The US wireless industry has chosen Long Term Evolution (LTE) as one of the major pathways for delivering fourth-generation data services. LTE will make data-intensive applications such as high-quality video streaming and fast file transfers possible. However, migrating beyond 3G to LTE (as well as WiMAX) presents two formidable challenges for wireless carriers: The need to handle the massive amounts of traffic in the uplink and downlink paths that these services will generate; and the task of upgrading their ability to "backhaul" this data from cell sites to the network hub.


The answer to the backhaul challenge lies in fiber, microwave and millimeter-wave links, either singly or together. The radios employed by the latter solutions require RF power devices and amplifiers with extremely high performance. The TGA4532-SM 1 W GaAs MMIC power amplifier from TriQuint Semiconductor has been designed to meet these requirements in the 17.7 to 19.7 GHz band allocated for wireless backhaul.

BACKHAUL IN FOCUS

The wireless industry has continuously demanded greater spectrum allocations especially since the emergence of third-generation access technologies such as HSPA and CDMA 2000 Rev. A; they received it in the 700 MHz FCC spectrum auction. LTE will be deployed there by Verizon Wireless and AT&T, which garnered most of the licenses in the US. Nevertheless, it is still possible that the massive amount of data traffic that LTE users will generate may saturate the capacity of current spectrum. Thus, the demands for more spectrum may likely continue.

Backhaul, although it accounts for 20 to 30 percent of carrier operating costs, hasn’t received much attention until lately. This is because leased T1 lines used almost exclusively for backhaul by US carriers have been adequate for voice traffic, which until 2008 accounted for most network volume. This situation changed dramatically in 2009, when data traffic exceeded voice traffic by 400 terabytes. This shift, along with the transition from Plesiochronous Digital Hierarchy (PDH)-based backhaul to IP-based solutions and the high cost of T1 leases, will soon make T1-based backhaul solutions inadequate.

Lightwave (fiber) systems, as well as microwave and millimeter-wave (radio) links, represent the two technologies with the bandwidth to accommodate the impending backhaul onslaught—each with strengths and limits. If fiber nodes were available at every cell site, that would be the obvious choice, as they eliminate interference and have massive bandwidth. However, only about 20 percent of the more than 250,000 US cell sites are served by fiber connections, and fiber deployment cost rivals that of copper, which is very high, particularly after the recent devastating earthquake in Chile, where approximately 30 percent of the world’s copper originates.

Microwave links typically deliver throughput ranging from 155 to 500 Mb/s, are less expensive to deploy and operate than copper lines (based on cost per transferred bit), and are extremely reliable. The high millimeter-wave FCC allocations for backhaul from 71 to 76 and 81 to 86 GHz can provide data rates in excess of 1 Gb/s (as they have greater channel bandwidths than lower-frequency allocations), but their cost is higher than microwave and low-millimeter-wave systems and have much shorter range, so they will likely be used only when the lower-frequency allocations have been saturated.

In Europe, microwave links currently handle 70 percent of wireless backhaul traffic because leasing costs for E1 (comparable to T1) lines are even higher than in the US. Globally, wireless backhaul accounts for about 50 percent of backhaul traffic. The consensus today is that fiber will be the choice where it is available, while microwave and millimeter-wave links will be used both where fiber is not located and to bring traffic to the nearest fiber node.

TGA4532-SM PERFORMANCE

The TGA4532-SM is the latest in a family of TriQuint GaAs MMIC power amplifiers designed exclusively for use in point-to-point microwave applications. It is based on one of the company’s GaAs PHEMT processes and operates from 6 VDC with quiescent current consumption of 816 mA. The TGA4532-SM is housed in a 4x4 mm leadless QFN package enabling easier assembly compared to leaded SMT packages.

Figure 1 IP3 (TOI at 22 dBm/tone), P1dB and Psat vs. frequency for TGA4532-SM at 6 V, 816 mA.

Figure 2 Gain and input/output losses vs. frequency for TGA4532-SM at 6 V, 816 mA.

TGA4532-SM represents best in class performance for a 1 W packaged power amplifier for the 17.7 to 19.7 GHz communication band. For example, the third-order intercept point is nominally 41 dBm at 22 dBm output power per tone. P1dB output power is 31 dBm; saturated output power is greater than 32 dBm (see Figure 1). The TG4532-SM delivers 23 dB of gain with an extremely flat response over the frequency band; the input and output return losses are well below 12 dB over most of the frequency range of interest (see Figure 2). Combining these RF performance characteristics with a DC power dissipation of less than 5 W, the TGA4532-SM offers an excellent solution for 18 GHz point-to-point applications (see Table 1).

Other devices in the family include the TGA2522-SM, which covers the same frequency range as the TGA4532-SM, provides P1dB output power of at least 27 dBm and Psat output of 28.5 dBm, third-order intercept of at least 36 dBm, gain of 21 dB, and is also housed in a 4x4 mm QFN package. It draws 712 mA from a 5 VDC supply. The TGA2706-SM, which delivers P1dB RF output power of 2 W over the 5.5 to 8.5 GHz point-to-point microwave band, is housed in a 5x5 mm QFN surface-mount package. It produces 32 dBm at P1dB with 42 dBm third-order intercept, small-signal gain of 31 dB, and a noise figure of 7 dB. Performance is extremely flat over its 3 GHz bandwidth.
In the bands above 23 GHz allocated for point-to-point microwave operation, bare die are commonly used to deliver the highest performance. TriQuint’s recently-introduced TGA4538 GaAs MMIC die offers excellent performance from 37 to 40 GHz. The device’s P1dB RF output power is at least 28 dBm (29.5 dBm Psat), third-order intercept performance is 38 dBm and gain is 24 dB. It draws 600 mA from a 5 VDC supply.

CONCLUSION

For manufacturers of point-to-point microwave and millimeter-wave radios and their components, the cellular backhaul market promises to grow dramatically in the next few years. The reason for that growth is simple: Along with lightwave systems, which are not available in a sizeable number of cell site locations, only point-to-point microwave offers the bandwidth to handle the massive volumes of data that LTE and WiMAX deployments will produce. Radio-based backhaul is also cost-effective both to deploy and operate, and has demonstrated its reliability in this as well as many other applications throughout the world. TriQuint will continue to introduce MMICs to serve this application for all allocated bands from 6 to 38 GHz, providing the linearity and other characteristics required to satisfy the demands of higher-order modulation schemes that point-to-point systems employ.

TriQuint Semiconductor,
Hillsboro, OR,
info-networks@tqs.com,
www.triquint.com.
RS No. 301