radio unit in a fixed wireless cellular system, the system communicating radio signals through a plurality of cells that are coupled to a fixed wireless controller, the method comprising the steps of: determining received signal strength indicators for received radio control channel signals from each of the plurality of cells; averaging, over time, the received signal strength indicators from each of the plurality of cells to generate a plurality of time averaged indicators; configuring a cell, having a largest time averaged indicator, as a home cell for the radio unit; determining a difference between the largest time averaged indicators and each of the plurality of time averaged indicators thus generating a plurality of differences; and comparing at least one of the differences to a predetermined difference parameter, each of the plurality of differences being compared until a difference being compared is greater than the difference parameter.