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Mini PCIe … A Standard Form Factor for High-Speed Radios

Introduction

When cellular radio modules first became available for M2M Applications in the old Analog AMPS days, these did not use a standard form factor. Each manufacturer designed their own circuit board, and implemented unique pin configurations, connectors, and data interface protocols for their Customers needs. Over the years, the module form factors and interface protocols evolved and changed along with the chipsets and wireless technologies, but most manufacturers of M2M radios continued using their own new form factor design (although there were some inter-company efforts to standardize on a format). Recently, the wireless radio industry has popularized a form factor: Mini PCI Express, also referred to as Mini PCIe, that is now a used for a variety of embedded radio applications. This is now a common standard form factor for high-speed 3G and 4G radio modules as well, although not all embedded cellular radio manufacturers have begun providing modules using it.

Where to Get Detailed Specifications

For the electrical and physical specifications of Mini PCIe, the PCI Special Interest Group (PCI-SIG) has provided all the details at the PCI-SIG web site, but this requires membership (and annual dues) to get access to the full, formal specifications. The module manufacturers also provide specifications with their products, of course. Edge connectors to use with these Mini PCIe modules are available from companies like Molex and others.

Brief Discussion of Mini PCIe

Popularity of Mini PCIe

Mini PCIe has become very popular in the PC industry for use in laptops and tablets. This physical and electrical format is ideal for extending the PCI Express busses (used in desktops and servers) for space-constrained uses, and providing a high-speed path to the processor and memory. Mini PCIe cards for a variety of short-range radio technologies, such as Bluetooth and WiFi short-range wireless embedded radios (including combo units with multiple technologies), are quite popular and commonly availableboth directly from laptop manufacturers and as add-on components.

Types of Mini PCIe Cards

The ultra-portable and tablet market has also driven a need for two types of Mini PCIe cards: Full Mini PCIe and Half Mini PCIe. The connectors on both formats are identical, but the board length has been reduced in half for Half Mini PCIe. This allows them to be used in even tighter, space-constrained applicationssuch as ultra-portable laptops and tabletsand many Bluetooth and WiFi Half Mini PCIe cards are commonly available. But this format is not as essential for many M2M Devices that are not space-constrained like tablets. Thus, today, most cellular implementations are Full Mini PCIe, although there are Half Mini PCIe designs in the worksthese are primarily intended for laptops and tablets where the higher cost of Half Mini PCIe radios can be more readily absorbed.

Antenna Connectors

In the Mini PCIe format, the antenna connectors on the boards are generally U.FL male connectors (these are surface-mount coaxial connectors from Hirose Electrical Group in Japan) designed to work up to 6GHz. Some manufacturers may also use IPX connectors from Lighthorse Technologies. These are designed to be 100% compatible with the U.FL connectors from Hirose. The location of the antenna connectors is next to the support holes (where screws should be used to hold down the Mini PCIe cards to the underlying circuit board). The cards may have multiple antenna connectorseither for diversity or Multiple-Input Multiple-Output (MIMO) antennas for 4G cellular technologies. The signal connections to the antennas are done with mini coaxial cablestypically ranging from 0.81mm to 1.32mm in overall thickness. Female U.FL connectors on typical antenna cables are designed for only a few insertions. These cables must be carefully inserted into the cardswithout repetitive twisting and the liketo avoid connection and signal loss problems. For M2M applications in high-vibration environments, it is important to design the enclosures to properly support the connections and cable attachments, so that they do not break or pop-off easilyembedded M2M Devices are expensive to service!

Signal Interface

In these cellular Mini PCIe cards, the interface is generally USB 2.0 High-Speed serial rather than the older RS-232 compatible serial (or, rather, the 5V CMOS equivalents of RS-232 serial). Thus, all external controllers that run the actual M2M Application must use USB 2.0 to communicate with Mini PCIe radios.

