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One of the challenges in today’s fixed outdoor wireless market is the ability to deploy and operate wireless connectivity while maintaining good performance and delivering high throughput in various geographical terrains. This is a challenge which many operators face today, whether due to Near Line of Sight (NrLOS) constraints or multi-path delay conditions. With modulation techniques like COFDM and OFDM however, most of these challenges can be solved.
Multi-path Delay and its Effects
Multi-path delay appears where the transmitted signal experiences reflection, diffraction and scattering due to various obstacles in the signal path. These conditions cause multiple echoes of the same signal to arrive at the receiver at the same time or staggered over an interval of time. This is due to the partial blockages caused by the obstacles between the transmitter and receiver. It causes the receiver to incur an additional time in identifying the incoming signals. The effect of multi-path on wireless communication is known as Inter Symbol Interference (ISI). |
White paper contributed by Anup Mangla, Technical Marketing Manager at smartBridges |
COFDM modulation is very robust against multi-path delay.
What is OFDM?
Orthogonal Frequency Division Multiplexing (OFDM) is a communications technique that divides a communications channel into a number of equally spaced frequency bands. A subcarrier carrying a portion of the user information is transmitted in each band. Each subcarrier is orthogonal (independent of each other) with every other subcarrier.
OFDM is a multi-carrier modulation technique that is unlike other modulation techniques. In OFDM, the carriers have substantial overlap. For each single high frequency carrier used, OFDM transmits multiple high data rates signals concurrently using sub carriers. The sub-carriers are orthogonal with each other and hence do not interfere with each other.
OFDM –Basic Concepts
- Multi-carrier signaling format
- Each Carrier = low rate
- Composite signal = high rate
Orthogonal Frequency Division Multiplexing (OFDM) offers benefits over spread spectrum in channel availability and data rate. Channel availability is significant because the more non-overlapping channels that are available, the more scalable the wireless network becomes. The high data rate is accomplished by combining many lower-speed subcarriers to create one high-speed channel.
OFDM-based radios distribute the data payload among many "subcarriers", each closely spaced. For 802.11 a/g applications, there are 52 subcarriers, 48 of which are used for data transmission. The remaining four subcarriers are used for pilot tones. Thus, the data rate on each individual OFDM subcarrier is much lower than on a comparable single carrier system. This enables a significantly longer symbol period. For 802.11 a/g, the symbol duration is 4 µec, or about 44 times longer than a CCK symbol.
OFDM at work against multi-path propagation effects
OFDM pulse contains a guard interval. It can be thought of as a prefix to the symbol containing redundant information that can be discarded at the receiver without affecting the ability to correctly decode the symbol. At first glance, the underlying reason for carrying this "excess baggage" might not be apparent. In fact, the guard interval is a crucial feature of OFDM systems. The length of the guard interval is selected specifically to be longer than the delayed paths encountered.
Why OFDM? –Advantages of OFDM
- More robust against multi-path propagation effects
- Less sensitive to timing errors
- High spectral efficiency
- Very high bandwidth efficiency
Additional Benefits of COFDM over OFDM
When OFDM is used in conjunction with channel coding techniques, it is described as Coded Orthogonal Frequency Division Modulation (COFDM). As the overhead of doing this in an already digital system is low, and the gains substantial, practical OFDM systems are all actually COFDM.
Although highly complex, COFDM gives high performance under even very challenging channel conditions.
By combining the OFDM technique with error-correcting codes, adaptive equalization and reconfigurable modulation, COFDM has the following resistance properties against:
- Link dispersion
- Slowly changing phase distortion and fading
- Frequency response nulls and constant frequency interference
- Burst noise
Benefits to Wireless service Provider with COFDM
- Better coverage and penetration due to NrLOS capabilities of COFDM
- Reduced operation and installation costs resulting from faster and simpler installation that do not dictate mandatory LOS conditions
- COFDM resistance to multi-path effects enables the wireless equipment to be used for wireless coverage in urban and suburban areas
Conclusion
COFDM is the key feature for any wireless equipment that ensures the most benefits for the wireless service providers and enterprises. smartBridges Nexus series of standards-based fixed outdoor wireless platform use COFDM modulation to yield the maximum benefits of this technique.
For more details on the Nexus Platform, please click here.
