Q and A: OFDM/OFDMA

Q&A: OFDM/OFDMA

What are OFDM and OFDMA?

What is the technical difference between OFDM and OFDMA?

When would OFDM be used versus OFDMA?

Where is OFDM in use today?

How spectrally efficient is OFDM/MIMO for different antenna configurations?

Do OFDM and OFDMA systems provide any technological advantage over HSPA, which is based on UMTS/WCDMA?

Is Long Term Evolution (LTE) an OFDMA technology and does it offer advantages over HSPA?

How will OFDMA be used in LTE?

What is the expected performance of OFDM as it would be implemented for LTE?

What makes OFDM receive signals in a less processing-intensive manner than CDMA?

What is the difference between OFDM and SC-FDMA, which is being considered by 3GPP for LTE (Long Term Evolution) as the air interface for the uplink?

What technologies are based on OFDM/OFDMA and why?

How are OFDM and OFDMA related to WiMAX and IEEE 802.20?

What is Flash OFDM?

What is Ultra Mobile Broadband or UMB and how is it related to OFDMA?

Can OFDM/MIMO be used in TDD and FDD modes?

How will 3GPP, 3GPP2 and other standards bodies adopt and introduce OFDM/MIMO?

Who are the main chipset vendors developing OFDM/MIMO technology?

Will OFDM be a more cost-effective means to deliver wireless broadband services?

What are OFDM and OFDMA?

Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) are two different variants of the same broadband wireless air interface that are often mistaken for one another. OFDMA is a form of OFDM, which is the underlying technology.

Both interfaces work by separating a single signal into subcarriers, or, in other words, by dividing one extremely fast signal into numerous slow signals. This signal division optimizes mobile access, as the sub-channels can then transmit data without being subject to the same intensity of multipath distortion faced by single carrier transmission. The numerous subcarriers are then collected at the receiver and recombined to form one high-speed transmission.

OFDM is sometimes referred to as discrete multi-tone modulation because, instead of a single carrier being modulated, a large number of evenly spaced subcarriers are modulated using some m-ary of QAM. This is a spread-spectrum technique that increases data communication efficiency by increasing data throughput because there are more carriers to modulate. In addition, problems with multi-path signal cancellation and spectral interference are greatly reduced by selectively modulating the "clear" carriers or ignoring carriers with high bit-rate errors.

What is the technical difference between OFDM and OFDMA?

The difference between OFDM and OFDMA is that OFDMA has the ability to dynamically assign a subset of subcarriers to individual users, making this the multi-user version of OFDM, using either Time Division Multiple Access (TDMA) (separate time frames) or Frequency Division Multiple Access (FDMA) (separate channels) for multiple users. OFDMA refers to supporting multiple simultaneous users by assigning them specific subchannels for intervals of time. Point-to-point systems are OFDM and do not support OFDMA. Point-to-multipoint fixed and mobile systems are the OFDMA form of OFDM.

When would OFDM be used versus OFDMA?

OFDM technologies typically occupy nomadic, fixed and one-way transmission standards, ranging from TV transmission to Wi-FI as well as fixed Wi-MAX and newer multicast wireless systems like Qualcomm’s FLO (Forward Link Only). OFDMA, however, adds true mobility to the mix, forming the backbone of many of the emerging technologies such as Long Term Evolution (LTE), UMB, and Mobile WiMAX.

Where is OFDM in use today?

OFDM is being used in a number of wireless and wire-line applications including WLAN, Digital Audio and Video Broadcast, Fixed WiMAX, ADSL, and ADSL2+.

How spectrally efficient is OFDM/MIMO for different antenna configurations?

Depending on the antenna configurations at the cell site and the mobile device, OFDM with MIMO can deliver on average 2.6 bits/sec/Hz a 2X2 antenna system (2 Tx and 2 Rx antennas). To put this number in perspective, Release 6 HSDPA is expected to deliver, on average, between .7 to .8 bits/sec/Hz.

Do OFDM and OFDMA systems provide any technological advantage over HSPA, which is based on UMTS/WCDMA?

