Femto base station and its radio frequency solution

The purpose of the third generation of mobile communications defined by 3GPP is to provide customers with a full range of mobile multimedia experiences. However, this goal has not been achieved under many conditions, especially in remote areas or densely populated areas.

A feasible solution is to deploy home base stations that can provide the maximum mobile data rate without the need for macro node-B base stations within the home. According to their power, they are called "femto" base stations.

• +45dBm: A macro base station that can cover approximately 5km outdoors
• +30dBm: micro-cell base station capable of covering approximately 0.5km
• +15dBm: A femto base station that can cover an indoor range of approximately 50 meters

Femto home base stations connect to the entire network through the public switched telephone network, which can usually use DSL to the home. The mobile phone communicates only with the femto base station in the home, completely disconnecting the user from the macro cell.

In the following paragraphs, we will review the basis for deploying femto home base stations and introduce many key advantages of Maxim's RF solutions.

General considerations for improving cellular coverage

The design of mobile communication systems must make a certain trade-off between the working time of mobile phone batteries, the computing power of mobile phones, and the user load of base stations. In addition, mobile communication standards are also changing with the continuous improvement of mobile terminal DSP computing performance. Mobile operators keep up with this trend and continue to provide more and better multimedia services in order to attract more users and increase revenue. Users will eventually gain "ubiquitous" mobile access capabilities. For example, the new supply chain established under the interest of 100 million new mobile phone customers each year is highly competitive and helps to maintain a very high cost performance.

Broadband Wireless and Cellular Communications

Broadband wireless networks also developed in parallel with cellular mobile communications. At first, some coffee shops provided 802.11 Internet access Wi-Fi services to customers who parked or stayed nearby. Later, some cities established experimental metropolitan Wi-Fi networks to provide broadband wireless access to the city core. The success of this model led to the emergence of another better broadband wireless system in 2004, namely 802.16 WiMAX (called WiBro in South Korea), which hopes to provide "metropolitan broadband wireless access" services.

By using OFDM instead of single carriers, these broadband systems can overcome multipath interference in cities, but require 4 times the transmission power. This is because the peak-to-average ratio (pk-avg) of 802.16d using OFDM is about 11dB, while the pk-avg of WCDMA 3GPP is only 5dB. Intel® supports WiMAX as a wireless connection method for ultra-mobile PCs (UMPCs). Ultra-mobile PCs are roughly the size of 6 mobile phones, so they can use WiMAX services, but for ultra-thin/flip phones, WiMAX requires too large a battery.

Transmission tower construction

The construction and land occupation of towers is an issue for both cellular and WiMAX. Land acquisition costs and municipal planning restrictions on "ugly" base stations mean that the pace of building new towers is the same for both systems. However, cellular operators started building towers 20 years before WiMAX appeared, so they have a head start in terms of tower coverage and new tower construction.

Where broadband wired access is suitable

Broadband wired access services compete directly with fixed access WiMAX, and typically include fiber-to-the-home (PON), telephone line DSL, and cable TV Internet access systems. In "first world" countries, these services benefit from a complex infrastructure that was previously established. PON is a new technology, but because the regional telecommunications operators (LECs) have already invested heavily in infrastructure construction, fiber systems are only considered when the old copper cable systems need to be replaced.

These access services are necessary for the deployment of femto base stations, but they are available free of charge, which we will discuss more in the following sections.

Implementation of Femto Base Station

Figure 1 compares the traditional cellular mobile communication system accessed through a node-B macro base station with a femto base station.


Figure 1. Traditional node-B macro cellular mobile access and femto base station connection

The left side of Figure 1 shows the traditional direct connection from the mobile phone to the base station. Here we use a wooden house as an illustration, which has relatively low RF signal loss and is assumed to be relatively close to the macro base station.

The right side of Figure 1 is a concrete high-rise apartment with a femto base station installed in the apartment. The femto base station accesses the network through wired DSL. As shown in the figure, the signal from the macro base station to the high-rise apartment is very weak. As a "private, dedicated base station", the femto base station does not communicate with the macro base station.

