Before the receiver can demodulate the subcarriers, it has to perform the synchronization task, since the OFDM systems can be extremely sensitive and vulnerable to synchroniza-tion errors. There are three major kinds of synchronizasynchroniza-tion tasks:
1. Symbol synchronization [12]
The purpose of it is to find the correct position of the fast Fourier transform (FFT) window. Any misalignment of the FFT window will result in an evolving phase shift in the frequency domain symbols, leading to BER degradation. If the timing errors are so high that the FFT window of the receiver includes samples outside the data and guard intervals of the current OFDMA symbol, then the consecutive OFDMA symbols interfere, severely affecting the system’s performance. Fig. 2.12(a) shows the correct FFT window. Fig. 2.12(b) shows an early FFT window that includes
Fig. 2.12: Positioning of the FFT window.
samples of the data segment and the guard interval. Fig. 2.12(c) depicts a delayed FFT window that overlaps with the next OFDMA symbol. The second case will not introduce any interference, but the third is detrimental to the performance.
2. Sampling clock synchronization
The purpose of it is to align the receiver sampling clock frequency to that of the transmitter. The sampling clock errors can cause ICI. In addition, the sampling clock frequency error can result in a drift in the symbol timing and can further worsen the symbol synchronization problems. In this thesis, we will assume that the sample clocks of the users and the base station are identical.
3. Carrier synchronization
Carrier frequency offset can give rise to a shift of all the subcarriers and results in not only ICI but also multiple access interference (MAI). It is caused by the difference in the local oscillators of the transmitter and the receiver, or the Doppler spread introduced by motion. Carrier synchronization is a complex problem in the UL system, since all users share the total number of subcarriers and each user has its own carrier frequency offset.
In our system, the synchronization scheme is subject to the specifications of 802.16a.
Thus we assume that after a successful initial synchronization and ranging, the mobile enters the time and frequency grid with a low offset in time and frequency [11]. Hence
no frequency synchronization is done in normal UL transmission. While this assump-tion may be suitable for fixed BS and SS, it is certainly debatable for multipath fading channels. However, for simplicity we leave it further consideration to future work.
2.6 UL Synchronization
The above discussion of the UL synchronization motivates our doing timing synchroniza-tion only. We now introduce the techniques used in our UL synchronizasynchroniza-tion, the detecsynchroniza-tion of symbol start time.
Our synchronization task is to find the first coming symbol. Different users’ transmit-ted signals may not arrive at the same time, but the correlation peak may occur between them, as shown in Fig. 2.13 for an example of three users. If we use the detected peak location as the symbol start time, the corresponding useful time will include a part of the guard interval of the next symbol for the earlier arriving signals. Therefore, we have to find the exact instant of the first arriving signal to avoid ISI.
Since the subchannels are comprised by the subcarriers which are orthogonal to one another, we can assume that the orthogonality property still exists among subchannels un-less the received signals from different users are subject to significantly different carrier
Fig. 2.13: Three UL signals arrive at different times, and the CP correlation peak may occur between them [11].
Fig. 2.14: Illustration of UL synchronization in time domain.
offsets. After passing through IFFT, the time domain signals which occupy different sub-channels in the frequency domain are uncorrelated if the channel has zero delay spread.
Since the first coming symbol is an all-pilot preamble, the BS knows the exact values of each user’s signals. Therefore, the signal transmitted by each SS in the UL preamble is deterministic and the BS can generate the same time domain signals as all SSs by taking IFFT. We show the block diagram depicting how the synchronization works in Fig. 2.14.
The received samples are correlated with the reference data string, which results from passing the preamble into the IFFT block. When the next sample arrives, we recompute the correlation. The start and stop times of the correlation are as illustrated in Fig. 2.15.
The start time is decided by when the BS turns to receive signals. Note that this time shall be in the TTG interval. As the user arrival time may vary as much as 50% of the guard interval, we stop the correlation up to 50% of the guard interval earlier than the corresponding detected useful time.
Then, the peak locations of different SSs are compared as follows. We can find the peak location of each correlator which uses a distinct preamble, then we can know the peak locations of different SSs. Finally, we compare all these peaks and get the start location of the first coming signal.
Fig. 2.15: The received samples and the time plan of the UL synchronization.