MIMO is applicable to all kinds of wireless communication technologies. However, the combination of MIMO and OFDM (Orthogonal Frequency Division Multiplex) has the following advantages. OFDM is adapted for multi-path propagation in wireless systems. The length of the OFDM-frames is determined by the Guard Interval (GI). This Gurad Interval restricts the maximum path delay and therefore the expansion of the network area. MIMO also uses the multi-path propagation.
OFDM is a wideband system with many narrowband sub-carriers. The mathematical MIMO channel model is based on a narrow band non-frequency selective channel. The latter is supported by OFDM as well. Fading effects in wideband systems normally occur only at particular frequencies and interfere with few sub-carriers. The data is spread over all carriers, so that only a small amount of bits get lost, and these can be
repaired by a forward error correction (FEC). OFDM provides a robust multi-path system suitable for MIMO. At the same time OFDM provides high spectral efficiency and a degree of freedom in spreading the time dimension of Space-Time Block Codes over several sub-carriers. This results in a stronger system based on the principle described previously
MIMO Standards:
Table 1 gives an overview of all current MIMO standards and their technologies. It is clear to see, that with the exception of 3GPP Release 7, all standards work with OFDM. The advantages of OFDM can obviously be linked to MIMO.
Table 1 MIMO Standards and the corresponding technology :
Standard Technology
WLAN 802.11n OFDM
WiMAX 802.16-2004 OFDM/OFDMA
WiMAX 802.16e OFDMA
3GPP Release 7 WCDMA
3GPP Release 8 (LTE) OFDMA
802.20 OFDM
802.22 OFDM
MIMO and OFDM
HSPDA - the next step for 3G
With 3G networks spreading throughout the globe, consumers are just now beginning to experience the true capabilities of 3G wireless services. Already, some fear that demands for high-speed data access will exceed the capability of today's UMTS networks. Fortunately, the wireless industry already has plans in place for enhanced data networks.
Among the next-generation contenders in the ring are Flarion's Flash OFDM (F-OFDM), WiMAX (IEEE802.16e) and CDMA2000 1XEV-DO. But it's high-speed downlink packet access, or HSDPA, that enables a smooth, cost-efficient upgrade to existing W-CDMA networks at minimal cost.
HSDPA's incremental UMTS network upgrade aims to increase user peak data rates and quality-of-service and improve spectral efficiency - much like EDGE and 1XRTT have done for 2G. Although UMTS enables streaming video, broadband Internet access and video conferencing, HSDPA offers peak downlink data rates of up to 14 Mbps - dramatically more than the 384 kbps that is typical of today's UMTS and the highest data rate of any available mobile WAN technology.
HSPDA works by moving important processing functions closer to the air interface. Although current UMTS networks perform network scheduling and retransmission in the radio network controller, HSDPA moves these functions to the base station (called Node B in UMTS systems), allowing scheduling priority to take account of channel quality and terminal capabilities. Retransmission also benefits from hybrid automatic retransmission request in which retransmissions are combined with prior signal transmissions to improve overall reception. HSDPA adds a channel-sharing mechanism that allows several users to share the high-speed air interface channel and other technological advances such as adaptive modulation and coding, quadrature amplitude modulation and channel quality feedback. These enhancements allow HSDPA to roughly double the total throughput capacity of a network.
For consumers, that translates into shorter service response times, fewer waits and faster connections. Wireless users can talk on the phone while simultaneously downloading packet data. Most important, they can use their wireless handsets to download Web pages, audio or video at speeds well above the performance they are accustomed to with landline-based DSL or even cable Internet connections.
What's good for the consumer ultimately is good for the operator, provided the costs and barriers to deployment are not too great. HSDPA significantly enhances W-CDMA with little hardware investment. It operates in 5 MHz channels and is backward-compatible with current W-CDMA networks. This allows network operators to introduce greater capacity and higher data speeds on the same carriers as with existing Release '99 W-CDMA services. A system may be upgraded incrementally to enhance performance for users of the latest handsets without losing network capacity or interrupting service to subscribers who rely on older handset technology.
As an extension of GSM, HSDPA can be deployed readily in the United States, Europe and Japan. In fact, some service providers may introduce pilot projects within the next year. By 2007, HSDPA likely will be a leading technology worldwide. This means that operators will be able to offer global roaming capabilities based on infrastructure already in place today and serving a sizable handset base, including older W-CDMA units.
HSDPA's incremental upgrade and dramatic performance benefits will serve as the best stepping stone to 4G for many operators. Texas Instruments is working within the 3G Partnership Project (3GPP) to enhance the standard in ways that will provide continuing business opportunities for wireless operators.
Among the developments we see down the road is high-speed uplink packet access (HSUPA), which will augment HSDPA to create a more symmetrical high performance system. We also expect ongoing improvements to boost network efficiency, reduce latency and increase overall network throughput.
All of this can be achieved through incremental upgrades to time proven W-CDMA technology. The HSDPA standard has been in place for a couple of years. Base station equipment is now starting to reach the market and handsets will follow, with widespread deployment anticipated in 2006 and 2007.
For consumers, HSDPA will help 3G technology fulfill its promise with more sophisticated data applications and better performance. For operators, this technology enables a highly competitive network with only incremental enhancements to existing infrastructure. HSDPA promises to provide a better return on investment and stronger revenues per megabit delivered compared to other avenues to very high-speed service.
WiMAX and Its benefits
WiMax (Worldwide Interoperability for Microwave Access) is a wireless broadband technology, which supports point to multi-point (PMP) broadband wireless access over a coverage area of 3 – 5 miles. WiMax is another name for the 802.16 IEEE wireless broadband standards. The wireless broadband technology, referred to as WiMax, can support access rates up to 2+ Mbps. WiMax can solve a problem called the “last-mile” problem, by connecting individual homes and business office’s communications.
The WiMax standard has a range of up to 30 miles, and can deliver broadband at around 75 megabits per second. This is theoretically, 20 times faster than a commercially available wireless broadband. WiMax can be termed partially a successor to the Wi-Fi protocol, which is measured in feet, and works, over shorter distances. WiMAX provides High-speed Internet access where it is currently unavailable. It also increase substantially data speeds for applications to include online gaming, streaming video, video conferencing, VoIP and location based services.
A key differentiator for WiMAX is the interoperability of WiMAX Forum Certified equipment, resulting in mass volume economy of scale and assurance for service providers that when buying equipment from more than one company, the technologies are interoperable. The WiMAX Forum has assembled an alliance of leaders in the communications and computing industries to drive a common platform for the global deployment of IP-based broadband wireless services. Other key elements include cost, coverage, capacity and standards for both fixed and mobile wireless usage models.
WiMAX Benefits:
1. Lower cost
A standards based platform for WiMAX technology drives down costs delivering volume economics to WiMAX equipment.
2. Wider coverage
The technology behind WiMAX has been optimized to provide excellent non-line-of-sight (NLoS) coverage. NLoS advantages are coverage of wider areas, better predictability of coverage and lower cost as it means fewer base stations and backhaul, simple RF planning, shorter towers and faster CPE install times. Thanks to techniques for improving NLoS coverage, such as diversity, space-time coding, and Automatic Retransmission Request (ARQ), coverage are increased.
3. Higher capacity
A key advantage of WiMAX technology is to use Orthogonal Frequency-Division Multiplexing (OFDM) over Edge, GPRS, HSPA to deliver higher bandwidth efficiency and therefore higher data throughput, with more than one Mbps downstream and higher data rates. Adaptive modulation also increases link reliability for carrier-class operation and the possibility to keep higher order modulation at wider distance extend full capacity over longer distances.
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