Multiband architecture for high-speed SerDes

Christian Weber, Jinjin He, Lizhi Charlie Zhong, and Huaping Liu

January 20, 2011

Christian Weber, Jinjin He, Lizhi Charlie Zhong, and Huaping LiuJanuary 20, 2011

Abstract
As the speed of serializer/deserializer (SerDes) increases (e.g., to 25 Gbps and above), the channel will cause more severe inter-symbol interference. Design of low-complexity transceivers for such high-speed SerDes faces many technical challenges. In this paper, we explore a multiband architecture for a 25 Gbps SerDes, where the channel in each sub-band is approximately frequency flat, eliminating need of an equalizer in the receiver. Since different bands experience different signal attenuations, the power level for each band can be adjusted accordingly to minimize the average transmission power. A multiband transceiver is designed, and analysis and simulation results for various choices of parameters (bands, modulations, etc.) are presented.

1. Introduction
The demand for narrower interfaces in 100G Ethernet drives the need for high speed serializer/deserializer (SerDes) running at 25Gbps or higher.

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However, there are many technical challenges in developing such a high-speed SerDes whose power consumption must be within acceptable limits.1 At rates as high as 25Gbps, the transceiver must be simple, since advanced signal processing at such speed is extremely power hungry. How to efficiently use the available bandwidth with low hardware complexity is therefore very important. The right signaling format or modulation scheme enables one to achieve such a goal.

Various modulation schemes could be employed for SerDes. Non-return-to-zero (NRZ) signaling has been used in most of the existing high-speed SerDes designs, and is the signaling format used by OIF CEI-11 and IEEE 10G Base-KR. However, there is controversy on if NRZ is the right signaling format for SerDes at 25Gbps and beyond.

First, the realistic channels one will be likely encounter are so lossy that a NRZ based receiver either cannot equalize them or requires an unrealistically complex equalizer which cannot be implemented within an acceptable power budget. The frequency responses of several channels contributed to IEEE802.3AP are shown in Figure 1. As one can see, they have loss much higher than 25dB at 12.5GHz. Although these are legacy channels designed for 10Gbps operation, they have relatively short trace lengths. Channels using new connectors designed for 25Gbps operation and low loss channel materials such as megtron 6 also have loss in the neighborhood of 35dB at 12.5GHz when the reach is 40 inches and longer. As a result, these channels are representative of the channels with long reach as well as the channels with shorter reach, but using lower cost (and therefore higher loss) channel materials.

One can also see from Figure 1 that as frequency increases, the attenuation to the transmitted signal increases nonlinearly. This indicates that the channel is highly frequency selective, which results in severe inter-symbol interference (ISI). At 25 Gbps, ISI could span more than ten symbols. Therefore, a complex equalizer at the receiver is essential to ensure a very low bit error rate (BER) (e.g., ≤10-15). Implementation of such a complex equalizer at 25G either is not feasible or incurs a huge power penalty.

Secondly, the circuit implementations for NRZ are very challenging. Design of circuits such as capture latches, analog-to-digital converter (ADC) and transmit driver running at such data rate is very challenging. Although by using parallel circuits each block could run at a lower rate (e.g., 12.5 G or 6.25 G), it comes at the cost of power consumption and circuit area.

Figure 1: Channel frequency response.

Click on image to enlarge.

There are many types of higher order modulation schemes that are also applicable to SerDes. Pulse amplitude modulation (PAM), duo-binary (DB), quadrature amplitude modulation (QAM), phase shift keying (PSK), and constant envelope modulation such as frequency shift keying (FSK) are a few examples. A drawback of amplitude modulation is that for a given maximum voltage limit, the minimum distance between symbols will be reduced as the modulation level increases. For example, for the same maximum voltage limit as NRZ, the minimum distance of PAM-4 is only one third of that of NRZ, resulting in a 9.5 dB of signal-to-noise ratio (SNR) penalty up front. Constant envelope modulation overcomes the voltage limit problem of amplitude modulations. However, in a single- band approach, it is not easy to accurately control the phase or frequencies at such a high speed as 25 Gbps (e.g., non-pulse-shaped QPSK would require the signal phases to be 90 degrees apart).

