UMTS TDD: The Other Broadband Standard


Fixed broadband Wi-Max (when it becomes available) will not directly compete with “3G” data services like Verizon’s EVDO. Cellular has mobility while WiMax will offer speed and lower connection costs. When 802.16e & 802.20 come around in couple of years, the environment could change.


Mobile Data Architectures
802.16e 802.20 3G
IP 802.16a mobility (more than 1Mbps) IP roaming & handoff (more than 1Mbps) Circuit-switched cell data (less than 1Mbps)
Extentions to MAC and PHY from 802.16a New MAC and PHY with IP and adaptive antennas W-CDMA & CDMA-2000
Backward compatible with 802.16a Optimized for full mobility Evolving GSM or IS-41
Between 2-6 GHz Licensed Bands below 3.5 GHz Licensed Bands below 2.7 GHz
Packet Architecture Packet Architecture Circuit Architecture
Low latency Low latency High latency

Meanwhile, another broadband wireless standard is out there; UMTS TDD. It’s not quite as mobile as EV-DO but it works in both cellular and MMDS frequencies. And it’s here. Now. Because it can use wider channels (on MMDS frequencies), it has the potential to deliver faster service than cellular-based EV-DO.

UMTS-TDD uses only one (simplex) channel for both upstream and downstream. UMTS TDD is one of 3 alternative air-interface (physical layer) standards of UMTS. Other alternatives are FDD W-CDMA (with two channels) and TD-SCDMA (which uses a syncronizing beacon and a single channel). It is favored by Chinese cellular carriers.

Four member companies of the TDD Alliance will speak at the WCA Conference, in Washington, DC this week. Axcera is demonstrating video conferencing over UMTS TDD at the conference.

The many successful commercial deployments of Axity3G and IPWireless technology have demonstrated to the marketplace that wireless Internet delivery can compete head-to-head with cable and DSL offerings, said Richard Schwartz, director of marketing and product management for Axcera.

The mobility aspect alone provides a strong differentiation from these traditional services, and with capabilities like VoIP and video conferencing, this technology has taken another step toward becoming the preferred IP delivery method.

One of the largest benefits of using TDD is that TDD supports variable asymmetry, meaning an operator can dictate how much capacity is allocated to downlink versus uplink.

It differs from WiMax in that it uses CDMA rather than COFDM modulation. Proponents of Wi-Max gear, which can also use both simplex and duplex channels, claim that COFDM is faster and more resistant to multipath. With the mobility of 802.16e, WiMax proponents claim they can deliver more users, more cost/effectively.

IP Wireless is a big proponent and is using the system in Vancouver, Canada. But Monet Mobile Networks, a Seattle-based wireless Internet service provider, went belly up deploying it. The service cost $40 per month, with a $100 modem and delivered 300 to 700 kbps. Data only. Monet leased channel capacity from cellular providers and provided a degree of mobility. But profit margins were too thin, apparently.

The CDMA-based 1xEV-DO standard uses regular 1.25 Mhz PCS channels with data speeds around 500kbps. But cellular carriers may make more money on voice. And cellular spectrum IS limited. Using the scarce 10-30 MHz PCS real estate for dedicated data channels is risky business. It may not pencil out if a carrier replaces a 3-4 $50/month voice users with one $80/month EV-DO data user. Sprint bet the company on CDMA and won. Will they bet the company on EV-DO, especially when they have all those MMDS freqencies? No way. They’d prefer to wait for EV-DV. Meanwhile, on MMDS, they could deploy UMTS TDD now or 802.16e later.

The TDD Coalition
Two different paired frequencies (duplex) are used for cellular and most 3G systems. One channel coming in, one going out. When 2.5-3 G cellular is used for data, a fully loaded downlink and a low loaded uplink is typical. Hence, most of the uplink spectrum cannot be used. It’s wasteful and expensive. Time Division Duplex (TDD), uses a single frequency band. Both voice and data can use TDD – you get an “effective” two channel duplex system – on one.

Smart antennas increase the spectrum efficiency, range and reliability of wireless networks and they’re easier to utilize using a single channel. When time division and smart antennas are applied at both the transmitter and the receiver, it’s called multiple-input, multiple-output (MIMO).

TDD operation is based on a periodically repeated TDMA frame in the radio channel. The frame is subdivided to a number of timeslots. Within this time domain subdivision the transmission direction from downlink to uplink can be changed and meet the required symmetry of any service – voice or data, low or high speed data.

Members of the TDD Coalition include the following “4G” hopefuls:

Aperto Networks ArrayComm BeamReach Networks Navini IPWireless Radiant Networks
Freq 5.8 GHZ MMDS MMDS 2.4/MMDS MMDS 28MHz
Speed 8 Mbps 1 Mbps 1.2 Mbps 1 Mbps 6 Mbps 25 Mbps MESH
Range 5 miles 5 miles 21 miles 6 miles 6 miles 20 miles?
Modulation COFDM Adaptive MultiBeam OFDMTM ? TD-SCDMA TD-CDMA QPSK, QAM16 or QAM64
Phased Tx? No Yes Yes Yes Yes No
Receive Ant? Outside Indoors Indoors Indoors Indoors Outdoors
Investors ? ? Verizon Intel/Nokia $120 M Intel Capital
Trials Operational Australia Ops Q3 2002 Sprint Operational BT Trials
5.8 GHz Band? Yes No No No No No
Other QOS TDD Other N-LOS Sprint trial No central tower

The TDD Coalition includes companies like Adaptive Broadband, Aperto Networks, ArrayComm, BeamReach Networks, CALY Networks, Clearwire Technologies, Harris Corporation, IPWireless, LinkAir, Malibu Networks, Navini, Radiant Networks, Raze Technologies and others.

Posted by Sam Churchill on .

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