Performance analysis of analog IF over fiber fronthaul link with 4G and 5G coexistence
Summary (2 min read)
Introduction
- In C-RAN architecture, the functionalities of the traditional base station are split between the baseband unit (BBU) and remote radio head (RRH) unit.
- Transmitting analog-domain radio signals at intermediate frequencies (IF) over fiber provides more flexibility and uses lower bandwidth optical components compared to radio/MMW frequency signal over fiber technique required for 5G signals fronthauling [5].
- Generalized frequency division multiplexing (GFDM) and universally filtered orthogonal frequency division multiplexing (UF-OFDM) have emerged as potential candidate waveforms for 5G mobile communication [9].
A. Orthogonal Frequency Division Multiplexing
- OFDM achieves high speed data transmission by dividing the serial data into multiple low speed parallel data channels modulated on different frequencies resulting in longer symbol periods.
- FFT operation on the symbols can simultaneously modulate and multiplex data as shown in Fig. 2(a).
- IFFT basically multiplies the incoming parallel QAM data with orthogonal frequency sinusoids and then adds to get one OFDM symbol [9].
- After the IFFT operation, NCP samples from the end of symbol are appended at the beginning, generating a cyclic prefix (CP) in order to avoid ISI due to fading [9].
- Moreover, OFDM suffers from a large amount of OOB emission, as evident from sinc pulse shape of subcarrier in the frequency domain, thus requiring large guard bands between multiple OFDM signals.
B. Generalized Frequency Division Multiplexing
- GFDM applies circular pulse filtering on each subcarrier to minimize the overlapping with adjacent subcarriers.
- The data of each stream is up-sampled by a factor of N , which essentially multiplies data with an impulse δ[n−mK] and introduces a time shift of mK samples, which moves each data symbol to the correct sub-symbol position in the GFDM block [17].
- Circular convolution is done with the transmitter prototype filter to remove the replicas generated as a result of upsampling.
- Equalization is performed to compensate for channel distortions, after downsampling, similar to that in OFDM.
- More details about GFDM and its relation with OFDM can be found in [9].
C. Universally Filtered Orthogonal Frequency Division Multiplexing
- UF-OFDM performs linear filtering on sub-bands of subcarriers, thus reducing the filter length as shown in Fig. 2(c) [9].
- Input QAM symbols are mapped to the allocated sub-bands of subcarriers while setting the subcarriers at the position of the remaining sub-bands zero.
- The UF-OFDM transmit signal is formed after superposition of the bandpass filtered sub-band signals.
- A single-tap equalization is done on the odd output bins, neglecting the even outputs of the FFT block, given perfect knowledge of the channel response [9].
- Details of waveform parameters used and corresponding spectra are discussed in section III.
C. Signal Generation
- Multiple bands of LTE signals are generated in Matlab, with 1200 subcarriers out of 2048 modulated using 16 QAM data and remaining subcarriers packed with zero.
- The effect of sub-band linear filtering used with UF-OFDM (Fig. 5(c)) is evident since the OOB emission is highly reduced in this case compared to both GFDM (Fig. 5(b)) and OFDM (Fig. 5(a)).
- The intermediate carrier frequencies for these bands are chosen to be located in the linear modulation region of the laser, which is below 2.25 GHz for 50 mA bias current.
- At the receiver, the signals are resampled and a 12th order Gaussian filter is used to extract each band for processing.
IV. RESULTS AND DISCUSSION
- The authors first evaluate the performance of multi-band LTE, UFOFDM and GFDM waveforms through the AIFoF link, when transmitted independently.
- The authors further compare this performance in the scenario where the multiple waveforms co-exist.
A. AIFoF for LTE
- RF carrier frequencies for LTE signal range from 700 MHz to 3600 MHz, so signals can be transmitted through the fronthaul network without IF conversion, with appropriate choice of lasers and detectors.
- Typically, the chosen guard-band in multi-bands transmission systems are selected to minimize the channel interference.
- Maximum received optical power is limited to -13 dBm in this case and lower EVMs could be obtained with the higher received power.
- In order to transmit the LTE signals with higher IF carrier frequencies, multiple bands were added on either side of the band centered at 2.5 GHz frequency, and transmitted through the fronthaul link.
- It is evident from the green curve that the performance of higher frequency bands significantly degrade.
B. AIFoF for 5G
- Wireless carrier frequency bands ranging from 6 GHz to 100 GHz are proposed for 5G [22].
- As in the case of LTE, the authors now transmit multiple bands of these 5G waveforms through the fronthaul link and evaluate their performance.
- Fig. 10 shows the EVM of the three waveforms in this multi-band scenario (with ten bands), as a function of the carrier frequency (similar to Fig. 8 for LTE), at two received optical power levels of -13 dBm and -17 dBm.
- It can be observed that GFDM performs worse than both UF-OFDM and OFDM due to the non-orthogonality of the GFDM subcarriers.
- Aspects related to synchronization of these waveforms are thoroughly investigated in [16] and [24].
C. AIFoF for coexistence of LTE & UF-OFDM
- The authors now demonstrate the co-existence of LTE and UFOFDM waveforms.
- Signals can be freely assigned independent of the bandwidth by maintaining sufficient guard bands with both neighboring bands.
- Effect on the performance of LTE in coexistence with UF-OFDM for transmission through same fronthaul link is demonstrated.
- Performance of these links can be further improved by using higher modulation bandwidth lasers or external modulation and higher saturation power APD’s.
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Citations
65 citations
Cites background from "Performance analysis of analog IF o..."
...tem instead of high-cost and low-spectral efficiency optoelectronics needed for A-RoF at mmWave frequencies [19]....