It should be pointed out that many baseband DSP suppliers now provide mature solutions that enable femto base stations to overcome interference from macro base stations and other femto base station signals, which is very important for femto base stations.

Femto base stations can overcome many shortcomings of cellular mobile communication systems

Capacity Limitations

Traditional macrocell base stations are usually capacity-limited when many users are trying to make calls. The system is designed based on statistical principles and uses a traffic queuing algorithm to handle a given number of calls in a given time. Therefore, long calls will reduce the total number of calls during busy call times.

In addition, when the base station serves mobile phones located near the border, the macro base station will have to increase the transmission power, which will affect the dynamic range of serving other customers. Therefore, mobile phone calls near the border will also reduce the total call capacity because they will compress the dynamic range of the base station transmission (see Figure 2 ).


Figure 2. The impact of call distance on macro base station transmission power

These two factors limit the user capacity of the macro base station, and therefore, also affect the operator's revenue and ultimately lead to a reduction in new users.

DSL-based home base station

According to statistics, about half of mobile phone calls come from indoors, and many users complain about poor signal reception in concrete apartments.

Figure 3 shows the path loss of two types of buildings. Since many 3G mobile phones require a receiving power higher than -110dBm to achieve reliable IP data services, it can be seen that in concrete buildings, the receiving sensitivity is so low that it cannot provide data services to customers.


Figure 3. Path loss of two types of buildings

At the same time, you can see that DSL has been adopted in Europe and is very mature in the United States. It is also widely used in the Asia-Pacific region including China. New DSL modems can be used with this service.

Many mobile phone users in the United States and Europe have cordless phones (POTS or IP phones), DSL or cable modem computer Internet access, TV, Wi-Fi, etc. at home. 3G mobile phone operators hope to launch the most competitive multimedia services to compete with these services.

Figure 4 shows the scenario where femto base stations coexist with home Wi-Fi to replace cordless phones.


Figure 4. Femto base stations coexist with home Wi-Fi access

Maxim's femto base station RF transceiver chipset

Maxim's femto base station reference design V8.0 can meet the TS25.104 requirements for WCDMA band class 1. The RF chipset includes two RF transceivers with Σ-Δ bit stream digital interfaces that can be directly connected to the baseband DSP/modem. The transceiver uses a reduced-amplitude LVDS interface, the filter is implemented in the baseband DSP/modem, and the RF section can be configured through software.

This is a complete reference design for the RF transceiver portion. The LNA integrated in the receiving chip supports the transmit monitor mode. Figure 5 is a photo of the reference design circuit board.


Figure 5. Maxim femto base station reference design

Can meet UTRA Band 1 femto base station standards (3GPP TS25.104)

The main requirements of Femto base stations are shown in Table 1. For WCDMA femto base stations, it is much more difficult to meet the requirements of TS25.104 for both transmit and receive dynamic range than to meet the requirements of TS25.101 for mobile phones. The gap is at least 10dB. However, due to the expected large production volume, the requirements for integration and BOM cost must be similar to those of mobile phones. Therefore, the ideal system is best designed with a low-cost, highly integrated mobile phone chipset, but maintains high performance in the receive/transmit frequency band opposite to that of the mobile phone (base station application).

