Multicarrier Access and Routing for Wireless Networking

A multicarrier access and routing system has been proposed for use in wireless networks. Users within each cell access a radio port (RP). All RPs are connected to a radio exchange node (REN) which routes the calls or packets. The uplink access is orthogonal multicarrier code-division multiple access (MC-CDMA) and the downlink transmission is multicarrier orthogonal code-division multiplexing (MC-OCDM). The REN contains a switch module which provides continuous routes between wireless terminals without demodulation/remodulation or channel decoding/reencoding. The switch module is nonblocking and has complexity and speed linearly proportional to its size. Also, the switch module does not introduce interference into the network. Any existing interference or noise in its input port is transferred to its output port. The input-output switch connections are assigned on demand by a control unit. A random input/output port assignment process can achieve maximum switch throughput.


INTRODUCTION
Multicarrier transmission methods have been widely accepted and used because of their advantages over singlecarrier transmission in broadband wireless links. The existing multicarrier systems however, such as the orthogonal frequency-division multiplexing (OFDM), are only defined for the physical layer, while wireless networks also require a method for multiuser access and routing. This paper focuses on the development of a multicarrier system that operates at physical, multiple access and routing level. The transmission and access is based on orthogonal multicarrier (MC) codedivision multiple access (CDMA), see [1,2,3,4,5]; while the routing scheme on code-division multiplexing [6].
The wireless network is assumed to have the configuration shown in Figure 1. It consists of a radio exchange node (REN) connected to a number of radio ports (RPs). All RPs are connected to the radio exchange node (REN) which routes packets or calls between RPs. Users within each cell access the corresponding radio port (RP) by an orthogonal multicarrier CDMA described in [5]. All uplink transmissions require synchronization. The downlink transmission is multicarrier orthogonal code-division multiplexing (OCDM) described in [4]. The REN contains the This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. switch module which routes packets and calls between radio ports without demodulation/remodulation or channel decoding/reencoding. Such a wireless network then provides a continuous mobile-to-mobile route that achieves high network throughput and spectral efficiency.
In Section 2 we present the system description and verify its functional correctness. In Section 3 we examine the network capacity and performance which includes the routing capacity and the network interference.

SYSTEM DESCRIPTION
The wireless access and routing system is shown in Figure 2. Wireless terminals within each cell access the corresponding radio port by orthogonal multicarrier code-division multiple access (MC-CDMA) [5]. The uplink transmission and reception is described below, in Section 2.1. The received signal at the REN is routed by the switch module to the destination output port. The source-destination information is supplied by the control unit. The switch module is an M-input, M-output, nonblocking, routing fabric which is described in Section 2.2. The output port signal is transmitted in the downlink and received by the wireless terminal as described in Section 2.3.

The uplink
The transmitter of wireless terminal in microcell m is shown in Figure 3. The input data stream x ( ,m) of rate R is  spread by the Hadamard sequence w with rate LR. Assuming that x ( ,m) represents a complex-valued signaling point, that is, x ( ,m) = α ( ,m) + jβ ( ,m) , the spread signal is The parallel outputs y ( ,m) n for n = 0, 1, . . . , L − 1 then enter a P/S converter where a guard time or cyclic prefix is added. The P/S converter output s ( ,m) (n) is converted into an analog signal s ( ,m) (t) which is up-converted into the uplink carrier f (m) u and then transmitted to the radio port (RP). At the REN the received signal at input port m is given by where r ( ,m) u (t) is the received uplink signal from terminal in microcell m, h ( ,m) u (t) is the impulse response of the corresponding uplink channel, and ( * ) denotes convolution (the channel is assumed noiseless at the moment). The analog signal r (m) u (t) enters the uplink receiver (or recovery circuit) shown in Figure 4, where is down-converted to baseband, digitized into signal z (m) u (n), and then decoded by the a MC-decoder (i.e., an OFDM-decoder). In the OFDMdecoder z (m) u (n) is S/P converted into L parallel data points z (m) u;n for n = 0, 1, . . . , L − 1, which enter an FFT given bȳ The post-FFT signal is given bȳ where H ( ,m) u;k is the uplink channel transfer function (CTF) of user in microcell m at subcarrier k. In the above we have used the assumption of perfect synchronization between transmitting signals in order to verify the functional correctness of the process. The L parallelZ (m) k points then enter a P/S converter, the output of which is first despread by g m,k to provide the signal The signal Z (m) u;k is then despread by the L Hadamard sequences w = [w ,0 , w ,2 , . . . , w ,L−1 ] in parallel in order to recover the data of each uplink transmission in microcell m. In particular, the output signal of despreader-1 is given by In the above we have made the assumption of frequency-flat channel, that is, H ( ,m) u for all k.

The switch module
Let G ( ,m) u denote the recovered signal of user at input port m; then There are L such signals at the output of the uplink recovery Tx .

