MSP430F5528: UART COMMUNICATION

Hi,

The system is first msp430 transmits a data then in some distance receiver receives that and does the work accordingly then it transmits an acknowledge signal it received by the msp430 receiver  and it glows a status led. The code is

#include <msp430.h>

int main(void)
{
  WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT
  P4SEL = BIT4+BIT5;                        // P4.4,5 = USCI_A1 TXD/RXD
  UCA1CTL1 |= UCSWRST;                      // **Put state machine in reset**
  UCA1CTL1 |= UCSSEL_2;                     // SMCLK
  UCA1BR0 = 6;                              // 1MHz 9600 (see User's Guide)
  UCA1BR1 = 0;                              // 1MHz 9600
  UCA1MCTL = UCBRS_0 + UCBRF_13 + UCOS16;   // Modln UCBRSx=0, UCBRFx=0,
                                            // over sampling
  UCA1CTL1 &= ~UCSWRST;                     // **Initialize USCI state machine**
  UCA1IE |= UCTXIE + UCRXIE;                         // Enable USCI_A0 RX interrupt

  __bis_SR_register(LPM0_bits + GIE);       // Enter LPM0, interrupts enabled
  __no_operation();                         // For debugger
}

// Echo back RXed character, confirm TX buffer is ready first
#if defined(__TI_COMPILER_VERSION__) || defined(__IAR_SYSTEMS_ICC__)
#pragma vector=USCI_A1_VECTOR
__interrupt void USCI_A1_ISR(void)
#elif defined(__GNUC__)
void __attribute__ ((interrupt(USCI_A1_VECTOR))) USCI_A1_ISR (void)
#else
#error Compiler not supported!
#endif
{
  switch(UCA1IV)
  {
  case 0:break;                             // Vector 0 - no interrupt
  case 2:                                   // Vector 2 - RXIFG
      P4DIR |= BIT7;
      P4OUT ^= BIT7;
    break;
  case 4:
      UCA1TXBUF = 19;
      break;                             // Vector 4 - TXIFG
  default: break;
  }
}

In this first msp430 is transmitting and it is being received in other side and an acknowledge signal also coming from other side but msp430 is not receiving. Please help....