For systems employing less than 10 MHz of bandwidth, the answer is largely "no." Because it transmits mutually orthogonal subchannels at a lower symbol rate, the fundamental advantage of OFDM is that it elegantly addresses the problem of intersymbol interference induced by multipath and greatly simplifies channel equalization. As such, OFDM systems—assuming they employ all the other standard techniques for maximizing spectral efficiency—may achieve slightly higher spectral efficiency than CDMA-based systems (such as UMTS/HSPA). However, advanced receiver architectures—including options such as practical equalization approaches and interference cancellation techniques—are already commercially available in chipsets and can nearly match this performance advantage.

It is with larger bandwidths (10 to 20 MHz), and in combination with advanced antenna approaches such as MIMO or Adaptive Antenna Systems (AAS), that OFDM enables less computationally complex implementations than those based on CDMA. Hence, OFDM is more readily realizable in mobile devices. However, studies have shown that the complexity advantage of OFDM may be quite small (that is, less than a factor of two) if frequency domain equalizers are used for CDMA-based technologies. Still, the advantage of reducing complexity is one reason 3GPP chose OFDM for its LTE project. It is also one reason newer WLAN standards, which employ 20 MHz radio channels, are based on OFDM. In other words, OFDM is currently a favored approach under consideration for radio systems that have extremely high peak rates. OFDM also has an advantage in that it can scale easily for different amounts of available bandwidth. This in turn allows OFDM to be progressively deployed in available spectrum by using different numbers of subcarriers.

In 5 MHz of spectrum, as used by UMTS/HSPA, continual advances with CDMA technology--realized in HSPA+ through approaches such as equalization, MIMO, interference cancellation, and high-order modulation--will allow CDMA to largely match OFDMA-based systems. Because OFDMA has only modest advantages over CDMA in 5 MHz channels, the advancement of HSPA is a logical and effective strategy. In particular, it extends the life of operators’ large 3G investments, reducing overall infrastructure investments, decreasing capital and operational expenditures, and allowing operators to offer competitive services.

Is Long Term Evolution (LTE) an OFDMA technology and does it offer advantages over HSPA?

LTE is an OFDMA technology and as such LTE can take better advantage of wider radio channels (for example, 10 MHz) by not requiring guard bands between radio carriers (for example, HSPA carriers).

Given some of the advantages of an OFDM approach, 3GPP has specified OFDMA as the basis of its LTE effort. LTE incorporates best-of-breed radio techniques to achieve performance levels beyond what will be practical with CDMA approaches, particularly in larger channel bandwidths. However, in the same way that 3G coexists with Second Generation (2G) systems in integrated networks, LTE systems will coexist with both 3G systems and 2G systems. Multimode devices will function across LTE/3G or even LTE/3G/2G, depending on market circumstances.

How will OFDMA be used in LTE?

LTE uses Orthogonal Frequency Division Multiple Access (OFDMA) on the downlink, which is well suited to achieve high peak data rates in high spectrum bandwidth. WCDMA radio technology is about as efficient as OFDM for delivering peak data rates in 5+5 MHz of bandwidth. However, achieving peak rates over 100 Mbps with wider radio channels would result in highly complex terminals and is not practical with current technology. It is here that OFDM provides a practical implementation advantage when an operator has 20 MHz of spectrum. Scheduling approaches in the frequency domain can also minimize interference, and hence boost spectral efficiency.

On the uplink, however, a pure OFDMA approach results in high Peak to Average Ratio (PAR) of the signal, which compromises power efficiency and ultimately battery life. Hence, LTE uses an approach called SC-FDMA, which has some similarities with OFDMA but will have a 2 to 6 dB PAR advantage over the OFDMA method used by other technologies such as IEEE 802.16e.

What is the expected performance of OFDM as it would be implemented for LTE?

3GPP had set the performance objectives for OFDM/MIMO based LTE to have a peak data rate of 100 Mbps, an average cell rate that is three to four times the spectral efficiency of Release 6 HSDPA, and a cell edge performance that is two to three times Release 6 HSDPA performance. Evaluation results from the 3GPP LTE standardization working group have shown that these objectives can be achieved with OFDM/MIMO. The standardization working group is in transition to lock now these performance objectives and move forward with detail LTE specifications. In fact, it is expected that LTE performance will exceed the goals initially developed by 3GPP.

