August 11, 2020

TD-LTE transmitter system design analysis


With the acceleration of data service requirements, 4G technology will replace 3G as the mainstream wireless communication technology in the next few years, and TD-LTE with China's independent intellectual property rights will become an important part of it. This article will introduce the current commonly used transmitter architecture , And analyze the system indicators of the TD-LTE transmitter, and combined with TI's chip solution, comprehensively introduce the system solutions that support TD-LTE.

Terminology: DPD (Digital Predistortion)


TD-LTE with China's independent intellectual property rights due to its high spectrum utilization rate (downlink: 5bit / S / Hz; uplink: 2.5bit / S / Hz); high rate (downlink: 100Mbps; uplink: 50Mbps); low latency ( 100ms control plane, 10ms user plane), and flexible spectrum usage (variable bandwidth, 1.4MHz; 3MHz; 5MHz, 10MHz, 15MHz, 20MHz) are increasingly favored by various operators, by February 2011, by China Mobile led the establishment of the TD-LTE Global Development Initiative (GTI) jointly with 7 operators. It has grown to 48 operator members and 27 vendor partners; currently 38 operators are planning to deploy or are conducting trials.

This article will analyze the challenges of TD-LTE transmitter systems with different architectures (including transmission and feedback), and finally decompose the system indicators based on the TD-LTE air interface indicator requirements, providing ideas and basis for TD-LTE transmitter design At the same time, combined with TI's program, the indicator requirements of key components of the transmitter link are analyzed.

1. TD-LTE transmitter architecture overview

In order to gain a deeper understanding of the system specifications of the base station transmitter system, this section will first introduce several architectures commonly used in base station systems and their respective advantages and disadvantages, so as to select different transmitter architectures according to different requirements.

In general, for TD-LTE, the biggest challenge is bandwidth. China currently has 190MHz (2500-2690) continuous bandwidth allocated to TD-LTE, and there are many multi-band requirements (such as F + A; 1880-1920MHz, 2010-2015MHz) frequency band, etc. have very high requirements on the transmitter, especially the feedback channel; strict out-of-band spur requirements, especially the F-band, is also a very big challenge for the transmitter system In addition, the requirement of high EVM and low noise floor is also a challenge to system design.

1.1 Zero IF transmission, zero IF feedback

Figure 1 shows the architecture of zero-IF transmission and zero-IF feedback. The input frequency of the modulator is zero, and the output frequency of the demodulator is zero. This is called zero-IF transmission and feedback.

This architecture has very great technical advantages:

1) The same local oscillator signal can be used for transmission and feedback, saving and simplifying system design and cost;

2) Reduced the sampling rate requirements of ADC and DAC, especially for the broadband TD-LTE system, because the ADC has export control, after adopting this architecture, the sampling rate requirement for ADC is reduced by half compared to the real intermediate frequency architecture;

3) Since the input and output frequency points of the modulator and demodulation are zero, the system does not have various spurious signals related to the intermediate frequency, which greatly reduces the need for various filters;

4) For ADC and DAC, the input / output frequency is low, and their performance is greatly improved, which is conducive to system design;

5) After adopting zero intermediate frequency, the flatness of the DPD signal can be ensured to be high, which is beneficial to DPD processing.

At present, more and more base station transmitter systems have begun to adopt this architecture, but the architecture also has its weaknesses:

1) Due to the zero-IF architecture, the local oscillator leakage and sideband signals are in the signal band. There is no way to suppress the local oscillator signal and the sideband through the filter. The algorithm is used to calibrate, so the algorithm requirements are compared. high;

2) The low-order harmonics (second and third) of the DAC will be in the band, and there is no way to suppress them through the filter, so it is necessary for the DAC itself to have better second and third harmonic performance, and the algorithm is also required for low The calibration of subharmonics will lead to complicated algorithms;

3) Since both the transmission channel and the feedback channel adopt a zero-IF architecture, when performing local oscillator leakage and sideband suppression, it is necessary to distinguish the local oscillator leakage signals of the transmission channel and the feedback channel, so a phase shifter needs to be added in the feedback channel To distinguish between the local leakage signal of the transmitted and the feedback signal.

Considering that the current algorithm has not been able to calibrate LO leakage, sideband signals, and various low-order harmonic signals very well, this kind of architecture has not yet been adopted for the MCGSM system.

1.2 Zero IF transmission, real IF feedback

Unlike the architecture of Figure 1, when the feedback channel uses a mixer instead of a demodulator, it is called real IF feedback.