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Cites background from "Performance analysis of analog IF o..."
...Table I summarizes the state-of-the-art solutions based on RoF, FiWi and FSO for high transmission capacity links [13]– [21]....
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...[13] demonstrated the transmission of different 4G and 5G services coexisting on the same RoF link in the 700 MHz to 3....
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33 citations
Cites background from "Performance analysis of analog IF o..."
...an analog mobile fronthaul system was presented in [34] for distributing LTE and 5G services concurrently over the same fiber link....
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29 citations
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References
882 citations
"Performance analysis of analog IF o..." refers background in this paper
...With the increasing demand for machine-to-machine communication, mobile networks must scale appropriately in order to provide higher speeds with increased flexibility to a variety of users [1]....
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809 citations
"Performance analysis of analog IF o..." refers background or methods in this paper
...It is known that GFDM suffers from some performance penalty compared with OFDM, which is due to the non-orthogonality of its subcarriers [17,18]....
[...]
...The data of each stream is upsampled by a factor ofN, which essentiallymultiplies datawithan impulse δ n − mK and introduces a time shift of mK samples, which moves each data symbol to the correct sub-symbol position in the GFDM block [17]....
[...]
...A block of input QAM symbols is split into N parallel sub-streams to fill K subcarrier frequencies andM time sub-symbols [17]....
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510 citations
"Performance analysis of analog IF o..." refers methods in this paper
...Analog-domain radio frequency signal over fiber (ARFoF) schemes increase the bandwidth efficiency and reduce the latency by avoiding the use of expensive A/D and D/A converters at the RRH for the digitization process, as required by CPRI [3]....
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...The digitized baseband signal over fiber (DBBoF) based common public radio interface (CPRI) mobile fronthaul scheme requires a 32 Gb/s optical line rate for serving a four-sector 8 × 8 MIMO antenna site for a 20 MHz bandwidth wireless channel [4]....
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...CPRI fronthauling architecture becomes more inefficient and complex for providing multiple services through the same fiber....
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497 citations
Additional excerpts
...spectral containment compared to OFDM [19]....
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245 citations
"Performance analysis of analog IF o..." refers background or methods in this paper
...A fair comparison of several 5G candidate multicarrier waveforms has been conducted in [9,16,23,24]....
[...]
...After the IFFToperation,NCP samples from the end of the symbol are appended at the beginning, generating a cyclic prefix (CP) in order to avoid ISI due to fading [9]....
[...]
...Generalized frequency division multiplexing (GFDM) and universally filtered orthogonal frequency division multiplexing (UFOFDM) have emerged as potential candidate waveforms for 5G mobile communication [9]....
[...]
...IFFT basically multiplies the incoming parallel quadrature amplitude modulation (QAM) data with orthogonal frequency sinusoids and then adds to get one OFDM symbol [9]....
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...asing before feeding the signal to the FFT block [9]....
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Frequently Asked Questions (12)
Q2. What is the frequency response of the avalanche photodiode?
A variable optical attenuator (VOA) is used to control the input optical power to the avalanche photodiode (APD - bandwidth of 10 GHz) which exhibits saturation close to -10 dBm.
Q3. How many bands of LTE and 5G are transmitted separately?
Seventy-five and ten bands of 20 MHz LTE and 201 MHz 5G candidate waveforms are transmitted separately over 25 km fronthaul network, respectively, showing the potential of link for providing services to multiple RRH sites.
Q4. How is the EVM observed for a single 20 MHz LTE signal?
For the transmission of a single 20 MHz LTE signal centered at 1.5 GHz of IF carrier, the EVM is observed to be well below the forward error correction (FEC) limit (12.5%) for 16 QAM data modulation.
Q5. What is the performance of the eight band transmission link?
This eight band transmission link offers an optical power budget of 18 dB considering 5 dB loss in the 25 km fiber and a transmitted power (Ptx) of ∼ 1 dBm.
Q6. What is the performance of LTE in coexistence scenario?
The performance of LTE degrades in this case as 7% EVM is observed at Prx = -13 dBm, which is also observed in the case of the transmissions of 75 LTE bands as shown in Fig.
Q7. What is the effect of GFDM on the frequency of the subcarriers?
Input QAM symbols are mapped to the allocated sub-bands of subcarriers while setting the subcarriers at the position of the remaining sub-bands zero.
Q8. What is the EVM performance for the 20 and 75 bands?
The EVM performance for the 20 and 75 bands as a function of carrier frequency at the received optical power of -13 dBm is shown in blue and red, respectively, in Fig.
Q9. What is the reason why GFDM has a larger EVM penalty than UF-?
For the same operatingconditions, GFDM exhibits a larger EVM penalty compared to UF-OFDM and OFDM for the same power and this is attributed to noise enhancement caused by the non-orthogonality of the GFDM subcarriers, as outlined in section II.B.
Q10. What is the EVM of the three waveforms in this multi-band scenario?
Fig. 10 shows the EVM of the three waveforms in this multi-band scenario (with ten bands), as a function of the carrier frequency (similar to Fig. 8 for LTE), at two received optical power levels of -13 dBm and -17 dBm.
Q11. What are the differences between the FDM and UFMC waveforms?
The performance differences between the FDM waveforms are only due to those arising in the electrical domain since the penalty introduced by the optical AIFoF fronthauling link is agnostic to these waveforms at the bandwidths considered.
Q12. What is the reason why GFDM suffers from performance penalty?
It is known that GFDM suffers from some performance penalty compared with OFDM which is due to the non-orthogonality of its subcarriers [17], [18].