Table 1. Main requirements of TS25.104

Uplink Requirements
Description	Specification	Condition
Frequency band	1920MHz to 1980MHz	Band 1
Rx sensitivity	-107dBm	12.2kbps data rate, BER shall not exceed 0.001%
Adjacent channel selectivity (ACS)	-101dBm	-38dBm, 5MHz offset WCDMA modulated interfering signal
Blocking (1900MHz to 2000MHz)	-101dBm	-30dBm (min), 10MHz offset WCDMA modulated interfering signal
Blocking (1MHz to 12,750MHz, except 1900MHz to 2000MHz)	-101dBm	-15dBm CW carrier
Intermodulation	-101dBm	-38dBm, 10Mhz offset CW signal and -38dBm, 20Mhz offset WCDMA signal
Downlink Requirements
Description	Specification	Condition
Frequency band	2110MHz to 2170MHz	Band 1
Maximum output power	Less than +24dBm	—
Adjacent channel leakage ratio (ACLR)	-45dB/-50dB	Offset frequency 5MHz/10MHz
Error vector magnitude	17.5%/12.5%	QPSK/16QAM, RMS

For example, in the receiving channel, the 5MHz in-band offset interference used to measure the ADC is required to be -38dBm, which is much more stringent than the -52dBm requirement for mobile phones. Similarly, the 10MHz in-band offset interference used to test channel selectivity is required to be -30dBm, while mobile phones only require -56dBm. Therefore, the receiver must have higher IIP2 and IIP3 performance.

The requirements for the linearity of femto base station transmitters are also more stringent than those for mobile phones. For WCDMA mobile phones, the ACPR gold requirement is -33dBm, but the TS25.104 femto base station requires -45dBm.

System performance testing and analysis

Receive sensitivity is an important system indicator, which is greatly affected by the signal quality of the RF channel and the DSP processing capability of the baseband modem. Under the condition of minimum input signal, the RF channel quality is affected by the receiver noise (ie NF).

Receive sensitivity can be calculated by the system NF. In the sensitivity calculation, a QPSK signal with a BER of 0.001% (baseband demodulator) requires an Eb _/ No _of 7.5dB. The sensitivity can be calculated as follows:

Reference sensitivity = KTB + NF + (Eb _/ No ₎ - PG

Where:
BW = 3.84MHz code rate
KTB = -174dBm/Hz + 10 log(3.84MHz) = -108.13dBm
PG = 12.2kbps bit rate relative to the extended band = 10log(3.84MHz/12.2kbps) = 25dB

If the NF is 10.5dB, the sensitivity is:
-108.13dBm + 10.5dB + 7.5dB - 25dB = -115.13dBm

The ACS and blocking performance can be calculated in the same way, measuring the system NF under the specified interference conditions. The poorer sensitivity here is because the interference adds noise.

The Tx performance is tested using the signal specified in TS25.141 Test Model 1 (TM1). The TM1 signal simulates a real communication scenario and may have a slightly higher peak-to-average ratio.

The maximum power is tested within the 3.84MHz WCDMA bandwidth and the ACLR is measured at a 5MHz offset. The transmit ACLR performance of this reference design depends on the external PA.

Femto Base Station Reference Design V8.0 UTRA Band 1 Performance Test Results

Table 2. V8.0 reference design performance

Uplink Requirements
Description	Specification	Maxim's Radio Performance
Frequency band	1920MHz to 1980MHz	Band 1
Rx sensitivity	-107dBm	Exceeds
ACS	-101dBm	Exceeds
Blocking (1900MHz to 2000MHz)	-101dBm	Passes
Blocking (1MHz to 12,750MHz, except 1900MHz to 2000MHz)	-101dBm	Passes
Intermodulation	-101dBm	Passes
Downlink Requirements
Description	Specification	Measured Performance
Frequency band	2110MHz to 2170MHz	Band 1
Maximum output power	Less than +24dBm	Passes
ACLR	-45dB/-50dB	Exceeds
Error vector magnitude	17.5%/12.5%	Exceeds

Figure 6. ACLR of TM1 64DPCH signal with output power of +17dBm


Figure 7. EVM of TM1 64DPCH signal with output power of +17dBm

in conclusion

Femto base stations will bring better data network experience to mobile phone users at home. In order to meet the stringent performance requirements of base stations at a cost close to that of mobile phones, circuit design will be more difficult. Maxim's femto base station chipset can meet the requirements of 3GPP TS25.104 key performance indicators and BOM cost requirements.