RE: stds-802-16: sub10. Proposal for the Duplexing Scheme

[Eli Pasternak <EliP@Bridgewave.com>]

When comparing TDD vs. FDD, a more thorough study is required. It may be
found that TDD is preferred in some frequency bands and for some
applications and FDD is preferred elsewhere. While TDD has the promise of
flexibility and economy, as discussed in Vicente Quilez message below, TDD
has its drawbacks, of which I am going to discuss briefly a few. 

TDD asymmetric bandwidth allocation flexibility is reduced in a multiple
cell environment, when T/R timing must be coordinated. 

TDD requires higher peak power than FDD, which is expensive at high
frequencies.

TDD needs a quiet period between transmit and receive for variance in time
of arrival of signals from terminals at different distances.

TDD is less proven in wireless broadband access and is not suitable to many
frequency bands around the wold as currently allocated and regulated.

The pros and cons of each method should be studied carefully before a
conclusion can be drawn.

Eli Pasternak, CTO
BridgeWave Communications Inc.
249 Humboldt Ct.
Sunnyvale, CA 94089
(408) 541-8633


		-----Original Message-----
		From:	vicente.quilez_sanchez@alcatel.es
[mailto:vicente.quilez_sanchez@alcatel.es]
		Sent:	Wednesday, December 22, 1999 3:09 AM
		To:	stds-802-16@ieee.org
		Subject:	stds-802-16: sub10. Proposal for the
Duplexing Scheme


		     When comparing TDD and FDD duplexing alternatives,
following 
		     considerations are relevant:
		     
		     1.  TDD allows asymmetrical traffic: When dealing with
asymmetrical 
		     traffic flow, as it is today for Internet-based data
traffic, in which 
		     the volume of information sent in the downlink
direction is much 
		     higher as compared to the corresponding volume sent in
the uplink 
		     direction, TDD allows to fully exploit the usage of the
resources 
		     (i.e. radiochannels), while in the FDD case this
asymmetry can not be 
		     matched in an efficient manner, given the inherent
symmetrical nature 
		     of FDD. This asymmetrical characteristic depends on the
type of 
		     application, but it is very significative, specially
for SoHo and 
		     Residential markets, because these market segments are
the ones which 
		     are reflected best in the client-server model
		     
		     2.  TDD provides more flexibility for spectrum usage:
Frequency 
		     spectrum is an increasingly scarce resource. More and
more often, it 
		     is difficult to obtain paired bands available for new
applications. 
		     TDD can make use of both paired and unpaired slots in
the spectrum, 
		     while FDD necessarily has to match paired bands. In
particular TDD is 
		     the chosen duplexing approach for the US MMDS band,
possibly the most 
		     important frequency band for IEEE 802.16sub10. 
		     
		     3.  TDD provides better economies: A switch is cheaper
than a 
		     duplexer, and this has had a direct impact for the cost
of the CPE 
		     equipment, the key element when computing the global
cost per user. 
		     Therefore TDD-based systems will inherently provide
cheaper solutions 
		     than FDD-based systems. Economy is always very
important, but it is 
		     particularly key for SoHo and Residential markets,
given the continuos 
		     trend of tremendous reduction costs of competing
technologies such as 
		     ADSL.
		     
		     4.  TDD allows to concentrate most of the air interface

		     related-process in the infrastructure, as channel
behaviour can be 
		     extrapolated from uplink into downlink and viceversa.
This functional 
		     partitioning open the possibility for further improving
the economies 
		     for CPE equipment
		     
		     
		     Based on the reasons above, it is proposed than IEE
802.6sub10 adopts 
		     a TDD duplexing scheme
		     
		     
		     Vicente Quilez
		     Alcatel
		     veq@alcatel.es