SC-OFDM-OFDMA SDR System: Transmitter Architecture

Transmitter Architecture

The block diagram shown in Figure 4.1 represents the transmitter architecture of the proposed SC-OFDM-OFDMA SDR system. In the following section, we will introduce some functional blocks of the transmitter architecture that are modified to be reconfigurable. Here, we will not introduce some functional blocks in detail that have been described in Chapter 2.

• Segmentation

Figure 4.1: Proposed SC-OFDM-OFDMA SDR transmitter architecture

As shown in Figure 4.1, the uncoded data is generated from a random source, consists of a series of zeros and ones. First, the uncoded data shall be passed to a

randomizer. Randomization is performed on each allocation, which means that for each allocation of a data block. After performing randomization, the randomized data shall be passed through the FEC block. The FEC block is performed by first passing the data in block format through the RS encoder and then passing it through the zero-terminating convolutional encoder. It is worthy of mention that the randomized data needs not first pass to the RS encoder in OFDMA transmission mode. After encoding, all encode data shall be interleaved by a block interleaver. After bit interleaving, the data bits are entered serially to the modulator and then mapped to form symbols. Modulation schemes used here are BPSK, QPSK, 16QAM, and 64QAM with gray coding in the constellation map. After constellation mapping, the modulated data will be processed by different transmit techniques such as SISO, STC, and AAS. But the different transmit techniques do not affect the data block size. The data block size is determined by the FEC block, modulator, and the number of subchannels allocated. In the following paragraphs, we will introduce the procedures to determine the uncoded data block size for different modes.

Single Carrier

In SC transmission mode, the receiver will perform frequency domain equalizer by utilizing the N-point FFT and IFFT. In order to adapt to the input size of FFT, the transmitter requires performing uncoded data block segmentation. In addition to the FFT size, we still require considering the FEC block and modulator. Under the whole consideration, the uncoded data block size per symbol can be written as

Uncoded data block size = (N m c p× × − ) 2− × (4.1) t where N denotes the FFT size, m is modulation order (QPSK: m=2, 16QAM: m=4), c is the code rate of CC encoder, p is the number of zero tail bits before CC encoder, and t is the number of data bits which can be corrected after RS encoder. As shown in Table 4.1, the FFT size is specified by 256 and each frame contains 10 symbols. The uncoded

data block size can be easily obtained by equation (4.1) in SC transmission mode.

Table 4.1: Uncoded data block size for SC mode

1024x10

CC code rate 1/2

(496,432,32)

CC code rate 1/2

(496,432,32)

In OFDM transmission mode, the mandatory FEC block consists of the concatenation of a Reed-Solomon outer code and a rate-compatible Convolutional inner code. The RS-CC coding rate 1/2 shall always be used as the coding mode when requesting access to the network and in the FCH burst. The FFT size is fixed to be 256 in OFDM transmission mode, consisting of 192 data subcarriers and 8 pilot subcarriers per symbol. In order to achieve the desired overall code rate, the RS encoder and CC code rate require adjustments. The uncoded data block size per symbol can be computed as

Uncoded data block size (192= × × −m c p) 2− × (4.2) t where m is modulation order (BPSK: m=1, QPSK: m=2, 16QAM: m=4), c is the code rate of CC encoder, p is the number of zero tail bits before CC encoder, and t is the number of data bits which can be corrected after RS encoder. ‘192’ represents the total number of data subcarriers. Table 4.2 gives the uncoded block sizes used for different modulations to achieve overall coding rate 1/2. In the case of BPSK modulation, the RS

encoder should be bypassed. When subchannelization is applied in the uplink, the FEC will bypass the RS encoder and the uncoded data block size can be computed by the number of allocated subchannels divided by 16 (total number of subchannels).

Table 4.2: Uncoded data block size for OFDM-256 mode

768x10

CC code rate 1/2

(480,352,64)

CC code rate 1/2

(480,352,64)

In OFDMA transmission mode, the mandatory coding scheme is convolution encoder. The encoding block size will depend on the modulation order and the number of subchannels allocated for the current transmission. As mentioned in Chapter 2, each subchannel is composed of 24 data subcarriers and 4 pilot subcarriers for PUSC permutation. According to the number of subchannels allocated and code rate, the uncoded data block size per symbol can be derived as

Uncoded data block size=N_sc×24 2× × × − (4.3) m c p where N denotes the number of subchannels allocated, m is modulation order _sc (QPSK: m=2, 16QAM: m=4, 64QAM: m=6), c is the code rate of CC encoder, p is the number of zero tail bits before CC encoder, and ‘2’ represents the number of spanned symbols for a slot in downlink PUSC. Although the FFT size can be scaled to adapt to different channel bandwidths in OFDMA mode, different FFT sizes still use the same equation (4.3) to determine the uncoded data block size. The uncoded data block sizes

for different FFT sizes are shown in Tables 4.3, 4.4, 4.5 and 4.6. The four tables provide the overall coding rate 1/2 in the case with the whole subchannels allocated in Major Group 0. The total number of subchannels in Major Group 0 is specified in Table 4.7 for different FFT sizes.

Table 4.3: Uncoded data block size for OFDMA-2048 mode

3456x5

CC code rate 1/2

1712x5 (bits)

CC code rate 1/2

1712x5 (bits)

CC code rate 1/2

848x5 (bits)

CC code rate 1/2

848x5 (bits)

Table 4.4: Uncoded data block size for OFDMA-1024 mode

Table 4.5: Uncoded data block size for OFDMA-512 mode

1440x5

CC code rate 1/2

704x5 (bits)

CC code rate 1/2

704x5 (bits)

Table 4.6: Uncoded data block size for OFDMA-128 mode

288x5 192x5

CC encoded data 96x5

48x5 48x5

Modulated data 48x5

1/2 1/2

CC code rate 1/2

128x5 (bits)

CC encoded data 96x5

48x5 48x5

Modulated data 48x5

1/2 1/2

CC code rate 1/2

128x5 (bits)

Table 4.7: Number of subchannels in Major Group 0 for different FFT sizes

PUSC Downlink

Once the uncoded data block size is decided and the uncoded data is encoded and modulated, the pilot subcarriers shall be inserted into each data block in order to constitute a symbol and they shall be modulated according to their locations within a symbol. The first symbol of the downlink transmission is the preamble. In OFDM and OFDMA transmission mode, each symbol will be fed into IFFT and transmitted in different formats by using various transmit techniques such as STC and AAS. SC transmission mode will bypass the IFFT block. Here, we omit the detail descriptions of other blocks that have been introduced in Chapter 2.

4.3 SC-OFDM-OFDMA SDR System: Receiver