KR19980083548A - Call Admission Control Scheme Adapted to Foreign Cell Environment in Direct Spread Code Division Multiple Access Cellular System - Google Patents

Abstract

The present invention relates to a call admission control method in a DS-CDMA cellular system.

The radio link capacity of the base station of the DS-CDMA cellular system is limited by the reverse link capacity. The capacity of the reverse link is determined by the amount of interference received from the neighbor cell and is closely related to the traffic load of the neighbor cell. When allocating a new call, the base station should ensure that the total number of users does not exceed the reverse link capacity. If the base station allocates a call exceeding the reverse link capacity, the mobile station in the cell boundary may be forcibly interrupted depending on the radio environment. .

Therefore, in the present invention, the average and the variance of the outer cell interference amount are estimated from the received signal strength and the total received interference amount used for the closed loop power control of the base station, and the base station calculates the reverse link capacity using the average and the variance of the estimated outer cell interference amount. Accordingly, a call admission control method adapted to an external cell environment in a DS-CDMA cellular system that can improve frequency usage efficiency and reduce call interruption probability is proposed.

Description

Call Admission Control Scheme Adapted to Foreign Cell Environment in Direct Spread Code Division Multiple Access Cellular System

The present invention relates to a call admission control method in a direct sequence code division multiple access (DS-CDMA) cellular system. In particular, a call adapted to an external cell environment in a DS-CDMA cellular system It relates to an admission control method.

In general, in a DS-CDMA cellular system, interference from inside and outside of the cell affects the call quality and sometimes causes interruptions in the call. Therefore, when it is impossible to calculate the reverse link capacity adapted to the outer cell environment, the call load must be controlled according to the reverse link capacity calculated on the assumption that the outer cell is at full load. If is increased, there is a problem that the call blocking probability due to the lack of reverse link capacity is increased.

In a DS-CDMA cellular system, a radio link capacity of a base station includes a link capacity for forward and a link capacity for reverse. The link capacity of the base station is mainly limited by the reverse link capacity, and the capacity of the reverse link is increased or decreased according to the traffic load of the neighboring cells. Thus, the base station should ensure that the total number of users does not exceed the reverse link capacity when attempting to accept a new call. If the call is allocated in excess of the reverse link capacity, the mobile station, especially at the cell boundary area, may be forced out of the call depending on the radio environment.

To solve this problem, Liu, Shin and Dziong proposed call control schemes adapted to the traffic environment of foreign cells in a recently published paper. Liu first proposed a call admission control algorithm that measures the signal-to-interference ratio (SIR) and adaptively controls the call according to the traffic load of the outer cell. However, this algorithm does not consider the voice activity rate and assumes perfect power control, which makes it difficult to apply in the real environment. Shin proposed an adaptive channel allocation algorithm that can calculate the remaining reverse link capacity by adding hardware devices that measure the total received power at the high frequency receiver of the base station. However, the total received power measured at the high frequency receiving end of the base station is random due to variables such as the number of users of the cell of interest and foreign cells, the voice activity rate, the distance variation and shadowing between the mobile stations in the foreign cell and the base station of interest. The total received power, which can be viewed as a variable and is a random variable, may change in each measurement even in the same external environment. Therefore, it is difficult to use the residual reverse link capacity calculated based on the variable instantaneous values for call control in the actual base station environment. Dziong has proposed a call admission control method considering the traffic environment of a foreign cell, but this method requires a message exchange for traffic state information and channel characteristics.

Accordingly, the present invention considers the voice activity rate, does not add a separate hardware device, and adapts to the interference environment according to the traffic load of the foreign cell without the enormous message exchange required to obtain the traffic state information of the foreign cell. The purpose of this paper is to propose a call admission control method adapted to an external cell environment in a DS-CDMA cellular system that can improve call quality by calculating link capacity so that the total number of users does not exceed the limit of reverse link capacity.

In the DS-CDMA cellular system according to the present invention for achieving the above object, a call admission control method adapted to an external cell environment includes the steps of receiving energy per Walsh symbol and interference per Walsh symbol through a demodulator; Calculating average and variance of the amount of foreign cell interference per Walsh symbol using data of energy per Walsh symbol and amount of interference per Walsh symbol, and calculating the average and the variance of the amount of foreign cell interference per Walsh symbol and determining whether a new call arrives. If the new call does not arrive, transitioning to the step of receiving, at the base station, the energy per Walsh symbol and the interference per Walsh symbol received from the demodulator; If a new call arrives as a result of the test, the average of the amount of foreign cell interference per Walsh symbol Calculating a reverse link capacity using an acid, checking whether the number of current users is less than a reverse link capacity after calculating the reverse link capacity, and checking whether the number of current users is less than a reverse link capacity. If the number is less than the reverse link capacity, accepting the call, performing a call processing, and ending; checking whether the current number of users is less than the reverse link capacity; Characterized in that it consists of a step to end after.

1 is a block diagram illustrating a process of collecting reverse received signal data at a base station.

2 is a flowchart illustrating a call admission control process adapted to an external cell environment in a base station according to the present invention.