Communication Protocol

With a few exceptions, of course, the modules are controlled using AT commands. While attempts have been made to standardize the actual AT commands, these are still unique to each manufacturer. However, most will extend the 3GPP TS 27.007 AT command set for User Equipment (UE) specification that was developed many years ago. (Some will extend the earlier 3GPP TS 07.07 AT command set for GSM Mobile Equipment (ME) specification, but this is effectively obsolete as of 2003, so this is not as likely.) Most importantly though, as mentioned, the specific modules implementations are usually extensions of the 3GPP AT command set, since the modules have unique capabilities that differ from manufacturer to manufacturer. This means that the M2M Application developer must read and understand the specific commands of their module choice carefullythe same AT command may operate a bit differently in each manufacturers implementation!

Examples of Mini PCIe Cellular Modules

Here are just a few examples of embedded radio modules using the Mini PCIe form factor:

Franklin Wireless

M600C CDMA

M600 CDMA/WiMAX Module

Novatel Wireless

Expedite E396 CDMA/HSPA+ Module

Expedite E362 LTE/CDMA Module

Sierra Wireless

AirPrime MC5728V EV-DO Rev A Module

AirPrime MC7750 LTE and EV-DO Module

Pros and Cons of Mini PCIe

Pros

This is, after all, a standard! Each module manufacturer can provide a number of modules with different capabilities and technologies. For example, Franklin Wireless (others have similar multiple product lineups) offers three Modules that differ only in the supported cellular technology:

  • M600 for use with CDMA and WiMAX networks.
  • M600C for use with CDMA networks only.
  • M600W for use with WiMAX networks only (not shown in this post).

And they have similar M720 LTE/CDMA and M710 LTE/HSPA Modules in the works.

Each is plug-and-play into the Mini PCIe edge connector and mounting holes. The location of the antenna connectors is interesting: the CDMA connectors on the single-technology M600C are identical to the location of the CDMA connectors on the dual-technology M600. Similarly, the location of the WiMAX antennas on the single-technology M600W are identical to the location of the WiMAX connectors on the dual-technology M600. This allows Customers to design a single base circuit board that can operate using a variety of possible cellular modulesparticularly from the same manufacturer, although the specific advantage of being able to use multiple sources is a distinct plus! Of course, the AT command set used will differ from manufacturer to manufacturer, so programming changes will be needed in the M2M Application firmware. But at least the base hardware will not need significant changesideally none at allwhen changing between manufacturers.

Cons

As far as I know (but could be wrong), Mini PCIe cellular modules have not as yet been extensively deployed in the M2M industry, i.e., outside the portable notebook, laptop and tablet markets. The more rigorous environmental requirements (such as temperature ranges, vibration, etc.) of some M2M Applications may cause problems for the edge connectors or antenna connectors on these cards. Automotive telematics applications also generally require wider temperature ranges than standard consumer products, although this is perhaps more of an issue for the radio modules rather than the physical/electrical form factor or edge connectors, etc. Special boards and connectors may be more easily designed for those harsher environmental requirements. Regardless, I do think that the advantages of using a standard format more than outweigh these Cons, so I expect to see more Mini PCIe modules in the future, as well as more M2M Applications that use them. Any M2M Application user who is concerned about the harsher environment can work with the manufacturer of their choice to improve the capabilities and specifications of the radios.

What Do You Think?

Id like to hear from developers who are using, or beginning to use, Mini PCIe cellular radio modules in their M2M Applications. What are your experiences with the format? What additional Pros and Cons would you add? Is there anything in your experience with Mini PCIe that has caused you to select or change to another form factor?

Copyright 2012 Aeris Communications, Inc. and Syed Zaeem Hosain. All Rights Reserved.

Note: Although I dont specifically identify Trademarks in my posts, they are implied. For example, Franklin Wireless, Novatel Wireless, Sierra Wireless, PCI, Mini PCI Express, Mini PCIe, PCI-SIG, etc., are Registered and Unregistered Trademarks of the respective holders.

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