What makes OFDM receive signals in a less processing-intensive manner than CDMA?

To deliver competitive broadband performance equivalent to fixed broadband offerings today, a higher device bit rate is absolutely required. In CDMA, as the bit rate increases in conjunction with the higher carrier bandwidth being used, the CDMA chip rate would have to be very fast leading to very short chip duration. Short chip duration could be problematic in a macro environment where the multi-path duration could spread across hundreds of chip periods. The computing power required to implement a time domain equalizer in the receiver to remove this potential distortion would be beyond the capability of the available technology today.

On the other hand, with an OFDM time domain signal, the sub-carriers’ magnitudes and phases can easily be detected using a very simple and well understood signal processing technique based on off-the-shelf Fast Fourier Transform (FFT) algorithms. Furthermore, OFDM signals are resistive to multi-path distortion and thus, OFDM receiver does not require complex equalizer implementation. Instead, multi-path distortion is completely eliminated just by simply provisioning for a slightly longer transmission time at the end of each symbol period by repeating a portion of the transmit signal know as the Cyclic Prefix or CP.

What is the difference between OFDM and SC-FDMA, which is being considered by 3GPP for LTE (Long Term Evolution) as the air interface for the uplink?

SC-FDMA stands for Single Carrier Frequency Division Multiple Access and is chosen by 3GPP LTE for the uplink because of its power amplifier (PA) friendly characteristics. The composite signal of a SC-FDMA modulated signal has 2-3dB lower peak to average power ratio (PAR) then an OFDM modulated signal. The lower PAR requirement leads to more efficient PA usage that translates into longer battery life for the end user devices.

Using signal processing, the modulated data gets spliced up into sub-carriers and a Discrete Fourier Transform (DFT) algorithm is applied. The output of DFT sub-carriers are mapped onto a portion of the transmission band or spread across the entire transmission band with similar characteristics to an OFDM sub-carrier before re-converted back to the time domain signal for transmission with an Inverse Fast Fourier Transform (IFFT) processing. By transmitting a single carrier in an OFDM fashion, the single carrier characteristic such as lower PAR can be preserved. In addition different users can be orthogonal multiplexed onto the same IFFT transformation.

What technologies are based on OFDM/OFDMA and why?

802.16/WiMAX, Flarion Fast Low-Latency Access with Seamless Handoff OFDM (Flash OFDM), 3GPP LTE, IEEE 802.11a, IEEE 802.11g, IEEE 802.20, Third Generation Partnership Project 2 (3GPP2) UMB, 3GPP2 Enhanced Broadcast Multicast Services (EBCMCS), Digital Video Broadcasting-H (DVB-H), Forward Link Only (FLO).

OFDM/OFDMA provides an effective approach for broadcast systems, higher bandwidth radio systems, and high peak data rates in large blocks of spectrum. It also provides flexibility in the amount of spectrum used and is well-suited for systems planned for the next decade.

How are OFDM and OFDMA related to WiMAX and IEEE 802.20?

OFDM technologies occupy nomadic, fixed and one-way transmission standards, ranging from TV transmission to Wi-Fi as well as well as Fixed WiMAX and newer multicast wireless systems like Qualcomm's Forward Link Only (FLO). OFDMA, however, adds true mobility to the mix, forming the backbone of Mobile WiMAX (802.20) and the 3GPP's new standards for 3G long-term evolution (LTE).

IEEE 802.20 is a mobile-broadband specification being developed by the Mobile Broadband Wireless Access Working Group of the IEEE. Initial contributions are similar in nature to IEEE 802.16e-2005, in that they use OFDMA, specify physical layer (PHY) and Medium Access Control (MAC) networking layers, address flexible channelization to 20 MHz, and provide peak data rates of over 100 Mbps. With vendors focused heavily on LTE, UMB, and WiMAX for next-generation wireless services, it is not clear whether there is sufficient momentum in this standard to make it a viable technology. At this time, no operator has committed to the possible standard.