Explanation of symbols for the main parts of the drawings

a: high frequency receiver b1 to bn: demodulator

c: base station controller

Hereinafter, with reference to the accompanying drawings will be described in detail the present invention.

1 is a block diagram illustrating a process of collecting reverse received signal data at a base station. In the DS-CDMA cellular system, the base station performs reverse power control for each user, and there are open loop power control and closed loop power control in the reverse power control. First, data used for closed loop power control entered through the high frequency receiver (a) is received. Closed loop power control measures the Walsh (walsh) and interference energy E _s per Walsh symbol per symbol I _s in the rake receiver (rake receiver) and the base station performs power control on the basis of that value. Data used for the closed loop power control received through the high frequency receiver a is demodulated in the demodulators b1 to bn. This data is collected by the base station controller (c) to estimate the average and variance of the amount of interference received from the foreign cell. E _s and I _s in the wake receiver for estimating the mean and variance of the interference amount are given by the following equations [1] and [2].

[Equation 1]

[Equation 2]

Where k _max is the index with the maximum value for the given Walsh symbol period, n _s is the number of Walsh symbols during the power control interval 1.25 mS, and d _{j, k} is the output of the diversity combiner during the j th Walsh symbol period. The kth determination value, M, is the number of signals represented by Walsh symbols. E _s and I _s are values used for power control at the base station, so data can always be collected. If the number of data of E _s and I _s obtained when the number of users in the base station is n is L, the mean and variance of E _s and I _s can be calculated using the following equations [3] and [4]. have.

[Equation 3]

[Equation 4]

When E _s and I _s are set as random variables, I _s is a sum of interference amounts received from external cells and other users in a cell except for a magnetic signal, and thus may be expressed as Equation [5].

[Equation 5]

Is a random variable having a value of 0 or 1 depending on the probability distribution of Equation [6].

[Equation 6]

In addition, I _{s, other} represents the amount of interference of the outer cell per Walsh symbol received from the outer cell. E _{s, i} is the energy per Walsh symbol received from the i-th user, it can be assumed that all have the same probability distribution by power control. The average and variance of the total interference amount I _s per Walsh symbol from Equation [5] can be expressed by the following Equations [7] and [8].

[Equation 7]

[Equation 8]

Using Equation [7] and Equation [8] _, the distribution of Is _{, other} is obtained as Equation [9] and Equation [10].

[Equation 9]

[Equation 10]

The base station substitutes Equations [3] and [4] into Equations [9] and [10] to estimate the average and variance of the interference amount I _{s, other} per Walsh symbol received from an external cell. can do. The base station can calculate the reverse link capacity adapted to the outer cell environment by using the average and variance of the estimated Is _{, other} .

The link capacity of the DS-CDMA system is different from that of frequency division multiple access (FDMA) or time division multiple access (TDMA) systems. Is determined. It is assumed that the call admission control method according to the present invention can accept a new call until the interrupt probability that does not satisfy the following equation [11] is less than 0.01.

[Equation 11]

Where R is the bit rate, E _{b, i} is the energy per bit of the i-th user, I _accept is the total allowed noise power density of the base station, W is the frequency bandwidth, k is the number of users at the base station of interest and I _other is received from the outer cell. Represents the amount of interference. N _o is the white noise density and the ratio to I _accept is given by Equation [12].

[Equation 12]

In this case, the reverse link capacity at the base station may be referred to as the maximum number of users k such that the probability of occurrence of interruption P _outage is 0.01 or less. In a system conforming to the IS-95 specification, one Walsh symbol is two bits. In order to represent Equation [11] in Walsh symbol units, Equation [13] can be obtained by dividing both sides by R / 2.

[Equation 13]

In Equation [13], the stopping probabilities represent the probabilities when Z is greater than A, and thus Equation [14].

[Equation 14]

Since Z can be assumed to have a normal distribution using a central limit theory, Equation [14] can be represented by Equation [15].

[Equation 15]

Here the Q (.) Function is a Gaussian error function defined by Equation [16].

[Equation 16]

In order to _calculate P _outage in Equation [15], E [Z] and Var [Z] should be obtained. First, the mean and the variance of Z in Equation [13] are obtained as Equation [17] and Equation [18].

[Equation 17]

Equation 18

The larger the energy E _s per Walsh symbol received at the base station, the larger the number of users in the cell and the greater the amount of external cell interference received from the outer cell. The ratio of bit energy to maximum permissible interference density, E _b / I _accept , measured at full load in urban environments, shows a lognormal distribution with an average of 7 dB and a standard deviation of 2.5 dB. Therefore, E _b / I _accept can be expressed by Equation [19].

[Equation 19]

Where X is a random variable with a normal distribution with a mean of 7 and a standard deviation of 2.5. Since I _accept is fixed by Equation [12], the energy distribution per Walsh symbol received by the base station in the full load state can be represented by Equation [20] using Equation [19].

