Multiband architecture for high-speed SerDes
3. Optimization of a Multiband Architecture for 25 Gbps SerDes
3.1. Transmitter processing
Depending on how many sub-bands are used and the modulation scheme adopted (e.g., binary or higher-order QAM), the signal in each sub-band might still experience slight amount of ISI because of the non-flat channel frequency response in each sub-band. Given the frequency-attenuation profile of the backplane channel as shown in Figure 1, the error performance of the system will remain to be ISI dominated. If minimizing this slight ISI still turns to be necessary, a pre-emphasis unit for each sub-band could be employed at the transmitter. This unit approximately equalizes the channel gain in each sub-band. Since the bandwidth of each of the bands is not very large (e.g., not exceeding 4 GHz or so), design of such unit in the analog doma.in with low complexity is practical.
The transmitter model with 4 sub-bands for the 25 G SerDes is shown in Figure 6, in which a pre-emphasis unit in the transmitter is assumed; the case without a pre-emphasis unit is a special case in which this unit has a constant gain at all frequencies in band.
Note that this specific configuration is only for illustration of the architecture; the number of sub-bands could be flexibly chosen to accommodate different settings. Indeed, in Section 4, we will evaluate the performance of two variations of this architecture: a system with four sub-bands but no pre-emphasis blocks, and a system with three sub-bands but with pre-emphasis blocks. Four sub-bands are considered because eye-diagram simulation results reveal that even when binary PAM is used in band 1 and QPSK is used in bands 2 to 4, the channel-induced ISI would be at a tolerable level; three sub-bands with a simple pre-emphasis unit are considered because the target BER performance can still be maintained without an equalizer at the receiver.
Figure 6: The 25G multiband transmitter model.
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Based on the channel model in Sections 1 and 2, we optimize modulation, bandwidth, guard-band, and power level for each sub-band. First, we design the pulse shape for the signals to be transmitted in each sub-band. Here, for baseband pulse shaping we adopt the raised-cosine filter (RCF), which is widely used in communication systems and standards. The impulse response of the RCF is expressed as:
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where T is the symbol period and β is the roll-off factor, which determines how much excess bandwidth is required over the minimum bandwidth (i.e., an ideal filter with a sinc impulse response) required to avoid ISI. The roll-off factors for each sub-band do not have to be identical.
To maintain zero-ISI, the desired pulse shape is the composite pulse in the receiver after receive baseband filtering and includes the effect of the channel. Since the channel will not have a perfectly flat frequency response for each sub-band, small amount of ISI exists.
To maximize the signal-to-noise ratio (SNR) at the receiver, it is common in practice to split an RCF as a pair of square-root-raised cosine filters (SRCFs), one at the transmitter and one at the receiver. The SRCFs at the transmitter and receiver are also known as the matched filters. The impulse response of an SRCF is expressed as:
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Note that for bandwidth not exceeding 3-4 GHz, implementation of very small passive analog filters that approximate the ideal frequency response of the SRCFs is practical.
As shown in Figure 6, one data stream that uses the lowest frequency band is transmitted using baseband signaling. As a result, for a transmitter with N bands, the transmitter only employs N-1 mixers.
In addition to approximately equalizing the channel gain over each sub-band, the pre-emphasis modules are also used to adjust the signal power in each sub-band. Since the channel behaves like a low pass filter, signals transmitted in a sub-band in the higher frequency region are attenuated more than those in the lower frequency area. To enhance the signal at high frequency area, we multiply the signal at each sub-band (except the baseband) with a coefficient Cpre_em. If f1 < f2 < f3, then the coefficients satisfy:
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