In the 3G era, three to five different frequency bands provided worldwide coverage for mobile phones. However, as spectrum worldwide is reclaimed from legacy analogue TV, government and military users, 4G services are being deployed across a wide frequency range from 450 MHz to 3.6 GHz, but in an increasingly fragmented and country-specific form.
This frequency band fragmentation (4G is already deployed in over 20 frequency bands worldwide) creates some significant technical challenges for handset manufacturers and network operators.
One of the key challenges introduced with the greater band fragmentation is a higher risk of interference. The fragmented nature of LTE spectrum allocations has meant that several LTE frequency bands are located in highly congested parts of the spectrum.
Certain bands have ended up in very close proximity to previously allocated bands, which has led to high interference risks. Operators with high-risk bands have to meet stringent regulatory performance metrics for interference – usually expressed in terms of Adjacent Channel Leakage Ratio (ACLR) – which may be up to 10x more stringent than the normal 3G/4G interference requirements. These requirements are currently achieved by applying Maximum Power Reduction (MPR) techniques to limit the RF output power from the handset, which results in decreased handset performance, call dropping, and ultimately poor quality user experiences.
High interference risks
There are several examples where these high interference risk situations occur. At higher frequencies there is a lot of spectrum congestion, particularly in the 2.3-2.7GHz range, with LTE bands 7,40 and 41 being very close to the ISM band used for WiFi and Bluetooth, and bands 7 and 38 adjacent to each other.
At lower frequencies a prime example is in the US 700 MHz network, where Verizon’s band 13 is directly adjacent to the nationwide public safety broadband network in band 14, which has been licensed with a view to creating an interoperable wireless circuit for first responders such as firefighters, police and paramedics. The close proximity of the two bands, with only a few MHz between them, has meant that interference risks are very high.
In all of these examples interference has the potential to severely degrade the quality of communications, or even to cause dropped calls. In the case of bands 13 and 14 this means there is also the potential for a serious threat to public safety as a result of interference risks.
Handsets and networks operating in these high-risk LTE bands are therefore required to meet stringent additional regulatory performance requirements for ACLR. The 3GPP specifications support network signalling messages (NS) which inform the handsets that additional interference mitigation is required, and handset vendors must also test their handsets against local specifications from regulatory organisations such as FCC and Ofcom.
Current solutions degrade performance
Currently the only practical way for handset manufacturers to mitigate the potential interference and achieve necessary ACLR levels in the handset is to apply MPR. This involves ‘backing off’ the transmitter by constraining the maximum output power from the handset power amplifier (PA) to less than the target 23dBm maximum output power level the standard demands.
For example, the NS_07 signalling used in the Band 13/Band 14 interference case allows the PA to be backed off by up to 12 dB, in addition to the 1-2 dB of MPR permitted for high bandwidth transmissions. This could result in a handset transmitting less than a tenth of the permitted power, resulting in LTE coverage significantly worse than a WiFi signal.
While MPR enables handsets to meet industry interference requirements, it is often at the expense of handset performance. Limiting output power in the handset diminishes the efficiency of the handsets, reducing battery life. It also results in decreased network coverage as reducing output power results in the network being uplink limited, whilst still posing a slight interference risk.
With traditional RF technology the only way of counteracting the problems introduced by MPR is for operators to deploy a larger number of basestations. Clearly this is not an ideal solution.
Envelope Tracking vs MPR
Envelope Tracking (ET) is a technology that has the potential to provide a solution to these network implementation issues by enabling handsets to meet and exceed industry interference specifications without the need for MPR.
ET is a power supply technique that replaces the fixed DC supply voltage to the RF PA with a very high bandwidth dynamic supply voltage, which closely tracks the amplitude, or “envelope” of the transmitted RF signal.
Although primarily intended as a technique to improve the efficiency of the RF PA in handsets, ET also improves the RF performance of the transmitter. The nature of ET systems means that the linearity of the PA is controlled digitally, enabling more flexibility in the trade offs between output power, efficiency and linearity. As a result of this greater control, ET-enabled PAs can deliver more output power, higher data rates and fully EVM/ACLR compliant waveforms at full output power. This eliminates the need for MPR with no risk of causing interference and no degradation in handset RF performance.
By eliminating the requirement for MPR, ET enables handset manufacturers and operators to increase coverage, data rates, and network capacity – delivering far better RF performance to users without the need for extra basestations.
Jeremy Hendy is VP Sales & Marketing, Nujira.