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To date, cellular phones have seen three generations of technology and a fourth generation is on the verge of deployment. This fourth-generation technology has implications for the proposed nationwide 700 MHz broadband network for first responders.
Generational history. In 1983, AT&T launched the first cellular-phone network in Chicago. This network was based on the Advanced Mobile Phone System (AMPS) developed by Bell Labs in the late 1970s. AMPS used frequency modulation and Frequency Division Multiple Access (FDMA). To enable this new service, the FCC allocated 666 channels, each 30 kHz wide, in two bands: 825-835 MHz for subscriber to base station and 870-890 MHz for base station to subscriber.
To promote competition, the FCC further split the cellular spectrum into two parts and offered licenses to two operators in each metropolitan or rural subscriber area. Each operator was licensed for 333 channels, 21 of which were used for control channels. AMPS employed a seven-cell frequency reuse pattern that created spectrum efficiencies much greater than any previous land mobile radio (LMR) system. After just a few years of service, cellular radio proved to be economically viable and the FCC released an additional 10 MHz of spectrum, which had been kept in reserve. This first-generation network was almost exclusively a voice service.
By the early 1990s, cellular radio had outgrown its spectrum allocation and no additional spectrum was immediately available. The wireless operators badly needed improved spectrum efficiency and they chose new digital modulation techniques to solve the problem. Initially, all U.S. operators agreed on a Time Division Multiple Access (TDMA) solution that offered six time slots per 20 millisecond (ms) frame. Vocoders of the time period required two time slots per frame, so the spectrum efficiency was improved by a factor of three over AMPS.
Before TDMA networks were fully deployed, a San Diego company called Qualcomm proposed a Code Division Multiple Access (CDMA) solution that promised capacity improvements of 20 times over AMPS. Unlike Europe, where all operators agreed on single TDMA standard called GSM, the U.S. operators split between the U.S. version of TDMA and Qualcomm’s CDMA. At the time, subscriber growth was 40% per year and technology improvements alone could not keep up.
Recognizing this problem, the FCC auctioned the first PCS spectrum (1,850-1,990 MHz) in 1995 and opened the door to several more wireless operators in each service area. For the most part, PCS licensees built networks based on GSM and CDMA. These second generation networks were still almost exclusively voice services.
In 1998, after prompting from the Japanese, the international standards bodies committed to developing third-generation (3G) wireless standards, with the primary goal of offering multimedia wireless services (voice, data and video). The Internet was just catching on and, for the first time, data was the primary motivation behind a new wireless standard. Interestingly, both technology camps chose CDMA as the basic third-generation technology, but with two incompatible flavors: UMTS for GSM operators and cdma2000 for CDMA operators.
Today’s third-generation wireless networks offer between 384 kb/s and 2 Mb/s, depending on proximity to the cell site and user mobility.
At the same time these expensive 3G cellular-phone technologies were being deployed, a cheap alternative called Wi-Fi was quietly sweeping the nation using license-free spectrum in the 2.4 and 5 GHz bands. Wi-Fi lacked a seamless nationwide network, but it offered much higher bit rates and often the service was free if one could find a “hot spot.” Wi-Fi radios are based on the IEEE 802.11 series of standards that employ Orthogonal Frequency Division Multiplexing (OFDM) and Carrier Sense Multiple Access with collision detection (CSMA-CD). Wi-Fi networks operate between 6 Mb/s and 54 Mb/s, depending on proximity to the base station, called the access point (AP).
From a user perspective, the choice in 2009 is free high-speed data with poor mobility and spotty coverage (Wi-Fi), or relatively expensive medium speed data with seamless nationwide coverage (3G). It’s not clear which is winning the wireless data war, but it is clear that wireless operators are under extreme pressure to offer cheap high-speed wireless data at data rates comparable to Wi-Fi. The operators’ response to this challenge is the fourth-generation (4G) standards process, begun in 2004.
Introducing LTE, the 4G standard.The first 4G standard to be published and adopted by a nationwide operator is called Long Term Evolution (LTE). LTE is fast, with peak data rates of 100 Mb/s downlink and 50 Mb/s uplink (assuming a 2 × 20 MHz channel). Downlink and uplink are decoupled for the first time in a cellular network. Third-generation and older systems use Frequency Division Duplexing (FDD), which means that one band of frequencies is used for the downlink (base station to mobile user) and another band of frequencies is used for the uplink (mobile user to base station). Such a system uses spectrum inefficiently when the traffic is unbalanced, i.e., when there is more traffic on the downlink than the uplink.
LTE offers both FDD and Time Division Duplexing (TDD), which means the uplink and downlink speeds need not be identical, so operators can better optimize their networks to use more uplink channels. LTE also is IP-based, so all traffic, including voice, is packetized. Advantages of LTE over earlier technologies include high throughput, low latency and a simple architecture resulting in low operating costs. LTE also supports seamless connection to existing 2G and 3G networks, including GSM, CDMA, UMTS, and cdma2000.
Spectrum for LTE.Most observers predict that wireless operators will use new spectrum for LTE, specifically the AWS band (2.1/1.7 GHz) and the 700 MHz band. LTE employs flexible channel bandwidths (1.25, 2.5, 5, 10, 15, and 20 MHz) to accommodate both existing and newly allocated spectrum. Table 1 summarizes some of the spectrum that is expected to be used for LTE worldwide. (Note: Not all bands are available in all countries.)
Modulation type and multiple-access method.LTE employs OFDM which is the same technique used by Wi-Fi and WiMAX. LTE devices are not necessarily interoperable with Wi-Fi or WiMAX, however. The waveform parameters for LTE are listed in Table 2. Like Wi-Fi, users share the broadband channel using a variation of CSMA-CD.
