Signal Strength checking Ardiuno Code

int8_t answer;
int x;

char SMS[200];

void setup(){

Serial.begin(9600);

power_on();

delay(3000);
Serial.println("Network...");
while( (sendATcommand("AT+CREG?", "+CREG: 0,1", 500) ||
sendATcommand("AT+CREG?", "+CREG: 0,5", 500)) == 0 );

delay(300);
sendATcommand("AT+CMGF=1", "OK", 1000);
delay(3000);
Signal();
delay(3000);

}

void loop(){

}

void power_on(){

uint8_t answer=0;

// checks if the module is started
answer = sendATcommand("AT", "OK", 2000);
if (answer == 0)
{


// waits for an answer from the module
while(answer == 0){ // Send AT every two seconds and wait for the answer
answer = sendATcommand("AT", "OK", 2000);
}
}

}

int8_t sendATcommand(char* ATcommand, char* expected_answer, unsigned int timeout){

uint8_t x=0, answer=0;
char response[100];
unsigned long previous;

memset(response, '\0', 100); // Initialice the string

delay(100);

while( Serial.available() > 0) Serial.read(); // Clean the input buffer

Serial.println(ATcommand); // Send the AT command

x = 0;
previous = millis();

// this loop waits for the answer
do{
// if there are data in the UART input buffer, reads it and checks for the asnwer
if(Serial.available() != 0){
response[x] = Serial.read();
x++;
// check if the desired answer is in the response of the module
if (strstr(response, expected_answer) != NULL)
{
answer = 1;
}
}
// Waits for the asnwer with time out
}while((answer == 0) && ((millis() - previous) < timeout));

return answer;
}

void Signal()
{
answer = sendATcommand("AT+CSQ", "+CSQ", 2000); // reads the first SMS

if (answer == 1)
{x=0;
memset(SMS, '\0', 300);
answer = 0;
while(Serial.available() == 0);
// this loop reads the data of the SMS
do{
// if there are data in the UART input buffer, reads it and checks for the asnwer
if(Serial.available() > 0){
SMS[x] = Serial.read();
x++;

if (strstr(SMS, "OK") != NULL)
{
answer = 1;
}
}
}while(answer == 0); // Waits for the asnwer with time out

SMS[x] = '\0';
Serial.println(SMS);
}

}