All the modulation schemes mentioned so far fall in the category of single-band approach. Because the channel is highly frequency selective in the band from 0 to 12.5GHz, all these schemes cannot avoid the use of a multi-tap DFE, which is very power hungry.

In this paper, we explore a multiband architecture for SerDes at 25Gbps or beyond. In this architecture, the frequency spectrum is divided into several smaller sub-bands, where parallel data streams use different sub-bands. This architecture has many advantages over the single-band approach:
  1. in each sub-band, the channel is approximately frequency flat, consequently no equalizer in the receiver is needed;
  2. the symbol rate in each sub-band is much lower, which makes the circuit design easier;
  3. and it may occupy a smaller overall bandwidth if higher order modulation schemes are used in the sub-bands.
The idea of multi-band communications has been around for many years. It has been used extensively in wireless for example. However, the data rates are much lower there. In recent years, multi-band architectures at data rates higher than 1Gbps have also been studied. For example, a16-carrier QAM-16 architecture has been studied in optical communications.6 Different multiband architectures. such as analog multitone (AMT) have also been proposed for high-speed SerDes 4, 5 AMT is the analog variation of the discrete multitone while it tries to eliminate the use of ADC. However, a multi-input, multi-output (MIMO) DFE is required.

2. Background
2.1. Channel model
We consider a backplane channel over which the system should achieve a target BER not higher than 10-15. The S-parameters obtained from measurements are provided by Intel (available in the endnotes).2 The test frequency ranges from 50 MHz to 15 GHz with a step size of 10 MHz. We extrapolate the data from 0 to 50MHz, where linear phase and 0 dB attenuation at ƒ = 0Hz are assumed. The resulting channel impulse response is shown in Figure 2.

Figure 2: Backplane channel impulse response obtained from S-parameters.

Click on image to enlarge.

If the data rate is 25 Gps, Figure 2 indicates that significant ISI lasts at least 12 symbol periods (about 0.48 ns) with NRZ signaling.

2.2. Multiband vs. single-band approaches
To assess how severe ISI is at different signaling rates, we simulate the received signal in the absence of additive white Gaussian noise (AWGN). The eye diagrams of NRZ signals with four different data rates passing through the channel are illustrated in Figure 3. It is observed that when the data rate increases to 10 Gbps, it is nearly impossible to achieve the target BER without an equalizer.

Figure 3: Eye diagrams of NRZ signals at four different data rates.

Click on image to enlarge.

If the signal occupies a smaller portion of the spectrum, it is possible that an equalizer might not be needed. Also, as shown in Figure 1, the attenuation to the transmitted signal at low frequencies is much lower than at high frequencies. Thus, it would be desirable to have the flexibility to adjust the transmission power levels of the signal that occupies different portions of the spectrum.

This could be achieved with a multiband approach. Figure 4 shows a general functional diagram of a multiband transmitter. The serial data are first converted into several sequences of parallel data streams. Then each sequence of data is modulated, using different modulation schemes, and pulse shaped by a low-pass filter (LPF). Pulse shaping filtering could be done in either the digital or the analog domain. After that, each sequence of data is moved to the desired sub-band. A traditional mixer is shown in Figure 4 for moving different data streams to different sub-bands. If the data rates for each band are not very high, it is possible to use digital methods for this conversion. Finally, the signals from different sub-bands are added together and transmitted.

Figure 4: Illustration of the multiband transmitter.

Click on image to enlarge.

The receiver of the multiband architecture is illustrated in Figure 5. The receiver operates reversely to the transmitter. Signals of all sub-bands are converted to the baseband with mixers and LPFs. After that, data in each sub-band are demodulated to recover the original bit sequence.

Figure 5: Illustration of the multiband receiver.


Click on image to enlarge.

In the next section, we will discuss the detail of the multiband architecture by using the example of a 25 Gbps SerDes transceiver.

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