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Demands on high-speed wireless systems have grown with the increasing numbers of smartphone users and their devices’ many functions, including wireless Internet access. As the capacities of global wireless-communications standards, such as Third-Generation (3G) cellular and Fourth-Generation (4G) Long-Term-Evolution (LTE) cellular technologies are increased, demands are created for wider available bandwidths and higher modulation rates.

Microwave backhaul systems now call for modulation levels to 1024-state quadrature amplitude modulation (1024QAM) and beyond. Such complex modulation schemes impose increased challenges at the system levels, both for transmitters and receivers. Due to the increased number of symbols, the systems require higher signal-to-noise ratios (SNRs) along with better linearity.

Cellular technologies and applications have grown steadily from the basic analog systems of First-Generation (1G) cellular standards to the high-speed and highly efficient 4G Third-Generation-Partnership-Program (3GPP) and LTE digital cellular standards. Many cellular markets are already eying the use of Fifth-Generation (5G) cellular systems and technologies.

Cellular and base-station systems must meet demands for the rapidly growing number of cell phone users, from making to routing calls; from secure data transfers to high-speed data downloading; and from efficient use of available frequency spectrum to handling multiple users at a time. These advancements and requirements have increased the capacity demand for base-station network equipment.

Numerous techniques have been introduced to effectively and efficiently use the available spectrum. More and more users are demanding access to high-speed data, so wireless operators require increased-capacity networks with higher-modulation-rate support capability. Base-station networks supporting microwave backhaul applications and operating at frequencies in the range of 6 to 42 GHz send data from numerous users to the central backbone network.

This data is an order of magnitude more than from one user or from one sector of users, requiring support capability at higher modulation levels. Low-noise and high-performance wireless infrastructure signal-chain solutions are available which are suitable for such emerging high-modulation requirements.

QAM Is Rising: 1024QAM And Beyond, Fig. 1

Figure 1 shows a simplified view of a cellular network, along with the central backbone network. Generally, it consists of a base transceiver station (BTS), base station controller (BSC), mobile switching center (MSC), and landline linked public switched telephone network (PSTN). User equipment (UE) or cellular telephones communicate directly with the BTS. The BTS is linked to a BSC, either via cable or wirelessly by means of microwave links.

The BTS and BSC are usually collocated, though occasionally a small number of BTS units are controlled by one BSC. The MSC is linked to the BSC. The MSC connects cells to a wide area network (WAN), manages call setup, implements call hand-over, and performs many more network operations. MSCs are also linked to the PSTN for landline calls and Internet access.

All BTS units are generally connected to a backbone network through MSCs. Backhaul functionality, which comprises this central backbone network, uses either wired communications, fiber-optic communications links, or microwave links. Fiber-optic links provide tremendous capacity, but require physical installation and can take more time to deploy than other wireless communications links. Also making them less desirable is the fact they are expensive, since land is required for hosting the link hardware, which must be rented or purchased. In addition, fiber-optic links necessitate digging the land to lay the fiber-optic line.

Microwave links, which are more easily and quickly deployed, are traditionally preferred for a cellular network. These links require line-of-sight communications transmission and are susceptible to the effects of atmospheric conditions, such as rainfall attenuation and fog.

QAM Is Rising: 1024QAM And Beyond, Table

Microwave backhaul links commonly use quadrature amplitude modulation (QAM) digital modulation.1 QAM is a highly developed digital modulation scheme where both the amplitude and phase of a high-frequency signal are modulated. The 2N QAM  level represents n bits/symbol. For example, 6 b/symbol represents 64QAM, and 12 b/symbol represents 4096QAM. Thus, in 64QAM, each symbol represents 6 b of information, whereas in 4096QAM, one symbol represents 12 b of information (see table).

Quite simply, higher QAM levels deliver higher capacity. The increase in capacity from a lower bit (nl) per symbol QAM level to a higher bit (nh) per symbol QAM level is given by Eq. 1. For example, an increase of 100% in system capacity occurs for a switch from 6-b 64QAM to 12-b 4096QAM1:

Increase in capacity (%) = [(nh – nl)/nl] × 100   (1)

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