This scheme is currently the most common architecture. Compared with architecture 1, the biggest difference is in the feedback channel, so its biggest advantages:

1) Since only the transmission channel uses zero intermediate frequency, the system's local oscillator leakage and sideband calibration are relatively easy to calculate;

2) For the feedback channel, due to the digital high-IF method, its local oscillator leakage and image signal can be filtered by a simple filter without any calibration;

3) Due to the real intermediate frequency scheme, only one feedback ADC is needed;

4) The sampling rate of the DAC is reduced, and the design of the anti-aliasing filter between the DAC and the modulator is simplified (only a low-pass filter is required).

The disadvantages of this architecture are:

1) The ADC sampling rate requirements for the feedback channel are very high, especially for the TD-LTE 190MHz bandwidth requirement;

2) The filter of the feedback channel will introduce DPD signal unevenness;

3) Two local oscillator signals are required. Most base stations currently use this architecture.

1.3 Complex IF transmission, complex IF feedback

Compared with the scheme in Figure 1, the difference is that the output signal of the DAC is a high intermediate frequency signal, and the input signal of the ADC is a high intermediate frequency signal. Its advantages are as follows:

1) The same local oscillator signal can be used for transmission and feedback, which simplifies and saves system design and cost;

2) LO leakage and sideband suppression can be suppressed by filters, which greatly reduces the algorithm requirements;

3) The low-order harmonic signals are out of band and can be suppressed by filters, which reduces the algorithm requirements;

4) Due to the complex IF architecture, the adoption rate of the feedback channel ADC needs to be reduced by half, especially for broadband TD-LTE systems.

Its disadvantages:

1) Since the signals of DAC and ADC are high-frequency signals, their performance is greatly affected;

2) The linear index of the demodulator will decrease;

3) The flatness performance of the transmission and feedback channels will be relatively poor, which will have a greater impact on the performance of the DPD. Since there is no need for DC and sideband calibration, and it can easily support broadband signals, it is relatively large on TD-LTE potential.

2. TD-LTE transmitter index analysis

This section will introduce the TD-LTE air interface index requirements and the corresponding system index allocation. According to the 3GPP TS 36.104 requirements, the TDLTE transmit air interface index requirements include: base station output power, base station output power dynamic range; transmitter on / off power; transmission The signal quality of the machine; the unwanted emission; the intermodulation of emission and others. The following will analyze and decompose the important indicators.

2.1 Base station output power

The base station output power refers to the average power per carrier of the antenna port. This index requirement is usually put forward by the operator. At present, TD-LTE usually requires a maximum power output of 8 antennas of 20W (43dBm). 3GPP requires its accuracy to be +/- 2dB, the accuracy is guaranteed to be +/- 2.5dB under extreme conditions, but in order to simplify the power amplifier design and good thermal design, the power fluctuation of the link is usually required to be +/- 1dB, in broadband signals, especially China TD-LTE 190MHz bandwidth, how to ensure that the power fluctuation of the link is within +/- 1dB.

When selecting a link device, it is necessary to ensure that the power fluctuation in the 190MHz band is as small as possible. Generally, the power fluctuation allocated to the DAC and modulation is within +/- 0.5dB. The current TI DAC34H8X + TRF3705 can guarantee +/- 0.5dB. In-band fluctuation requirements.

In addition to ensuring better fluctuations in the selection of link devices, closed-loop power calibration is also necessary. The following table is an example of requirements for closed-loop calibration.

2.2 Transmitter on / off power

The transmitter switch is specifically an indicator requirement for TD-LTE. Since TD-LTE uses the same frequency reception / transmission, after the transmitter is turned off, its noise floor is required to control the impact of the receiver to a certain range. 3GPP requires it to After shutdown, the power is lower than -85dBm / MHz, after normalization: -85dBm / MHz =-(85 + 10 * lg10 ^ 6) =-145dBm / Hz, considering the gain, it is allocated to the noise floor of the DAC modulator Make sure that it is around -155dBm / Hz.

2.3 Transmitter signal quality

The signal quality of the transmitter includes: frequency error; EVM (vector amplitude error); delay synchronization between multiple antennas.

2.3.1 Frequency error

3GPP requires that the frequency deviation of the modulated carrier frequency within a subframe period (1ms) should not exceed +/- 0.05ppm. In the TD-LTE transmitter system, since the baseband system is fully synchronized, there is no long-term frequency error, so Its frequency error mainly comes from the contribution of clock and frequency synthesis.