Spectrum efficiency compared to 3G.The capacity improvements created by LTE result from several features, including packet-access, TDD, OFDM, and Multiple In and Multiple Out (MIMO) receivers. Manufacturers and wireless operators have conducted capacity analyses and network simulations to quantify the improved capacity. Early simulations indicate that a 1 MHz LTE channel (using an 8 kb/s vocoder) may support up to 105 simultaneous voice calls, which is three-fold improvement over 3G-UMTS. Nortel’s analysis and simulation show a three- to five-fold improvement over 3G networks.
LTE and public safety.With few exceptions, public-safety radio and cellular radio have followed separate development paths and use different vendors for their voice networks. The reliability and robustness needed from a public-safety voice radio are at odds with the inexpensive cellular phone device and its shared, public network. In contrast, however, first responders commonly employ wireless data in ways identical to commercial entities and many subscribe to 3G wireless data networks. The advantages of commercial wireless data networks are obvious — the economies of scale created by 270 million subscribers mean powerful features at low cost.
But first responders generally prefer to control their own networks. So, how can first responders exploit such economies of scale and still own the network? By employing industry standards in frequency bands adjacent to commercial wireless networks. In doing so, public safety creates an attractive customer base for the wireless industry hardware vendors who generally refuse to develop custom products for small markets.
Recognizing these facts, four influential public-safety organizations — the Association of Public-Safety Communications Officials (APCO), the National Emergency Number Association (NENA), the National Public-Safety Telecommunications Council (NPSTC) and the Public-Safety Spectrum Trust (PSST) — have endorsed LTE as the technological standard for the proposed 700 MHz national broadband network for first responders.
Jay Jacobsmeyer is president of Pericle Communications Co., a consulting engineering firm located in Colorado Springs, Colo. He holds bachelor’s and master’s degrees in electrical engineering from Virginia Tech and Cornell University, respectively, and has more than 25 years experience as a radio-frequency engineer.
POTENTIAL LTE SPECTRUM
|E-UTRA band||Uplink (UL) BS receive UE transmit
F(UL) low-F(UL) high
|Downlink (DL) BS transmit UE receive
F(DL) low-F(DL) high
|1||1,920 MHz-1,980 MHz||2,110 MHz-,2170 MHz||FDD|
|2||1,850 MHz-1,910 MHz||1,930 MHz-1,990 MHz||FDD|
|3||1,710 MHz-1,785 MHz||1,805 MHz-1,880 MHz||FDD|
|4||1,710 MHz-1,755 MHz||2,110 MHz-2,155 MHz||FDD|
|5||824 MHz-849 MHz||869 MHz-894 MHz||FDD|
|6||830 MHz-840 MHz||875 MHz-885 MHz||FDD|
|7||2,500 MHz-2,570 MHz||2,620 MHz-2,690 MHz||FDD|
|8||880 MHz-915 MHz||925 MHz-960 MHz||FDD|
|9||1,749.9 MHz-1,784.9 MHz||1,844.9 MHz-1,879.9 MHz||FDD|
|10||1,710 MHz-1,770 MHz||2,110 MHz-2,170 MHz||FDD|
|11||1,427.9 MHz-1,452.9 MHz||1,475.9 MHz-1,500.9 MHz||FDD|
|12||698 MHz-716 MHz||728 MHz-746 MHz||FDD|
|13||777 MHz-787 MHz||746 MHz-756 MHz||FDD|
|14||788 MHz-798 MHz||758 MHz-768 MHz||FDD|
|17||704 MHz-716 MHz||734 MHz-746 MHz||FDD|
|33||1,900 MHz-1,920 MHz||1,900 MHz-1,920 MHz||TDD|
|34||2010 MHz-2025 MHz||2,010 MHz-2,025 MHz||TDD|
|35||1850 MHz-1910 MHz||1,850 MHz-1,910 MHz||TDD|
|36||1,930 MHz-1,990 MHz||1,930 MHz-1,990 MHz||TDD|
|37||1,910 MHz-1,930 MHz||1,910 MHz-1,930 MHz||TDD|
|38||2,570 MHz-2,620 MHz||2,570 MHz-2,620 MHz||TDD|
|39||1,880 MHz-1,920 MHz||1,880 MHz-1,920 MHz||TDD|
|40||2,300 MHz-2,400 MHz||2,300 MHz-2,400 MHz||TDD|
|Source: Rhode & Schwartz|
LTE WAVEFORM PARAMETERS
|Channel bandwidth||1.4 MHz||3 MHz||5 MHz||10 MHz||15 MHz||20 MHz|
|Resource blocks (1 resource block=180kHz)||6||15||25||50||75||100|
|Modulation schemes|| Downlink: QPSK, 16QAM, 64QAM
Uplink: QPSK, 16QAM, 64QAM (optional for handset)
|Multiple access|| Downlink: OFDMA (Orthogonal frequency division multiple access)
Uplink: SC-FDMA (Single-carrier frequency division multiple access)
|MIMO technology|| Downlink: Wide choice of MIMO configuration options for transmit diversity, spatial multiplexing and cyclic delay diversity (max. 4 antennas at base station and handset)
Uplink: Multi-user collaborative MIMO
|Peak data rate|| Downlink: 150 Mb/s (UE category 4, 2×2 MIMO, 20 MHz)
300 Mb/s (UE category 5, 4×4 MIMO, 20 MHz)
Uplink: 75 Mb/s (20 MHz)
|Source: Rhode & Schwartz|