ELECTRONICS FEATURE
NO NEED TO COMPROMISE
Nujira's Jeremy Hendy explains why TD-LTE needs Envelope Tracking more than FD-LTE
E
nvelope Tracking (ET) is rapidly becoming a standard component in
the RF front end of FD-LTE handsets. Conversely, some have suggested that ET will not be required for handsets on TD- LTE networks. I disagree. In fact Nujira believes that ET is actually more integral to improving the battery life and RF performance of TD-LTE handsets than it is for FD-LTE. The main benefit of TD-LTE for mobile
handsets is that the transmit and receive bands are not operated simultaneously. As a result there is no chance for desensitisation of the receiver due to leakage of out-of-band noise from the PA, via the duplex filter, as experienced in FD-LTE. However, for handset OEMs and chipset vendors TD-LTE still presents some significant technical challenges. Currently, operators, including China
Mobile, are expected to deploy TD-LTE with an asymmetric timeslot allocation of 4:1 (download:upload) to reflect the typical user data traffic loading. This has the advantage of maximising overall network capacity, but limits the available uplink capacity and bandwidth to 20% of the channel. For example, a 20 MHz TD- LTE channel only provides the equivalent of 4 MHz in the uplink direction, shared between all users in the cell, providing a user experience that is more 3G than 4G. Both TD and FD-LTE networks allocate spectral resources using Resource Blocks (RBs) in both frequency and time. With only 20% of the TD-LTE timeslots available for transmissions from handsets, for a given throughput the network must allocate 5x more RBs to each user in the frequency domain (i.e. a 5x increase in bandwidth).
In LTE networks, the RF transmission
power used is directly proportional to the bandwidth of the transmission, i.e. the number of RBs allocated during the burst. This maintains the same signal- to-noise ratio at the base station by ensuring a constant energy-per-RB. The 5x increase in instantaneous bandwidth therefore results in a fivefold increase in instantaneous transmit power, a 7 dB increase in RF terms. This significantly increases the peak current consumed by the RF Front End, in particular the RF Power Amplifier. This corresponding increase in peak current consumption can be double that of FD-LTE. For handset OEMs and chipset vendors
this is a major engineering headache, but makes the case for ET in TD-LTE mode even more compelling. Our testing has shown that ET typically reduces the peak current consumption of the RF PA by 30%.
However, using ET with a TD-LTE system imposes several challenging requirements on the ET IC implementation. For instance, ET ICs for TD-LTE must be able to support rapid mode switching between ET and low power standby to minimise power consumption during the receive timeslots. More importantly the ET IC must also
be able to support high output power to maximise power savings. Since transmit power is almost always expressed on a logarithmic scale, the PA consumes much more energy at high RF power levels. Based on the statistical distribution across bandwidth and transmit power, we analysed the energy consumed by the RF PA from the battery as a function of
/ IRISHMANUFACTURING
Envelope Tracking is fast becoming a standard component in the RF front end of FD-LTE handsets
bandwidth, using measurements of LTE PA efficiency made at varying power levels. From this analysis it became clear that around 40% of the energy consumed by the PA is at low bandwidths (5 MHz / 25 RB or less), and around 60% of the energy is consumed at high bandwidths (15 MHz / 75 RB or more). With most energy consumed at high bandwidths, the most significant power savings from ET will only be achieved through a solution that can support very high bandwidths in ET mode. Lower performance ET solutions have to fall back to Average Power Tracking (APT) mode at high bandwidths, eliminating the performance advantage of ET and complicating the software development for chipset vendors. Despite being much simpler to develop,
5 MHz, or even 10 MHz, ET components will not deliver much benefit in a TD-LTE system. Chipset vendors and handset OEMs really need ET solutions that can support the maximum 20 MHz bandwidth in order to optimise the RF front end. With TD-LTE networks being deployed
in an asymmetric configuration for improved network efficiency, uplink capacity and throughput are getting squeezed in comparison to FD-LTE networks. ET is the key to ensuring that performance is not compromised in the uplink - in many ways making the benefits of ET even more relevant to TD- LTE than FD-LTE. By enabling the high bandwidth, high
power transmissions required by TD-LTE networks ET can also deliver a significant additional benefit. Even though the peak power consumption during TD-LTE transmission can be double that of FD- LTE, by modelling the overall handset power consumption we can see that the TD-LTE configuration is more energy efficient for a given throughput than an FD-LTE implementation. Indeed our testing shows that you can achieve a 40% saving in energy-per-bit because the RF front end is actually significantly more efficient when transmitting at high power and high bandwidth. So although TD-LTE pushes transmit bandwidth up by 5x and doubles peak power consumption, ET restores the balance, making TD-LTE more energy efficient than FD-LTE, not less. However, it must be remembered that a high performance ET IC is critical in order to gain full advantage of these efficiency and performance benefits. Low bandwidth ET solutions simply aren’t sufficient.
Nujira
www.nujira.com Enter 206
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