Carrier Frequency Offset Estimation in Frequency

Domain for DOCSIS 3.1 Uplink System

Kwanwoong Ryu1, Haechan Kwon1 and Joon-Young Jung1

1 Electronics and Telecommunications Research Institute,

Daejeon, Korea

{kwryu0730, khc1201,jungjy }@etri.re.kr

Abstract. We present an efficient carrier frequency offset (CFO) estimation

algorithm in frequency domain for DOCSIS 3.1 Uplink System. After

performing the CFO algorithm in time domain for DOCSIS 3.1 Uplink receiver,

the residual CFO can be remained. The residual CFO is degraded performance

of the system. To correct residual CFO, we propose CFO algorithm in

frequency domain using pilot and complimentary pilot in DOCSIS 3.1 Uplink

System. The simulation results show that the proposed CFO algorithm is

significantly reduced residual CFO.

Keywords: CFO, DOCSIS 3.1

1 Introduction

Data-Over-Cable Service Interface Specification (DOCSIS) is a global standard for

cable communication systems [1]. The DOCSIS 3.1 specifications introduce a number

of advancements, including: orthogonal frequency division multiplexing (OFDM) to

improve network flexibility and utilization; large channel sizes (up to 192 MHz) to

improve cost effectiveness at higher bandwidths; low density parity check (LDPC)

FEC to improve performance; and adaptive modulation to adjust to different network

conditions.

In upstream channel, DOCSIS 3.1 occupies a bandwidth of 96 MHz. There are two

modes of operation for a DOCSIS 3.1 OFDM system depending on the size of the fast

Fourier transform (FFT) operation: 2048 (2K) and 4096 (4K) modes. Note that in this

example, the system operates in the 4K mode. There are a total of 4096 subcarriers

inside the channel and the spacing between two adjacent carriers is 25 kHz. This

spacing between subcarriers is different but fixed for each mode: it is 25 kHz in the

4K mode and 50 kHz in the 2K mode. DOCSIS 3.1 also supports high-order

modulation schemes of up to 4096 QAM, and therefore theoretically can provide up

to 10 Gbits/s downstream and 1 Gbits/s on the upstream.

In this paper, we consider CFO estimation in frequency domain for residual CFO

correction after CFO estimation in time domain. Although there have been many

studies on the topic of CFO estimation based on OFDM, none of the existing methods

are not directly applicable to DOCSIS 3.1 systems [2]-[3]. Therefore, the main

objective of this research is to develop effective residual CFO estimation algorithms

Advanced Science and Technology Letters Vol.146 (FGCN 2017), pp.60-63

http://dx.doi.org/10.14257/astl.2017.146.12

ISSN: 2287-1233 ASTL Copyright © 2017 SERSC

in frequency domain for the DOCSIS 3.1 upstream signal. The proposed methods are

evaluated under an AWGN using computer simulation.

2 DOCSIS 3.1 Uplink receiver System Model

Fig. 1 describes the DOCSIS 3.1 uplink receiver system for residual CFO estimation

in frequency domain. The synchronization of the system compensates the symbol

timing offset and carrier frequency offset in the time domain and then performs FFT.

After the FFT is performed, a frequency offset and a channel value are estimated

using a pilot signal in the frequency domain, and then a complex value of

complimentary pilot is determined by compensation to the estimated frequency offset

and channel value. After that, the residual frequency offset is estimated and

compensated using the pilot and the determined complimentary pilot. In channel

estimation 2, channel is estimated, and then channel compensation is performed in the

equalizer. The signal that has passed through the equalization performs demodulation

and signal detection in the signal detection.

Fig. 1. Proposed system with residual CFO Estimation in DOCSIS 3.1 Uplink.