2.3.2 EVM (Vector Magnitude Error)

The requirements of 3GPP for various modulation signals are shown in the following table. Usually, designers need to reserve a margin of 2-3%.

The contribution of EVM to the entire two-way transmission mainly comes from several parts: digital processing and link part.

1: CFR (peak cut processing), in order to improve the efficiency of PA, peak cut is currently the most important technology combined with DPD, it has a very important impact on the EVM of the system, different peak cut technology performance is different, usually hard peak cut It is relatively simple, but the impact on the system, especially the EVM, will be relatively large. The current commonly used method is peak cancellation, although it requires more resources, but it is optimal for the performance of the entire system. The specific impact of CFR on EVM, but need to be clear that CFR has a very important impact on EVM.

2: Link part

As shown in the following general diagram of the transmission link, we can list 7 factors that affect EVM. Without detailed derivation, this article directly applies the conclusion (2).

A: I / Q path is unbalanced, set the gain difference to δ dB

in case

Then you can get the following relationship between EVM and gain error

B: Influence of the phase shift error of the local oscillator of the quadrature modulator on β on EVM

C: The leakage of the local oscillator £ Impact on the EVM

D: LO phase noise? Effect on EVM

Let G? Be the power spectral density of the phase noise, then

In addition to the above main effects, the distortion of the amplitude and frequency characteristics of the channel filter, the nonlinear phase distortion of the channel filter, and the nonlinear distortion of the link will affect the EVM of the transmitter. Equation (5) gives the calculation formula of the total EVM of the link

2.3.3 Multi-antenna synchronization delay error

TD-LTE In addition to the phase requirements between multiple antennas required by smart antennas, 3GPP also requires that the phase error between multiple antennas should not be greater than 65ns, and the delay allocated to the transmitter signal link is usually not greater than 30ns. The link usually requires a common local oscillator for the modulator and a common clock for the DAC, and because the FIFO is integrated in the DAC, the DAC is usually required to be synchronized. At the same time, if the phase delay between multiple antennas is fixed, compensation is usually required. There is uncertainty in the phase delay between, and the uncertainty needs to be kept as small as possible.

2.4 Un-wanted emissions (Stray radiation requirements)

Un-wanted emissions includes two parts: out-of-band radiation and spurious radiation. Out-of-band radiation refers to the radiation requirements within the range of the transmitter's operating bandwidth from the lower limit frequency point to within a range of 10MHz, and the emission requirements within the range of 10MHz above the upper limit frequency , With UMTS as the column, the transmitter's operating bandwidth is 2110-2170MH, so out-of-band spurs refer to the spur requirements in the range of 2100-2110MHz and 2170-2180MHz; except for signals in the frequency range required for out-of-band radiation, other The regional spurs are attributed to the requirements of the spurious radiation index.

2.4.1 Operating band unwanted emissions

Operating band unwanted emissions is a type of Un-wanted emission. The near-end contribution mainly comes from the residual nonlinear products of the power amplifier after DPD and the phase noise of the local oscillator signal, while the far-end contribution mainly comes from the transmit link The noise floor of the power amplifier, as well as the residual nonlinear products of the power amplifier after the DPD, the spurs of the transmission link falling in the band also have a relatively large impact on the far-end radiation.

As shown above, it is the in-band spurious template. The blue color is the FCC requirement. When offset by 1MHz, it needs to meet 22dBm / MHz, while China Mobile only needs to meet the requirements of 3GPP cat A, and when offset by 1MHz, it needs to meet -11dBm / MHz requirement, but in order to support beam forming, it needs to increase the requirement of 9dB, that is, when offset by 1MHz, it needs to meet the -20dBm / MHz requirement, set the amplifier output power to 20W (43dBm), for 20MHz TD-LTE In terms of signals, in order to meet 3GPP requirements.

0-1MHz: 43dBm-10 * log (18MHz)-(-11dBm / MHz) = 40.5dBc

1MHz-10MHz: 43dBm-10 * log (18MHz)-(-13dBm / MHz) = 42.5dBc

10MHz: 43dBm-10 * log (18MHz)-(--15dBm / MHz) = 44.5dBc

According to the above results, it can be assigned to each level such as DUC, CFR, modulator, power amplifier, etc., to ensure that the cascade result cannot be lower than the above value.

