• 沒有找到結果。

4 Diversity Reception and Phase Noise Compensation in

4.1.3 Cyclic delay Diversity

Before referring to CDD, we introduce delay diversity (DD) and phase diversity (PD) whose concepts are similar to that of CDD [2]-[5]. Usually, DD and PD are applied at multiple-antenna transmitter side.

Delay Diversity

In DD, the transmitted multi-carrier modulated signal differs only with the specific delay δm at the mth antenna, where m = 0,…, M-1. The block diagram of an OFDM system with the spatial transmit diversity applying DD is shown in Fig. 4.2. In order to achieve constructive and destructive superposition of the signals, the delay δm has to fulfill the condition

1,

m m

δ ≥ B ∀ (4.3)

where B is the channel coherence bandwidth. Therefore, frequency selectivity is

generated, and long burst errors induced from deep flat fading are avoidable. Fig. 4.3 shows the effect of DD over a typical indoor channel. But the disadvantage of DD is that the additional delays increase the total delay spread at the receiver antenna, i.e. the effective cyclic prefix length is reduced relatively. To overcome this issue, phase diversity scheme is presented.

Phase Diversity

In PD, the transmitted multi-carrier modulated signal differs only with the specific phase offset. The block diagram of an OFDM system with the spatial transmit diversity applying PD is shown in Fig. 4.4. In order to achieve constructive and destructive superposition of the signals, the phase offset Φm k, has to fulfill the condition

, the cyclic prefix. Therefore, frequency selectivity is generated. But the disadvantage of PD is that the scheme operates in the frequency domain, which needs the M OFDM modulation blocks. To prevent additional hardware induced from PD and the effective cyclic prefix length from being shortened by DD, the CDD is exploited in place of PD and DD.

Cyclic Delay Diversity

In CDD, the signal is not truly delayed but cyclically shifted. The basic idea behind the new scheme is to convert time-selectivity of the channel into frequency-selectivity so that channel coding which is applied mainly in frequency domain in the DVB-T system is more effective. The equivalence between PD and CDD is a property of DFT and can directly be seen from the length Nc IDFT definition

1 2

complex-valued signals in time and frequency domain respectively with l, k. δ cy stands for cyclic time shift. From (4.5), the operation for PD has to be done before the OFDM modulation. So for an M-antennas system with PD, the M OFDM modulation blocks have to be constructed. On the other hand, only one OFDM block is required in CDD. Thus the implementation of CDD is more efficient, and has the same functionality as those of DD/PD. However, all schemes of DD, PD and CDD are often applied in the transmitter. To implement them in the receiver side, we derive the equivalence between the transmitter and receiver with CDD. The received signals with CDD applied at the transmitter side can be written as

(1) (1) (1)

And the received signals with CDD applied at the receiver side can be written as

(1) (1) (1)

where Rx denotes the receive antenna. From (4.6a) and (4.6b), the equivalence is mainly based on the linear characteristics of convolution and delay. Fig. 4.5 shows the equivalence model of the transmitter and receiver using CDD. Thus, CDD can be applied in the receiver. From Fig. 4.5, we know that the implementation of CDD only needs an adder and a man-made cyclic delay. The receiver structure remains unchanged compared with the single antenna case after FFT. In addition to lower computational complexity, CDD can also incorporate other diversity schemes if hardware loading can be increased. Finally, we summarize CDD as follows:

(1) Standard compatibility, i.e. CDD can be applied without changing the transmitter.

(2) No necessity of the multiple channel estimators; therefore lower the receiver complexity.

(3) The number of receive antennas is arbitrary.

(4) Low implementation complexity, due to the simple cyclic shifts in the time domain.

(5) CDD can incorporate other diversity schemes to enhance the system performance.

Maximal Ratio Combining with Cyclic Delay Diversity

As discussed above, it is easy to combine CDD with other diversity reception schemes [5], e.g. MRC with CDD, as shown in Fig 4.6. Fig. 4.7 shows the BER performances of MRC, CDD, and MRC with CDD in a DVB-T system with cyclic delay varied as a parameter. We observe that the value of cyclic delay is important, which should be larger than RMS delay spread of a channel model. The RMS delay spread of Rayleigh fading channel model in a DVB-T system is about 1.2689μs (see Section 4.3). Therefore, the system with the cyclic delay δcy =1.8μs gets better performance. On the other hand, MRC with CDD outperforms conventional MRC. This is because MRC with CDD not only gets full diversity gain, but also rejects long burst errors.

4.1.4 In-Viterbi Selective Diversity

In wireless communication, channel fading and random noise effects make points on the constellation shift and thus cause decision errors. In Fig.4.8, we can know that the probability of the effects making signals of different branches shift toward error areas simultaneously is small. Therefore, we exploit the diversity scheme to increase freedom of add-compare-select (ACS) in Viterbi to help suppress the deep fading and random noise effects as shown in Fig.4.9 [6]-[7]. By the Viterbi algorithm, the branch metrics associated with the state transitions are computed and then added to the previous path metrics. The contending path metrics are compared and the path with the largest metrics is as the survivor. With the aid of diversity scheme, the path metrics are computed for each diversity branch, and then compared to select the survivor and the criterion is given by

ˆ arg min j

j= j d (4.7)

where dj denotes the Euclidian distance at the jth diversity path. Moreover, IVSD incorporating interleaver can achieve better performance. This is because interleaver combating deep fading helps IVSD more concentrate on random noise. Therefore, IVSD can suppress deep fading and random noise effects effectively.

相關文件