Bringing Together LTE and WiMAX

Crack open any 3G phone and you’ll find multiple RF PAs. The fundamental differences between WCDMA, EDGE, and GSM/GPRS signaling demand architectural variations in PA design that limit the practicality of functional integration. To date, there are no integrated 2G/3G PA devices.

However, the evolution of broadband wireless communications to WiMAX and LTE brings, for the first time, some level of commonality in RF signaling. This invites the possibility of a single PA architecture supporting functional integration while attaining the output power and ACP performance required by high-capacity, high-data-rate wireless networks.

The ultimate challenge is to achieve convergence in small, power-efficient PA devices with performance meeting the expectations of broadband wireless consumers. The first step in designing a converged architecture is a full understanding of LTE and how it is similar to and different from WiMAX, especially in terms of frequency, duplexing, power, linearity requirements, and modulation.

Many different bands are currently available for LTE operation. The table shows frequency bands for uplink operation defined in 3GPP TS 36.101. (For the mobile terminal, the uplink band is the band of interest, since this is the transmit frequency.)

While many bands are available, and both FDD and TDD options are shown, the main bands of interest for North America are bands 13 and 14 (700-MHz bands) and band 4 (1710 to 1755 MHz). In Europe, band 7 is expected to be widely used, with operation from 2500 to 2570 MHz. In Japan, it is likely that band 1 (1920 to 1980 MHz) will be deployed first for LTE.

In terms of mobile WiMAX, worldwide regulators are allocating spectrum in the 2.3- to 2.4-GHz and 2.5- to 2.7-GHz regions, as well as the 3.4- to 3.6-GHz band. India, for instance, recently allocated 2.3- and 2.5-GHz bands for mobile WiMAX.

With WiMAX operating at 2.5 GHz and LTE using 1.7 or 2.5 GHz, it is certainly possible to create a single PA module to handle these frequencies. This type of design would be analogous to what is being done with GPRS/EDGE/GSM quad-band PAs (which are currently offered by numerous vendors, such as RF Micro Devices, Skyworks, and TriQuint), where the PAs required for each band have the same basic calibration scheme and voltage.

While both TDD and FDD are possible in LTE and WiMAX, it is widely expected that initial LTE networks will be FDD systems while WiMAX will initially use TDD. For a converged PA architecture, this means the design will need to be more complex, including additional filtering for FDD and the ability to switch on and off without incident for TDD.

From a PA design perspective, the greatest complexity lies with an FDD system. Since it transmits and receives simultaneously with Tx and Rx on different frequencies, significant filtering is required to ensure that the transmit energy does not leak into the receiver, desensitizing it. This requires large and lossy duplex filters. (Typical losses are 2.5 to 3 dB.)

In contrast, there is no duplexer in a TDD system. Only a switch is required to move between Tx and Rx modes, and switch losses are much lower than those from duplexers, at approximately 0.5 dB.

Another challenge with FDD is that the uplink and downlink ratios are fixed and cannot be easily adjusted. Note in the table that the FDD-paired spectrum is always symmetric, with equal bandwidth allocated to uplink and downlink. Since data usage is often highly weighted toward the device downlink, the FDD spectrum may not be used efficiently.

In contrast, a TDD system can change the uplink to downlink ratio easily. To ensure that adjacent cells all remain synchronized, though, all cells in the network would normally standardize on one particular ratio.

With WiMAX and LTE systems diverging on their choice between TDD and FDD, it would certainly be desirable to develop a handset that could support both TDD and FDD operation. However, significant advancements will need to be made in TDD/FDD coexistence for this to occur.

On the plus side for integration, LTE and WiMAX will likely be similar in both output power and linearity requirements. We expect required transmit power to be on the order of +23 to +24 dBm for mobile WiMAX and LTE devices. EVM requirements for LTE systems are still being defined, but they will likely be similar to the WiMAX EVM requirements of –30-dB EVM for 64QAM and –24-dB EVM for 16QAM.