Wireless terminals
Radio exchange node Figure 2: The wireless access and routing system.
Multicarrier encoder (by OFDM) The signals at the outputs of the destination encoders are then summed up over all channels in each port and over all ports to provide the signal Each destination port m then recovers its corresponding channels from the signal G k,n (taking it from the switch bus) by using the port decoder (shown in Figure 5), which despreads G k,n with the port destination sequence w m . The output of the port decoder-1 is then given by Hence, the signal at the output of port decoder-m (output port-m ) is given bȳ The above indicates that each output port is a sum of up to L channels which may originate in different input ports.

The downlink
The signalḠ (m ) d;k then enters the downlink transmitter (or downlink encoder) shown in Figure 6 Figure 7. The received downlink signal from the REN is given by where s (m ) d (t) is the sum of all downlink signals , that is, x ( ,m ) Multicarrier decoder (by OFDM) Figure 7: The wireless terminal receiver.
The post-FFT signalZ (m ) d;k is given bȳ where H (m ) d;k is the transfer function of the downlink channel in microcell m at subcarrier k and H ( ,m) u is the transfer function of the uplink channel of terminal in microcell m, (which has been assumed to be constant at all subcarriers). In the above we have used (8) and (12).Z (m ) d;k is then despread with the PN-sequence g m ,k to provide the signal The desired signal of a wireless terminal is then recovered by despreading Z (m ) k with the sequence w , that is, In the above we have made the assumption that the downlink channel in microcell m is frequency flat, that is, H (m ) for all k. The purpose of the above analysis is to verify the functional correctness of the process and therefore it does not include the effects of noise or interference. The effects of interference and noise are examined in the performance section.

The routing capacity
We consider a switch module with M input and M output ports, and L access channels per port. The switch will then provide a capacity of ML × ML simultaneous connections. This capacity is achieved when the REN is fully equipped. A fully equipped REN has M multicarrier uplink receivers (uplink recovery circuits) and M multicarrier downlink transmitters. The switch module has ML destination encoders in the input (L in each input) and L port decoders at the output (L in each output port). Therefore the required circuitry of the switch module is linearly proportional to the its size M (this may be compared to a crossbar switch that has M 2 crosspoints). Given the above assumptions the switch fabric is nonblocking for any incoming call to an input port. That is, there is always a connection available to a destination output. The number of active calls at an input port i or output port j must be less than L, that is, M j=0 t i j ≤ L and M i=0 t i j ≤ L, where t i j is the number of calls between i and j. A call may be blocked by the input-and/or output-port capacity limit L.
The input-output connections are assigned on demand by the control unit (CU). That is, the CU receives an inputoutput call request via a demand or control channel and makes the requested connection in the switch module which is used to route the call. The assignment input-output connection in the switch module by the CU is made upon availability (at random) without rearranging the on-going calls. This simple approach is shown to achieve maximum switch throughput for the type of code-multiplexed switch module presented here [6]. This is not the case in time-multiplexed switching which requires more complex routing control algorithms to achieve maximum throughput [7].
The speed or the clock rate of and M × M switch module is M times the rate of the incoming signal. This is MLR; where L is the number of access channels per port, and R is the symbol rate per access channel (when there is no demodulation, i.e., no phase detection and symbol recovery at the REN). If we consider demodulation of M-ary symbols at the REN, the speed of the switch will increase by a factor log 2 M (i.e., (log 2 M)ML).

The network interference
Let n (m) u;k represent the sum of uplink multiple access interference (MAI) and AWGN of cell m in frequency bin k, that is, where I (m) u;k represent the intercell interference. The intracell interference is zero if we assume all uplink transmissions are perfectly synchronized. The received uplink signal then is The signal at the output of the uplink recovery circuit then is where The signal at the switch bus is where The downlink signal at the output port m in frequency bin k is given by where The received downlink signal of cell m in frequency bin k is The signal at the output of the wireless terminal receiver then is where I;x is due to MAI and the term N (m ) n;x is due to noise (x → u/d or d). As we observe the variance of the MAI and noise has the following terms (in the order they appear): the uplink (transferred; u/d) MAI, the downlink MAI, the cross-product of the uplink MAI and the downlink MAI, the cross-product of the uplink MAI and the downlink noise, the cross-product of the uplink noise and the downlink MAI, the uplink noise, and the downlink noise. There are seven terms of interference and noise instead of the typical two terms (MAI and AWGN) in a single-hop point-to-point system.
In the above analysis we have assumed no demodulation at the REN, that is, no phase detection and symbol recovery and no channel decoding at the REN. If we had assumed demodulation, that is, making a hard decision after MC-decoding and before MC-reencoding, that is, x ( ,m) = α ( ,m) + jβ ( ,m) → {−1, 1}, then that would effectively decouple the downlink from the uplink. In this case the interference transfer terms (cross-terms) will not appear. The speed of the switch however will increase by a factor log 2 M (i.e., (log 2 M)ML). Therefore the cross-terms appear as a result of coupling between uplink and downlink in the case of no demodulation at the REN.

The signal amplitude distribution
[−MLK j , MLK j ]; where K j is the amplitude of the input signal and L is the number of access channels per port. The input-output switch connections are assigned on demand by the control unit (upon availability of input/output ports). A random input/output port assignment process can provide maximum switch throughput.