2.4.2 ACLR (Adjacent Channel Leakage Power Suppression Ratio)

ACLR refers to the ratio of the average power of the main signal and the power of the lead signal. Generally, 3GPP cat A requires that it can take a relative ratio of -45dB or -13dBm / MHz as the absolute value of the lead power. It is lighter; cat B can be lighter in relative 45dB or absolute -15dBm / MHz. Compared with the in-band spur requirements, ACPR requirements are lower. It is usually used to evaluate the nonlinear indicators of the transmitter system. In the system, the PA driver, the nonlinearity of the modulation, the phase noise of the local oscillator signal, and the nonlinearity index of the DAC all affect the ACLR of the system.

ACLR is a system indicator, corresponding to PA driver, the non-linear indicator of the modulator is OIP3, and the rough conversion can be expressed as:

1 Carrier ACPR = IIM3I-3dB

2 Carrier ACPR = IIM3I-9dB

4 Carrier ACPR = IIM3I-12dB

Therefore, OIP3 can be calculated according to IM3, and the OIP3 requirements for each level of device can be calculated according to the cascade formula of OIP3.

2.4.3 Transmitter spurious emissions

Transmitted spur refers to the 9KHz to 12.5GHz frequency band, except for the spur requirements in all frequency ranges except the in-band spurs. This is a mandatory requirement. The general spur indicators are summarized as follows. Scattered requirements, coexistence spurious indicators, co-site requirements such as co-site addresses, co-existence, co-site sites do not list all frequency ranges, but only list some representative parts, the specific design needs to compare with all listed in 3GPP Spurious index requirements for the frequency range.

Especially for the signal interference of the receiver, you need to ensure that -96dBm-10 * log (100KHz / Hz) = -146dBm / Hz, which is not suppressed by the filter, you need to ensure that the output power is extremely low after the transmitter is cut off; For the -98dBm spurious requirement of the common site, it is necessary to suppress the corresponding common site frequency when designing the duplexer.

At the same time, it is necessary to consider that various possible intermodulation products fall within the corresponding frequency range (there are many tools for calculating intermodulation), and it is necessary to consider various possible signals, such as clock frequency and harmonics, PLL frequency and harmonics, and Intermodulation products between them.

2.4.4 Transmit intermodulation

The transmit intermodulation requirement is used to test the ability of the transmitter to suppress its own nonlinearity. The interference signal tested is a 5MHz E-UTRA signal with an average power of 43dBm-30dB = 13dBm, and its position is located +/- 12.5MHz away from the main signal. , +/- 7.5MH At +/- 2.5MHz, the intermodulation products must meet the spurious emission template shown in Figure 6.

3. TI TD-LTE launch key device requirements

The second section analyzes the specifications of the transmitter in detail. In this section, the requirements of key devices (DAC + modulator, clock, frequency synthesis) will be introduced based on its specifications and the TI solution.

3.1 DAC + modulator

High bandwidth: supports 190MHz BW, requires a DPD bandwidth of at least 600MHz, and a DAC data rate of 750MHz.

(750 * 0.8 = 600MHz) DAC sampling rate 750 * 2 = 1.5Gpbs, DAC34SH84 sampling rate 1.5GHz, supports a maximum signal bandwidth of 600MHz.

Low noise: Considering the requirements of the far-end noise mask, leaving 13dB of margin, the DAC + modulation noise requirements should be lower than: -22dBm / MHz-13dB-(-43dBm-(-14dBm))-10lg1MHz = -152dBm / Hz, DAC34SH84 noise The noise floor is -156dBm / Hz, and the TRF3705 noise floor is -157dBm / Hz, so the total noise is approximately -153dBm / Hz.

Flatness: Figure 8 shows the flatness of DAC34SH84 + TRF3705, combined with frequency calibration, it can meet the flatness requirements.

Figure 8 DAC34SH84 + TRF3705 flatness

SFDR: Consider a single carrier signal with a bandwidth of 20MHz. For the in-band spurious index requirements, assuming that 10% of the contribution comes from DAC34SH84 + TRF3705, SFDR needs to be better than: [43dBm-10lg (18MHz)]-[(-22dBm)-(10lg1MHz )]-10lg0.1 = 63dB

Figure 9 DAC34SH84 + TRF3705 SFDR

to sum up

This article details the system requirements and decomposition of the transmitter, the system requirements of various indicators, and the selection of key components. Combined with TI's solution DAC34SH84 + TRF3705, it provides a set of ideas for the analysis of the transmitter system solution for TD-LTE And the basis for device selection.

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