[Equation 20]

Here, β = (ln10) / 10 and the random variable Y has a normal distribution with mean mY of 10 log (2I _accept ) +7 and standard deviation Y of 2.5. The average and the squared mean of E _s are obtained from Equation [20], as shown in Equation [21] and Equation [22].

[Equation 21]

[Equation 22]

Given the average and variance of the outer cell interference I _{s, other} per Walsh symbol received at the base station, assume that the number of users in the cell is full loaded. At this time, the average and variance of Z can be calculated by substituting Equation [21] and Equation [22] into Equation [17] and Equation [18]. Given the condition of I _{s, other} , the reverse link capacity k of the base station is a maximum integer that satisfies the condition that P _outage is less than 0.01. The maximum integer k means that k users can be accommodated at the same time, considering current outer cell interference.

As can be seen from equations [17] and [18], E [Z] and Var [Z] are functions of k. In order to find k directly, the equation [15] can be solved in reverse to obtain the equation [23].

[Equation 23]

After substituting Equations [17] and [18] into Equation [23] and arranging for k, Equation [24] can be obtained.

[Equation 24]

Here, B, C and D are defined by equations [25], [26] and [27].

[Equation 25]

[Equation 26]

[Equation 27]

The link capacity is obtained from Equation [24] as shown in Equation [28].

[Equation 28]

From here Represents the largest integer less than X. That is, when the base station estimates the interference amount _{s, other} per Walsh symbol received from the outer cell, the reverse link capacity is adapted to the outer cell environment by using Equations [25], [26], [27], and [28]. k can be calculated. The base station needs to control the call so that it does not exceed k, since P _outage may be greater than 0.01 and call interruption may occur when the number of users n exceeds k.

2 is a flowchart illustrating a call admission control process adapted to an external cell environment in a base station according to the present invention. The base station continuously receives data of E _s and I _s from the demodulator while the call has not arrived yet (21). Then, using the data of E _s and I _s , calculate the mean and variance of I _{s, other} (22). After calculating the mean and variance of I _{s, other} , we check the arrival of a new call (23). If the new call does not arrive as a result of the new call arrival check, the process proceeds to step 21 where the data of E _s and I _s is continuously received. If a new call arrives as a result of the new call arrival test _, the reverse link capacity k is calculated using the mean and the variance of I _{s, other} (24). After calculating the reverse link capacity k, it is checked if n is less than the reverse link capacity k when the current number of users is n (25). If the current number of users is less than k as a result of the test, processing is allowed by the call (26). If n is greater than k as a result of the test, the call is rejected (27) and then terminated.

As described above, according to the present invention, since the reverse link capacity is calculated to adapt to the traffic load environment of the outer cell, call admission control is performed so that the reverse link capacity is the worst unless the actual outer cell is in a full load state. As compared to the case, the call blocking rate and call interruption can be reduced, and the frequency use efficiency can be improved. In addition, it considers the voice activity rate to be applied to the real environment, does not require a separate hardware device, there is an excellent effect that does not require a separate message exchange to grasp the environment of the external cell.

Claims (4)

Receiving energy per Walsh symbol and interference per Walsh symbol through the demodulator, Calculating average and variance of the amount of outer cell interference per Walsh symbol by using the received data of the energy per Walsh symbol and the amount of interference per Walsh symbol; Calculating the average and variance of the amount of outer cell interference per Walsh symbol and checking whether a new call arrives; Transitioning to receiving the energy per Walsh symbol and the interference per Walsh symbol when the new call does not arrive as a result of the check of the arrival of the new call; Calculating a reverse link capacity using an average and a variance of the amount of foreign cell interference per Walsh symbol when a new call arrives as a result of the arrival test of the new call; Calculating the reverse link capacity and checking whether the number of current users is less than the reverse link capacity; Checking that the current number of users is less than the reverse link capacity, and if the current number of users is less than the reverse link capacity, accepting and processing a call and ending; If the number of current users is greater than the reverse link capacity, the current number of users is greater than the reverse link capacity. If the current number of users is larger than the reverse link capacity, the DS-CDMA cellular system adapts to the foreign cell environment. Call admission control method. The method of claim 1, wherein the average of the amount of foreign cell interference per Walsh symbol is represented by the following equation. [Equation] Where ν _i is the i th voice activity rate, E _{S, i} is the energy per Walsh symbol received from the i th user, α is the probability distribution of ν, and n is the number of users. The method of claim 1, wherein the variance of the amount of foreign cell interference per Walsh symbol is represented by the following equation. [Equation] Where ν _i is the i th voice activity rate, E _{S, i} is the energy per Walsh symbol received from the i th user, α is the probability distribution value of the voice activity rate, and n is the number of users. 2. The method of claim 1, wherein the reverse link capacity is represented by the following equation. [Equation] In this case, B, C and D are defined by the following equation. Where α is the probability distribution of speech activity rate, E_{S, i}Is the energy per Walsh symbol received from the i th user, P_outageIs the probability of interruption of the call, and Q (.) Is the Gaussian error function. I also_acceptTo Total allowable noise power density of base station, η is white noise density and I_acceptWhen W is a frequency bandwidth, A is defined as follows. [Equation]