3 Residual CFO Estimation for DOCSIS 3.1 Uplink System

In DOCSIS 3.1 Uplink System, subcarriers that are assigned to a specific CM are

grouped into minislots. A minislot is the data carried by Q contiguous subcarriers

distributed across K OFDMA symbols in a single frame. The value of Q is either 8

subcarriers in 2k mode or 16 subcarriers in 4k mode. The value of K can range from 6

to 36 OFDMA symbols in 2k mode, or from 6 to 18 OFDMA symbols in 4k mode. In

this system, we assume that 2K FFT mode with Q=8 and K=8. In addition, OFDMA

symbol contain a mixture of 64 QAM-mapped data symbols, BPSK pilot symbols

and/or QPSK complementary pilots. The pilot pattern use 2 as shown in Fig.2.

Advanced Science and Technology Letters Vol.146 (FGCN 2017)

Copyright © 2017 SERSC 61

(a) Edge Minislot (b) Body Minislot

Fig. 2. Minislot structure with pilot pattern 2

As shown in FIG. 2, the phase rotation between the first and third symbols can be

obtained as follows using the pilots of the first symbol and the third symbol in the

time domain.

1

0

3

*

1

1

1 ][][tan2

1ˆ

N

i

kYkY

(1)

2/ˆˆ1 (2)

The phase rotation is compensated by the second symbol , the third symbol

2 , and the mth symbol by )1( m . The estimated values ][ˆ6 pC and ][ˆ

8 pC of

the 6th and 8th complimentary pilots can be obtained as follows.

ˆ52

66 ][][ˆ jepCpC (3)

ˆ72

88 ][][ˆ jepCpC (4)

where ][ˆ6 pC , ][ˆ

8 pC denotes the 6th and 8th complimentary pilots after the FFT and

p denotes the frequency domain position of the complimentary pilot. After detecting

complimentary pilot, we can compensate iteratively carrier frequency offset using

pilot and complimentary pilot.

4 Simulation Results and Conclusions

In this simulation, we show an efficient carrier frequency offset (CFO) estimation

algorithm in frequency domain for DOCSIS 3.1 Uplink System. We evaluate the

algorithm performance for an AWGN channel by assuming that the minislot structure

with Q=8 and K=8 is available in a DOCSIS 3.1 Uplink System. The system

parameters for simulation are shown in Table 1.

Advanced Science and Technology Letters Vol.146 (FGCN 2017)

62 Copyright © 2017 SERSC

Table 1. System parameters for simulation

Parameter 2k Mode Sampling rate(fsu) 102.4 MHz Elementary Period rate(Tsu) 1/102.4 MHz Channel Bandwidths 95MHz IDFT size 2048 Subcarrier spacing 50 kHz FFT duration(Useful symbol duration) 20us Maximum number of active subcarriers 1900 Upstream Cyclic Prefix 2.5 us(256 * Tsu) Roll-off period 0

(a) Conventional System (b) Proposed System

Fig.3. Performance of proposed system with Data (64QAM), Complimentary Pilot(QPSK), and

Pilot(BPSK).

Fig.3(a) shows the system performance according to increase residual CFO. As

shown in fig.3(a) , the system performance is degraded according to increase residual

CFO. Fig.3(b) shows the performance after Residual CFO Compensation. The results show

that the system performance is improved after Residual CFO Compensation.

Acknowledgments. This work was supported by Institute for Information &

communications Technology Promotion (IITP) grant funded by the Korea

government (MSIP) (No. 2016-0-00106, Development of the RF-signal over IP

Technology for the Smart Media Services based on Optical IP Network).

References

1. “Data-over-cable service interface specifications DOCSIS 3.1-physical layer specification,”

Cable Televis. Lab. (CableLabs), Louisville, CO, USA, Tech. Rep. CM-SP-PHYv3.1-I09-

160602, Jun. 2016.

2. M. Li and W. Zhang, “A novel method of carrier frequency offset estimation for OFDM

systems,” IEEE Trans. Consumer Electron.,vol. 49, no. 4, pp. 965-972, Nov. 2003.

3. D. C. Chang, “Effect and compensation of symbol timing offset in OFDM systems with

channel interpolation,” IEEE Trans. Broadcasting, vol. 54, no. 4, pp. 761-770, Dec. 2008.

Advanced Science and Technology Letters Vol.146 (FGCN 2017)

Copyright © 2017 SERSC 63