"Mobile WiMAX, or 802.16-2005, is really misnamed," said Mark Whitton, vice president and general manager for WiMAX at Nortel Networks. "802.16-2005 is an ideal solution for mobile, portable and fixed implementations of WiMAX, and it is essentially a superset of 802.16-2004, with significant performance advances like MIMO and scalable OFDMA."

What is Flash OFDM?

Flash OFDM is a proprietary wireless-networking technology developed by Flarion Technologies. Qualcomm purchased this company for a reported $600 to $800 million in 2006. A number of operators in Asia and Europe have trialed Flash OFDM. The first commercial network was launched in Slovakia in 2005 by T-Mobile Slovakia using frequencies released from NMT analog service in the 450 MHz band. Another deployment commitment is in Finland, where the government has granted an operating license in the 450 MHz band for a nationwide network.

Flash OFDM is based on OFDM in the 1.25 MHz radio channels. It employs frequency hopping in the tones (subchannels), which provides frequency diversity and enables 1/1 reuse. The network is all IP-based and implements voice functions using VoIP. Flarion claims typical downlink speeds of 1 to 1.5 Mbps and average uplink speeds of 300 to 500 kbps, with typical latency of less than 50 msec.

From a spectral efficiency point of view, Flash OFDM claims to achieve approximately the same downlink value as HSPA, in combination with mobile receive diversity, and approximately the same uplink value as HSUPA. Because the technology is proprietary, details are not available for an objective assessment. Although Flash OFDM has a time-to-market advantage in that its equipment is already available, it has major disadvantages in that support is available only from a limited vendor base and the technology is not based on open standards.

What is Ultra Mobile Broadband or UMB and how is it related to OFDMA?

Ultra Mobile Broadband or UMB is the CDMA-based evolution beyond EV-DO Rev A and Rev B. 3GPP2 has defined EV-DO Rev B as allowing the combination of up to 15 1.25 MHz radio channels in 20 MHz—significantly boosting peak theoretical rates to 73.5 Mbps. More likely, an operator would combine three radio channels in 5 MHz. Such an approach by itself does not increase overall capacity, but it does offer users high peak data rates. No operators have yet publicly committed to EV-DO Rev B; beyond Rev B, UMB will be based on an OFDMA approach like LTE. UMB supports radio channels from 1.25 to 20 MHz. In a 20 MHz radio channel, using 4X4 MIMO, UMB will deliver a peak data rate of 280 Mbps. UMB and LTE are being developed basically simultaneously, so it is logical to assume that both technologies will exploit the same advances in wireless technology. Both UMB and LTE are more recent than other OFDMA technologies, so it is also logical to assume that their capabilities will exceed initial OFDMA designs.

Can OFDM/MIMO be used in TDD and FDD modes?

Yes, both implementations are possible. Early WiMAX OFDM/MIMO-based implementation will use TDD even though the pre-dominant spectrum allocation around the world is for FDD systems.

How will 3GPP, 3GPP2 and other standards bodies adopt and introduce OFDM/MIMO?

3GPP2 will adopt an OFDMA/MIMO variant for its Revision C release. 3GPP LTE (Long Term Evolution) is opting for the OFDMA/MIMO in the Downlink and the more Power Amplifier (PA) friendly and battery efficient SC-FDMA/MIMO in the Uplink.

Who are the main chipset vendors developing OFDM/MIMO technology?

We expect many chip vendors to be involved in this market. OFDM/MIMO will be the baseline for all future evolutions of wireless technologies. Particularly for 3GPP LTE, OFDM/MIMO is being embraced by all the 3GPP partners and will be supported by the usual group of vendors both on the infrastructure and chipset sides. Given the popularity of OFDM/MIMO, there is the potential for leveraging an even larger economy of scale with a common OFDM/MIMO chipset across all new technologies. However, LTE OFDM/MIMO has the advantage of being the mainstream wireless technology with broad industry, governmental and user support to ensure a smooth standardized and cost effective network migration from 3G to IMT Advanced and beyond.

Will OFDM be a more cost-effective means to deliver wireless broadband services?

For large bandwidth solutions, OFDM allows simpler receivers in handsets which could lead to cheaper devices; however, it is noted that volume and scale are also key components which need to be included when considering comparisons to UMTS/HSPA solutions.

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